Ultrasonic Cavitation In Supercritical Carbon Dioxide

The substance will be in supercritical state with the temperature and pressure above thefluid critical point. The supercritical fluid is different greatly from the gas and liquid inphysicalproperties. The related technologyofsupercritical fluid has been widely developed indifferent fields. However, it is still unknown whether cavitation can be induced in thesupercritical fluid. Therefore, the exploration of cavitation in supercritical fluid becomes ahotspot in both supercritical fluid technology and ultrasound technology.In this paper, with Rayleigh’s cavitation theory and physical parameters of supercriticalcarbon dioxide, the threshold pressure is analyzed to induce cavitation in supercritical carbondioxide, which is similar to the critical pressure due to the absence of vapor pressure.Furthermore, it is easier to induce cavitation by introducing the gas that has the upper criticalpoint to form artificially the nucleus.It is necessary to consider the fluid compressibility during discussing the cavitation ofsupercritical fluid because of the higher compressibility than the liquid such as water.Therefore, the paper analyzes the movement of a cavitationbubble in the supercritical fluid byapplying the dynamic model including the fluid compressibility, namely Gilmore equation.Since the solution of Gilmore equation depends on the exact relationship between fluidpressure and density, which is the state equation of fluid. It is necessary to obtain therelationship of the pressure and density which is convenient to calculate in the Gilmoreequation with the improved accuracy. The paper proposes a state equation based on the modelof state equation of the expansion liquid, according to the experimental data of density andpressure of supercritical carbondioxide provided by the U.S. BureauofStandards. Finally, theradius versus time relationship of a cavitation bubble is obtained by the solution of Gilmoreequation using the the Runge-Kutta algorithm on the MATLAB platform. Then, three modelsof Flynn, Lastman-Wentzell and Tilmann considering the fluid small compressibility iscompared withthe Small Mach-number model which is compared with the Gilmore equationconsidering the larger compressibility at the same time. It is concluded that Gilmoreequation is more suitable to simulate the motion of bubble in supercritical carbon dioxide.In this paper, the cavitation bubble is analyzed by considering many factors, includingthe supercritical fluid compressibility, the initial radius of gas bubble, the frequency andpressure of ultrasonic. It is found that the compressibility attenuates largely the amplitude ofmotion. The initial bubble radius has the weaker effect. The larger the initial radius is, thegreater the maximum radius of the bubble expands and the longer oscillation period. The sound pressure is the greatest impact on the bubble motion, whose tiny increase can arousethe sharp expansion of bubble motion. The ultrasonic frequency influences less. During thereduction of the frequency, the motion of the bubble is attenuated and the cycle extends.