Turbocharger Aero-Thermal Performance under Realistic Flow Conditions: Pulsating Flow and Heat Transfer Considerations

Abstract

Differences between turbocharger performance on an engine and on a gas stand have been observed throughout decades of turbocharging studies. These differences have been attributed, in part, to various phenomena, the most important of which include heat transfer and pulsating exhaust gas flows that occur on-engine, and that are not commonly captured in experimental turbocharger test gas stands. In early stage design, turbocharger performance maps are regularly used to provide a boundary condition to the engine in gas flow simulations. These maps are typically produced from steady-state gas stand tests, which only cover a limited range of data. Engine performance predictions are therefore determined by air flow and thermodynamic parameters from steady-state compressor maps, which in turn influence the parameters for exhaust gases entering the turbine. If heat transfer effects are not properly accounted for, errors of varying severity will occur in the characterisation of the compressor and the performance prediction of the engine as a consequence. The steady flow conditions of the gas stand tests also determine the turbine map without accounting for the effects of the unsteady pulsating flow that exists under realistic engine conditions, this leads to further errors in the characterisation of the turbine, and subsequently the engine performance prediction. Today, these predictions are being supplemented with CFD analysis, although these simulations are expensive in terms of the hardware and time requirements; efforts to model turbines in this manner are currently outstripping the data available to validate their accuracy. This chapter describes the phenomena associated with heat transfer and pulsating flows in turbomachinery, how they can be modelled, and what their impact is on the turbocharger performance predictions used during early stage engine design.