Experimental study on energy transformation of static liquid pool during flash evaporation

Abstract

Energy transformation during static liquid pool flash evaporation of pure water is examined on the basis of flashing experiments with superheats between 2 and 38 °C, initial water height ranging from 50 to 170 mm, and an initial liquid pool temperature from 50 to 85 °C. Energy balance link the flashed mass with the liquid pool temperature and then measure the variable parameters influence on several quantities: cooling rate, flashing time, and liquid mass evaporated. The non-dimensional numbers, Jakob number (Ja) and Non-Equilibrium Fraction (NEF) are applied to evaluate the heat transfer process. In addition, parametric study is performed to address the effect of superheat, initial temperature, and depressurization on the flashing phenomenon. Finally, a comparison of energy transformation during normal cooling and flash cooling is proposed. Results suggested that at higher initial temperature and lower initial pressure, a lower value of the equilibrium temperature obtained, and a more rapid temperature drop occurs. The maximum deviation obtained between the experimental and calculated ones is 11 % for larger vacuum tank pressure. The cooling rate and the flashing time also affected by these flashing parameters. With the increase of initial temperature by 15 °C, the cooling rate is enhanced by 41 %, and by dropping the vacuum tank initial pressure by 34.4 kPa, the cooling rate is improved by 56 %. A combined effect of different initial conditions on the flashing process has been shown to better understand their impact on evaporated mass. The present procedure is useful in designing the flashing system for a variety of thermal applications.