Numerical Analysis of Energy Flow Paths in Exhaust Gas Turbochargers by Means of Conjugate Heat Transfer

Heat transfer effects play a significant role in assessing the performance of automotive turbochargers. Thermal effects are becoming increasingly relevant due to reduced machine sizes and increased exhaust gas temperatures. In this work, a study of the individual energy flows is conducted by simulation of a complete turbocharger comprising compressor (d_C = 51 mm), turbine, and bearing housing using conjugate heat transfer. Special focus is given to the analysis of the various heat flows occurring in the machine aiming to identify the major heat transfer paths and their sensitivity with respect to varying operating conditions. Cooling of the bearing housing is shown to be a powerful thermal isolator mitigating the heat transferred to the compressor by up to 60%. Moreover, the rotating speed largely dictates the amount of heat transfer in the compressor and the direction of the heat flow: Whereas at low speeds (22% of max. speed), 117 W are introduced into the fluid and 338 W are being discharged from the fluid at maximum speed. At high speed operation, the heat transfer is shown to be insignificant compared to the aerodynamic work. At low speeds, however, it can reach up to 35% of the aerodynamic work. While the turbine inlet temperature largely governs the overall heat that is lost from the exhaust gas passing the turbine (from 630 W at 300 °C up to 3.72 kW at 1050 °C), only a minor effect on the compressor heat transfer is detected.