In-Situ Measurement of Liberation of a Dissolved Gas in Unsteady Cavitating Flow in Water

Abstract

Cavitation is a phenomenon in which phase change occurs in a liquid by pressure decrease due to flow acceleration. The phase change is caused by mainly evaporation of the liquid but sometimes by liberation of dissolved non-condensable gas in the liquid. In particular, unsteady cavitation causes vibration, noise, erosion and performance deterioration, which has been a serious problem in the development of fluid machinery. Therefore, it is important to research the characteristics of cavitation generation and develop methods to suppress or control it. In the current CFD (computational fluid dynamics) model of cavitating flow, the saturated vapor pressure has been used as a criterion for determining the cavitation generation or disappearance based on the idea of phase equilibrium, however it is well known that these calculation results don’t agree well with experimental results. For example, it is reported that the cavitation inception pressure is higher than its saturated vapor pressure in water. This is predicted to be resulting from the generation of gaseous cavitation which is caused by liberation of dissolved air, however this has not been taken into consideration in the current CFD model. Here, liberation of non-condensable gas is supposed to be treated by MD (molecular dynamics) then it is not suitable for CFD. Thus, in order to develop a more accurate CFD model for cavitating flow, it is necessary to develop a macroscopic and coarse-grained model of liberation should be developed, which may be related to flow dynamic-stimulation of the unsteady flow field with cavitation. In the present study, we focus attention on relationship between liberation of dissolved gas and unsteadiness of cavitation. Experiment is conducted in high-temperature water cavitation tunnel in which in-situ measurement of the amount of dissolved oxygen can be performed during the operation with cavitation. The variation of dissolved oxygen is used as one of the indexes of liberation of dissolved non-condensable gas during the experiment. The degree of cavitation unsteadiness is judged by calculation based on the FFT (Fast Fourier Transform) of the downstream fluctuation pressure and the RMS (root mean square) of brightness value using images taken with a high-speed camera. In addition, in order to eliminate the factors of dissolved gas liberation other than cavitation unsteadiness, the mainstream pressure, the mainstream temperature and volume of the cavity are made to be equal, respectively. Under the above preconditions, the time evolution of dissolved oxygen amount is measured in several kinds of cavitating flow fields around NACA0015 and NACA16012 hydrofoils.