Aerodynamic Design to Increase Pressure Ratio of Centrifugal Compressors for Turbochargers

Abstract

This paper describes the ways to increase pressure ratio of centrifugal compressors on condition of fixed impeller diameter. The compressors discussed in the paper were designed for turbochargers applied to 500kW class Diesel engines and their impeller diameters were about 140mm. Pressure ratio was increased from 3.6 to 4.8 at first and then increased from 4.8 to 5.7. The pressure was raised by means of increase in rotational speed together with additional increase in pressure coefficient. The pressure coefficient was increased by means of increase in work coefficient. The increases in rotational speed and work coefficient lead to increase in relative Mach number at inducer. However, the relative Mach number at inducer of the compressor with a pressure ratio of 3.6 already exceeded 1.3. Therefore, it was expected that complicated flow features related to the occurrence of shock wave would deteriorate the new compressors’ efficiency and operating range. In order to avoid this problem, following new measures were taken with conventional design methods. (1) Reduction of a blade loading — an increase in number of blade by means of double splitter (two adjoining splitters). (2) Introduction of technologies to enhance an operating range — recirculation device. (3) Selection of a proper diffuser to convert kinetic energy at an impeller exit to static pressure — vaned diffuser with elliptic leading edge. Newly designed compressors realized expected performance, stage pressure ratio and efficiency. Suitability of these measures was confirmed.