HKU Scholars Hubhttp://hub.hku.hkThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Fri, 24 Jan 2020 00:06:45 GMT2020-01-24T00:06:45Z50651- On the spreading of viscous dense liquid under surface waveshttp://hdl.handle.net/10722/32043Title: On the spreading of viscous dense liquid under surface waves
Authors: Fu, Sau-chung.; 傅秀聰
Mon, 01 Jan 2001 00:00:00 GMThttp://hdl.handle.net/10722/320432001-01-01T00:00:00Z
- One-step simulation of thermoacoustic waves in two-dimensional enclosureshttp://hdl.handle.net/10722/270363Title: One-step simulation of thermoacoustic waves in two-dimensional enclosures
Authors: Kam, E. W.S.; So, R. M.C.; Fu, S. C.
Abstract: © 2016 Elsevier Ltd This paper reports on a one-step simulation study of the generation and propagation of thermoacoustic waves in a two-dimensional enclosure using a finite-difference lattice-Boltzmann-type method (FDLBM) with a single relaxation time and one equilibrium particle distribution function (EPDF), and a direct aeroacoustic simulation (DAS) technique that solves the primitive Navier–Stokes (N–S) equations. Conventional expansion of the EPDF is not adopted; instead, it is expanded in terms of the particle velocity alone, and the expansion coefficients are determined by requiring the FDLBM to fully recover the N–S equations. The expansion coefficients are found to depend on the flow and thermal properties and their nonlinear interactions. Thus formulated, physical boundary conditions can be specified for the FDLBM. This EPDF has been validated against simple aeroacoustic problems and good agreement with DAS results is obtained. In this paper, the EPDF is further validated against three thermoacoustic cases: 1) a sudden increase of temperature on the left vertical wall of the enclosure; 2) a sudden increase of temperature on the left vertical wall coupled by a sudden decrease of temperature on the right vertical wall in the enclosure; 3) a gradually heated up left vertical wall of the enclosure. Comparisons with DAS simulations show that FDLBM and DAS give essentially identical results; hence, the validity and extent of the EPDF is ascertained for the calculation of these thermoacoustic waves. The FDLBM results are also compared with known theoretical and numerical results. These known solutions are found to be less accurate compared to the FDLBM results. The discrepancy could be attributed to the partially linearized equations solved in the theoretical analysis, and the inadequacy of the numerical scheme to replicate the nonlinear effects accurately in the generation and propagation of thermoacoustic waves.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/10722/2703632016-01-01T00:00:00Z
- Stochastic finite difference lattice Boltzmann method for steady incompressible viscous flowshttp://hdl.handle.net/10722/270398Title: Stochastic finite difference lattice Boltzmann method for steady incompressible viscous flows
Authors: Fu, S. C.; So, R. M C; Leung, W. W F
Abstract: With the advent of state-of-the-art computers and their rapid availability, the time is ripe for the development of efficient uncertainty quantification (UQ) methods to reduce the complexity of numerical models used to simulate complicated systems with incomplete knowledge and data. The spectral stochastic finite element method (SSFEM) which is one of the widely used UQ methods, regards uncertainty as generating a new dimension and the solution as dependent on this dimension. A convergent expansion along the new dimension is then sought in terms of the polynomial chaos system, and the coefficients in this representation are determined through a Galerkin approach. This approach provides an accurate representation even when only a small number of terms are used in the spectral expansion; consequently, saving in computational resource can be realized compared to the Monte Carlo (MC) scheme. Recent development of a finite difference lattice Boltzmann method (FDLBM) that provides a convenient algorithm for setting the boundary condition allows the flow of Newtonian and non-Newtonian fluids, with and without external body forces to be simulated with ease. Also, the inherent compressibility effect in the conventional lattice Boltzmann method, which might produce significant errors in some incompressible flow simulations, is eliminated. As such, the FDLBM together with an efficient UQ method can be used to treat incompressible flows with built in uncertainty, such as blood flow in stenosed arteries. The objective of this paper is to develop a stochastic numerical solver for steady incompressible viscous flows by combining the FDLBM with a SSFEM. Validation against MC solutions of channel/Couette, driven cavity, and sudden expansion flows are carried out. © 2010 Elsevier Inc.
Fri, 01 Jan 2010 00:00:00 GMThttp://hdl.handle.net/10722/2703982010-01-01T00:00:00Z
- 交错角与半径比对组合型垂直轴风轮影响仿真http://hdl.handle.net/10722/270370Title: 交错角与半径比对组合型垂直轴风轮影响仿真; Numerical simulation study of impact of stagger angle and radius ratio on performance of combined type VAWT
Authors: Fu, Xiucong; Liang, Xiaoting; Ou, Baoxing; Wu, Chili; Zhao, Ruheng; Li, Yuepeng; Pi, Kaihong; Xiao, Shuai
Abstract: Darrieus-Savonius组合型垂直风轮可有效改善单一型垂直轴风力机性能。为探究半径比和交错角对组合风轮性能的影响和其最优参数组合,对多组半径比和交错角参数组合下的组合风轮的气动特性进行仿真研究。数值模拟计算基于二维非定常不可压缩流体,采用Realizable k-ε湍流模型。结果表明,半径比越大,周期内输出的正力矩值(力矩值≥0)所占比例越大;在不同的半径比下,交错角存在最优值使得组合风轮运行在高尖速比区域,并功率系数最高。; © 2016, The Editorial Board of Journal of System Simulation. All right reserved. Darrieus-Savonius combined rotor could effectively improve the performance of sole type vertical axis wind turbine. In order to investigate the impacts of radius ratios and stagger angles on the combined rotor, and their best combination, aerodynamic simulations of different radius ratios and stagger angles were studied. The numerical simulation is based on 2D unsteady incompressible air model and realizable k-ε turbulence model. The result indicates that the radius ratio represents the occupied swept area of Savonius rotor, the larger radius ratio, the higher proportion of the positive momentum outputs (M≥0) in a rotation period. In addition, there exists an optimal stagger angle under a specific radius ratio making the turbine operate in higher tip speed ratio zone with maximum power coefficient.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/10722/2703702016-01-01T00:00:00Z
- A Lattice boltzmann and immersed boundary scheme for model blood flow in constricted pipes: Part 1 - Steady flowhttp://hdl.handle.net/10722/270341Title: A Lattice boltzmann and immersed boundary scheme for model blood flow in constricted pipes: Part 1 - Steady flow
Authors: Fu, S. C.; Leung, W. W.F.; So, R. M.C.
Abstract: Hemodynamics is a complex problem with several distinct characteristics; fluid is non-Newtonian, flow is pulsatile in nature, flow is three-dimensional due to cholesterol/plague built up, and blood vessel wall is elastic. In order to simulate this type of flows accurately, any proposed numerical scheme has to be able to replicate these characteristics correctly, efficiently, as well as individually and collectively. Since the equations of the finite difference lattice Boltzmann method (FDLBM) are hyperbolic, and can be solved using Cartesian grids locally, explicitly and efficiently on parallel computers, a program of study to develop a viable FDLBM numerical scheme that can mimic these characteristics individually in any model blood flow problem was initiated. The present objective is to first develop a steady FDLBM with an immersed boundary (IB) method to model blood flow in stenoic artery over a range of Reynolds numbers. The resulting equations in the FDLBM/IB numerical scheme can still be solved using Cartesian grids; thus, changing complex artery geometry can be treated without resorting to grid generation. The FDLBM/IB numerical scheme is validated against known data and is then used to study Newtonian and non-Newtonian fluid flow through constricted tubes. The investigation aims to gain insight into the constricted flow behavior and the non-Newtonian fluid effect on this behavior. © 2013 Global-Science Press.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10722/2703412013-01-01T00:00:00Z
- A Lattice boltzmann and immersed boundary scheme for model blood flow in constricted pipes: Part 2 - Pulsatile flowhttp://hdl.handle.net/10722/270340Title: A Lattice boltzmann and immersed boundary scheme for model blood flow in constricted pipes: Part 2 - Pulsatile flow
Authors: Fu, S. C.; So, R. M C; Leung, W. W F
Abstract: One viable approach to the study of haemodynamics is to numericallymodel this flow behavior in normal and stenosed arteries. The blood is either treated as Newtonian or non-Newtonian fluid and the flow is assumed to be pulsating, while the arteries can be modeled by constricted tubes with rigid or elastic wall. Such a task involves formulation and development of a numerical method that could at least handle pulsating flow of Newtonian and non-Newtonian fluid through tubes with and without constrictions where the boundary is assumed to be inelastic or elastic. As a first attempt, the present paper explores and develops a time-accurate finite difference lattice Boltzmann method (FDLBM) equipped with an immersed boundary (IB) scheme to simulate pulsating flow in constricted tube with rigid walls at different Reynolds numbers. The unsteady flow simulations using a time-accurate FDLBM/IB numerical scheme is validated against theoretical solutions and other known numerical data. In the process, the performance of the time-accurate FDLBM/IB for a model blood flow problem and the ease with which the no-slip boundary condition can be correctly implemented is successfully demonstrated. © 2013 Global-Science Press.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10722/2703402013-01-01T00:00:00Z
- A lattice Boltzmann method based numerical scheme for microchannel flowshttp://hdl.handle.net/10722/270397Title: A lattice Boltzmann method based numerical scheme for microchannel flows
Authors: Fu, S. C.; Leung, W. W.F.; So, R. M.C.
