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The roughness level of the turbine housing’s internal surfaces can affect the total pressure loss, and hence the efficiency, of turbine stages for automotive turbochargers significantly. The actual surface roughness achieved in the housing hardware depends on the manufacturing process. Improving the volute surface quality will in most cases be more expensive. The potential benefits in turbine efficiency therefore have to be balanced with the additional costs. In the present article, the aerodynamic effects due to changes of volute surface roughness are assessed. Pressure loss measurements in simple pipes are conducted in order to calibrate a computational fluid dynamics (CFD) sand grain roughness model against measured roughness values. Two test cases involving flow simulations of the complete turbine stage are presented in order to validate the calibrated model and to show that Reynolds-averaged Navier Stokes (RANS) CFD simulations are generally able to predict the effects of surface roughness on the turbine performance. The results of the test cases show a significant reduction in turbine efficiency with increased volute roughness levels. As expected, the sensitivity of efficiency due to changes in surface roughness of the volute is largest in areas of high near-wall velocity, i.e., at the volute exducer and in the small volute cross-sections close to the tongue.

The effect of volute surface roughness on the performance of automotive turbocharger turbines