EXPERIMENTAL RESEARCH OF A MICROJET CAVITATION

Abstract: The paper presents some results of a cavitation research behind a micro-orifice. Investigated were the conditions of the origin of cavitation represented by parameters such as upstream pressure, downstream pressure, liquid temperature and cavitation number. Presented are also images of a cavitating microjet made by the high speed high definition camera RedLake Y3. Dimensions of a microjet are: diameter 0,3 mm; length 0,5 mm.


INTRODUCTION
Recently a great attention is paid to a research of the cavitation in microdevices.It is related to a development of microfluidic applications and also with an exploration of the cavitation as a useful phenomenon: surfaces cleaning, material disintegration, cold disinfection, etc.We dealt in our institute with the research of cavitation in micro-orifices for purpose of development of a device intended to a munition disposal and rock drilling.Results of the research are applicable in a jet optimization for purpose of maximization of required effect of cavitation.This paper presents a test rig where the research was done and some results as well.

RESEARCH METHODOLOGY
The research consisted of three main parts: CFD simulation, flow visualization, and study of other effects such as acoustic (ultrasound) emissions.This paper dealt with the experimental research of cavitation behind a micro-orifice at very low cavitation numbers when a cavitation cloud is drifted from the space of the orifice and collapses at some distance behind it (supercavitation).Respect to velocities in a micro-orifice (up to 100 m/s) and respect to its dimensions the flow was photographed by a high speed high definition camera.Erratic length of the cavitation cloud was analysed from the camera images.Acoustic effects were recorded by an ultrasound sensor of acoustic emissions.As a liquid was used water with known temperature.

TEST RIG DESCRIPTION
In the Fig. 1 is the scheme of the test rig.A base part is a high pressure pump supplying water to the microfluidic chamber where is placed the micro-orifice.Water then flows to the drain.The space behind the orifice is photographed by a high speed HD camera RedLake Y3.Pulsed light source is placed under the transparent chamber.The camera is connected to the computer where the images are saved.Following parameters were also measured: Input pressure P1 -piezoresistive pressure sensor (BD Sensors), Range: 0-600 bar, Output: 0-10 V; Output pressure P2 (in chamber) -piezoresistive pressure sensor (BD Sensors), Range: 0-1 bar, Output: 0-10 V. Mass flow rate: Weighing machine UWE GM-11K, sensitivity 1 g.Acoustic (ultrasound) emissions were taken from a chamber wall with the sensor Ultrasound Sensor Amplifier (Physical acoustic corporation) Model 2/4/6C, Amplification: 20/40/60 dB.

CAVITATION NUMBER
Dimensionless number quantifying the cavitation is called cavitation number and is usually denoted as ı.Cavitation number for nozzles and orifices may be written according classical definition as follows: In a case of the orifice is as reference pressure p r taken the downstream pressure sufficiently far behind the orifice opening and as a reference velocity v r is taken a velocity in the orifice opening.p v is the vapor pressure of the liquid, Ǐ is a liquid density.

RESULTS OF EXPERIMENTS
The goal of the visualization was an estimation of a flow pattern at different values of cavitation number.From the pictures shot by the camera it is possible to estimate a cavitation cloud length which is changing within a certain range.

Figure 1 :
Figure 1: Scheme of the test rig

Figure 5 :
Figure 5: Typical power spectrum and amplitude histogram for supercavitation.

Figure 6 :
Figure 6: Typical power spectrum and amplitude histogram for cavitation.

Figure 7 :
Figure 7: RMS dependence on a cavitation number ı