The Effect of Volute Design On The Performance Of A Turbocharger Compressor

An investigation has been carried out to assess the effect of volute design on the performance of a centrifugal compressor. Three volutes, based on a turbocharger compressor volute, have been fabricated and tested. These volutes provided two that were larger than the original design, and one that was smaller. The alternative volute designs were fabricated from sheet steel and had a square cross-section. A common centre core, which provided the impeller cover, was copied from the original cast design and manufactured as a separate piece. This single centre core was used with each of the fabricated volutes. A scroll design, which was 25% larger than the original cast design, gave improved performance throughout the operating range. Scroll designs with a reduced discharge area did not perform well at high flow rates; this was attributed primarily to excessive flow separation from the leading edge of the tongue. The volute design also modified the performance of the upstream vaneless diffuser. The pressure recovery coefficient for the vaneless diffuser with the original cast design showed a clear maximum, whilst with the fabricated designs the pressure recovery coefficient increased throughout the flow range. NOMENCLATURE B4 Vaneless diffuser discharge passage height R4 diffuser discharge radius Cθ4 Tangential component of velocity at diffuser discharge r Radius of volute passage Cr4 Radial component of velocity at diffuser discharge α Absolute flow angle qv Volume flow rate θ Azimuth angle INTRODUCTION It is well established that volutes of centrifugal compressors cause a circumferential pressure distortion around the impeller at all flow rates other than that at design. The design objective, Eck(1), is to achieve a uniform flow at volute inlet. At off-design conditions the volute is either too small or too large and a pressure distortion develops circumferentially around the volute passage. At low flow rates the pressure increases with azimuth angle while at high flow rates the pressure decreases. These circumferential pressure distortions are transmitted back to the impeller discharge and have been observed at impeller inlet, Fink et al(2). These circumferential pressure distortions reduce the stage performance and have a direct impact on diffuser and impeller flow stability. Roberts and Whitfield (3) showed that the flow rate at which the peak efficiency operating point of the compressor occurred was closely related to the flow rate at which minimum pressure distortion was generated by the volute. A similar observation was made by Young(4) where he concluded that volute circumferential pressure distortion, which is influenced by tongue location, has a strong affect on efficiency. Mishina and Gyobu(5) showed that the effect of the cross-sectional shape of the flow passage was small, whilst the loss coefficient was a function of the volute area ratio and radius ratio. Reducing the radius ratio to provide an overhung volute, typical of that used for turbocharger compressors, led to an increase in the loss coefficient. Whitfield and Eynon (6) observed that the volute of a turbocharger compressor provided a significant pressure rise at the best efficiency point, and at flow rates down to full surge. They showed that the volute performance plays a significant role in setting both the best efficiency flow rate and the operating range of the compressor. The purpose of the present study was to develop a technique to fabricate volute so that alternative designs could be quickly made and tested. Three volutes, based on a turbocharger compressor volute, have been fabricated and tested. These volutes provided two that were larger than the original design, and one that was smaller. VOLUTE DESIGN AND FABRICATION The design of the fabricated volutes was based on the existing turbocharger volute design. The basic volute design procedure followed that given by Eck(1).Eck showed that the radius of the volute passage for a circular section is given as a function of azimuth angle, θ, by