Parametric study on the sensitivity and influence of axial and radial clearance on the performance of a positive displacement hydraulic turbine

Abstract

Positive Displacement hydro turbine (PDT) is a special class of hydraulic turbine which can be useful in recovering micro and Pico hydro power from conventional as well as unconventional resources. This class of turbine is very uncommon and most effective in low specific speed applications. Flow leakage through the clearance gaps plays a vital role in influencing the performance of positive displacement type of machines. The present work illustrates the effect of axial and radial clearance on the overall performance of the PDT. Experimental studies were conducted with PDT having different combination of axial and radial clearances and their influence on the performance of the PDT was examined. The designed PDT had rotor diameter and width of 118 mm and 91 mm respectively. A parametric study was undertaken using Computational Fluid Dynamics (CFD) to find the sensitivity of these clearances in influencing the performance of PDT in the range of 0.025–0.6 mm. Also, the effect of reduction in clearance was analyzed. From the analysis, it was inferred that radial clearance was sensitive for clearance range of 0.15–0.6 mm and axial clearance in the range of 0.025–0.15 mm. The leakage flowrate was very high and was close to one-third of the total flowrate through the turbine at higher clearance of 0.6 mm, although the clearance volume was close to 4% of the total turbine volume. Reduction in clearance from 0.15 mm to 0.025 mm caused an increase of 9.27 and 10.4% in hydraulic and volumetric efficiency respectively at rated condition of rotational speed ratio (SR) = 112.4 and effective head ratio (HER) = 0.7. At the rated condition, the turbine with axial and radial clearance of 0.2 mm generated 7.15 kW of power with a hydraulic efficiency of 66.2% from the experimental study.