Nozzle Entry Effects and Cavitation Inception in Crossflow Hydroturbines

Abstract

Crossflow hydroturbines are simple and cheap to manufacture. This work is part of a program to improve the design methodology of small crossflow hydroturbines for remote power systems in developing countries. Adhikari and Wood (2018) showed that the exit arc—the circumferential extent of the flow exiting the runner—is typically half the length of the entry arc, θ_s, of the flow entering the runner. Negative gauge pressures are thus required in the exiting flow to achieve high efficiency. Here, we investigate increasing θ_s to reduce the negative pressures and the influence on cavitation which begins on the blades near the exit. The influence of θ_s is studied using computational fluid dynamics simulation of two turbines whose performance has been measured experimentally. No evidence of cavitation was found in the 0.53 kW turbine (efficiency η = 88%, flow rate Q = 46 lps, and head H = 1.337 m) at any operating speed, whereas significant cavitation was found at maximum efficiency (η = 91%) in the improved design of the 7 kW turbine (original η = 69%, Q = 105 lps, and H = 10 m). The extent of cavitation decreased as θ_s was increased but this also decreased η from 91% to 87%. Changes in radius ratio and inner blade angle decreased significantly the size of cavitation inception region with only a small decrease in maximum efficiency. Finally, redesigning the turbine for reduced H = 8 m and Q = 94 lps avoided cavitation while achieving a similar maximum efficiency of 89.3%. Nevertheless, it may be possible to avoid cavitation through investigation of an optimum combination of geometrical parameters but this would require a considerable computational effort which probably should be guided by experiment.