Stability Improvement Of Turbocharger Centrifugal Compressor By Asymmetric Vaneless Diffuser Treatment

Turbocharging is one of the core technologies for meeting the futureemission legislations as well as the demand of energy saving and CO2reductionfor internal combustion engines. The stability of a centrifugal compressor is oneof the key issues for the advanced turbocharging technology.At off-design conditions, the nonaxisymmetrical volute causes anasymmetric flow field in the centrifugal compressor. Based on the hypothesisthat the flow is symmetrical, the conventional flow control methods fail to takeinto account of the intrinsic asymmetric flow in the centrifugal compressor,which is far away from the real. Considering of this situation, TsinghuaUniversity proposed a breakthrough idea known as “asymmetric flow controlmethod” in order to extend the flow range of centrifugal compressors forturbochargers. Based on this idea, the stability enhancement mechanism byasymmetric diffuser is studied and the corresponding asymmetric flow controlmethod is explored, with the support of the Natural Science Foundation of China(No.51176087) and the International Cooperation Projects (No.74&No.84).Both numerical simulation and experiments are conducted to analyze thedetailed asymmetric flow field as well as the performance of the centrifugalcompressor. It is found that the asymmetric flow induced by the volute affectsthe stability of the centrifugal compressor in different ways respectively at highand low pressure ratio (PR) conditions: at high PR condition, the asymmetricflow in the impeller causes large flow separation firstly appear in a certaininducer passage, and the low momentum flow generated further induces the stallof the whole impeller and finally results in the surge of the compressor; while atlow PR condition, the asymmetric flow in the vaneless diffuser make thebackflow emerge first at some section along the circumference, which cause thestall of the diffuser and finally results in the surge of the compressor.A novel asymmetric vaneless diffuser treatment with circumferentiallynonaxisymmetrical diffuser width is first proposed to enhance the stability ofthe centrifugal compressor. Furthermore, two asymmetric diffuser designprinciples are developed. For high PR condition, the objective of the asymmetric diffuser design is to make the circumferential static pressure distribution in thediffuser more uniform, which requires that the phase of the maximal diffuserwidth is coincident with the phase of the minimum static pressure. While forlow PR condition, the objective of the asymmetric diffuser design is to make thecircumferential absolute flow angle distribution more uniform, which requiresthat the phase of the minimum diffuser width is coincident with the phase of themaximum absolute flow angle.Experiments are conducted on a turbocharger centrifugal compressor with thedesigned asymmetric diffuser case based on the proposed principles. The results showthat the stability of the centrifugal compressor at whole conditions (including high PRand low PR conditions) is enhanced by the developed asymmetric diffuser. Specifically,the stable flow range is extended by28.3%compared with the prototype usingsymmetrical diffuser at designed speed.