Turbuelence Modeling and Valication to Predict Velocity Profiles in Close-Coupled Five-Gore Elbows

Abstract

This research work aims to validate advanced turbulence modeling techniques used to predict complex flows in close-coupled five-gore elbows and similar complex geometries using data from 2016 ASHRAE RP-1682 (Study to Identify CFD Models for Use in Determining HVAC Duct Fitting Loss Coefficients). The study in question conducted experimental measurements of friction factor, pressure loss coefficient, and detailed velocity profiles in two close-coupled five-gore elbows. Using this data, this research will test the validity of specific CFD models in the case of turbulent flow in a Z-shape duct. The models, namely Reynolds Stress Model, Large Eddy Simulation, ζ-f Model and Wall-Modeled Large Eddy Simulation (WMLES) are analyzed, validated and compared using experimental data from ASHRAE RP-1682 [1], [2] and [3]. The effect of separation distance (Lint/D) is also investigated; to assess and identify the capabilities and limitations of each turbulence model in predicting such complex flow; and to probe the influence of the numerical grid size and quality on the accuracy of the CFD predictions. The study is comprehensively discussing the framework of the current LES model with an eddy viscosity subgrid-scale model. The dissertation focused on the issues encountered by RSM in properly capturing flow behavior dominated by flow separations. The LES simulation has shown some limitations in the flow separation and re-attachment regions. This dissertation finds that the turbulent kinetic energy production in ζ equation of ζ-f model is reproduced much more easily and accurately than with other models. Mean velocity gradient as well as local turbulent stress terms are also much easier to resolve properly. The ζ-f model was found to be both more efficient in terms of computational power and better able to predict the mean flow velocity profile results than the RSM model, despite both models being coupled steady-state RANS models. ζ-f model also performed better in the numerical resolution of flow separation and re-attachment regions compared to the RSM model. WMLES model is employed to investigate the SGS model impact on the small eddies dissipated from the large eddies. Moreover, WMLES model produces much better results than the LES model with much less computational time, however the SGS viscosity give further undesired damping effect to the flow energy.