Abstract: Conventional lattice Boltzmann method (LBM) is hyperbolic and can be solved locally, explicitly, and efficiently on parallel computers. The LBM has been applied to different types of complex flows with varying degrees of success, and with increased attention focusing on microscale flows now. Due to its small scale, microchannel flows exhibit many interesting phenomena that are not observed in their macroscale counterpart. It is known that the Navier-Stokes equations can still be used to treat microchannel flows if a slip-wall boundary condition is assumed. The setting of boundary conditions in the conventional LBM has been a difficult task, and reliable boundary setting methods are limited. This paper reports on the development of a finite difference LBM (FDLBM) based numerical scheme suitable for microchannel flows to solve the modeled Boltzmann equation using a splitting technique that allows convenient application of a slip-wall boundary condition. Moreover, the fluid viscosity is accounted for as an additional term in the equilibrium particle distribution function, which offers the ability to simulate both Newtonian and non-Newtonian fluids. A two-dimensional nine-velocity lattice model is developed for the numerical simulation. Validation of the FDLBM is carried out against microchannel and microtube flows, a driven cavity flow, and a two-dimensional sudden expansion flow. Excellent agreement is obtained between numerical calculations and analytical solutions of these flows. Copyright © 2009 by ASME.
Thu, 01 Jan 2009 00:00:00 GMThttp://hdl.handle.net/10722/2703972009-01-01T00:00:00Z
- Water droplets as agents to enhance the removal efficiency of an acoustic aerosol removal systemhttp://hdl.handle.net/10722/270402Title: Water droplets as agents to enhance the removal efficiency of an acoustic aerosol removal system
Authors: Yuen, Wai T.; Fu, Sau C.; Chao, Christopher Y.H.
Abstract: © 2015 International Society of Indoor Air Quality and Climate. All rights reserved. An acoustic aerosol removal system with the addition of water droplets was examined in this paper. Previous studies used a radiating plate, attached to an ultrasonic transducer, to create stationary waves in an air duct. The acoustic effects were found to aid the aerosol deposition process. In this study, the radiating plate was subject to an inflow of water, which was vibrated and micron size water droplets were generated. It was found that these extra micro size droplet lowered submicron incense smoke particle concentration by 34%. The micron size agglomerates formed with liquid droplets with aerosols were found to be easily deposited. This process has the potential to disinfect airborne bacteria or viruses with selected antimicrobial agent as the aerosolized liquid droplets.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/10722/2704022015-01-01T00:00:00Z
- Resuspension of solid aerosols from dusty surfaces in an indoor environment using a centrifugal methodhttp://hdl.handle.net/10722/270386Title: Resuspension of solid aerosols from dusty surfaces in an indoor environment using a centrifugal method
Authors: Cheung, Yung Shan; Fu, Sau Chung; Wu, Chili; Chao, Christopher Y.H.
Abstract: © Healthy Buildings Europe 2017. All rights reserved. Resuspension of deposited particles is a pollutant source in indoor environments. Previous studies of resuspension have shown a deviation from actual real life situations and the cleanliness of the surfaces in the studies has been considered as the major factor leading to the deviation. This research studies the detachment behavior of polyethylene (PE) particles on dusty surfaces using a centrifugal method. The dusty surface was produced by settling Arizona test dust (ATD) on a cleaned Poly(methyl methacrylate) (PMMA) substrate by gravitational settling inside a deposition chamber. It was found that the magnitude of force required to remove the target PE particles from the dusty surface decreased significantly compared to that on the cleaned substrate. Also, the removal force decreased linearly with the increase of dust density.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/10722/2703862017-01-01T00:00:00Z
- Resuspension of particles on dusty surfaces in a fully developed turbulent channel flowhttp://hdl.handle.net/10722/270385Title: Resuspension of particles on dusty surfaces in a fully developed turbulent channel flow
Authors: Lee, Hau Him; Fu, Sau Chung; Qin, Daoding; Chao, Christopher Y.H.
Abstract: © Healthy Buildings Europe 2017. All rights reserved. Particle resuspension is a common occurrence in environmental processes and it is an important factor in indoor air quality. To achieve a heathy building, the indoor air quality must be maintained at a good level. So, it is crucial to investigate the resuspension of particles in different environments. Dusty surfaces are commonly encountered environments because these increase the rate of resuspension and are thus more potentially hazardous. The objective of this paper is to study resuspension of different sizes of particles on a dusty surface in a fully developed turbulent channel flow. The result shows that larger particles required smaller friction velocity to become resuspended. The estimated friction velocities, evaluated by a rolling mechanism, have good agreement with the experimental results for large particles.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/10722/2703852017-01-01T00:00:00Z
- Modeled Boltzmann equation and its application to direct aeroacoustics simulationhttp://hdl.handle.net/10722/270316Title: Modeled Boltzmann equation and its application to direct aeroacoustics simulation
Authors: Fu, S. C.; So, R. M C; Leung, R. C K
Abstract: The Bhatnagar, Gross, and Krook modeled Boltzmann equation has been applied to simulate different fluid dynamics problems with varying degrees of success. However, its application to direct aeroacoustic computation is less successful. One possible reason couldbe its inability to recover the state equation correctly for a diatomic gas and hence an inaccurate determination of the speed of sound. The present study reports on the development of an improved modeled Boltzmann equation for aeroacoustics simulation. The approach is to modify the Maxwellian distribution normally assumed for the equilibrium particle distribution function. Constraints imposed are the exact recovery of the state equation for a diatomic gas and the Euler equations without invoking the small Mach number assumption. Thus formulated, a distribution function consisting of the Maxwellian distribution plus three other terms that attempt to account for particle - particle collisions is obtained. A velocity lattice method is used to solve the improved modeled Boltzmann equation using an equivalent lattice equilibrium distribution function. The simulations are validated against benchmark aeroacoustic problems whose solutions are deduced from a direct numerical simulation of the Euler equations. The results of the improved modeled Boltzmann equation obtained using a smaller computational domain are in excellent agreement with those deduced from direct numerical simulation using a larger computational domain, thus verifying the viability and correctness of the modified equilibrium distribution function.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/10722/2703162008-01-01T00:00:00Z
- Linearized-Boltzmann-type-equation-based finite difference method for thermal incompressible flowhttp://hdl.handle.net/10722/270334Title: Linearized-Boltzmann-type-equation-based finite difference method for thermal incompressible flow
Authors: Fu, S. C.; So, R. M.C.; Leung, W. W.F.
Abstract: This study reports on further development of a finite difference method formulated on the basis of a linearized-Boltzmann-type-equation for thermal incompressible flows with external body force effect. In classical lattice Boltzmann methods, a pressure-density relation, and/or a finite Mach number, no matter how small, are required in the solution of the linearized Boltzmann-type equation, thus generating inherent compressibility error unavoidably. In the present approach, the pressure field is determined by a pressure-correction method to ensure incompressibility, thus the approach is valid for both liquid and incompressible gas flows. A variety of thermal laminar incompressible flows, such as Couette flow, falling thin liquid film flow, fluid flow through porous plates, and two- and three-dimensional natural convection flow are simulated. The results compared extremely well with analytical solutions and other known numerical simulations of the thermal incompressible flows investigated. © 2012 Elsevier Ltd.
Sun, 01 Jan 2012 00:00:00 GMThttp://hdl.handle.net/10722/2703342012-01-01T00:00:00Z
- Modeled Boltzmann equation and its application to shock-capturing simulationhttp://hdl.handle.net/10722/270318Title: Modeled Boltzmann equation and its application to shock-capturing simulation
Authors: So, R. M C; Leung, R. C K; Fu, S. C.
Abstract: A modified equilibrium distribution function for the Bhatnagar-Gross-Krook- type modeled Boltzmann equation has recently been proposed. The function was deduced using acoustics scaling to normalize the equation and allowed a correct recovery of similarly normalized Euler equations. It is a combination of a Maxwellian distribution plus three other terms that are moments of particle velocity. The lattice counterpart of the modified equilibrium distribution function also led to an exact recovery of the Euler equations; therefore, there is no Mach number limitation in the entire approach. This lattice counterpart was able to replicate aeroacoustics problems involving vorticity-acoustic and entropy-acoustic interactions correctly, and the simulations were carried out using a finite difference lattice Boltzmann method employing only a two-dimensional, nine-velocity lattice. Thus formulated, the numerical scheme has no arbitrary constants and all calculations were carried out using one single relaxation time and a set of constants derived from the analysis. This paper investigates the validity and extent of the formulation to capture shocks and resolve contact discontinuity and expansion waves in one- and two-dimensional Riemann problems. The simulations are carried out using the same two-dimensional, nine-velocity lattice, and identical set of constants and relaxation time; they are compared with theoretical results and those obtained by solving the Euler equations directly using Harten's first-order numerical scheme. Good agreement is obtained for all test cases. However, the modified equilibrium distribution function is not suitable for shock structure simulation; for that, an exact recovery of the Navier-Stokes equations is required.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/10722/2703182008-01-01T00:00:00Z
- A lattice Boltzmann method based numerical scheme for microchannel flowshttp://hdl.handle.net/10722/270320Title: A lattice Boltzmann method based numerical scheme for microchannel flows
Authors: Fu, S. C.; Leung, W. W.F.; So, R. M.C.
