Droplet Evaporation to Boiling in Van Der Waals Fluid

Superheated droplets vaporized in a vacuum or vapor chamber was studied using smoothed particle hydrodynamics method. The two-phase fluids of vapor and liquid with a diffused interface were modeled by Navier-Stokes-Korteweg equations, with the van der Waals equation of state. The liquid-vapor separation was processed with different initial temperature and density. Simulation was firstly validated by the proper equilibrium states with analytical data from the binodal line of Maxwell construction, as well as the droplet surface tension compared with experimental and other groups’ numerical data in a good agreement. Droplet morphology, density, temperature and entropy increment were then carefully examined during the dynamic process from evaporation to boiling. Four phase transitions patterns were concluded, namely from surface evaporation, internal bubble, fragment and explosion, to flash boiling with increasing equivalent superheat degree that is defined by the non-dimensional superheat temperature over surface tension. Results showed that droplet temperature and density decreased slightly at surface evaporation when equivalent superheat degree was lower than 1/3. Above this criterion, the internal bubble could be sustained. Growth of new liquid nucleates and connected secondary liquid ring were calculated by domain growth theory. We found that droplet continuous to expand at high superheat but withdrawn at equivalent superheat degree equal to or lower than 0.52. At equivalent superheat degree higher than 1, the growth and expansion of droplet resulted in liquid fragment and explosion to many small secondary droplets. Mean mass and diameter of these droplets were found to have power-law dependency on the expansion rate. Finally, fast flash boiling occurred at equivalent superheat degree higher than 3. The vapor pressure in the chamber showed a negative linear correlation between the overpressure and the atmospheric pressure. We also found out that comparing to increase the pressure value, dense gas had better efficiency on retarding the propagation of shock wave.