Validation of a Theoretical Model for the Correction of Heat Transfer Effects in Turbocharger Testing through a Quasi-3D Model 2020-01-1010
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 1D procedure, usually adopted for the engine-turbocharger matching. Actually, it is common practice in automotive applications to use simulation codes, which require as an input the value of efficiency. 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.
The practical purpose of an adiabatic test program is to obtain an accurate measurement of the work transfer, and of the real efficiency of compressor and turbine (unaffected by internal and external heat transfer rates). In fact, the heat flow leads to an apparent increase of the power absorption and an apparent drop in efficiency of the compressor. However, lack of understanding of the heat transfer effects as well as the high costs associated with testing facilities often discourages efforts on this topic and manufacturer maps frequently consider the compression and expansion process within turbochargers to be adiabatic.
The aim of the present paper is to measure the heat transfer internal to the turbocharger in order to provide a general understanding of heat transfer mechanism occurring in turbochargers and relationships for heat transfer rate useful to derive the real diabatic efficiency. The approach proposed makes use of the experimental definition of compressor steady flow performance maps 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. A mathematical model, developed by the University of Genoa, for the correction of compressor steady flow maps is adopted and compared to the quasi-adiabatic condition. Besides, a quasi3D approach, developed by Politecnico di Milano, is applied to the fluid dynamic simulation of turbocharging devices for internal combustion engines, focusing on the compressor side. The 3Dcell is based on a pseudo-staggered leapfrog method applied to the governing equation of a 1D problem arbitrarily oriented in space. This approach has been conceived for acoustic and fluid dynamic simulations of intake and exhaust systems of ICE and then extended to the modeling of turbochargers. In the context of this work the adiabaticity 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 coeffiencient have been taken from the literature and implemented in the code. The 3Dcell model is then used to simulate the turbocompressor both in adiabatic and diabatic condition and validated against the experimental results, confirming the validity of the mathematical model used to correct the maps.
Silvia Marelli, Gianluca Montenegro, Matteo Tamborski, Augusto Della Torre
Universita Degli Studi di Genova, Politecnico di Milano