Non-stationary collective effects in a caviting jet


Аuthors

Biryukov D. A.1, Gerasimov D. N.2, Peretiatko V. V.1*

1. Joint Institute for High Temperatures of the Russian Academy of Sciences, 13, Izhorskaya str., Moscow, 125412, Russia
2. National Research University “Moscow Power Engineering Institute”, 14, Krasnokazarmennaya str., Moscow, 111250 Russia

*e-mail: peretyatkovika2001@gmail.com

Abstract

The paper discusses the determination of the speed and frequency of a cavitating flow by statistical processing of a number of photographs obtained. A pressure wave in a glycerin flow was detected and its speed was determined.
Cavitating flow occurs when liquid flows through a constricting device made of organic glass. The smallest element of a constricting device is a channel with a diameter of 1 mm and a length of 1 mm. When a dielectric flows through such constricting devices, a glow occurs in a 1 mm channel. The study of hydrodynamic processes occurring when liquid flows through a constricting device is a part of a study aimed at interpreting the phenomenon of hydroluminescence.
The non-stationary dynamics of vapor cavities leads to non-linear effects: the growth and collapse of bubbles in one region of the channel lead to a change in pressure in the liquid, which, in turn, affects the dynamics of bubbles in another part of the channel. This work is devoted to the study of these collective effects.
Usually, contact research methods are used to measure velocities, but in fast processes their use is complicated by the introduction of disturbances into the flow, so it becomes necessary to involve other methods. In fact, bubbles are convenient natural tracers for diagnostics by the PIV method, but in this case, there is a number of features.
When studying the cavitating flow, high-speed shooting of the narrowing device was performed. The device was located between the camera and the light source, so the cavitating bubbles in the resulting frames were darker than the flowing liquid, which made the analysis possible. Water at 20 ℃ was used as the liquid under study. The inlet pressure was set equal to 40 atm. The shooting was performed at a frequency of 77,000 fps. In the resulting frames, pixels were selected on the same section of the frame and their brightness values were recorded.
The study examined the flow of glycerin. During the study, pressure waves arising in the experimental section were detected. The pressure wave is identified by a short-term increase in brightness along the channel. Such a change in brightness was visually detected in the glycerin flow starting from an input pressure of 70 atm, the outlet pressure was assumed to be constant and equal to atmospheric. Using the previously described technique, it was found that the speed of the pressure wave was 300 m/s.
The study of the dependencies of brightness changes over time made it possible to determine a number of important effects. Collective processes in a two-phase cavitating flow have a pronounced periodic nature, and the frequency of these processes is very high and cannot be correlated with any external parameters, in particular, with the frequency of the pump. A pattern of bubble appearance in a 1 mm channel and the appearance of a shock wave in the channel has been revealed. The results obtained seem to be very promising for the clarifying the physical mechanisms operating in a medium with a non-stationary gas phase and studying hydroluminescence.

Keywords:

cavitation, high-speed photography, correlation analysis, hydroluminescence, hydraulic shock, cavitating jets, collective processes

References

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