Experimental Evaluation of Turbocharger Turbine Performance Under Pulsating Flow Conditions

Abstract

The steady and pulsating performance results of a turbocharger mixed-flow turbine are presented. The results are taken at an equivalent speed of 70% (42,000rpm) for a pulse frequency range of 20 to 80 Hz. All instantaneous parameters required for unsteady performance evaluation are measured and discussed. Significant improvements to the measurement of instantaneous actual power have been carried out. Large variations in the operating point of the turbine occur in each pulse cycle, a velocity ratio range of 0.43 to 1.28 is seen for a 20 Hz pulse, this range reduces as the pulse frequency increases and unsteady effects become more prominent. During periods of turbine freewheeling, negative efficiencies can arise due to momentum transfer from the turbine to the working gas, although detrimental to the efficiency the energy content in these regimes are low. The use of a modified Strouhal number (MSt.) and a pressure modified Strouhal number (PMSt.) has proved useful in assessing when the onset of unsteadiness of the flow will become significant, a value of 0.1 has been used as an appropriate limit to steadiness. The results suggest that for a typical engine speed range the rotor may be considered quasi-steady whilst the turbine stage is predominately operating in an unsteady regime. Inference from the experimental data would suggest it is adequate to capture the performance of a turbine under pulsating flow using a ‘quasi-steady’ model when the MSt. < 0.1, and a ‘filling and emptying’ code when a PMSt. < 0.1 and above this value a ‘wave action’ model is more appropriate.