Abstract
The purpose of this paper is to investigate experimentally the influence of the cavitation extent on the pressure and velocity fluctuations in a small convergent–divergent channel. The mean cavity length is determined from high-speed photography images. The mean pressure and the intensity of the pressure fluctuations are obtained from the transient pressure signals recorded by two pressure transducers at the inlet and outlet of the test section. The statistical turbulence quantities are derived from the instantaneous velocity fields measured by the laser-induced fluorescent particle image velocimetry (PIV-LIF) technique. The experimental results show that the decrease of the cavitation number (the increase in the extent of cavitation) leads to a rise in the turbulent fluctuations in the wake region due to the impact of vapour clouds collapsing, while the presence of a vapour phase is found to reduce the streamwise and cross-stream velocity fluctuations in the attached cavity. It might be attributed to two mechanisms: the presence of a vapour phase modifies the vortex-stretching process, and the cavitation compressibility damps out the turbulent fluctuations. Similar effects of cavitation are also observed in the pressure fluctuations.
Highlights
Hydrodynamic cavitation is a complex phenomenon involving mass and heat transfer between liquid and vapour phases at nearly constant temperatures
We investigated the cavitating flows developed in a 2D convergent–divergent channel at different flow rates and cavitation numbers through the techniques of high-speed photography, transient pressure measurement and particle image velocimetry (PIV)-laser-induced fluorescent (LIF) measurement
This result is consistent with the measurements of Jahangir et al [21] in an axisymmetric converging–diverging nozzle. Another parameter of the pressure ratio Pr = Pout /Pin is plotted as a function of the cavitation number shown in Figure 7c, and it is observed to be independent of the inflow velocity
Summary
Hydrodynamic cavitation is a complex phenomenon involving mass and heat transfer between liquid and vapour phases at nearly constant temperatures. It typically occurs in some widely used hydraulic machines, such as turbines, pumps and propellers, when the local pressure is reduced below the vapour pressure. In order to obtain a better physical understanding of this complex phenomenon, the simple geometry venturi channel or hydrofoil are often used for investigation instead of real hydraulic systems. As reported by Leroux et al [1], when the cavity length increased to be larger than half the hydrofoil chord length, the cavity was transformed from
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