Abstract: Lattice Boltzmann method (LBM) has been recently developed into an alternative and promising numerical scheme for modeling fluid physics and fluid flows. The equation is hyperbolic and can be solved locally, explicitly, and efficiently on parallel computers. LBM has been applied to different types of complex flows with varying degree of success, but rarely to micro-scale flow. Due to its small scale, micro-channel flow exhibits many interesting phenomena that are not observed in its macro-scale counterpart. It is known that the Navier-Stokes equations can still be used to treat micro-channel flows if a slip wall boundary condition is assumed. The setting of boundary conditions in LBM has been a difficult task, and reliable boundary setting methods are limited. This paper reports on the development of an algorithm to solve the Boltzmann equation with a splitting method that allows the application of a slip wall boundary condition. Moreover, the fluid viscosity is accounted for as an additional term in the equilibrium particle distribution function, which offers the ability to simulate both Newtonian and non-Newtonian fluids. An LBM based numerical scheme, which is suitable for micro-channel flows, is proposed. A two-dimensional nine-velocity lattice model is developed for the numerical simulation. Validation of the numerical scheme is carried out against micro-channel, micro-tube and driven cavity flows, and excellent agreement is obtained between numerical calculations and analytical solutions of these flows. Copyright © 2008 by ASME.
Thu, 01 Jan 2009 00:00:00 GMThttp://hdl.handle.net/10722/2703202009-01-01T00:00:00Z
- Modeled lattice boltzmann equation and the constant-density assumptionhttp://hdl.handle.net/10722/270321Title: Modeled lattice boltzmann equation and the constant-density assumption
Authors: Fu, S. C.; So, R. M.C.
Abstract: An approach is proposed to recover the incompressible NS equation based on a constant-density assumption. It is observed that the recovered NS equation is identical to the continuum equation. The constant-density assumption can be used to evaluate the pressure and there is no need to postulate a density distribution function or to assume that density is related to density. A splitting method similar to Toro's proposal is used to solve the Lattice-Boltzmann Equation (LBE) where simulations are validated against 2-D unsteady flow problems. The starting point of the present formulation is the unsteady incompressible isothermal NS equation. The objective of the approach is to recover the first and second equation from a BGK-type modeled BE.
Thu, 01 Jan 2009 00:00:00 GMThttp://hdl.handle.net/10722/2703212009-01-01T00:00:00Z
- Finite difference lattice boltzmann method applied to acoustic-scattering problemshttp://hdl.handle.net/10722/270322Title: Finite difference lattice boltzmann method applied to acoustic-scattering problems
Authors: Kam, E. W.S.; So, R. M.C.; Fu, S. C.; Leung, R. C.K.
Abstract: This paper reports on an attempt to simulate acoustic waves scattering using a finite-difference lattice Boltzmann method based on an alternative lattice equilibrium particle distribution function constructed for compressible thermal fluids. The studies focus on acoustics scattering by a zero-circulation vortex and by an isolated thermal source with no heat gain/loss. Two limiting cases of each type of scattering are examined; one is the case of an incoming acoustic wave with a short wavelength, and the other has a relatively long wavelength compared with the characteristic dimension of the obstacle. These scattering problems have been treated previously using a conventional lattice Boltzmann method and a gas-kinetic scheme. The results showed that these methods were only able to simulate the short wavelength limit case with fair accuracy for the two types of acoustics scattering considered. Because the present approach is able to recover the compressible Navier-Stokes equations with correct fluid properties, the finite-difference solution of the proposed alternative modeled lattice Boltzmann equation allows the limiting cases of the acoustics scattering problems to be calculated without numerical instability. The results thus obtained are in agreement either with analysis or with results obtained from direct aeroacoustics simulations employing the compressible Navier-Stokes equations. Copyright © 2009. by E.W. S. Kam.
Fri, 01 Jan 2010 00:00:00 GMThttp://hdl.handle.net/10722/2703222010-01-01T00:00:00Z
- Finite difference lattice boltzmann method for compressible thermal fluidshttp://hdl.handle.net/10722/270324Title: Finite difference lattice boltzmann method for compressible thermal fluids
Authors: So, R. M.C.; Fu, S. C.; Leung, R. C.K.
Abstract: A finite difference lattice Boltzmann method based on the Bhatnagar-Gross-Krook-type modeled Boltzmann equation is proposed. The method relies on a different lattice equilibrium particle distribution function and the use of a splitting method to solve the modeled lattice Boltzmann equation. The splitting technique permits the boundary conditions for the lattice Boltzmann equation to be set as conveniently as those required for the finite difference solution of the Navier-Stokes equations. It is shown that the compressible Navier-Stokes equation can be recovered fully from this approach; however, the formulation requires the solution of a Poisson equation governing a secondorder tensor. Thus constructed, the method has no arbitrary constants. The proposed method is used to simulate thermal Couette flow, aeroacoustics, and shock structures with an extended thermodynamics model. The simulations are carried out using a high-order finite difference scheme with a two-dimensional, nine-velocity lattice. All simulations are performed using a single relaxation time and a set of constants deduced from the derivation. It is found that the finite difference lattice Boltzmann method is able to correctly replicate viscous effects in thermal Couette flows, aeroacoustics, and shock structures. The solutions obtained are identical either to analytical results, or obtained by solving the compressible Navier-Stokes equations using a direct numerical simulation technique.
Fri, 01 Jan 2010 00:00:00 GMThttp://hdl.handle.net/10722/2703242010-01-01T00:00:00Z
- A discrete flux scheme for aerodynamic and hydrodynamic flowshttp://hdl.handle.net/10722/270327Title: A discrete flux scheme for aerodynamic and hydrodynamic flows
Authors: Fu, S. C.; So, R. M.C.; Leung, W. W.F.
Abstract: The objective of this paper is to seek an alternative to the numerical simulation of the Navier-Stokes equations by a method similar to solving the BGK-type modeled lattice Boltzmann equation. The proposed method is valid for both gas and liquid flows. A discrete flux scheme (DFS) is used to derive the governing equations for two distribution functions; one for mass and another for thermal energy. These equations are derived by considering an infinitesimally small control volume with a velocity lattice representation for the distribution functions. The zero-order moment equation of the mass distribution function is used to recover the continuity equation, while the first-order moment equation recovers the linear momentum equation. The recovered equations are correct to the first order of the Knudsen number (Kn); thus, satisfying the continuum assumption. Similarly, the zero-order moment equation of the thermal energy distribution function is used to recover the thermal energy equation. For aerodynamic flows, it is shown that the finite difference solution of the DFS is equivalent to solving the lattice Boltzmann equation (LBE) with a BGK-type model and a specified equation of state. Thus formulated, the DFS can be used to simulate a variety of aerodynamic and hydrodynamic flows. Examples of classical aeroacoustics, compressible flow with shocks, incompressible isothermal and non-isothermal Couette flows, stratified flow in a cavity, and double diffusive flow inside a rectangle are used to demonstrate the validity and extent of the DFS. Very good to excellent agreement with known analytical and/or numerical solutions is obtained; thus lending evidence to the DFS approach as an alternative to solving the Navier-Stokes equations for fluid flow simulations. © 2011 Global-Science Press.
