Thaithacha Sudsuansee, Udomkiat Nontakaew and Yodchai Tiaple

pp. 51 - 60

Abstract

Cavitation caused by phases exchange between fluids of large density difference occurs in a region where the
pressure of water falls below its vapor pressure. The density of water in a water-vapor contact area decreases dramatically.
As a result, the flow in this region is compressible, which affects directly turbulent dissipation structures. Leading edge
cavitation is naturally time dependent. Re-entrant jet generated by liquid flow over a cavity is a main actor of cavity shedding.
Simulation of unsteady leading edge cavitation flows through a 4-blade runner bulb turbine was performed. Particular attention was given to the phenomena of re-entrant jet, cavity shedding, and cavitation vortices in the flow over turbine blade.
The Reynolds-Average Navier-Stokes equations with finite volume discretization were used. The calculations were done
with pressure-based algorithms since the flow possesses a wide range of density change and high complexity turbulence.
The new formula for dilatation dissipation parameter in k-ω model was introduced and the turbulent Mach number was
calculated from density of mixture instead. 2-D and 3-D hydrofoils based on both numerical and experimental results accomplished a validation. The results show that re-entrant jet, shedding of cavity, and cavitation vortices can be captured. In
addition, this paper also calculates the cycle frequency of torque generated by the runner and vapor area evolution on the
blade surface. The cycle frequency varies with cavitation number. At normal operation of this turbine (σ = 1) it is found that
both of them have a frequency of 46 Hertz.

Keywords

• cavitation
• turbulent
• computational fluid dynamics
• dilatation dissipation
• bulb turbine