Thermal Tip Clearance Control for Centrifugal Compressor of an APU Engine

Abstract

To master today’s demand for efficiency and compactness of centrifugal compressor components for small gas turbine engines the main attention must not only be given to the aerodynamic design of the impeller and diffuser components, but also to the installation situation of the surrounding parts. A vital aspect is the tip clearance control between impeller and shroud casing over the total operating range. Using the radial compressor for a small gas turbine engine, developed at BMW Rolls-Royce, the importance of tip clearance control is demonstrated. The possibilities for influencing and optimizing passive tip clearance control by design features are described; transient expansion processes must be considered when using a thermal tip clearance control. The results of the design calculations are compared with the results on the test stand and the engine itself. An effort is made to find a qualitative influence of tip clearance to the engine power output at operating conditions. This qualitative description is substantiated by test results with different tip clearances at the compressor teststand.

Aerodynamics of a Centrifugal Compressor Impeller With Transonic Inlet Conditions

“…This diffuser ensures a wide flow range and is only applied for the initial tests that mainly deal with rotor flow. Typically, compressors of this kind are equipped with vaned diffusers having higher pressure recovery than vaneless diffusers (Japikse, 1992, Yoshinaka, 1994, Rodgers, 1982, Simon et al, 1986, Eisenlohr and Chladek, 1993. The vaneless diffuser will be replaced by a vaned type diffuser when the studies are extended to the whole stage.…”

Experimental study of centrifugal compressor tip clearance and vaneless diffuser flow fields

“…It has been universally long known that increasing the tip clearance decreases the compressor efficiency and pressure ratio. 16–18 For example in a recent study, Hong and Abhari 19 got a slope of 0.37 for the efficiency drop due to the increasing tip clearance, meaning approximately one percentage point drop in efficiency for every three percentage point increase in the relative tip clearance. In a study of Turunen-Saaresti and Jaatinen 20 in which an array of studies was looked through, it was noted that the slope for the efficiency drop varied between 0.2 and 1.05 for different compressors.…”

Effect of tip clearance on impeller discharge flow and vaneless diffuser performance of a centrifugal compressor

“…Chriss et al 3 and Harthaway and Wood 4 emphasized the importance of throughflow wake, which is largely fed by the tip clearance flow when the centrifugal impeller is throttled. Eisenlohr and Chladek 5 described the possibilities for optimizing passive tip clearance control by design features and suggested that transient expansion processes must be considered when using a thermal tip clearance control.…”