Sensitivity studies on the effect of axial clearances on the turbine efficiency

Small values of clearances between the rotating and stationary elements of high speed turbomachinery involve a potential risk of rubbing. This occurs under the combined action of pressure, thermal and centrifugal loading and can lead to a system failure. It is preferable to keep the clearances on the higher side in order to avoid such risks without compromising the performance of the hardware. An experimental study was taken up to investigate the influence of the axial gaps between the stationary and rotating elements of a liquid rocket engine axial turbine on its performance. A subsonic, two row, axial flow turbine was used for the study. A turbine with provision for varying the axial gaps between the rotor and stator were realized. Tests were done with gaseous nitrogen as the medium and a dynamometer was used tor control the turbine speed as well as to absorb the power generated by the turbine. The axial gap between the stationary and rotating elements was varied from 1.4 mm to 3.5mm. For a given test, the gap was kept constant at the selected value and the test speed was varied from 70% to 130% of the nominal value. The overall efficiency of the turbine was evaluated and was used for comparison of turbine performance. At each value of the axial clearance, two tests were carried out to confirm the results. It is observed that there is no significant loss of performance due to the increase in the axial clearance within this range. The highest value of the axial clearance is preferred as it involves least risk of rubbing without any loss of performance.