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

Abstract

Impeller recirculation is a loss which has long been considered in 1 D modelling, however the full extent of its impact on stage performance has not been analysed. 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 analysed 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 was to apply a much more physical treatment to the impact of impeller exit recirculation (and the aerodynamic blockage associated with it), and hence realise an improvement in the 1 D 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 characterise its extent. A combination of validated 3 D 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 six compressor stages under investigation. With an understanding of the extent of the region of recirculating flow realised, 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 1 D 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 minimised.

Experimental and Numerical Assessment of the Impact of an Integrated Active Pre-Swirl Generator on Turbocharger Compressor Performance and Operating Range

“…The approach used for the baseline CFD configuration was that as presented by Stuart et al [8], representing a typical single passage analysis comprising two stationary domains (inlet and vaneless diffuser) and one rotating domain (impeller). Consequently, for the sake of brevity, the current publication will not enter into a detailed description of this method.…”

“…In order to minimize the impact of heat transfer on the measured efficiency values obtained from the test rig, steps were taken to control the levels of heat transfer to and from the turbocharger [8]. With the current data set, a rigorous procedure of thermal matching across the turbocharger was employed at all points to control the internal heat transfer levels (characterized by a maximum variation of ±50 • C between compressor discharge, turbine inlet, and bearing oil inlet temperatures), while the turbocharger was insulated to control the external heat transfer.…”

“…Aside from the aforementioned wide operating range of automotive turbocharger compressors, one particular consequential flow phenomenon could also act as a limiting factor for the success of inlet swirl devices. Recent investigations [7,8] have highlighted that, at inlet in particular, shroud side recirculation can occupy up to 60% of the passage for near-surge-operating points, and remain a prominent flow feature for a large proportion of the compressor-operating range. As this region of recirculation can extend for a number of pipe diameters upstream of the inducer, the resulting complex interaction involves not only the flow emanating from an IGV system, but also the reverse flow from the impeller impinging on the IGV's [9].…”

A Marine Turbocharger Retrofitting Platform

“…Another contribution to the 1D model was made by Japikse [7], who assumed that the jet-wake structure exists in the impeller passage. A different approach was made by Stuart et al [8,9] who developed the three zone model by showing that impeller exit recirculation influences compressor work input.…”

“…In the present work, Galvas [4] single zone centrifugal compressor mean line code is used, employing Rodgers [6] correlations for surge and choke limits prediction. Stuart et al [8,9] is used in order to develope an empirical correlation for calculating the exducer to inducer area ratio based on impeller flow coefficient.…”