Investigation On Efficient Droplet Spray Cooling

When droplets impact the solid wall,they will break down and deform rapidly,and then spread on the solid wall.When there is a temperature difference between the droplet and the substrate,the heat transfer will occur.Therefore,the droplet impingement and spreading involve complex phenomena in the terms of fluid mechanics,thermodynamics,surface chemistry and so on;In addition,droplet impingement and spreading are also a common phenomenon in daily life.It is also widely used in power generation,inkjet printing,power electronics and agriculture.Therefore,it is not only of great theoretical significance,but also of great practical value to explore the spreading process of droplets.In this thesis,the impact spreading process of droplets on solid substrates will be studied by numerical calculation.The gas-liquid interface in spreading process is captured with Level Set Method,a two-dimensional axisymmetric model is established.The dynamic contact angle of the droplet spreading stage is described with Molecular Kinetic Theory,in which the contact angle is related to the velocity of moving contact lines.After comparing the numerical simulation results with the experimental results,it is found that the numerical model based on molecular dynamics theory agree well with the actual droplet spreading process,while the numerical model with constant angle has much deviation.Next,based on the numerical model obtained above,the effects of the We,Re and Ca numbers of droplets on the spreading process of droplets are analyzed and compared.Then,it is found that gas cushion will form beneath the liquid droplet at the moment of droplet impacting on the solid wall.Then,the process of droplet spreading with cavitation is deeply studied.The results show that the presence of gas cushion at the impact point promotes the flow of the fluid at that location,thus increasing the spreading speed of the droplets with gas cushion,and ultimately affecting the impact spreading process of the droplets.Finally,the process of droplet spreading with gas cushion under different We and Re numbers is compared and analyzed.The results show that the trend of We number and Re number on the spreading process of droplets with and without gas cushion are identical.