Numerical study of pressure losses in close-coupled fittings

Commercial computational fluid dynamics (CFD) software CFX5.5 was used to study the pressure losses in close-coupled fittings. Turbulent flows in fittings with one elbow, two elbows (U-shape, Z-shape, and T-shape), and three elbows (S-shape) were studied by solving three-dimensional Reynolds-averaged Navier-Stokes (RANS) equations. The shear stress transport (SST) turbulence model provided much better results than the standard k-epsilon model in prediction of the loss coefficient in close-coupled fittings, in which flow experienced massive separation. The calculations confirmed the experimental observations: for separation distance less than 20 pipe diameters, the pressure loss coefficient was less than that of the equivalent uncoupled elbows, while for separation distance larger than 20 pipe diameters, the pressure loss coefficient was the same as that of the uncoupled fittings. The calculated velocity vectors associated with various fittings were also studied to reveal the mechanism of the pressure loss and its correlation with the fitting configurations of different separation distances.