Study of hydrodynamic luminescence in a cavitation liquid medium

Keywords: cavitation, visualization, hydrodynamic luminescence, throttle, triboluminescence, cavitator, visualization offluid flow, temperature, viscosity, cavitation number, sonoluminescence, fluidflow rate.

Modem hydraulic equipment operates at significant working pressures, which can reach 400 bar. In hydraulic devices, hydro systems as shutoff and regulating elements are widely used different types of chokes, nozzles, throttles, etc. [1, 2]. In the diaphragm and cylindrical chokes, the flow of the working fluid has a pronounced turbulent character, and in the compressed section, high speed causes cavitation and the active allocation of bubbles of undissolved air and steam [2-5] associated with it. The phenomenon of cavitation is often harmful to the operation of hydraulic systems. Since the appearance of cavitation is observed in a stream only under certain conditions, and it may be unstable when changing, it becomes necessary to create systems that will provide a stable cavitation regime [2-7]. Cavitation leads to the appearance of a number of physicochemical processes, may be accompanied by cavitation chemical reactions, oxidation, destruction and luminescence. State of the problem. The urgency of studying the phenomena of hydroluminescence is due to the widespread use of dielectric pipelines and local resistance. In addition to simulation, the study of the flow of fluid in the cavitator can be carried out by involving experimental methods of research. Experimental methods help visualize the movement of the fluid in the nozzle to further analyze the processes of the medium [5].

Observation of the origin of cavitation processes can be accomplished by visualizing a fluid stream with a highspeed video recording of the work process in a cavitator with a transparent material casing [10-12]. Thus, in the indicated way, it is possible to analyze the process of occurrence of cavitation bubbles and the emission of light, as well as to obtain empirical calculations of dependence.

After passing through the rarefaction zone, in the zone of high pressure formed bubbles are instantly locked up, which causes the emergence of powerful spherical waves and cumulative jets, which leads to erosive destruction of the material of the walls of the channel and the emergence of active acoustic processes (Fig. 1).

Fig. 1. The emergence of cavitation in the contraction region of the flows

Problem statement. The phenomenon of hydroluminescence is widely used as a method of visualization of cavitation. With the help of hydrolumnescence, ionize nanoparticles and electrify liquids, for example, varnishes or paints in order to achieve a more uniform coating of parts with paint. In this case, the liquid acquires a positive charge, and the walls of the channels are negative. In missile carriers, unauthorized cavitation with the appearance of hydrolumnescence in the fuel system (pump) may lead to an explosion of the fuel system of the rocket carrier [3-5]. The question remains of the physics of the process, which is due to the occurrence of hydroluminescence and its estimation in accordance with the theories proposed. To study cavitation processes and managing them it is necessary to: clarification of the discharge coefficient for the throttle of a hydrodynamic cavitator, determination of cavitation regimes in which hydroluminescence is observed, and factors influencing its intensity and managing them.

Purpose of the paper. The presented paper is aimed at is a study of the process of occurrence of cavitation phenomena and an experimental study of hydrodynamic cavitation accompanied by hydroluminescence.

Problem setting. In accordance with [12], for some processes it is necessary to determine the nature and conditions for the occurrence of cavitation processes, which are accompanied by hydroluminescence.

For study cavitation processes, a turbulent throttle model was created with the possibility of changing the channel cross sectional area. The screw is made from ebonite - a material with good cavitation emission of electrons.

Before the experiment, the temperature and pressure sensor were calibrated with a reference thermocouple and a manometer, and the digital flow meter calibrated with volumetric flow meter.

Cavitation intensity was estimated using a graphical editor, determining the weighted average brightness level of the image pixels obtained by multiplying each brightness level by the number of pixels of a given level, and then divided by the total number of brightness levels.

For an in-depth study of light emission, a schematic diagram of an experimental stand was developed (Fig. 2). The stand is built on the basis of a hydraulic station, and measuring equipment for recording the differential pressure on the model and the val-

Fig. 2. Basic hydraulic diagram of the test bench (1 - cavitation nozzle, 2 - speed camera)

For the study of cavitation processes, a model of a hydrodynamic cavitator with a regulated orifice was created. The element that controls the cross-section - the screw is made of brass. (Fig. 3).

In the fluid moving in the stream, with the growth of the velocity vector, cavitation bubbles begin to appear (Fig. 4), the size of which increases with increasing pressure drop.

At a flow velocity of more than 40 m / s, visually the cavitation has a pronounced appearance with a well-formed cavitation torch and the appearance of light radiation (Fig. 4). In addition to the appearance of cavitation bubbles, it was observed that there were light effects in the narrow channel. The light emission was observed at a pressure drop of 1,8 MPa and was manifested in the form of sparks that slip along downstream in the central part of the canal.

The discharge coefficient of working fluid through the nozzle was determined:

where Qa - the actual flow rate, Qt - the theoretical flow rate.

Visually managed to trace the process of nucleation and development of the cavi- tational zone up to the moment of hydroluminescence [11; 12]. The study of the cavitation process was carried out for mineral oil of category H-LP and domestic mineral oil «Leol M20», in which the cavitation in the stream was more intense. No light emission was observed. It can be assumed that the dependence of light radiation on the properties of the base of mineral oil, the quantity and type of gas, water, the pressure of its saturated vapor and the composition of the additive package.

Fig. 4. Visualization of the occurrence of cavitation (t = 50°С, pressure drop, (P1-P2) = 4 МPa, flow rate, Q = 1,1 l/min, diameter of the throttle, d = 0,6 mm)

To prevent cavitation or its use at the certain level, it is proposed to create a design of a hydrodynamic cavitator, which will automatically adjust the flow rate in the throttles and maintain the required number of cavitation numbers to account for the use of a thermosensitive element.