Turbulent Forced Convection in a Plane Asymmetric Diffuser: Effect of Diffuser Angle

Simulation of two-dimensional turbulent forced convection in a plane asymmetric diffuser is performed and the effect of diffuser angle on the flow and heat transfer is reported. This geometry is common in many heat exchanging devices and the convective heat transfer in it has not been examined. The flow field in this geometry, however, has received significant attention already and the results show that the υ²–f turbulence model provides a better comparison with measured velocity distributions than the k–ε turbulence models. This improvement in predictions is due to selecting different turbulent velocity scale and time scale for the eddy viscosity than what is used in conventional two-equation turbulence models. The results show that the diffuser angle influences significantly the flow field (separation and reattachment) and consequently, it must influence significantly the heat transfer. The υ²–f type turbulence models have been shown to provide good heat transfer results for separated flow, and for that reason a k-ε-ζ (or υ²–f type) turbulence model is used in this study. FLUENT 6.2.16 is used as the platform for these simulations and User Defined Functions (UDF) are developed to incorporate this turbulence model (which is not included in the commercial version of the FLUENT code at this time) into this CFD code. The UDF for the k-ε-ζ turbulence model is validated by comparisons with available measured velocity data in an asymmetric diffuser and with available measured heat transfer and velocity data in a backward facing step flow, and with heat transfer data for a normally-impinging jet flow with very good agreement between simulated and measured results.