Sat, 01 Jan 2011 00:00:00 GMThttp://hdl.handle.net/10722/2703272011-01-01T00:00:00Z
- Analysis of ultrafine particle emission and sink characteristics from side-stream cigarette smokehttp://hdl.handle.net/10722/265111Title: Analysis of ultrafine particle emission and sink characteristics from side-stream cigarette smoke; 香烟侧流烟中超细颗粒物释放及衰减特性
Authors: Wu, CL; Zhang, CH; Guo, CH; Fu, SC; Chao, YHC
Abstract: 超细颗粒物（Ultrafine Particles，UFPs）能够进入人体肺泡组织进而危害人体健康，为了解香烟侧流烟中UFPs数浓度和表面积浓度释放及衰减特性，结合环境舱和现场实验结果对不同品牌香烟颗粒进行了分析.结果表明：燃烧过程中UFPs数浓度和表面积浓度峰值粒径分别为47.5~66.3 nm和85.6~100.5 nm，衰减2 h后，数浓度和表面积浓度峰值粒径仍小于200 nm；不同品牌香烟侧流烟中UFPs数浓度和表面积浓度释放速率分别为4.22×1012~5.23×1012个·min-1和5.95×1010~6.90×1010 μm2·min-1，根据该释放速率模拟得到的不同换气率条件下UFPs数浓度衰减趋势与实验结果相符；通风和实验环境均会影响UFPs的衰减速率，随着换气率由0.7次·h-1增至7次·h-1，侧流烟中UFPs的衰减时间由3 h降低至40 min，吸附作用的存在亦会加快UFPs的衰减速率.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/10722/2651112018-01-01T00:00:00Z
- 實度對垂直軸風輪功率系數影響的數值模擬分析http://hdl.handle.net/10722/256311Title: 實度對垂直軸風輪功率系數影響的數值模擬分析; Effect of Solidity on the Performance of a Vertical Axis Wind Turbine by Numerical Simulation Analysis
Authors: Chen, YM; Fu, SC; Zhang, JS; Ou, BX; Wu, CL; Chao, YHC
Abstract: 垂直軸風輪是風能轉換的主要設備之一,CFD方法是對其復雜繞流場進行數值模擬的有效手段。實度是影響垂直軸風輪性能的主要參數之一,由葉片數、翼型弦長及轉子半徑三個參數共同決定。為研究不同參數組合工況下垂直軸風輪的氣動力學行為,采用SST-DDES方法對垂直軸風輪二維模型進行了數值模擬,并與已有研究結果對比分析,評估了SST-DDES預測垂直軸風輪氣動性能的準確性。其次,對模擬結果有關鍵影響的計算域、網格精度以及時間步長進行了無關性研究,減少計算資源消耗并增進數值模擬的可靠性。最后,對多組不同實度的工況進行數值模擬,得到每種工況下功率系數與尖速比的關系曲線。結果表明,風輪實度在0.2~0.5之間時其功率系數峰值最大;兩葉片風輪比多葉片風輪具有更高的功率系數峰值和更寬的有效尖速比范圍;增大風輪的半徑可顯著提高風輪的功率峰值系數;風輪實度及半徑一定時,不同葉片數和弦長組合具有相似的功率系數曲線。
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/10722/2563112017-01-01T00:00:00Z
- A computational study of the effects of the radius ratio and attachment angle on the performance of a Darrieus-Savonius combined wind turbinehttp://hdl.handle.net/10722/255991Title: A computational study of the effects of the radius ratio and attachment angle on the performance of a Darrieus-Savonius combined wind turbine
Authors: Liang, Xiaoting; Fu, Sauchung; Ou, Baoxing; Wu, Chili; Chao, Christopher Y.H.; Pi, Kaihong
Abstract: © 2017 Elsevier Ltd Small wind turbines as energy saving devices can be applied in many fields and are in great demand. There are two common types of rotors in wind turbines. Savonius rotors require a low torque for starting, but their efficiency is low. Darrieus rotors have high efficiency, but they are difficult to start up. A Darrieus-Savonius combined rotor aiming at a high aerodynamic performance with a low start-up requirement has drawn the attention of many researchers. The radius ratio and the attachment angle between the two types of rotors are important factors, but have not been studied systematically. In this paper, the effects of these two parameters on the efficiency and starting performance are investigated by a computational fluid dynamics approach. Steady simulations solving the Reynolds averaged Navier-Stokes equations with realizable k-ε turbulence model were conducted to obtain the static torque. Unsteady simulations were performed to calculate the power coefficient of the combined rotor at different tip-speed-ratios. A number of configurations were tested and the optimum attachment angle and radius ratio for the combined rotor was obtained with a maximum power coefficient of 0.363 and required starting torque below 0.1 Nm at a wind speed of 2 m/s.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/10722/2559912017-01-01T00:00:00Z
- Airborne particles in indoor environment of homes, schools, offices and aged care facilities: The main routes of exposurehttp://hdl.handle.net/10722/255995Title: Airborne particles in indoor environment of homes, schools, offices and aged care facilities: The main routes of exposure
Authors: Morawska, L.; Ayoko, G. A.; Bae, G. N.; Buonanno, G.; Chao, C. Y.H.; Clifford, S.; Fu, S. C.; Hänninen, O.; He, C.; Isaxon, C.; Mazaheri, M.; Salthammer, T.; Waring, M. S.; Wierzbicka, A.
Abstract: © 2017 The Authors It has been shown that the exposure to airborne particulate matter is one of the most significant environmental risks people face. Since indoor environment is where people spend the majority of time, in order to protect against this risk, the origin of the particles needs to be understood: do they come from indoor, outdoor sources or both? Further, this question needs to be answered separately for each of the PM mass/number size fractions, as they originate from different sources. Numerous studies have been conducted for specific indoor environments or under specific setting. Here our aim was to go beyond the specifics of individual studies, and to explore, based on pooled data from the literature, whether there are generalizable trends in routes of exposure at homes, schools and day cares, offices and aged care facilities. To do this, we quantified the overall 24 h and occupancy weighted means of PM10, PM2.5and PN - particle number concentration. Based on this, we developed a summary of the indoor versus outdoor origin of indoor particles and compared the means to the WHO guidelines (for PM10and PM2.5) and to the typical levels reported for urban environments (PN). We showed that the main origins of particle metrics differ from one type of indoor environment to another. For homes, outdoor air is the main origin of PM10and PM2.5but PN originate from indoor sources; for schools and day cares, outdoor air is the source of PN while PM10and PM2.5have indoor sources; and for offices, outdoor air is the source of all three particle size fractions. While each individual building is different, leading to differences in exposure and ideally necessitating its own assessment (which is very rarely done), our findings point to the existence of generalizable trends for the main types of indoor environments where people spend time, and therefore to the type of prevention measures which need to be considered in general for these environments.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/10722/2559952017-01-01T00:00:00Z
- A semi-Analytical model for the thermal conductivity of nanofluids and determination of the nanolayer thicknesshttp://hdl.handle.net/10722/255939Title: A semi-Analytical model for the thermal conductivity of nanofluids and determination of the nanolayer thickness
Authors: Tso, C. Y.; Fu, S. C.; Chao, Christopher Y.H.
Abstract: Nanofluid shows a huge potential to be the next-generation heat transfer fluid since the nanoparticles can suspend in the base fluids for a long time and the thermal conductivity of the nanofluid can be far above those of convectional solid-liquid suspension. It has long been known that liquid molecules close to a solid surface can form a layer which is solid-like in structure, but little is known about the connection between this layer and the thermal properties of the suspension. In this study, a semi-analytical model for calculating the enhanced thermal conductivity of nanofluids is derived from the steady heat conduction equation in spherical coordinates. The effects of nanolayer thickness, nanoparticle size, volume fraction, thermal conductivity of nanoparticles and base fluid are discussed. A linear thermal conductivity profile inside the nanolayer is considered in the present model. The proposed model, while investigating the impact of the interfacial nanolayer on the effective thermal conductivity of nanofluids, provides an equation to determine its nanolayer thickness for different types of nanofluids. Hence, different relationships between the nanolayer thickness and the nanoparticle size are found for each type of nanofluid. Moreover, based on the present model's prediction, it is found that the effective thermal conductivities of nanofluids show the same result as the Maxwell model when the nanolayer thickness value approaches to zero. Lastly, the effective thermal conductivities of different types of nanofluids calculated by the present model is in good agreement with the experimental results and the prediction is much better than the Maxwell model and Bruggeman model. © 2013 Elsevier Ltd. All rights reserved.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/10722/2559392014-01-01T00:00:00Z
- A numerical investigation of effects of a moving operator on airborne contamination removal in a cleanroomhttp://hdl.handle.net/10722/255963Title: A numerical investigation of effects of a moving operator on airborne contamination removal in a cleanroom
Authors: Cherniakov, Evgeny; Fu, Sauchung; Sze To, Ginnam; Chao, Christopher Yh
Abstract: This study deals with the influence of inlet airflow velocity on airflow distribution in a unidirectional type cleanroom taking into account a moving operator. Computational fluid dynamics was employed. The standard k-ε turbulence model was used to resolve the flow field and the trajectory of contaminants which were represented by a large number of particles was predicted by a Lagrangian approach. Dynamic meshes were applied to simulate the movement of the moving operator. A characteristic time to quantify the stabilization of the airflow was introduced. Local turbulence and recirculation zones were observed around the operator and the worktable due to the movement of the operator, which may trap contaminants and affect the cleanliness of the cleanroom. The contamination removal efficiency can be improved by increasing the airflow velocity from 0.2 m/s to 0.5 m/s, but the improvement becomes insignificant when the velocity is over 0.6 m/s.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/10722/2559632014-01-01T00:00:00Z
- An energy efficient air filtration technique with acoustic radiation force and acoustic streaminghttp://hdl.handle.net/10722/255964Title: An energy efficient air filtration technique with acoustic radiation force and acoustic streaming
Authors: Yuen, Wai T.; Fu, Sau C.; Chao, Christopher Y H
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/10722/2559642014-01-01T00:00:00Z
- Differential gene expression in Escherichia coli during aerosolization from liquid suspensionhttp://hdl.handle.net/10722/256005Title: Differential gene expression in Escherichia coli during aerosolization from liquid suspension
Authors: Ng, Tsz Wai; Ip, Margaret; Chao, Christopher Y.H.; Tang, Julian Wei; Lai, Keng Po; Fu, Sau Chung; Leung, Wing Tong; Lai, Ka Man
Abstract: © 2018, Springer-Verlag GmbH Germany, part of Springer Nature. Comparative transcriptome analysis was used to determine the differentially expressed genes in Escherichia coli during aerosolization from liquid suspension. Isogenic mutant studies were then used to examine the potential part played by some of these genes in bacterial survival in the air. Bioaerosols were sampled after 3 min of nebulization, which aerosolized the bacteria from the liquid suspension to an aerosol chamber (A0), and after further 30 min of airborne suspension in the chamber (A30). Bacteria at A0 showed 65 differentially expressed genes (30 downregulated and 35 upregulated) as compared to the original bacteria in the nebulizer. Droplet evaporation models predicted a drop in temperature in the bioaerosols, which coincides with the change in the expression of cold shock protein genes—cspB and cspG in the bacteria. The most notable group of differentially expressed genes was sorbitol transport and metabolism genes (srlABDEMR). Other genes associated with osmotic stress, nutrient limitation, DNA damage, and other stresses were differentially expressed in the bacteria at A0. After further airborne suspension, one gene (ypfM, which encodes a hypothetical protein with unknown function) was downregulated in the bacteria at A30 as compared to those at A0. Finally, isogenic mutants with either the dps or srlA gene deleted (both genes were upregulated at A0) had lower survival than the parental strain, which is a sign of their potential ability to protect the bacteria in the air.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/10722/2560052018-01-01T00:00:00Z
- Comparison of the resuspension behavior between liquid and solid aerosolshttp://hdl.handle.net/10722/255937Title: Comparison of the resuspension behavior between liquid and solid aerosols
Authors: Leung, W. T.; Fu, S. C.; Sze To, G. N.; Chao, C. Y.H.
