Study On Heat And Mass Transfer During Rapid Evaporation Of Multi-Component Droplet Under Negative Pressure

With the development of technology,droplet evaporation processes are involved in many industries.The systems and equipment for handling liquids are updated constantly for the purpose of saving resources,reducing pollutants generated by emissions,and protecting the natural environment.For example,studying the evaporation characteristics of liquid fuel in the cylinder can optimize the efficiency of the engine,boiler wastewater can be evaporated to remove heavy metal ions,seawater droplets can use multi-stage flash evaporation technology to obtain fresh water,and studying the evaporation process of multi-component droplets in a rapid depressurization environment can improve the performance of space suits.Therefore,studying the evaporation process and characteristics of droplets of different components is of positive significance for the sustainable development of our country’s industry and agriculture.Based on the Fick law and the VOF(Volume of Fluid)method to capture the gas-liquid interface,this paper establishes an evaporation model suitable for a single droplet in a environment of rapid depressurization,and sets the corresponding thermophysical model according to the fluid phase state.In this paper,the discrete method of the control equations,the method of solving the flow field and the corresponding program modules of the numerical model are described in detail,and the morphological changes and heat and mass transfer characteristics of the droplet evaporation process under the negative pressure environment are analyzed.The droplet suspension method was used to build an experimental system for the depressurization and evaporation process of a single droplet.The experimental data of the droplet center temperature during the evaporation of pure water droplets,anhydrous alcohol droplets,multi-component alcohol droplets and brine droplets are used to verify the model.The article uses cloud images to show the morphological changes of stationary droplets during the phase transition and the changes in the flow field,temperature field,and vapor distribution inside the droplets over time.The results show that in the initial stage of evaporation,the environmental pressure decreases rapidly,the diameter of the droplet rapidly decreases.Due to the velocity field at the interior and boundary changes with time,the droplet shape and the distribution of vapor in the environment will also change during the depressurization evaporation process.The article also analyzes the changes in the center temperature and evaporation rate of the droplet when the ambient pressure is reduced from atmospheric pressure to 540Pa～2000Pa and the droplet diameter is 1mm～2mm.For single-component droplets: the center temperature of the droplet is directly proportional to the change in the final environmental pressure,and the rate of rise of the center temperature of the droplet is proportional to the initial temperature of the droplet;and the final environmental pressure has a greater impact on the evaporation rate of the droplet in the initial stage.The initial droplet diameter and initial temperature have a greater impact on the later evaporation rate changes.For multi-component droplets: the higher the initial alcohol concentration,the lower the center temperature of the droplet is,the faster the temperature rise in the later stage of evaporation is,and the faster the droplet evaporation speed is;but the greater the initial salt concentration,the higher the center temperature of the droplet will evaporate is,the slower the temperature rise in the later period is,the slower the droplet evaporation speed is.