Direct evaluation of turbine isentropic efficiency in turbochargers: Cfd assisted design of an innovative measuring technique

Turbocharging is playing today a fundamental role not only to improve automotive engine performance, but also to reduce fuel consumption and exhaust emissions for both Spark Ignition and Diesel engines. Dedicated experimental investigations on turbochargers are therefore necessary to assess a better understanding of its performance. The availability of experimental information on turbocharger steady flow performance is an essential requirement to optimize the engine-turbocharger matching, which is usually achieved by means of simulation models. This aspect is even more important when referred to the turbine efficiency, since its swallowing capacity can be accurately evaluated through the measurement of mass flow rate, inlet temperature and pressure ratio across the machine. However, in the case of a turbocharger radial inflow turbine, isentropic efficiency, directly evaluated starting from measurement of thermodynamic parameters at the inlet and outlet sections, can give significant errors. This inaccuracy is mainly related to the difficulty of a correct evaluation of the turbine outlet temperature due to the non-uniform distribution of flow field and temperature at the measuring section. This work is the follow up of a previous publication where an intensive measurement campaign was performed to obtain a reliable measurement of the turbine outlet temperature. To this aim, a hand-made 3-hole probe (unlike most of the measuring probes available on the market, which are considered as intrusive ) was adopted to perform measurement of the flow field, pressure and temperature downstream the turbine with special reference to different radial and tangential positions in two sections located near and far from the outlet machine, allowing the evaluation of the efficiency through local enthalpy fluxes across the turbine in cold and hot conditions upstream the turbine. The comparison between results obtained through the local measurements and those achieved through a direct measurement of turbine outlet temperature by three probes inserted in pipe with a different protrusion, have highlighted that heat transfer effects across the pipes and across the turbocharger components play an important role on the estimation of temperature profile at the outlet section. In order to put some light on this aspect, CFD simulations have been performed to estimate the impact of the heat transfer and flow distribution on the estimation of the isentropic efficiency. The OpenFOAM® code has been adopted to simulate the actual turbine geometry resorting to multi reference frame (MRF) strategies, instead of mesh motion strategies, to characterize the flow pattern downstream of the turbine. Moreover, CFD analysis was used to design a specific device, whose goal was the dissipation of flow structures dominated by vorticity, achieving in this way a uniform distribution of the flow and temperature fields at the measuring section. This will result in a much more reliable evaluation of the turbine efficiency