Experimental Study On The Static Flash Evaporation Of Solution

The flash evaporation is a phase change process,which is accompanied by intense heatand mass transfer.It has been widely used in many industrial fields,such as the multi-stage flashing in seawater desalination,the drying and purification process in food and beverage production,the salt production,the wastewater treatment and the cooling of electronic and aerospace devices.In this paper,a static flash evaporation experiment unit is designed and built to study the homogeneous flash evaporation process of solution.The imbalance phenomenon in the flash evaporation process and the dynamics of flash evaporation wave are analyzed.In addition,the nucleation and the growth of the single bubble in the superheat solution is analyzed experimentally and theoretically to explore the mechanism of flash evaporation.The main works and conclusions of this paper are summarized as follows:The temperature,pressure,NETD and NEF of pure water,Na Cl solution and ethanol solution are measured during their flash evaporation process by taking into account different superheats,initial vacuum chamber pressures and pressure reduction rates.The results show that the decrease of superheat,the increase of the initial pressure of the vacuum chamber,the increase of the depressurization rate,the decrease of the mass fraction of the Na Cl solution and the increase of the volume fraction of the ethanol solution all decrease the NETD in the flashing evaporation process,respectively.On the other hand,the depressurization rate,the mass fraction of the Na Cl solution and the volume fraction of the ethanol solution play the same roles on NEF as on NETD.However,both the increase of superheat and the decrease of the initial pressure of the vacuum chamber will decrease the NEF.The experimental correlations of NETD as a function of the operating parameters are obtained from the experimental data.In order to explore the mechanism of the flash evaporation phenomenon,the experimental studies on the growth of a single bubble in the flashing evaporation process of pure water and Na Cl solution are conducted.The theoretical solutions of the bubble growth are also obtained by solving the momentum equation,energy conservation equation and massdiffusion equation inside and outside the bubble.The results show that the theoretical solutions fit the experimental data very well.It is concluded that the bubble growth rate in pure water and Na Cl solution increases with the increase of superheat and the decrease of the initial pressure of the vacuum chamber,and decreases with the increase of the mass fraction of Na Cl solution.This conclusion is consistent with the imbalance analysis from the above experimental studies.The experimental studies on the evolution of flash wave during the flashing evaporation process for pure water and Na Cl solution are finally conducted.The results show that the increase of the superheat,the decrease of the initial pressure of the vacuum chamber,the increase of the depressurization rate,and the increase of the mass fraction of the Na Cl solution all increase the maximum depth and the velocity of the flash wave.