A procedure for predicting pressure loss coefficients of duct fittings using CFD (RP-1493)

Knowing loss coefficients of duct fittings is crucial for designing a duct network for HVAC systems. Traditionally, the coefficients have been obtained using experimental measurements according to AHSRAE Standard 120, a process which is time consuming and expensive. An alternative is to use CFD, but this has uncertainty due to the approximations used in modeling turbulence. This study first validated three turbulence models by comparing the predicted loss coefficient of an elbow with the corresponding experimental data from the literature. The Standard k-ε model and Reynolds stress model could accurately predict the loss coefficient; however, the more advanced LES model failed. The study found that the surface roughness of the straight duct connected to the elbow had a significant influence on the predicted pressure loss and that the accurate surface roughness could be determined. Then, this study applied the same procedure and turbulence models to predict the pressure loss coefficient of a lateral and a tee junction with both converging and diverging flows. The results show again that the surface roughness of the straight duct connected to the junctions was very important and that the best value could be estimated. The pressure loss coefficients predicted were accurate when compared with the experimental data available to us after the simulations. Some discrepancies between the calculated and measured results exist that could be attributed to the approximations used in the simulations or the errors in the experimental measurements.