Experimental Study Of Heat And Mass Transfer Of Flash Evaporation

Flash evaporation is widely applied to seawater desalination, waste heat recovery, steam accumulator and low temperature refrigeration. To enhance flash evaporation experimental study, not only can lead to a better understanding of the internal mechanism of flash evaporation, and also can provide a theoretical support to further promote the industrial application of flash evaporation.This paper has improved a new flash experimental system can be applied to conditions of high pressure and vacuum pressure of flash experiments, which using high-speed video camera observation system to observe and record the whole process of flashing,vertical-placed temperature sensor in whole flash chamber can precise measurement the temperature distribution of Vapor-liquid phase space, pressure sensors and data acquisition system real-time measure and record the experimental data of flash evaporation.In this paper, flash evaporation experiment was carried on using deionized water in pressure - vacuum conditions, the experiment conditions were as follows: initial temperature of 55?100?, superheat 3?45?, initial level height 400mm ?500mm,the differential pressure 24 ?89kPa. According to flash image recording by the High-speed cameras,the causes of the phenomenon of violent vaporization is discussed, and the characteristic parameters of flash phenomena including swelling height, the maximum of penetration depth and penetration velocity were quantitative analysed, at last empirical formula by fitting the initial conditions for the characteristic parameters of flash evaporation is derived. In the same time, image gray analyses method is used to deal with images during flash evaporation, the average gray value of the flash image and the initial temperature, as well as the relationship between initial superheat and initial level were studied, which lead to the conclusion that average value of flash gray decline rate increases with the degree of superheat and the initial temperature and faster, and the initial water level changes in its little effect. It demonstrates the proposed initial conditions influence on the characteristic parameters from the perspective of image analysis, it also provide a new idea to flash evaporation process visualization.This paper also studied the influence of the initial conditions of flash evaporation on the characteristic parameters NEF of flash evaporation, including the impact of the initial water temperature, initial level of height and initial superheat. Besides, the idea of integral curve was used to analyze NEF curve versus time, the parameter, the flash factor, for quantitative description of the whole process of flash was proposed. And the variation of the flashing coefficient under different initial conditions were studied. It has been found that flashing coefficient increases with the initial temperature and superheat decreases, the initial level of changes had little influence. By the multi-point thermocouple installed in the flash chamber,the vertical distribution of temperature in the flash chamber was studied. It found that vapor liquid space occured apparent temperature difference. Especially the variation of gas-liquid interface temperature was different with vapor and liquid space temperature. And the influence of initial conditions on water temperature was studied. Simultaneously, the variation of pressure in the flash process Under the different initial temperature and different initial water level was studied. The smaller the initial temperature, the faster the pressure drop.This article describes the internal variation of flash process based on the heat and mass transfer mechanism, and quantitative study of the influence of the initial conditions on the flash evaporation. The study provides a theoretical support for flash evaporation to further industrial application, and there are a good guide for solving the problems in the practical application of flash evaporation.