Flow analysis in eccentric bucket of Micro Pelton turbine: Multiphase modelling with transient state condition

Abstract

Micro hydro power plants use Pelton wheel to generate electrical power. Generally, bolted buckets are used in this type of power plants as it is small in capacity and size which makes easier to the operator to replace or repair old buckets whenever required. In this study, the effect caused by eccentricity of those bolted buckets on the operation of turbine is studied with the help of formulating mathematical model and conducting flow analysis in Computational Fluid Dynamics (CFD) code by ANSYS (CFX). Mathematical model has been developed to find the torque generated by eccentric bucket about three global axes, X, Y and Z. Torque has been obtained by calculating unbalanced forces in whirl and flow direction and compared with non-eccentric bucket condition to find the deviation. Numerical simulation has been carried out with outlet from nozzle maintained at constant 5.345 × 10-3 m3 /s flow rate to study the flow pattern in eccentric bucket and compared to that of non-eccentric bucket at five different angular positions showing velocity contour around the flow. The velocity distribution analysis revealed that the velocity is minimum at the bucket exit and maximum at the region near the splitter with values of 27.724 m/s, 28.688 m/s and 29.444 m/s for non- eccentric, 1° eccentric and 2° eccentric bucket respectively. Multiphase analysis at transient state condition has been conducted with complete 140 bucket rotation. The VOF scheme used for the multiphase analysis presented the deflection of water jet due to the eccentric splitter is increased with the increase in eccentricity. Torque has been monitored for single bucket and duplicated by using MATLAB software to obtain total torque generated. Maximum Pressure exerted on bucket has been obtained from numerical simulation and found to be 107.682 kPa, 125.765 kPa and 137.248 kPa at front side and 145.905 kPa, 160.059 kPa and 193.466 kPa at back side of non-eccentric, 1° eccentric and 2° eccentric bucket respectively. The pressure distribution analysis on back side of the bucket revealed that the maximum pressure occurred at 0 m from the splitter for non-eccentric condition and it goes increasing at 0.00378 m and 0.00599 m from the splitter for 1° and 2° eccentric condition respectively. The accuracy of the results was conformed by comparison of mathematical and numerical outputs by calculating the error. The error is found to be less than 6.017% in all non-eccentric, 1° eccentric and 2° eccentric buckets for torques generated about X, Y and Z-axis.