Parasitic pressure losses in diffusers and in their downstream piping systems for fluid flow and heat transfer

Abstract

The need for precise design of piping systems and of ducts which interconnect fluid flow and heat transfer equipment of different dimensional scales depends on the availability of basic fluid flow information. One of the most ubiquitous features in a system which transports fluids is the occurrence of changes in flow cross section. In particular, enlargements of the flow cross section give rise to parasitic pressure losses both in the enlargement fitting and in the piping system that is downstream of the enlargement. The most commonly used enlargement fittings are diffusers. Although there is some information about pressure losses that are caused by flows in diffusers, it is highly contradictory and, equally significant, the available information takes no account of the heightened pressure losses in the downstream piping which are due to the highly disturbed flow that exits the diffuser. This investigation was undertaken to provide definitive information on diffuser-created parasitic pressure losses that occur in response to all possible operating conditions, including the Reynolds number at the diffuser inlet, the diffuser opening angle, and the ratio of the exit and inlet diameters of the diffuser. This information is cast in dimensionless form and is presented as incremental departures from a simply calculated baseline case. In addition, the lengths of pipe downstream of the diffuser that are required to dampen the irregularities of the diffuser exit flow are presented as a necessary prerequisite for the proper use of the incremental pressure drop results. It was found that the incremental pressure drop increases monotonically with increasing diffuser opening angle. In general, the incremental pressure drop values for a given diffuser opening angle decrease with increasing Reynolds number and tend to level off as the Reynolds number further increases.