Centrifugal compressor casing flow field analysis and optimisation

Abstract

Turbocharging has become a fundamental technology to realize engine downsizing, which is an attractive strategy for low carbon vehicles in the near term. The stable operation of turbocharger compressors at low and high flow rates is crucial to provide peak torque demand and rated power for turbocharged automotive engines.

The volute is a key component in centrifugal compressors as it affects the compressor efficiency and operating range. The presence of this component results in a distorted pressure field upstream which can contribute to stall in the impeller, inducing surge. As the flow inside the volute is fully three dimensional and turbulent, a better understanding of flow mechanisms inside the volute is required to enable a better design methodology.

In this research a centrifugal compressor volute has been studied numerically and experimentally in order to identify the parameters that contribute to the main flow losses. By solving Reynolds average Navier-stokes (RANS) equations with k-Epsilon turbulence model, the three-dimensional flow field model of the compressor has been obtained using a commercial code. Based on this analysis, an optimized compressor volute was presented. The progress to obtain the final model can be divided into two main groups; comparative analysis and flow field understanding & new concept design respectively.

A detailed study of the baseline compressor has been carried out, this allowed the creation of a well validated CFD model and it also allowed identification of the key areas for improvement. The analysis focused on the three most important points of engine operation. Three critical areas within the diffuser-volute system affecting the compressor performance were studied: A/R, volute shape and exit pipe angle. The loss mechanisms were identified. The results have shown that the total to total isentropic efficiency and surge margin have been improved by 1.5% and 4.5% at 100% rpm speed line with the new design, both numbers have been confirmed by experiments. The flow field inside the new volute has been also captured and compared to the numerical analysis carried out from the baseline.

Surge margin is a central need in turbocharger compressors, much effort has been employed to increase the operating range of a compressor. A new design has been presented in this thesis with the aim of increasing the surge limit while incurring the lowest possible losses. This new concept consisted of an annular "cavity'' covering the entire circumference of the shroud. Four concepts are explored located in two areas of the compressor shroud (main blade leading edge and splitter leading edge) and with two different cavity geometries.

The results showed surge enhancement both experimentally and numerically for the four cavity concepts. The best concept (Blade Mach) achieved a surge margin improvement of 13.5% at 30Hz, 8.4% at 43Hz, 6.1% at 66.67Hz, 11.7% at 80Hz and 8.4% at 100Hz. Along with this enhancement there as an efficiency deterioration (ED) of -7.8% at 30Hz, -3.7% at 43Hz, -3% at 66.67Hz, -3.6% at 80Hz and -2.9% at 100Hz. But the Blade Mach in regions of maximum compressor efficiency achieves better values than the original compressor (43Hz to 100 Hz), which further increases the potential shown by this design.