Study On Heat Transfer Characteristics Of Spray Cooling And Microchannel Heat Sink For High Heat Flux Electronic Equipment

With the increasing integration and power of electronic equipment,the heat flux increases sharply.Heat dissipation has become one of the technical bottlenecks restricting the development of electronic equipment.Spray cooling and microchannel cooling utilize the latent heat of phase change of refrigerant,which has the advantages of high heat transfer coefficient,good temperature uniformity,small working fluid demand,and are often used to solve the heat dissipation problem of electronic equipment with high heat flux.However,there are many factors affecting spray cooling and microchannel cooling,and the heat transfer mechanism is very complex.In order to meet the heat dissipation needs of different types of electronic devices with high heat flux,it is necessary to further explore the enhanced heat transfer methods of spray cooling and microchannel.The flow and heat transfer characteristics of spray cooling and microchannel cooling were numerically analyzed and experimentally studied.The specific research contents are as follows:（1）Visualization study on atomization characteristics and heat transfer performance of flash spray coolingA visual experimental platform was established on the basis of flash spray cooling system.The atomization characteristics,liquid film behavior and atomization angle of flash and subcooled spray were observed by high speed camera.The effects of superheat,spray distance and nozzle orifice diameter on atomization characteristics and cooling performance were studied.The difference of heat transfer characteristics between flash spray and subcooled spray was compared and analyzed with visualization results.The results showed that superheat can improve the atomization characteristics,reduce the droplet size,thin the liquid film and improve the cooling performance.As the dimensionless superheat increased from 0 to 0.47,the wall temperature decreased from 57.9℃to 52℃.Compared with subcooled spray,flash spray can shorten the optimal spray distance,which was beneficial to the design of compact spray cooling device.Finally,the effects of nozzle diameter（0.5 mm,0.7 mm and 0.9 mm）on atomization and cooling performance of flash spray were analyzed.Nozzles should be selected in combination with parameters such as inlet temperature and flow rate.The optimum nozzle diameter is not only conducive to the formation of fine droplets,but also ensures that sufficient droplet flux is involved in wall heat transfer.（2）Numerical simulation of heat transfer characteristics of spray coolingA numerical model of spray cooling was established based on Euler-Lagrange method,including turbulence model,species transport model,discrete phase model and liquid film model.Firstly,continuous phase flow field was calculated by turbulence and species transport;Then,the behavior of droplet aggregation,droplet transport,particle splashing,particle separation and adhesion was calculated by Lagrange liquid film model;Finally,continuous phase and discrete phase were coupled to solve the problem.The effects of heat flux,flow rate,droplet velocity and overload acceleration on spray cooling heat transfer characteristics were studied,and the characteristics of liquid film velocity,liquid film thickness and wall temperature distribution were analyzed.The results showed that the droplet velocity in the center of atomization region was fast,and the impact on the central liquid film on the wall surface was strong,which led to the fast oscillation flow of the central liquid film on the wall surface,and the slow flow of the outer liquid film.At the same time,the liquid film flowed along the radial direction,resulting in thick liquid film around the wall surface.The average wall temperature decreased with the increase of spray flow rate.The average wall temperature decreased from 34.9℃to 30.9℃when the spray flow increased from 0.0035 kg/s to 0.0125kg/s.By comparing the distribution characteristics of liquid film and average wall temperature under different overload acceleration conditions,it was found that overload acceleration has little effect on spray cooling heat transfer performance.The spray cooling performance was improved with the increase of droplet velocity.When the droplet velocity increased from 12 m/s to 20 m/s,the average wall temperature decreased from 42.9℃to 39℃,and the heat transfer coefficient increased from 28 k W/（m²·℃）to 32 k W/（m²·℃）.（3）Study on spray cooling performance of compact space spray coolingA micro nozzle was designed,a compact spray cooling device was developed,and a flash spray cooling system was established.The heat transfer performance of subcooled spray cooling,jet cooling and flash spray cooling in compact space was compared.The temperature fluctuation characteristics of jet cooling and flash spray cooling were analyzed.The results show that when the inlet temperature is higher than the saturation temperature of the spray chamber,the temperature of the heating surface was reduced in compact spray cooling device,which indicated that flash occurred in compact space,and the flash effect promoted the enhancement of heat transfer due to the rapid utilization of latent heat.With the increase of heat flux,the cooling performance of flash cooling was better than that of subcooled spray cooling and jet cooling.It was also found that flash spray cooling was more stable than jet cooling.（4）Experimental study on spray cooling of high power moduleAiming at the heat dissipation requirements of high power modules,a flash spray cooling device with nozzle array was developed.The effects of spray flow rate,inlet temperature and spray chamber pressure on heat transfer characteristics were investigated experimentally.The experimental results showed the highest CHF of 349 W/cm² can be achieved by the flow rate of 1.4 L/min.Increasing the inlet temperature or decreasing the spray chamber pressure was beneficial to the performance of flash atomization,reducing the particle size of atomized droplets,increasing the specific surface area of R1336mzz droplets,and improving the heat transfer efficiency of phase change.The maximum heat transfer coefficient of 48.2°C was increased by 85.6%and 36.1%compared with that of 26.5°C and 34.3°C,respectively.While the spray chamber pressure decreased from 66 k Pa to 52 k Pa,the wall temperature decreased by 16.9%.The heat dissipation ability of spray cooling to high power module was verified.（5）Numerical study of novel microchannel for high power device arrayBased on the topology optimization method,the microchannel structure was designed with the objective function of minimizing flow power dissipation and average temperature.The flow and heat transfer characteristics of topological microchannel heat sink and conventional straight microchannel heat sink were compared by using conjugate heat transfer model.The results showed that the topological microchannel has the structural characteristics of bifurcation and confluence,which can destroy the flow boundary layer,promote the full mixing of cold and hot fluids and improve the local convective heat transfer coefficient.Compared with the conventional straight microchannel,the topological microchannel has the advantages of good temperature uniformity and low pressure drop.When the flow rate was2.2×10^-4 kg/s,the maximum temperature of the wall surface of the topological microchannel was 4℃lower than that of the conventional straight microchannel,and the pressure drop of the topological microchannel was 0.593 k Pa lower than that of the conventional straight microchannel.（6）Experimental study on microchannel cooling of high power device arrayAccording to the heat dissipation requirements of high power device array,combined with the advantages of good integration and high heat transfer efficiency of microchannel cooling and jet cooling,a microchannel-jet hybrid heat sink was developed.The effects of inlet flow rate and heat flux on the flow and heat transfer characteristics of the microchannel-jet hybrid heat sink were studied using working fluid R1336mzz.The results showed that the cooling performance of the microchannel-jet hybrid heat sink increases with the increase of the flow rate,and the maximum cooling capacity of 1537.9 W/cm² can be achieved.At the same time,the increase of flow rate will also cause the increase of pressure drop.In the single-phase heat transfer region,the increase of heat flux did not cause significant change of pressure drop.In the two-phase heat transfer region,the pressure drop increased with the increase of heat flux.In addition,the boiling stability was also analyzed.When the heat flux approached the critical point,the pressure fluctuation amplitude increased sharply and the boiling stability decreased.The pressure fluctuation amplitude increased sharply and the boiling stability decreased at high heat flux.The boiling stability and temperature distribution uniformity can be improved by increasing the flow rate appropriately.