Abstract: Resuspension of an aerosol from solid surfaces is an important phenomenon. The resuspension behaviors of solid aerosols and liquid aerosols are not necessarily the same. A whole solid particle detaches from the surface when the removal force is sufficient, while a portion of a droplet may detach even if the removal force is insufficient to detach the whole droplet. The objective of this article is to compare the resuspension behaviors between liquid and solid aerosols from a solid surface. Polystyrene particles and glycerol in micron sizes were generated and deposited on substrates. Two types of experiments, centrifugal detachment and vibrational resuspension, were carried out. In the centrifuge experiment, a constant removal force field was provided. Larger droplets split into smaller portions during detachment. In terms of the fraction remaining, the adhesion of the liquid aerosol has the same order of magnitude to that of the solid particle. A theoretical analysis analogous to the case of pendent drop was carried out, and the theoretical prediction agreed well with the experimental result. In the vibration experiment, a sinusoidal force field was applied. A same fraction of the solid particle detached with a much smaller force in vibration experiment than in the centrifuge experiment, whereas no resuspension was observed for liquid droplets. The adhesive forces of the liquid and solid aerosols have the same order of magnitude in the centrifuge case, but in the vibration case their adhesive forces have much greater difference. It poses a necessity for further investigation. Copyright © American Association for Aerosol Research.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10722/2559372013-01-01T00:00:00Z
- Detachment of droplets by air jet impingementhttp://hdl.handle.net/10722/255984Title: Detachment of droplets by air jet impingement
Authors: Leung, W. T.; Fu, S. C.; Chao, Christopher Y.H.
Abstract: © 2017 American Association for Aerosol Research. Surface cleaning using air jets is an appropriate method to remove particles from surfaces especially when cleaning by mechanical methods is not suitable. The detachment behavior of droplets using an air jet is not necessarily the same as solid particles and there is a lack of studies regarding this behavior. In this article, the detachment of droplets on a plastic substrate by air jet impingement was investigated experimentally. Droplets of two different size ranges were impinged by an air jet with different impinging angles. For micrometer-sized droplets, a smaller horizontal velocity was required to detach large droplets. Moreover, the horizontal velocity required to detach 50% number fraction of droplets decreased when the air jet impinging angle increased. Millimeter-sized droplets split into many portions. Most portions remained on the substrate and only a few were resuspended. The remaining portions were distributed in a fan shape, with larger droplets traveling further on the substrate. A linear lower bound of traveled distance was observed. Due to the splitting and the small fraction of resuspension, it should not be expected that air jet cleaning of droplets is the same as that for solid particles. Copyright © 2017 American Association for Aerosol Research.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/10722/2559842017-01-01T00:00:00Z
- Detachment of droplets in a fully developed turbulent channel flowhttp://hdl.handle.net/10722/255965Title: Detachment of droplets in a fully developed turbulent channel flow
Authors: Fu, S. C.; Leung, W. T.; Chao, Christopher Y.H.
Abstract: Liquid aerosols deform and detach from solid surfaces under an external force. It is a familiar phenomenon in many engineering applications. This article experimentally investigates the deformation and detachment of liquid droplets on three different solid surfaces in a fully developed turbulent channel flow. It is shown that the droplets either are compressed or elongate under the turbulent flow. The elongation of the droplet due to the turbulent flow is measured and presented. When the friction velocity of the flow exceeds a critical value, the droplets slide along the surface. The critical friction velocity is found empirically to be inversely proportional to the square root of the contact diameter. The sliding velocity after detachment is also reported. It has been observed by many researchers that, when the external force is gravity or a simple shear flow, the retention force of the droplet is proportional to the difference between the cosines of the receding and advancing contact angles. As the shape of a deformed droplet is much more complex under a turbulent flow, this article discusses the applicability of the same relation to the turbulent channel flow.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/10722/2559652014-01-01T00:00:00Z
- Detachment of droplets from surfaces due to turbulent flowhttp://hdl.handle.net/10722/255962Title: Detachment of droplets from surfaces due to turbulent flow
Authors: Leung, Wingtong; Fu, Sauchung; Chao, Christopher Y.H.
Abstract: Detachment of droplets from surfaces due to turbulent flow was studied experimentally. A wind tunnel generating a fully developed channel flow was constructed. Droplets on three different materials were tested in the wind tunnel. With plastic and glass substrates, the whole droplet moved when the velocity of the air current exceeded a critical value. For a stainless steel substrate, the droplet split into portions; one portion migrated and the other portion stayed at the initial position. When a droplet came to the end of the three tested substrates and reached the edge of a step, it moved along the step. Under the tested flow condition, all droplets either remained in the place or slid on the substrate, but no resuspension to the airborne state was observed.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/10722/2559622014-01-01T00:00:00Z
- Effect of human movement on airborne disease transmission in an airplane cabin: Study using numerical modeling and quantitative risk analysishttp://hdl.handle.net/10722/255955Title: Effect of human movement on airborne disease transmission in an airplane cabin: Study using numerical modeling and quantitative risk analysis
Authors: Han, Zhuyang; Sze To, Gin N.; Fu, Sau C.; Chao, Christopher Yu Hang; Weng, Wenguo; Huang, Quanyi
Abstract: © 2014 Han et al. Background: Airborne transmission of respiratory infectious disease in indoor environment (e.g. airplane cabin, conference room, hospital, isolated room and inpatient ward) may cause outbreaks of infectious diseases, which may lead to many infection cases and significantly influences on the public health. This issue has received more and more attentions from academics. This work investigates the influence of human movement on the airborne transmission of respiratory infectious diseases in an airplane cabin by using an accurate human model in numerical simulation and comparing the influences of different human movement behaviors on disease transmission. Methods: The Eulerian-Lagrangian approach is adopted to simulate the dispersion and deposition of the expiratory aerosols. The dose-response model is used to assess the infection risks of the occupants. The likelihood analysis is performed as a hypothesis test on the input parameters and different human movement pattern assumptions. An in-flight SARS outbreak case is used for investigation. A moving person with different moving speeds is simulated to represent the movement behaviors. A digital human model was used to represent the detailed profile of the occupants, which was obtained by scanning a real thermal manikin using the 3D laser scanning system. Results: The analysis results indicate that human movement can strengthen the downward transport of the aerosols, significantly reduce the overall deposition and removal rate of the suspended aerosols and increase the average infection risk in the cabin. The likelihood estimation result shows that the risk assessment results better fit the outcome of the outbreak case when the movements of the seated passengers are considered. The intake fraction of the moving person is significantly higher than most of the seated passengers. Conclusions: The infection risk distribution in the airplane cabin highly depends on the movement behaviors of the passengers and the index patient. The walking activities of the crew members and the seated passengers can significantly increase their personal infection risks. Taking the influence of the movement of the seated passengers and the index patient into consideration is necessary and important. For future studies, investigations on the behaviors characteristics of the passengers during flight will be useful and helpful for infection control.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/10722/2559552014-01-01T00:00:00Z
- Evaporation of Al2O3-water nanofluids in an externally micro-grooved evaporatorhttp://hdl.handle.net/10722/255981Title: Evaporation of Al2O3-water nanofluids in an externally micro-grooved evaporator
Authors: Fu, Sauchung; Tso, Chiyan; Fong, Yicksau; Chao, Christopher Y.H.
