Characterizing the Influence of Impeller Exit Recirculation on Centrifugal Compressor Work Input

Abstract

Impeller recirculation is a loss which has long been considered in one-dimensional (1D) modeling; however, the full extent of its impact on stage performance has not been analyzed. Recirculation has traditionally been considered purely as a parasitic (or external) loss, i.e., one which absorbs work but does not contribute to total pressure rise across the stage. Having extensively analyzed the impact of recirculation on the impeller exit flow field, it was possible to show that it has far-reaching consequences beyond that of increasing total temperature. The overall aim of this package of work is to apply a much more physical treatment to the impact of impeller exit recirculation (and the aerodynamic blockage associated with it) and hence realize an improvement in the 1D stage performance prediction of a number of turbocharger centrifugal compressors. The factors influencing the presence and extent of this recirculation are numerous, requiring detailed investigations to successfully understand its sources and to characterize its extent. A combination of validated three-dimensional computational fluid dynamics (CFD) data and gas stand test data for six automotive turbocharger compressor stages was employed to achieve this aim. In order to capture the variation of the blockage presented to the flow with both geometry and operating condition, an approach involving the impeller outlet to inlet area ratio and a novel flow coefficient term were employed. The resulting data permitted the generation of a single equation to represent the impeller exit blockage levels for the entire operating map of all the six compressor stages under investigation. With an understanding of the extent of the region of recirculating flow realized and the key drivers leading to its creation identified, it was necessary to comprehend how the resulting blockage influenced compressor performance. Consideration was given to the impact on impeller work input through modification of the impeller exit velocity triangle, incorporating the introduction of the concept of an “aerodynamic meanline” to account for the reduction in the size of the active flow region due to the presence of blockage. The sensitivity of the stage to this change was then related back to the level of backsweep applied to the impeller. As a result of this analysis, the improvement in the 1D performance prediction of the six compressor stages is demonstrated. In addition, a number of design recommendations are presented to ensure that the detrimental effects associated with the presence of impeller exit recirculation can be minimized.

A New Approach to Modeling Slip and Work Input for Centrifugal Compressors

“…However, traditional models derived from the relative eddy approach, such as Wiesner's model, are dependent only on the impeller geometry and cannot capture the variation of slip factor with operating conditions driven by equations ( 14) and (15). Some recent relative eddy approaches have incorporated the impeller exit flow coefficient to account for the variation of slip factor with operating conditions [9,21], but these methods dilute the influence of the machine Mach number in equation (15). The variation of the exit flow coefficient throughout the compressor map is usually small relative to the range of machine Mach numbers available to high-speed compressors, and furthermore, the machine Mach number is raised to a larger power than the exit flow coefficient.…”

“…Stodola's [1] original method used geometry to predict the size of the relative eddy, and by extension, the slip factor associated with a given impeller design. Most models available in the literature have followed Stodola's approach and primarily rely on the impeller geometry to predict the behavior of a highly unsteady, three-dimensional flow field at the impeller exit [2,9,11,12,14,21]. Due to errors and inconsistencies in predicting the behavior of a complex flow field chiefly from geometry, as well as limited success in predicting slip at off-design conditions, there is significant room for improvement in the prediction of slip factor.…”

On the Effect of Side Clearance in the Vaned Diffuser of a Centrifugal Compressor

“…Tests were conducted using a turbocharger test rig located in Queen's University Belfast (QUB); further details on the test rig can be found in Harley et al [5] and Stuart et al [6]. The QUB test rig conformed to the SAE J1826 standard for hot gas stand testing of turbochargers [7]; this is the industry standard for the testing of passenger vehicle turbochargers.…”

A New Approach to Modeling Slip and Work Input for Centrifugal Compressors

“…( 14) and (15). Some recent relative eddy approaches have incorporated the impeller exit flow coefficient to account for the variation of slip factor with operating conditions [9,21], but these methods dilute the influence of the machine Mach number in Eq. (15).…”

“…Stodola's [1] original method used geometry to predict the size of the relative eddy, and by extension, the slip factor associated with a given impeller design. Most models available in the literature have followed Stodola's approach and primarily rely on the impeller geometry to predict the behavior of a highly unsteady, three-dimensional flow field at the impeller exit [2,9,11,12,14,21]. Due to errors and inconsistencies in predicting the behavior of a complex flow field chiefly from geometry, as well as limited success in predicting slip at off-design conditions, there is significant room for improvement in the prediction of slip factor.…”