Abstract
Variations in cross-sectional areas may lead to pressure drops below a critical value, such that cavitation and air release are provoked in hydraulic systems. Due to a relatively slow dissolution of gas bubbles, the performance of hydraulic systems will be affected on long time scales by the gas phase. Therefore predictions of air production rates are desirable to describe the system characteristics. Existing investigations on generic geometries such as micro-orifice flows show an outgassing process due to hydrodynamic cavitation which takes place on time scales far shorter than diffusion processes. The aim of the present investigation is to find a correlation between global, hydrodynamic flow characteristics and cavitation induced undissolved gas fractions generated behind generic flow constrictions such as an orifice or venturi tube. Experimental investigations are realised in a cavitation channel that enables an independent adjustment of the pressure level upstream and downstream of the orifice. Released air fractions are determined by means of shadowgraphy imaging. First results indicate that an increased cavitation activity leads to a rapid increase in undissolved gas volume only in the choking regime. The frequency distribution of generated gas bubble size seems to depend only indirectly on the cavitation intensity driven by an increase of downstream coalescence events due to a more densely populated bubbly flow.
Highlights
Hydrodynamic cavitation in liquid flows can be produced by a pressure drop behind a flow constriction
Variations in cross-sectional areas may lead to pressure drops below a critical value, such that cavitation and air release are provoked in hydraulic systems
Existing investigations on generic geometries such as micro-orifice flows show an outgassing process due to hydrodynamic cavitation which takes place on time scales far shorter than diffusion processes
Summary
Hydrodynamic cavitation in liquid flows can be produced by a pressure drop behind a flow constriction. Since most liquids used in technical applications contain dissolved gases both vaporous cavitation and degassing processes may take place simultaneously at different intensity depending on the pressure conditions and solubility of gases in liquids. The two phase flow has an effect on the density, speed of sound and compressibility of the flowing liquid This can lead to noise, vibrations and fluctuation of the mass flow rate in hydraulic systems and lower the efficiency of the system. Most flow cavitation experiments are realised in small scale and/or rectangular cross sectional channels, e.g. A rectangular channel cross section enables a better optical access but leads to topology related corner flow and cavitation behaviour. Due to this fact, the experiments described in this paper are restricted to rotationally symmetric cavitation flows in generic geometries. Decreasing cavitation numbers correspond to an increase in cavitation probability or intensity, respectively
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