Experimental Study of the Effect of Blade Roughness on the Efficiency of Laboratory Scale Pelton Turbines

Abstract

Pelton turbines are widely used in high-head and low-flow hydropower systems because the kinetic energy of a water jet can be converted directly into runner torque. However, deterioration of the bucket surface can disturb the water sheet, increase local friction, and reduce momentum transfer. This study experimentally investigated the effect of controlled bucket surface roughness on the shaft power and efficiency of a laboratory scale Pelton turbine. Four roughness levels were prepared by polishing, graded sanding, light sand blasting, and heavy sand blasting, producing average Ra values of 0.8, 1.6, 3.2, and 6.3 micrometers, respectively. The tests were conducted on a single nozzle closed loop Pelton turbine rig at an effective head of 10.0 m and discharge of 5.45 L/s. Discharge, head, rotational speed, torque, and roughness were measured using calibrated instruments, and the uncertainty of efficiency was estimated using propagation of measurement errors. The results show that increasing roughness consistently reduced performance. The maximum shaft power decreased from 419.7 W at Ra = 0.8 micrometers to 314.9 W at Ra = 6.3 micrometers, while efficiency decreased from 78.5% to 58.9%. The combined relative uncertainty of efficiency was estimated at approximately 2.2%. The findings confirm that bucket surface finishing and roughness control are important for maintaining Pelton turbine performance in laboratory-scale and small hydropower applications.