Mechanism And Experimental Research Of Droplet Flash Evaporation In Circulating Spray Flash

When a liquid is at a temperature above the saturation temperature for its pressure, it is superheated and is out of thermodynamic equilibrium. One of the ways to make a liquid at its saturation temperature and pressure gain superheat is that let the liquid be depressurized rapidly so that its thermal inertia ensures the internal temperature remains nearly constant and above its new saturation temperature. That is named flash evaporation. Since flash evaporation is applied to so many fields such as desalination, refrigeration and combustion engines, it is important to study its mechanism and promote its applications.In this paper, the methods above intergrading numerical simulation and experiment are applied to study the mechanism of flash evaporation and cycle spray flash evaporation. Works are mainly focusing on following aspects:The equations that describe the heat and mass transfer of a droplet are present by establishing diffusion controlled evaporation model. Then MATLAB is used to build a mathematic model of that process and the change laws of diameter and temperature are found in this way. The paper find out that influence factors are initial temperature of liquid, pressure of surrounding and initial diameter of drop. In addition, the internal thermal resistance is studied in this paper.Geometric model of droplet is built by Auto CAD and mesh generation is finished by ICEM CFD. To accomplish the numerical modeling, the software choose in this paper is FLUENT. The turbulence model is Large Eddy Simulation(LES). VOF method is used to catch the interface of two phase. Phase change process is described by programming and inserted by user-define-function. Above all, the feasible numerical simulation method is set up.The numerical simulation results of static droplet show the curve that vaporization rate of liquid changing with time. It turns out that this change rule is matched with the result of chapter two. That means this method is reasonable. By studying dynamic droplet, Marangoni Effect caused by surface tension is discovered in the drop. Comparing static droplet and dynamic droplet, it is found out that enhancing convection can affect vaporization but do little favor to vaporization rate.The experimental apparatus of circulating spray flash was set up by adding a flash tank to the heat recovery boiler. It can increase the amount of steam and decrease the temperature of exhaust gas. Since when liquid get through the spray, they can be atomized to drops. At this point of view, the phenomenon of droplet flash evaporation can be observed. The results present that vaporization rate changes with initial temperature and environment pressure, which validates the reliability of numerical method.