Validation of a Theoretical Model for the Correction of Heat Transfer Effects in Turbocharger Testing through a Quasi-3D Model

In the last few years, the effect of diabatic test conditions on compressor performance maps has been widely investigated, leading some Authors to propose different correction models. The accuracy of turbocharger performance map constitute the basis for the tuning and validation of a numerical method, usually adopted for the prediction of engine-turbocharger matching. Actually, it is common practice in automotive applications to use simulation codes, which can either require measured compression ratio and efficiency maps as input values or calculate them "on the fly" throughout specific sub-models integrated in the numerical procedures. Therefore, the ability to correct the measured performance maps taking into account internal heat transfer would allow the implementation of commercial simulation codes used for engine-turbocharger matching calculations. In the paper the main results of a wide experimental activity are reported to provide a general understanding of heat transfer mechanism occurring in turbochargers and relationships for heat transfer rate useful to derive the adiabatic efficiency. The compressor steady flow performance maps were measured at different operating temperatures for compressor and turbine, with and without water-cooling and under quasi-adiabatic condition achieved by maintaining the lubricating oil average temperature equal to compressor outlet temperature and turbine inlet temperature to minimize internal heat fluxes. Furthermore, a mathematical model for the correction of compressor steady flow maps, developed by the University of Genoa, is adopted and compared to the quasi-adiabatic condition. In the context of this work a quasi-3D CFD code developed at Politecnico di Milano has been extended and applied to simulate the flow inside the compressor. To this purpose the adiabatic assumption has been removed and the heat transfer between the gas and the stationary and rotating components has been taken into account. Correlations for the heat transfer coefficient have been taken from the literature and implemented in the code. The quasi-3D model is then used to simulate the compressor both in adiabatic and diabatic condition. The quasi-3D CFD code was validated against the experimental results, confirming also the validity of the mathematical model used to correct the maps.