Study On Heat Transfer Enhancement In Compact Spray Cooling System

The power density of electronic components grows continuously,and the subsequent heat accumulation and temperature increase inevitably affect stability,reliability and service life of electronic equipment.Therefore,how to achieve efficient cooling in a limited space has become a key problem in the development of electronic devices with high performance and high integration.Spray cooling is a novel and efficient cooling method,compared with the conventional cooling methods,it has many advantages such as high heat dissipation capacity,rapid temperature response,small contact resistance,small flow rate and low superheat.In consequence,spray cooling has broad application prospects in thermal control of electronic components.This paper focusing on practical compact spray cooling system,then mainly analyzes the heat transfer performance and influence parameters of spray cooling in a compact spray cooling system,and explores efficient and stable heat transfer enhancement methods suitable for the compact systems.Firstly,the paper studied the compactness and the arraying of atomizing nozzle,core component of compact spray cooling system.Aiming at the features of compact spray cooling system,the design scheme of a novel compact was presented.The corresponding spray characteristics of the compact nozzle were studied with experiments,in which spray cone structure and optimizing droplet distribution of spray field were analyzed.The results showed that this novel compact nozzle could meet requirements of the compact spray cooling system.Based on the novel nozzle,variant compact multi-nozzle arrays were developed according to the features of different application systems and different using environment,including the plate multi-nozzle array appropriate for large area heat source,the annular multi-nozzle array for large heat flux and the linear multi-nozzle array for dispersing heat sources.This part of study laid the foundation for further study of compact chamber and integrated systems.Using the plate multi-nozzle array,a compact spray cooling system for computer chip was established,and its heat transfer characteristics were systematically studied.The system used a plate multi-nozzle array to form a compact spray chamber with 26mm thickness,and to cool a heating surface of square analog chip with 30mm X 30mm.The results showed that the system could achieve heat dissipation higher than 90W/cm2 in the single phase regime of spray cooling,and temperature nonuniformity of cooled surface increased with the increasing of surface heat flux,and volume flow rate obviously increased the heat flux,while spray height had optimum range.Using the annular multi-nozzle array,the ultra-high power spray cooling system for neutron target of simulated reactor was established.The system used an annular nozzle array to form a large compact chamber to cool a ring heat source with 450mm inner diameter and 470mm outer diameter.With optimizing the arrangement of nozzle array and improving the dispersion of spot source,the system successfully achieved the cooling of 80kW high power heat source while the temperature of heating surface was below 140℃,which met the cooling requirements of large heat flux neutron target.The results also showed that the compact spray cooling system could not only be used in cooling of relatively lower-power heat sources such as electronic components,but also in cooling of high-power and large-area heat sources.Based on the studies of compact nozzle and compact chamber,a compact spray cooling system for plug-in integrated enclosure was established,and the heat transfer performance and heat transfer enhancement of the system were systematically studied.In the system,the effects of flow rate,inclination angle and surface structure on spray cooling heat transfer using HFE7100 and PF5060 were studied experimentally.The results showed that the maximum heat fluxes of PF5060 and HFE7100 were 79.1W/cm2 and 79.1 W/cm2 respectively.And it was found that the flow rate significantly affected the heat transfer,while the temperature imbalance was mainly related to flow rate and flow resistance.In transient heat transfer,spray cooling response quickly and the inclination angle affected the response time.The temperature of pulse heating was fast and the equilibrium time of heat transfer was affected by surface heat flux.Moreover,the reasonable inclination angle could improve heat transfer performance of the system.Under the experimental conditions,spray cooling heat transfer achieved the best cooling capacity at 75° inclination angle.It was considered that the working fluid utilization and the fluid management of spray cooling were improved at this inclination angle.The enhanced surface also could improve heat flux of the enclosure.Under the experimental conditions,heat transfer performance of straight fins surface was superior to that of others surfaces.The reason was considered that enhanced surface could increase the contact area,prolong the contact time and improve the fluidity of liquid film.In addition,in order to analyze the practicability and applicability of spray cooling system,a comprehensive evaluation method for spray cooling system was presented.In order to further explore efficient and stable heat transfer enhancement methods in the compact space,the paper studied the influence of high-alcohol surfactants on spray cooling heat transfer on the basis of the system compactness.Four kinds of high-alcohol surfactants,1-heptanol,1-octanol,isooctanol and n-decanol,were used in the system.The results showed that the surfactants had obvious enhancement on transient and steady spray cooling heat transfer.In transient heat transfer,heating surface temperature could reduce in a short period of time more obviously,and higher transient heat flux was achieved.In steady heat transfer,the surfactants enhanced heat transfer especially in the two phase regime.Besides,it was found that surfactant solution had optimum concentration,and the optimum concentrations of 1-heptanol and 1-octanol were 0.3‰,while that of isooctanol and n-decanol were 0.5‰ and 0.1 ‰ respectively.Because the high-alcohol surfactants have many advantages compared with the traditional ionic additives,such as less demand,no corrosion,no crystallization and no clogging,this study are of great significance to practical application of spray cooling and corresponding heat transfer enhancement.Finally,the effect of high-alcohol surfactants on spray characteristics was studied based on the aspects of physical properties,and the enhancement mechanism of the surfactant on spray cooling heat transfer was further analyzed.The relationship between the surfactant concentration and the physical properties,such as saturated vapor pressure,surface tension,contact angle and dynamic viscosity,were studied experimentally,as well as the corresponding correlations were proposed.Moreover,the effects of the surfactant on spray cone structure and atomization parameters were analyzed.The results showed that the surfactant decreased saturated vapor pressure of surfactant solution,while the effect was not obvious at low temperature.Surface tension and contact angle changed significantly with added concentration,which conformed to Traube’s Rule.And dynamic viscosity varied with the surfactant’s type and added concentration were not significant.Besides,according to the testing of PDA,it was found that the surfactant had an obvious effect on droplet flux,including expanding the entrainment region and moving out of the mainstream region of spray cone,but it had little impact on angle of the mainstream region.Droplet diameter decreased at the central of spray cone and increased at the edge of spray cone.Droplet velocity had little change by the surfactant.Additionally,combined with physical properties and added concentration,the predictive correlation of droplet diameter and droplet velocity were obtained in the paper,and accordingly,heat transfer enhancement mechanism of surfactant in spray cooling system was also analyzed.