Study On Vertical Spray Cooling Of Closed Single Nozzle

Spray cooling is widely used in electronics,energy,transportation,manufacturing,aerospace,national defense and other industries because of its advantages such as low flow demand,high heat transfer coefficient,good temperature uniformity and high critical heat flux.In this paper,based on the traditional refrigeration cycle system,the spray cooling method is used instead of the traditional evaporator cooling method.The cooling medium enters the nozzle after being throttled and depressurized by the condenser,and is atomized into fine droplets through the nozzle,which are sprayed on the plane needing heat exchange for evaporation heat exchange.This heat exchange process not only reduces the consumption of cooling medium,but also makes its distribution more uniform and has better heat exchange potential.After a great deal of literature collection and consulting,as well as experimental simulation and analysis,this paper set up a closed single nozzle vertical spray cooling heat transfer experiment table,and established the physical model and mathematical model of the experiment,and comprehensively analyzed the closed single nozzle vertical spray cooling heat transfer through experiment and simulation.The main contents of this paper are as follows:1.The physical model,mathematical model,turbulent model,liquid film model on heating surface and discrete phase model of closed single nozzle vertical spray cooling are established.Fluent software is used to simulate the heat transfer process of closed single nozzle vertical spray cooling.The heat transfer characteristics of spray evaporation under different influencing factors such as nozzle outlet pressure,nozzle height and heating surface power were studied.The simulation results show that with the decrease of nozzle height,the temperature of heating surface decreases,and the area of low temperature area of heating surface increases,the heat transfer coefficient increases and the heat transfer capacity increases.With the increase of the power of the heating surface,the temperature difference between the heating surface and the droplets sprayed by the nozzle is larger,the heat conduction is faster,the droplet evaporates and absorbs heat faster,the liquid film produced on the heating surface is thinner,the heat transfer coefficient is larger and the heat transfer effect is better.With the increase of nozzle outlet pressure,the droplet velocity reaching the heating surface increases,the impact force between the droplet and the heating surface is also greater,the contact surface is larger,and the spray heat transfer performance is better.2.Set up a visual closed single-nozzle vertical spray cooling heat exchange test bed,using R134 a as the cooling medium,and the cooling medium is sprayed on the heating surface through a nozzle with a diameter of 0.4mm in the spray cavity for endothermic evaporation.The spray state of the cooling medium can be clearly judged through the visual spray cavity.Experimental findings:（1）Selecting the effective flow rate of 0.0023～0.0042kg/s,with the increase of the effective flow rate of droplets from the nozzle reaching the heating surface,the more droplets participate in heat exchange,and the greater the heat absorbed by droplets during evaporation,the better the spray heat exchange effect will be.With the increase of the effective flow rate of 0.0001Kg/s,the heating surface temperature will decrease by 2-10℃.（2）The nozzle height is selected to be 70 ～ 103 mm.With the increase of the nozzle height,the distribution of circular droplets on the heating surface sprayed by the nozzle changes from incomplete coverage to full coverage.The lower the height of the nozzle,the greater the axial velocity of the droplets ejected from the nozzle,and the greater the impact force on the heating surface.The mutual collision between the droplets strengthens the disturbance of the droplets to the liquid film on the heating surface and strengthens the convection heat transfer.（3）When the nozzle outlet pressure is 0.3～0.6MPa,the experimental results show that the temperature of the heating surface decreases with the increase of the nozzle outlet pressure.The reason is that the higher the nozzle outlet pressure is,the faster the droplets reach the heating surface,the larger the contact area between the droplets participating in heat exchange and the heating surface,and the more droplets participating in heat exchange,the better the effect of spray cooling heat exchange.（4）Comparing the simulation results with the experimental results,it is found that the error between the simulation value and the experimental value of the closed single nozzle vertical spray cooling is within the allowable range,and the simulation model has a good guiding significance for the operation of the spray cooling system.