Study of turbulent jet in coflow : a theoretical model of vehicular plume exhaust

Abstract

Traffic exhaust is the major pollutant source in urban areas that adversely affects air quality and the living standard of stakeholders. Round free jet is an appropriate platform that helps elucidate the turbulence and transport processes in the wake of a vehicular tailpipe. There is increasing evidence that turbulence structures in the whole flow are all affected by jet initial conditions, such as Reynolds number at the jet exit. However, most of the studies focused on round free jet in the stagnant environment or with extremely low coflow strength, which cannot be used to characterize the effect of background wind. Therefore, it is necessary to study the downstream emission plume being influenced by the interaction between the vehicular exhaust jet and the ambient wind.

This research applied laboratory measurement techniques with the assistance of an axisymmetric mathematical model. In the laboratory, a 1-meter high and 1-meter wide test rack was built, on which a pipe nozzle and several measurement sensors were mounted. Pipe nozzles of different diameter were employed to control the mean velocity also. The Reynolds number at the jet exit in this study ranged from 6000 to 28000. A constant-temperature hot-wire anemometer with Xprobe and a resistance temperature detector (RTD) were installed on the test rack to detect the mean value and turbulent quantities in the wake of the jet. Due to the difficulty of simulating ambient wind in the laboratory platform, an axisymmetric computational domain was created in the cylindrical coordinates; it was solved by the standard k–ε model and was validated by lab measurement results to examine the flow characteristics in a turbulent round jet in coflow environment.

Initial conditions are very significant factors for turbulence development. The critical value of Reynolds number for mean flow field is 10,000 and is little related to the jet exit diameter. The mean velocity evolves into self-similarity faster than turbulent intensity. The critical Reynolds number for turbulent flow field is larger than the critical value of Reynolds number for mean flow field and is closely related to jet exit diameter. The relationship between critical Reynolds number for turbulent velocity field and jet exit diameter has been achieved by measurement data. The asymptotic value of turbulent intensity is also linearly related to long pipe nozzle diameter, instead of other initial conditions.

Coflow strength is another significant factor for turbulence development. The normalized axial excess velocity decay is strongly affected by coflow strength. The mean velocity decay rate is not constant in the flow (except in the near-field region). The velocity field in the far-field region is dominated by the background wind. So the effect of coflow strength on turbulence development in the axial direction varies with coflow strength. Meanwhile, the spreading rate is linearly dependent on coflow strength that means coflow strength has the same effect on the turbulence development in the radial direction. Coflow strength also inhibits the turbulent intensity along the centerline in the intermediate-field and far-field regions. With higher coflow strength, the turbulent flow gets into self-similarity more quickly.