Study to identify computational fluid dynamics models for use in determining HVAC duct fitting loss coefficients

This article presents results from a systematic study to establish whether computational fluid dynamics techniques are capable of predicting pressure drop in close-coupled five-gore elbows having nominal diameters of 203mm (8 in.) and turning radii r/D = 1.5. The close-coupled elbow combinations comprised either a Z-shape or a U-shape. In every instance the duct length separating the center-points of the elbows was systematically varied. An experimental program was likewise conducted to verify the computational fluid dynamics predictions, and data from the measurements are included. Zero-length pressure loss coefficients were predicted using five two-equations Eddy Viscosity Models including the standard k-epsilon, the Realizable k-epsilon, RNG k-epsilon, standard k-omega, and SST k-omega models, as well as the Reynolds Stress Model, and compared to the experimental data. The two-equation turbulence models predicted incorrect trends when applied to flow in U- and Z-configuration ducts. However, the Reynolds Stress Models with enhanced wall treatment was generally able to correctly predict elbow loss coefficients with less than 15% of error.