Abstract: Copyright © 2017 ASHRAE. Nanofluid is a mixture of liquid and solid phase nano-sized particles that shows a high thermal conductivity compared to its base fluid. With micro-grooves being a good method to increase the surface area for heat transfer, there is potential for nanofluid together with microgrooves to improve the performance of cooling systems. This article aims to study the effect of an externally micro-grooved surface on evaporation of aqueous nanofluids and discuss its potential application. In order to prepare a stable nanofluid, the duration time required for ultrasonication, which is a common technique in the preparation process of nanofluid, was first investigated. Next, experiments were conducted to investigate the effect of the micro-grooved surface on the evaporation rate of the nanofluids under different environmental conditions. Results show that the advantage of the enhanced thermal properties of nanofluids can only be manifested by the presence of micro-grooves. The use of a heat exchanger with nanofluids together with micro-grooves shows a better heat transfer performance than a heat exchanger without micro-grooves using water as the coolant. The enhancement is dependent on the vapor pressure and the largest improvement can be up to 44.6%.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/10722/2559812017-01-01T00:00:00Z
- Finite-difference lattice Boltzmann simulation on acoustics-induced particle depositionhttp://hdl.handle.net/10722/255972Title: Finite-difference lattice Boltzmann simulation on acoustics-induced particle deposition
Authors: Fu, Sau Chung; Yuen, Wai Tung; Wu, Chili; Chao, Christopher Yu Hang
Abstract: © 2015 Académie des sciences. Particle manipulation by acoustics has been investigated for many years. By a proper design, particle deposition can be induced by the same principle. The use of acoustics can potentially be developed into an energy-efficient technique for particle removal or filtration system as the pressure drop due to acoustic effects is low and the flow velocity is not necessary to be high. Two nonlinear acoustic effects, acoustic streaming and acoustic radiation pressure, are important. Acoustic streaming introduces vortices and stagnation points on the surface of an air duct and removes the particles by deposition. Acoustic radiation pressure causes particles to form agglomerates and enhances inertial impaction and/or gravitational sedimentation. The objective of this paper is to develop a numerical model to investigate the particle deposition induced by acoustic effects. A three-step approach is adopted and lattice Boltzamnn technique is employed as the numerical method. This is because the lattice Boltzmann equation is hyperbolic and can be solved locally, explicitly, and efficiently on parallel computers. In the first step, the acoustic field and its mean square fluctuation values are calculated. Due to the advantage of the lattice Boltzmann technique, a simple, stable and fast lattice Boltzmann method is proposed and verified. The result of the first step is input into the second step to solve for acoustic streaming. Another finite difference lattice Boltzmann method, which has been validated by a number of flows and benchmark cases in the literature, is used. The third step consists in tracking the particle's motion by a Lagrangian approach where the acoustic radiation pressure is considered. The influence of the acoustics effects on particle deposition is explained. The numerical result matches with an experiment. The model is a useful tool for optimizing the design and helps to further develop the technique.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/10722/2559722015-01-01T00:00:00Z
- Influence of sinusoidal airflow and airflow distance on human thermal response to a personalized ventilation systemhttp://hdl.handle.net/10722/256004Title: Influence of sinusoidal airflow and airflow distance on human thermal response to a personalized ventilation system
Authors: Xie, Yongxin; Fu, Sauchung; Wu, Chili; Chao, Christopher Y.H.
Abstract: © 2016, © The Author(s) 2016. Since the concept of personalized ventilation was introduced in the late 1990s, many studies on thermal comfort have been conducted and a number of parameters identified. In this research, the influence of three parameters, the airflow speed, airflow fluctuating period and a parameter which has drawn less attention in previous studies – the airflow distance between the human subject and the nozzle of the personalized ventilation device on air movement perception, thermal sensation and thermal comfort – are studied. The combinations of fluctuating period and airflow amplitude were selected based on the Power Spectrum Density method. Then 25 human subjects participated in the thermal comfort experiment, each of them underwent 54 tests of different experimental conditions and expressed their thermal feelings by completing the survey questionnaire. Our findings showed that a longer airflow distance could lead to cooler thermal sensation, but not cause any difference in thermal comfort. Changing the fluctuating period of the sinusoidal airflow from 10 s to 60 s did not cause an influence on thermal sensation, but a shorter fluctuating period could result in a higher air movement perception. When dealing with thermal comfort issues, a joint effect with airflow speed and fluctuating period occurs and this should also be considered.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/10722/2560042018-01-01T00:00:00Z
- Modeling a solar-powered double bed novel composite adsorbent (silica activated carbon/CaCl2)-water adsorption chillerhttp://hdl.handle.net/10722/255940Title: Modeling a solar-powered double bed novel composite adsorbent (silica activated carbon/CaCl2)-water adsorption chiller
Authors: Tso, Chi Yan; Fu, Sau Chung; Chao, Christopher Y H
Abstract: During the past few decades, the growing demand for air conditioning has caused a significant increase in demand for primary energy resources. Adsorption cooling system is one of the technologies which could be powered by renewable energy. This study aims to improve the performance of a solar-powered adsorption chiller by applying a novel composite adsorbent, a mixture of activated carbon, silica gel and calcium chloride. Modeling is established to investigate the cooling performance of a composite adsorbent based adsorption chiller driven by flat-type solar collectors with three different configurations of glaze: (1) single glazed cover; (2) double glazed cover and (3) transparent insulation material (TIM) cover. The simulation results show that the coefficient of performance (COP) and the specific cooling power (SCP) of the adsorption chiller depend hugely on the solar collector temperature. It is found that a double glazed cover shows the best cooling performance and 30 m2is the most optimized solar collector area. Two to three hours of pre-heating time is required to initiate the desorption process of the adsorber in a day of operation. This newly developed silica activated carbon/CaCl2composite material as adsorbent used in the adsorption chiller could achieve a high mean COPscof 0.48. Its satisfactory performance suggests that this novel composite material has a potential to be used in the adsorption chiller system even if it is powered by unstable solar energy. © 2014 Tsinghua University Press and Springer-Verlag Berlin Heidelberg.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/10722/2559402014-01-01T00:00:00Z
- Mass transport in a thin layer of Bi-viscous mud under surface waveshttp://hdl.handle.net/10722/75803Title: Mass transport in a thin layer of Bi-viscous mud under surface waves
Authors: Ng, CO; Fu, SC; Bai, YC
Abstract: The mass transport in a thin layer of non-Newtonian bed mud under surface waves is examined with a two-fluid Stokes boundary layer model. The mud is assumed to be a bi-viscous fluid, which tends to resist motion for small-applied stresses, but flows readily when the yield stress is exceeded. Asymptotic expansions suitable for shallow fluid layers are applied, and the second-order solutions for the mass transport induced by surface progressive waves are obtained numerically. It is found that the stronger the non-Newtonian behavior of the mud, the more pronounced intermittency of the flow. Consequently, the mass transport velocity is diminished in magnitude, and can even become negative (i.e., opposite to wave propagation) for a certain range of yield stress.
Tue, 01 Jan 2002 00:00:00 GMThttp://hdl.handle.net/10722/758032002-01-01T00:00:00Z
- Numerical study on merging and interaction of jet diffusion flameshttp://hdl.handle.net/10722/265274Title: Numerical study on merging and interaction of jet diffusion flames
Authors: Ho, TC; Fu, SC; Chao, YHC; Gupta, S
Abstract: A high velocity jet fire can cause catastrophic failure due to flame impingement or radiation. The scenario becomes more complicated when multiple jet fires exist following ignition of release from pressure relief valves (PRV) as the thermal effect not only distorts the individual jet flame but also changes the flame height and temperature profile and such kind of high velocity jet flames have not been studied in the past. Therefore, prediction of the flame shape including the merging and interaction of multiple jet fires is essential in risk analysis. In this paper, fire interaction of two high velocity (>10 m/s) jet fires is investigated using computational fluid dynamics (CFD) techniques. Different radiation models are analyzed and validated by experimental data from the literature. Based on the simulation result, the merging of high velocity jet fires is divided into three stages. An empirical equation considering the fire interaction for the average flame height with different release velocities and separation distance is developed. The flame height increases dramatically when the separation distance decreases resulting in a shortage of oxygen. So, part of the methane is reacted in a higher height, which explains the change in the merging flame height and temperature.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/10722/2652742018-01-01T00:00:00Z
- Particle resuspension in a wall-bounded turbulent flowhttp://hdl.handle.net/10722/255927Title: Particle resuspension in a wall-bounded turbulent flow
Authors: Fu, S. C.; Chao, C. Y H; So, R. M C; Leung, W. T.
