Experiment And Investigation Into The Performance Of A Closed-loop Aircraft Spray Cooling System

With the increasing requirements for maneuvering characteristics,stealth performance and defensive ability of aircrafts,the technologies of high power laser beam and micro-electronic components develop rapidly.Directed energy weapon as the standard weapon for the fighters of the next generation,is one of the key technologies which should be solved.Power in the magnitude of megawatt can be produced within several seconds after the launch of the directed energy weapon,which leads to high heat load and high heat flux of several hundreds and thousands of watts per square centimeter on the weapon surface.The heating dissipation problem has drawn the attention of the researchers.How to solve the problem is of great importance and has significant value for improving the combat ability of next generation fighters in our country.For its advantages of high heat transfer coefficient,no boiling hysteresis and uniform surface temperature,spray cooling owns an application potential in the field of aircraft directed energy weapon cooling.In this paper closed-loop experiment bench is established,and then the influences of nozzle parameters,spray height,heating power,cooling medium flow rate and cooling medium charge volume are investigated.The system operating conditions are also analyzed.Breakthroughs are achieved in the fields of dimensionless correlations and mathematical model simulation,and common conclusions are obtained to provide experimental and theoretical reference for the application of spray cooling on the field of aircraft weapon cooling.Major contents are as follows:(1)Numerical simulation of spray cooling characteristicsThe droplet impact model is established by the VOF model and total spray atomizing model is established based on the DPM model.On the basis the influences of the overload condition,the droplet parameters and multi-nozzle spray are all researched.The results indicate that the influence of overload acceleration to spray cooling performance is limited.The liquid film distribution of two-nozzle spray is more uniform than that of single-nozzle spray.Heat transfer performance of spray cooling can be enhanced by the increase of droplet velocity and droplet diameter as well as the decrease of droplet distance,droplet temperature and liquid film thickness.(2)Investigation on spray cooling performance with R22 as the cooling mediumThe influences of nozzle parameters,spray height,spray flow rate,heating power and refrigeration charge are mainly studied.Three kind of nozzles are compared under different heating power and the optimum spray height of nozzle type 1/8GG-SS1,1/8GG-SS1.5 and 1/8GG-SS3002.5 is obtained.Taking the optimized nozzle 1/8GG-SS1.5 as the research object,the influences of spray mass flow rate and the refrigerant charge are experimentally investigated.The results indicate that Spray cooling performance increases with the increase of spray mass flow rate due to the increase of droplets involved in the heat transfer.Surface heat flux,heat transfer coefficient and surface temperature increase with the increase of heating power when other parameters are unchanged.The heat transfer performance improves when the refrigerant charge increases while the surface temperature does not decrease with the increase of refrigerant charge because the spray mass flow rate and the chamber pressure both increase with the refrigerant charge.Moreover,the variations of the parameters are analyzed from the aspect of dimensionless number.The variations of We,Re,Ja and ? with spray mass flow rate promote the increase of the heat transfer coefficient.When the charging pressure is less than 0.55 MPa the variations of Re,Ja and ? promote the increase of the heat transfer coefficient.When the charging pressure is more than 0.55 Mpa,the variations of We,Re,Ja and ? all promote the increase of the heat transfer coefficient.Moreover,Experiment results obtained in this experiment are compared with the results using water as the cooling medium.The application ranges of R22 and water are divided as follows: R22 is more suitable when heating power is less than 1000 W and surface temperature should be controlled accurately by spray cooling in the boiling region.Besides water is more suitable under heating power above 1000 W.(3)Investigation on the critical state and the failure state of the spray coolingThe heat transfer performance in the critical state under different charging pressures from 0.35 MPa to 0.70 MPa.The heat transfer performance deteriorate rapidly after the critical state and surface temperature increases rapidly to more than 200?.Meanwhile the heat transfer coefficient decreases rapidly to the value of 0.17-0.22W/(cm ?K).Further study indicates that when reducing the heat flux in the failure state,the boiling heat transfer ability recovers and the heat transfer coefficient begins to increase.It can be concluded that a gas film is forming which makes the droplets isolated from the heating surface in the critical state.From the variations of the dimensionless numbers it can be found that the increase of heat transfer coefficient in the boiling region is mainly influenced by the increase of Re.When the increasing rate of Re decreases,Ja and ? become the main influencing factor,making the heat transfer coefficient decrease slowly.When the CHF is achieved,the rapid variations of We,Re,Ja and ? make a shape decline of the heat transfer coefficient.In the heat dissipation process the decrease of Ja and the increase of ? are the key influencing factors to the rapid increase of the heat transfer coefficient.(4)Investigation on system parameters and operating conditionsThe influence of system parameters such as the sub-cooling degree of the condenser,the superheat degree of the compressor and the bypass volume are investigated.Meanwhile the system operating conditions are studied.From the experiment results it can be concluded that the sub-cooling degree can promote the improve of heat transfer performance,but has no effect on the phase change heat transfer.The superheat degree nearly has no influence on the heat transfer performance.Besides,the open of the bypass valve will reduce the cooling performance of the system.When the charging pressure is above 0.7MPa,throttling effect of the nozzle disappears with the increase of chamber pressure which is unbeneficial for the spray cooling performance and the system operation.With the increase of refrigerant charge the evaporating temperature and the condensing temperature both increases,but the increasing rate decreases gradually.Meanwhile the sub-cooling degree increases and the superheat degree decreases.COP increases and then decreases with the increase of refrigerant charge,the maximum value of which(8.86)is achieved with the refrigerant charge of 0.60 MPa.High value of COP and the cooling efficiency can be achieved when the refrigerant charge is 0.55 MPa,while the critical heat flux of 0.55 MPa is 157.6W/cm2,which is a little less than the max value 162.3 W/cm2.Therefore refrigerant charge of 0.55 MPa is preferred for this system.(5)Fitting of the dimensionless correlations in the critical state and the failure state4 dimensionless parameters are defined which are Reynolds number Re,Weber number We,Jacob number Ja and the evaporation intensity ?.On the basis dimensionless correlations are obtained.During the fitting process the influence of each parameter to the fitting accuracy is analyzed.It can be concluded that the Weber number We could not be ignored.Compared with the evaporation intensity ? Jacob number Ja is more suitable to reflect the temperature variations of the droplets and the heating surface during the boiling process of spray cooling.During the failure state We plays the major role to spray cooling performance.