Assessment of unsteady behavior in turbocharger turbines

The flow in turbocharger turbines is highly unsteady in nature as it responds to the exhaust manifold of an internal combustion engine. This paper investigates the significance of unsteadiness by examining first its relevance to real engine situations and then its effect on turbocharger turbine operation. The engine simulations carried out show the relevance of the Strouhal number effect for real turbocharger applications, which has been demonstrated experimentally on a turbine stage test stand. Therefore, for realistic multiple-cylinder-engine configurations with different exhaust gas pipe lengths and firing frequencies the importance of the actual unsteady behavior needs careful assessment. The effect upon the turbine itself is examined by modeling the laboratory arrangement to replicate the test stand configuration and operation using a one-dimensional wave action code. The ID model is validated against experimental results obtained using a new permanent magnet eddy-current dynamometer for a mixed flow turbine suitable for a medium-sized automotive application covering an equivalent speed range of 50-100%, U2/C_is of 0.3-1.1 and a pulse frequency of 20-80 Hz. The turbine model has been refined using unsteady experimental data and so enables the capture of unsteady effects in engine design codes. The beneficial effect of the ability of this model to predict turbine mass flow is discussed.