Pressure losses and flow maldistribution in ducts with sharp bends

Pipelines and ducting systems used in process industry applications are characterized by large sizes, sharp bends and successive bends connected by short straight sections. Systematic studies of such flow situations have not been reported in the literature and the design of such ducting systems is based on rules of thumb and scaled-model testing. Taking advantage of the potential size-insensitivity of CFD-based calculations, a large number of calculations of typical flow situations has been carried out in the present study to address such issues as the optimum location of guides vanes, the influence of an upstream bend on the loss coefficient and the scaling laws for proper extrapolation of laboratory results to industrial scale applications. It is shown that the ideal radial location of a vane for a square duct is given by (RiRo )0.5 and that the influence of an upstream bend on the loss coefficient is beneficial in most cases. Thus, the calculation of bend losses treating each bend as being isolated is likely to be conservative. The velocity profile is uniformly improved by the use of vanes. The loss coefficient is not very sensitive to the fluid scaling, but the effect of wall roughness has to be considered carefully in scaled-down conditions. © 2004 Institution of Chemical Engineers.