Experimental and Numerical Investigation Into Turbocharger Centrifugal Compressor Surge Inception

Abstract

In order to meet stricter emission regulations and decarbonisation targets, the transportation sector places increasing demands on the performance and reliability of internal combustion engines. The stability of turbocharger centrifugal compressors plays a decisive role in determining engine power density and operating safety. However, stall and surge have long constrained the stable operation of centrifugal compressors. Despite progress in recent research, accurately and rapidly predicting compressor instability remains a major challenge. In this thesis, three ported shroud equipped turbocharger centrifugal compressors of the same type but with minor local geometry variations, are studied. The stability characteristics are investigated experimentally and numerically. Building on the insights gained into fundamental surge precursors, a numerical simulation framework is proposed to predict the compressor stability limit at acceptable computational costs, thereby enabling rapid determination of the surge boundary for new designs at early stage. The main contributions of this thesis are listed as follow: • Experimental tests demonstrated that minor geometry variations, such as the length of ported shroud struts and size of diffuser recess, had a significant influence on compressor map width, despite identical designs of core components such as impeller and vaneless diffuser. Enlarging the diffuser recess broadened the map width, whereas shortening the ported shroud struts was detrimental to compressor stability. • A comprehensive analysis of the impeller and the vaneless diffuser was conducted using a high-fidelity numerical model. The impeller analysis revealed that at low flow rates, the recirculated flow from the ported shroud was reinjected into the upper 20% span at the impeller inlet. The variations in ported shroud struts length introduced different pre-swirl conditions. Long struts induced a negative pre-swirl flow, which increased the blade loading at inducer tip and thereby enhanced the impeller stability. Based on these insights, a modified impeller diffusion factor (DF) was proposed to quantify the impeller stability. Examination of the impeller DF demonstrated that the dominant component governing the compressor instability switched from the impeller to the vaneless diffuser at high speedlines. • The diffuser analysis revealed that the enlarged diffuser recess provided a buffer to the impeller discharge flow and improved the flow condition near diffuser hub side. As a result, the separation on diffuser hub side was suppressed and the diffuser stability was enhanced. In addition, a non-uniformity parameter was proposed to effectively quantify the influence of circumferential and axial non-uniformity in the vaneless diffuser. Combining with the diffuser inlet flow angle distribution, the inference that the vaneless diffuser became the dominate component governing the compressor instability at high speedlines was further confirmed. • Dynamic pressure measurements gathered during the experimental test campaign unearthed three distinct compressor instability behavior, which were governed by the change in impeller stability. The underlying mechanism was attributed to the growth of the recirculating bubble at impeller exducer tip and its interaction with the bleed slot of the ported shroud. Finally, building on the insights gained, a numerical simulation framework was proposed to predict the compressor surge boundary. The framework employed the modified impeller diffusion factor and the stability parameter of individual compressor components as the criteria for identifying the last stable operating point (surge point). Validation with a representative case demonstrated the feasibility of the framework. Although certain limitations remained, particularly poor performance at high speedlines, the framework nonetheless represented a promising step towards practical surge boundary prediction. This thesis deepens the understanding of stability characteristics of turbocharger centrifugal compressors, and explores the potential of using numerical simulations for surge boundary prediction, providing a strong support for the future development of reliable surge prediction methodologies.