Research On Bubble Growth In The Process Of Droplet Evaporation During Depressurization

The liquid flash evaporation is widely used in modern industry, and there are practical research values of liquid flash evaporation in aerospace and safety production of the national economy. The study of bubble growth within a single droplet during depressurization is a necessary part of the research on liquid flash evaporation. Aimed at the bubble growth in the process of droplet evaporation during depressurization, this thesis took the study by using the methods of empirical research and theoretical analysis.In the experimental research, a n experimental system which can heat droplets was designed and built by the droplet suspension method, which had a good testing repeatability and high-accuracy showed by the experimental data. The experiment used distilled water and pentane as the working medium, the init ial temperature of the droplets was during 20~40℃, the environmental pressures were from 1 atm to 200~2000Pa. The results show that to improve initial temperature, the droplets are more likely to burst, and the thermal state is closer to the saturation line, but still not into the overheating zone. With a higher fina l environmental pressure, the time that the bubble from generated to rupture is longer. After the water droplet burst, the remaining liquid on the thermocouple node continue to evaporate because the low environment pressure, so the temperature measured by thermocouple keeps dropping till the leftover moisture evaporate completely. However, the pentane droplets have different phenomenon with water droplet because the low boiling point and low viscosity of pentane. When the environmental pressure decreases suddenly, there is bubble generates within the pentane droplet and the droplet shaking severely. With the bubble becomes larger, the droplet will fall from the thermocouple and the temperature measured by thermocouple raises quickly.In the theory research, a mathematical model was developed to simulate the bubble growth process within a single droplet during flash evaporation. The model was based on several reasonable simplifying assumptions and the droplet flash model, and coupled the momentum equation for bubble growth and the energy equation for droplet flash evaporation. The calculated results show that the process of bubble growth can be divided into initia l stage, transition stage and accelerating stage; the bubble growth is influenced b y the pressure difference, the surface tension and the viscous stress of bubble interface. The differential pressure can promote the growth of bubbles, but the surface tension and viscous stress inhibit it. The different stages during bubble growth process are the outcomes of combined action of these three forces. The temperature of bubble interface can be divided into remained stage, gradually decline stage and rapidly drop stage, which is corresponding to the three stages of bubble growth. But the droplet surface temperature stays at a near-constant numeric, because the bubble growth rate is much faster than the interface motion caused by droplet surface evaporation. With the increase of final environmental pressure, the time when the bubble begins to grow is delayed and the bubble growth rate and bubble growth acceleration are decreased. The higher the droplet init ial temperature is, the shorter the initial stage of the bubble growth will be, and the greater the bubble growth rate and bubble growth acceleration will be. A bigger droplet initial size causes a smaller bubble radius and then the bubble growth rate and bubble growth acceleration are smaller.