Abstract: Resuspension is of common occurrence in a wide range of industrial and environmental processes. Excessive resuspension in these processes could have a severe impact on human safety and health. Therefore, it is necessary to develop a practical, yet reasonably accurate model to describe the resuspension phenomenon. It has been identified that rolling is the dominant mechanism for particle resuspension in the presence of an air stream, be it laminar or turbulent. Existing models predict the resuspension rate by regarding particles as being resuspended once they are set in motion; only a few of these models attempt to describe the full scenario, including rolling motion and the effect of turbulence. The objective of this paper is to propose a stochastic model to simulate the resuspension rate in the presence of a near-wall turbulent stream, and where the rolling mechanism is assumed to dominate the resuspension process. The fluctuating part of the angular velocity of a rolling particle is modeled by the Langevin equation (i.e., an Ornstein-Uhlenbeck process); thus, the overall angular velocity is modeled as a diffusion process. A free parameter of the proposed resuspension model is determined using data obtained from a Monte Carlo (MC) simulation of the problem. Once determined, the parameter is found to be universal for different materials and different sizes of particles tested. The modeling results obtained using this parameter are found to be in good agreement with experimental data, and the model performs better compared to other models. © 2013 by ASME.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10722/2559272013-01-01T00:00:00Z
- On detachment of micron droplets using a centrifugal methodhttp://hdl.handle.net/10722/256023Title: On detachment of micron droplets using a centrifugal method
Authors: Fu, S. C.; Leung, W. T.; Chao, C. Y.H.
Abstract: Liquid drops adhering to and dislodging from solid surfaces are of common occurrence in a diverse range of industrial and environmental processes. Although tremendous effort, both experimental and theoretical, has been spent on studying the fundamental mechanisms of particle resuspension, most studies concentrated on solid particles. The detachment behaviour of drops is different from that of solid particles; drops may deform and split into smaller portions during detachment and those studies are lacking in literature, especially for small droplets (in the range of micron size). This paper studies the detachment of a droplet from a plastic substrate by a centrifugal method. Monodisperse glycerol droplets in the sizes of micron ranges were generated and deposited on the substrate. Owing to the small in size, an ultracentrifuge was employed to generate the removal forces, in both normal and tangential directions. The detachment behaviours were found different in different force directions. For the normal direction, larger droplets split into smaller portions during detachment; some portions are detached and the remains form smaller droplets on the substrate. The volume fraction that remained or detached against the removal forces was determined and the result was compared with that of the solid particle. For the tangential direction, the droplet is stationary until a high enough force field is applied at which the whole droplet is detached. The phenomena are explained by the retention force model which was previously employed in a larger drop and this research shows that the same model is potential to be applicable to a small droplet in micron size. Copyright © 2013 by ASME.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10722/2560232013-01-01T00:00:00Z
- Performance analysis of a waste heat driven activated carbon based composite adsorbent - Water adsorption chiller using simulation modelhttp://hdl.handle.net/10722/255919Title: Performance analysis of a waste heat driven activated carbon based composite adsorbent - Water adsorption chiller using simulation model
Authors: Tso, C. Y.; Chao, Christopher Y.H.; Fu, S. C.
Abstract: This study aims at improving the performance of a waste heat driven adsorption chiller by applying a novel composite adsorbent which is synthesized from activated carbon impregnated by soaking in sodium silicate solution and then in calcium chloride solution. Modeling is performed to analyze the influence of the hot water inlet temperature, cooling water inlet temperature, chilled water inlet temperatures, and adsorption/desorption cycle time on the specific cooling power (SCP) and coefficient of performance (COP) of the chiller system with the composite adsorbent. The simulation calculation indicates a COP value of 0.65 with a driving source temperature of 85 °C in combination with coolant inlet and chilled water inlet temperature of 30 °C and 14 °C, respectively. The most optimum adsorption-desorption cycle time is approximately 360 s based on the performance from COP and SCP. The delivered chilled water temperature is about 9 °C under these operating conditions, achieving a SCP of 380 W/kg. © 2012 Elsevier Ltd. All rights reserved.
Sun, 01 Jan 2012 00:00:00 GMThttp://hdl.handle.net/10722/2559192012-01-01T00:00:00Z
- The effect of aerosol size distribution and concentration on the removal efficiency of an acoustic aerosol removal systemhttp://hdl.handle.net/10722/255983Title: The effect of aerosol size distribution and concentration on the removal efficiency of an acoustic aerosol removal system
Authors: Yuen, W. T.; Fu, S. C.; Chao, Christopher Y.H.
Abstract: © 2016 Noninvasive aerosol removal processes are sought after in manufacturing industries that utilize powder product recovery. Traditionally, cyclone separators are commonly deployed, however, this method is energy intensive; space demanding and inefficient for particles with a diameter less than 5 μm. While acoustically induced aerosols depositions was proposed as an alternative aerosol removal method which addressed such limitations, the underlying parameters determining the performance of such technique are yet to be explored. This study examines those underlying parameters determining the efficiency of acoustic aerosol removal, such as; the removal efficiencies, varying aerosol inputs, particle initial concentrations and size distributions. Experimental results showed that given the same initial particle number concentrations, aerosols containing two particles sizes, larger (seed) and smaller (test) particles, improved the removal efficiencies for the test particles. A higher seed to test particle concentration ratio further enhanced this effect. For particle number concentrations ranging above 2.0×104#/cm3, the increase in concentrations enhanced the removal efficiency of aerosols. Further analysis conducted by numerical simulations demonstrated that the deposition of particles depended on their sizes and their initial positions in the acoustic field. A new parameter, deposition cell ratio was proposed to quantify the deposition probability for particles at a given size and in a given domain. Results suggested that the deposition probability of a group of aerosols may be a function of the sum of the probabilities of each individual particle.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/10722/2559832017-01-01T00:00:00Z
- Study of residue patterns of aqueous nanofluid droplets with different particle sizes and concentrations on different substrateshttp://hdl.handle.net/10722/255980Title: Study of residue patterns of aqueous nanofluid droplets with different particle sizes and concentrations on different substrates
Authors: Lee, H. H.; Fu, S. C.; Tso, C. Y.; Chao, Christopher Y.H.
Abstract: © 2016 Nanofluid droplet evaporation has attracted great interest due to its applications such as in painting, coating and patterning. In most studies, either the particle size or the concentration of nanofluid is considered as a factor in the formation of the residue pattern. This work aims to investigate the effect of both particle size and concentration on the residue pattern. A comprehensive study was made of the residue patterns of Al2O3and TiO2aqueous nanofluid droplets on different substrates (i.e. glass, stainless steel and Teflon). It was found that a ring-shaped pattern was formed at low concentrations and small particle sizes, while a uniform pattern was formed at high concentrations and large particle sizes for Al2O3nanofluids. In addition, only ring-shaped residue patterns were observed for all concentrations of TiO2nanofluids. In the case of different substrates, on a material with a high contact angle with water, it was difficult to form a ring-shaped pattern. The widths of the ring-shaped pattern were analyzed as well. The results showed that the width of the ring-shaped pattern was larger for small particles. The materials of substrate and nanoparticle also influenced the width.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/10722/2559802017-01-01T00:00:00Z
- Studies on detachment behavior of micron sized droplets: A comparison between pure fluid and nanofluidhttp://hdl.handle.net/10722/255998Title: Studies on detachment behavior of micron sized droplets: A comparison between pure fluid and nanofluid
Authors: Fu, S. C.; Cheung, Y. S.; Lee, H. H.; Kwan, Joseph K.C.; Chao, Christopher Y.H.
Abstract: © 2018 American Association for Aerosol Research. Resuspension is considered as a source of indoor air pollutants. These airborne pollutants can be in the form of liquid or solid. It has been previously found that the detachment mechanism of liquid droplets is different from the solid particles on the poly(methyl methacrylate) (PMMA) surface. Liquid droplets detach by portion when they are under an increasing normal force field while droplets detach completely when under a tangential force field. In this research, droplet detachment experiments are extended to different substrate materials, which are PMMA, glass, and stainless steel by the means of centrifuge. Also, the differences in detachment between pure glycerol-water (pure fluid) and a glycerol solution with the addition of nanoparticles (nanofluid) are investigated under different substrate materials. It is found that liquid droplets, again, detach by portion under normal force for all the substrate materials. For tangential force, the droplets detach completely if the exerted force was sufficiently large and the threshold values are material dependent, which is further elaborated by retention theory. After the addition of nanoparticles, a higher removal force was required compared to the droplets of pure fluid within the same size range. Also, solid residues with a negligible amount of fluid were found on the substrate after each removal of droplets under both normal and tangential force. The involvement of nanoparticles could be the pioneer work for future studies on commonly found liquid pollutants, which are prone to be contaminated by solid particles, such as in salivary excretion. Copyright © 2018 American Association for Aerosol Research.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/10722/2559982018-01-01T00:00:00Z
- The correlation between acoustic streaming patterns and aerosol removal efficiencies in an acoustic aerosol removal systemhttp://hdl.handle.net/10722/255976Title: The correlation between acoustic streaming patterns and aerosol removal efficiencies in an acoustic aerosol removal system
Authors: Yuen, W. T.; Fu, S. C.; Chao, Christopher Y.H.
