| Advances in supercomputer,avionics,lasers and many other applications have resulted in a dramatic increase of heat flux dissipation density in electronic systems.The thermal control systems of these electronic devices require high heat flux cooling capacity,tight temperature control and high reliability,since the performance of electronic devices are very sensitive to working temperature.Compared with other high heat flux cooling techniques,spray cooling has numerous advantages such as high heat flux removal capacity,high cooling efficiency,excellent comprehensive performance,and so on.In order to further understand the heat transfer mechanism of spray cooling,improve the heat transfer performance of spray cooling and solve some crucial problems in the practical application of spray cooling,this paper focuses on studying the effect of spray droplet characteristics(including the droplet flash characteristics,piezoelectric spray characteristics and droplet impacting dynamics)on heat transfer enhancement in spray cooling.Firstly,considering the vacuum environment in the thermal control of avionics,a comprehensive mathematical model of flash evaporation spray cooling was presented in this paper.The effects of flash evaporation on spray droplet characteristics and spray cooling heat transfer performance were studied.The influence mechanism and inner law of various parameters on spray cooling heat transfer mechanism were obtained.The droplet flash evaporation was modelled based on diffusion-controlled evaporation model,while the film flash evaporation was modelled based on film penetration theory.The model was in good agreement with the experimental results.It was found that the initial spray temperature,spray flow rate,spray height and spray angle were very important parameters affecting the heat transfer performance of flash evaporation spray cooling.The flash evaporation had a great enhancement effect on spray cooling heat transfer.After taking droplet flash into account,the temperature of spray droplets and the heating surface decreased a lot,the spray cooling heat flux increased significantly,and the ratio of droplet-wall impaction heat transfer increased remarkably.The latent heat utilization was very high in flash evaporation spray cooling,which can achieve excellent cooling performance at low flow rate.The droplet-wall impaction and film flash heat transfer were dominant in the flash evaporation spray cooling.This study provides a theoretical basis for the engineering application and optimization design of flash evaporation spray cooling.Based on above analysis,it was found that the spray flow rate and spray droplet characteristics have great influence on spray cooling heat transfer performance.However,the commonly used pressure swirl nozzles have disadvantages such as uncontrollable spray characteristics,large spray flow rate,uneven atomization and the need of pressure pump.In order to improve the spray droplet characteristics at low flow rate,a novel piezoelectric atomizer applicable in spray cooling system was designed in this paper.Spray cooling system based on piezoelectric atomizer was established and the spray characteristics and spray cooling heat transfer performance of piezoelectric atomizer were analyzed in this paper.It was found that the piezoelectric atomizer could achieve excellent atomization at low flow rate,and has advantages such as controllable spray characteristics,needless of pump and low power consumption.Compared with pressure swirl nozzle,the spray distribution of piezoelectric atomizer was more uniform and there was no hollow zone in the center.The surface temperature was low in the center and high at the edge corresponding to the spray distribution.Compared with pressure swirl nozzle,the temperature distribution of piezoelectric atomizer was more uniform.With the increase of micropore diameter,the flow rate and the heat flux increased,but the spray cooling efficiency decreased as a sacrifice.According to the experimental results,correlations between spray cooling efficiency/heat flux and flow rate were obtained,and could provide reference value for the engineering application and system optimization of piezoelectric spray cooling.As droplet-wall impaction heat transfer was very important in spray cooling,this paper proposed a novel method of increasing dynamic Leidenfrost temperature through addition of high-alcohol surfactant and studied the dynamic behavior of droplet impacting on superheated surface using high-speed photography.The results showed that dynamic Leidenfrost temperature was significantly increased by high-alcohol surfactant and the maximum increase value was 65?.The dynamic Leidenfrost temperature and maximum spreading factor increased with the increase of surfactant concentration and droplet impact momentum.A possible explanation for the dynamic Leidenfrost temperature increase caused by surfactant was proposed based on bubble bursting and bubble coalescence.According to above experimental data and mechanism analysis,empirical correlations of dynamic Leidenfrost temperature and maximum spreading factor were obtained based on Webber number and Ohnesorge number.These empirical correlations could provide reference value for the future research.Finaly,after fully understanding the heat transfer mechanism and influencing factors of spray cooling,in order to take full advantages of spray cooling,a high energy efficiency thermal control system for supercomputer center was proposed based on spray cooling.The cooling efficiency,energy saving effect and economic efficiency were evaluated based on Dawning 5000A supercomputer in China.The novel cooling system was composed of a plug-type spray cooling system and a waste heat driven absorption chiller.The spray cooling system was used to cool computer racks,while the absorption chiller was used to not only reuse the recycle water in spray cooling system but also partially eliminate the heat load of air conditioner.It was found that the novel cooling system is much efficient than the original cooling system.The energy saving effect was highly affected by inlet temperature of spray cooling.At the optimal design,the energy efficiency of novel cooling system was very high,with the cooling load factor(CLF)as low as 0.22,energy saving efficiency(ESE)as high as 49%and power usage effectiveness(PUE)within best practice scenario of 1.44.What's more,the novel cooling system was of high economic value with a payback period of 1.5 years. |
PDF Full Text Request