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Nozzle Entry Effects and Cavitation Inception in Crossflow Hydroturbines

  • R. C. Adhikari
  • D. H. WoodEmail author
Conference paper
Part of the Springer Proceedings in Energy book series (SPE)

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, θs, of the flow entering the runner. Negative gauge pressures are thus required in the exiting flow to achieve high efficiency. Here, we investigate increasing θs to reduce the negative pressures and the influence on cavitation which begins on the blades near the exit. The influence of θs 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 θs 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.

Keywords

Crossflow turbine Efficiency Entry arc RANS simulation Cavitation inception 

Nomenclature

β1

Runner entry flow angle (rad or degrees)

β1b

Outer blade angle (rad or degrees)

β2b

Inner blade angle (rad or degrees)

H

Turbine head (m)

h0

Nozzle throat (m)

h(θ)

Nozzle rear wall from the runner tip, R(θ)–R1, (m)

Nb

Number of blades

Q

Flow rate through turbine (lps)

Qmax

Maximum (design) flow rate (lps)

R(θ)

Radius of nozzle rear wall (m)

R1

Outer radius of the runner (m)

R2

Inner radius of the runner (m)

p

Local static pressure (Pa)

pv

Saturated vapor pressure (Pa)

U0

Nozzle inlet velocity (m/s)

W

Nozzle and runner width (m)

W˙

Turbine power (kW)

ω

Runner angular speed (m/s)

θs

Entry arc (rad or degrees)

η

Turbine efficiency (%)

ρ

Water density (kg/m3)

Notes

Acknowledgement

The authors acknowledge the funding from the Schulich Research Chair in Renewable Energy at the University of Calgary, Canada. We also acknowledge WestGrid Canada for providing high-performance computers to perform flow simulations.

References

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Copyright information

© Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.Department of Mechanical and Manufacturing EngineeringUniversity of CalgaryCalgaryCanada

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