Modelling the process of superheated steam drying using computational fluid dynamics approaches

Abstract

Using steam to replace heated air for the drying of heat-sensitive materials like fruits and natural fibres could preserve their quality, conduct sterilization and prevent surface hardening problems in many real-life applications such as in food industry. Steam at superheated stage can has better dehydration property than heated air because specific heat capacity (cp) and thermal conductivity (k) of superheated steam are usually higher than air at the same temperature. Thus, using steam could make the drying process to have a higher efficiency. In addition, steam drying is a clean process and does not include any oxidation, explosion and emission of hazardous substances. These are the key factors for some industries to use steam dryers in their manufacturing plants. Lots of effort have been played onto the design of steam drying equipment for different industrial applications, however, most of the design have based upon the jet impingement approach. The research as report has focused on the pressurized steam drying method. The materials to be dried could be natural fibres like cotton and wool, and synthetic fibres like nylon and viscose. The water absorption rate would be different across different materials that always cause different drying characteristics. Aims of the research are to study fabric drying characteristics under superheated steam. The factors used for the control of the drying process could be examined through series of experiments. The studying factors have included the steam temperature, pressure, water flow velocity, water viscous resistance, material density and material porosity. Further analysis of the drying process has been carried out using Computational Fluid Dynamics (CFD) approaches that would be an alternative analytical method to simulate the process under different combination of the studying factors. In the study, a CFD computing tool, ANSYS was used to assist the simulation of coupling heat exchange between the water in the fabric material and the supplied superheated steam. Results from the CFD approach can be further used to compare with the experimental finding, and also be employed to determine the water removal behaviour at each drying cycle.