Abstract: © 2016 Taylor & Francis Group, LLC. Recently, researchers proposed that a second-order effect, acoustic streaming, induced aerosol depositions as a noninvasive aerosol removal technique. However, the acoustic streaming patterns, which determined the deposition efficiencies, were merely observed but not investigated. This article studied this correlation by both experimental and numerical methods. By keeping the sound frequency and sound pressure level constant, the acoustic streaming field was varied by the dimensions of the air duct and ultrasonic radiating plate. The numerical model was validated by the experimental results on three accounts, acoustic streaming patterns, velocity vectors, and deposition trends. Two new parameters, acoustic deposition ratio and near wall vorticity magnitude, were introduced to measure and predict the correlation between variation of the acoustic fields and deposition efficiencies. It was found that the geometry that produced a high vorticity magnitude near the deposition surfaces provided the condition to induced high occurrence for aerosols depositions (up to 3.5 times the rate caused by nature deposition). The finding in this study provided a method to quantify acoustic streaming fields, and the correlation between such quantity and the related deposition performance. This provides a guideline for designing acoustic aerosol manipulation devices, which aims to cause deposit with a noninvasive method.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/10722/2559762016-01-01T00:00:00Z
- The use of nonlinear acoustics as an energy-efficient technique for aerosol removalhttp://hdl.handle.net/10722/256024Title: The use of nonlinear acoustics as an energy-efficient technique for aerosol removal
Authors: Yuen, W. T.; Fu, S. C.; Kwan, Joseph K.C.; Chao, Christopher Y.H.
Abstract: Copyright © American Association for Aerosol Research. This article is a feasibility study on using nonlinear acoustic effects, acoustic streaming and acoustic radiation pressure, for aerosol removal in an air duct. Unlike previous research, which used acoustics solely to cause aerosol agglomeration prior to aerosol removal in traditional duct collection systems, this article considers the acoustic streaming effect, which is significant but was previously neglected. Monodispersed polystyrene spheres with diameters ranging from 0.3 to 6 μm were tested. The proposed system removed 12-20% of the submicron aerosols and 25-32% of the micron aerosols when the airflow rate was approximately 90 L/min. Acoustic streaming introduces stagnation points on the surface of the air duct and removes the aerosols by deposition. Acoustic radiation pressure causes aerosols to form agglomerates. This enhances inertial impaction and/or gravitational sedimentation, which further enhances the removal efficiency of micron aerosols. The particle-removal efficiency is proportional to the duration that the aerosols are exposed to the acoustic field. The pressure drop due to the nonlinear acoustic effects is negligible; thus, power consumption is minimal. This system has the potential to be developed into an energy-efficient technique for aerosol removal.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/10722/2560242014-01-01T00:00:00Z
- Resuspension of infectious particle: Detachment of an artificial saliva droplet from a plastic substratehttp://hdl.handle.net/10722/255936Title: Resuspension of infectious particle: Detachment of an artificial saliva droplet from a plastic substrate
Authors: Leung, W. T.; Fu, S. C.; Szeto, G. N.; Chao, Christopher Y H
Abstract: This paper experimentally studied the detachment of an artificial saliva droplet from a plastic substrate. Monodisperse glycerol droplets in micron sizes were generated and deposited on the substrates. The adhesive force between the droplets and the substrates was determined by using the centrifugal method. Normal removal force was applied to the droplets by a centrifuge and the force distributions of droplets of different sizes were obtained. Large droplets were detached from the substrate at a lower rotational speed than the smaller droplets. Unlike solid particles, larger droplets may split into smaller portions during detachment. Some portions were detached and the remaining formed smaller droplets on the substrate. The volume fraction resuspended against removal forces was determined. This study provides a basis for the prediction of the infection risk of disease transmission by airborne route which increases with the volume of infectious particles resuspended.
Sun, 01 Jan 2012 00:00:00 GMThttp://hdl.handle.net/10722/2559362012-01-01T00:00:00Z
- Modeling a novel composite adsorbent based adsorption chiller driven by solar energyhttp://hdl.handle.net/10722/255941Title: Modeling a novel composite adsorbent based adsorption chiller driven by solar energy
Authors: Tso, C. Y.; Fu, S. C.; Chao, Christopher Y H
Abstract: This paper aims to study the performance of a solar-powered adsorption chiller with a novel composite adsorbent material (silica activated carbon/CaCl2) operating during some typical months in Hong Kong. Modeling is established to investigate the cooling performance of this adsorption chiller driven by flat-type solar collectors with three different configurations of glaze: 1) single glazed cover; 2) double glazed cover and 3) transparent insulation material (TIM) cover. The simulation results show that the higher the solar collector temperature is, the better the coefficient of performance (COP) and the specific cooling power (SCP) of the adsorption chiller are. It is suggested to select a double glazed collector with a small value of the lumped capacitance for this adsorption chiller. Seasonal effects are discussed in which the solar COP achieves its highest value during autumn. However, the cooling capacities in spring, summer and autumn are similar. All in all, this newly developed composite material as adsorbent used in the adsorption chiller could achieve a mean solar COP of 0.36 and SCP of 94W/kg on a typical summer day of operation. Copyright © 2013 by ASME.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10722/2559412013-01-01T00:00:00Z
- Investigation of flame height from multiple liquefied natural gas firehttp://hdl.handle.net/10722/255982Title: Investigation of flame height from multiple liquefied natural gas fire
Authors: Ho, T. C.; Fu, S. C.; Chao, Christopher Y.H.
Abstract: © Copyright 2016 by ASME. The research of multiple liquefied natural gas fires is limited due to the unique condition of temperature and velocity. Therefore, the effect of the distance between multiple liquefied natural gas jet fires on the characteristics of the flame is investigated by numerical methods in this paper. Grid analyses have been carried out to establish sufficient resolution of the grid and to ensure the domain size is appropriate for jet fire cases. The temperature, velocity and concentration of the reactant inside the fire were calculated, and the shape of the flame was also studied. By changing the distance between the jets, the size and height of the fires are changed. Simulations of jet fires separated by different distances were carried out. Investigation of three discharge velocities (30, 40, 50 m/s) with different separation distances (0-28 m) were done. When the jets were close enough fires were observed to merge and were enhanced due to interaction. Both the flame height and the temperature increased. When the jets were separated by a significant distance, the flame height returned to the height of a single jet fire. The distance effect of multiple jet fires was investigated and reported in this paper.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/10722/2559822016-01-01T00:00:00Z
- Investigation of particle size on gasification process for solid waste treatmenthttp://hdl.handle.net/10722/255957Title: Investigation of particle size on gasification process for solid waste treatment
Authors: Ho, T. C.; Fu, S. C.; Chao, Christopher Y.H.
Abstract: Copyright © 2014 by ASME. Gasification is a technologically advanced and environmentally friendly process for solid waste treatment. The chemical reactions in the gasification process highly depend on the agents' flow rates which, due to fluid dynamics and thermodynamics, are in fact functions of particle size and structure. Therefore, in order to obtain a better prediction model, it is important to determine the effect of particle size on the operation of a gasification system. The purpose of this research is to investigate the effect of particle size of some common solid waste on the gasification process. Specimens including starch and polyethylene of different sizes are investigated experimentally. To achieve the aim, the gasification processes are monitored by a thermal gravimetric analysis system. The mass change and the heat flow are measured in real time during the reaction. Comparison between the experimental results and different gasification models are made. Based on the experimental results, the effect of particle size was studied and the importance of the porous structure was revealed. The relationship between particle size and porous structure during gasification was developed.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/10722/2559572014-01-01T00:00:00Z
- Evaporation kinetics and residue patterns of a nanofluid droplethttp://hdl.handle.net/10722/255977Title: Evaporation kinetics and residue patterns of a nanofluid droplet
Authors: Lee, H. H.; Fu, S. C.; Chao, Y. H.Christopher
Abstract: Copyright © 2015 by ASME. This paper studies how the nanoparticle size affects nanofluid droplet evaporation kinetics and residue patterns. An experiment is set up to investigate the evaporation rate of a sessile nanofluid droplet under ambient conditions using different particle sizes, with diameters of 9nm, 13nm, 20nm, 80nm and 135nm. The smaller the particle size, the higher the evaporation rate of the nanofluid droplet. After evaporation, a residue pattern is left on a substrate. The experiment shows that different particle sizes and concentrations have various residue patterns. For smaller size particles (diameters up to 13nm), a ring-shaped pattern is observed after evaporation. A uniform pattern appears for particles bigger than 13nm.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/10722/2559772015-01-01T00:00:00Z