Effect of the Back-face Gap to the Performance of a Scalloped Radial Turbine

In a radial-type turbine rotor, scallop has a good effect on the rotor"s quick responsibility and reduction of the axial thrust. However, it can be accompanied by a decrease in efficiency of the turbine. In addition, an increase of the back-face gap decreases its efficiency. Hence, it is necessary to set the optimal gap of the back-face with a consideration of the minimum tolerance for assembly. However, the gap of the back-face can be varied depending on the operating condition. Therefore, it is necessary to investigate the performance of the turbine for the various gaps. In this study, a computational study was conducted to investigate the performance of the turbine for the various gaps. In addition, the axial thrust was studied for the various gaps. In conclusion, the efficiency of the turbine decreased by 2% when the back-face gap increased by 10%. An optimal gap, which can minimize the axial thrust, could be existed depending on the rotational speed, and the wider gap increased the axial thrust.