Numerical Characterization of the Inlet Flow for Eleven Radial Turbomachines

Abstract

Successful modeling of a turbomachine stage requires attention to many details, including a realistic understanding of the inlet flow field. Various levels of modeling, from 1-D to 3-D viscous, plus various levels of measurement, from a simple inlet shroud pressure tap to complex inlet surveys, are considered in design and development work. In this study, a careful review is made of measurement and calculation options for inlet modeling. Historical practice places a static pressure tap on the shroud just upstream of the impeller leading edge for experimental characterization of centrifugal turbomachines. Previously developed statistics based meanline models rely in part on this measured data. However, the location of the tap may be vulnerable to high gradients which would decrease the dependability of the developed models. Computational Fluid Dynamics (CFD) and Multi-Stream Tube (MST) analyses were performed to test the appropriateness of the historically placed static pressure tap location and to characterize the inlet flow of typical radial flow turbomachines. Eleven ConceptsNREC machines were chosen for investigation to provide a wide variety of inlet geometric and flow conditions. The results derived from the CFD and MST analyses suggest that the historically placed static pressure tap location is an inappropriate anchor point for model development. While the focus of this work is not intended to reveal why the inlet behaves as it does, it does reveal that for a wide variety of inlet configurations and impeller sizes, the presumed inlet tap location should no longer be used in experimental work. Steep gradients in the static pressure indicate that a relatively minor movement of the static pressure tap would significantly alter the experimental measurements and generate noise in statistical modeling. While large variations in the pressure field are apparent near the impeller leading edge for all machines considered, the study results show that the flow field is uniform and very predictable when well upstream of the impeller leading edge. A point approximately 3 blade heights upstream from the impeller leading edge appears to be a sound location to anchor 1-D meanline model development, as well as for future experimental investigation.