A Comparison of the Predicted and Measured Performance of High Pressure Ratio Centrifugal Compressor Diffusers

Abstract

A novel analysis of the hub and shroud wall boundary layer growth through the diffusing system of a centrifugal compressor is proposed to model the physical processes. It is shown that the diffuser throat blockage and total pressure loss characteristics can be accurately predicted for a 6:1 PR stage. The static pressure effectiveness and stalling limit are successfully predicted qualitatively, but are underestimated and overestimated by 14 and 12 percent respectively. It is argued that diffuser performance is largely controlled by the combined effect of the boundary layer conditions on the hub and shroud walls at impeller exit and the diffusion required to the diffuser throat. For this reason, it is contended that, for best performance at high pressure ratio (≃ 12:1), impeller exit Mach number must be minimized by employing zero to negative prewhirl at impeller entry which in turn maximizes impeller entry shroud relative Mach number. Performance maps are presented for a single-stage centrifugal compressor based on this premise with specific speed = 90. At 15, 12 and 101 PR, 72, 75 and 76.8 percent efficiency, respectively, were attached at 2.6 lb/sec.

The Stability of Pumping Systems—The 1980 Freeman Scholar Lecture

“…For the vaned diffuser there are also a number of correlations that exist. One of these is by Kenny [76], who used the criteria that the nondimensional static pressure rise coefficient from impeller tip to diffuser throat = 0.45 as an indication of stage stall. Another method is given in [77], in which correlations are developed to predict the flow range as functions of impeller exit conditions and diffuser geometries.…”

Discussion: “Performance of Three Vaned Radial Diffusers With Swirling Transonic Flow” (Baghdadi, S., and McDonald, A. T., 1975, ASME J. Fluids Eng., 97, pp. 155–160)

“…The data shown in Fig. 24 indicate that the inlet-to-throat pressure coefficient may be a correlating parameter (such as proposed by Kenny [22]). There is evidence to indicate that a universal correlation exists between this pressure coefficient (C P 2*_i) and the aerodynamic blockage factor at the throat.…”

“…Fig. 5 may be somewhat misleading in' that the vane-island duffuser geometry of these tests is compared with pipe diffuser geometry data of Kenny [22]. The data are compared at the vane leading edge station where appreciable differences in the axial variation of flow properties might be expected between the It would be enlightening in Fig.…”

The PT6 Engine: 30 Years of Gas Turbine Technology Evolution

“…The novel pipe diffuser was first introduced on the PT6 engine and is now standard company design practice because of its demonstrated superior performance and compactness (Kenny, 1973). Manufacturing technology has developed to meet stringent diffuser aerodynamic design requirements while new designs are done using 3D viscous codes to enhance compressor surge margin.…”