Experimental And Theoretical Study On Spray Cooling And Its Heat Transfer Enhancement

At present, electronic parts and components have been becoming miniaturized and highly integrated, and meanwhile their heating power has been increasingly higher. Failure to remove the heat promptly may negatively influence the performance and working life of the electronic parts and components, and even the safety of the working environment. Compared with the existing heat removal technologies including air cooling, compulsive water cooling, heat pipe, thermoelectric cooler, and micro-channel as well, spray cooling possesses higher unit heat removal capacity, and is enabled to meet the heat removal demands of electronic parts and components with higher heat flux density. Therefore, it has drawn considerable attention in heat transfer researches at home and abroad in recent years, having broad prospect of engineering application. However, researches on mechanism of spray cooling are still scarce and the understanding of its influencing factors still incomplete. This study intended to investigate the heat transfer characteristics and mechanism of spray cooling technology, so as to provide theoretical grounds and guiding principles for its engineering application.In the first place, an in-depth experimental study and theoretical analysis on the heat transfer characteristics on smooth heat sink surface were made. Based on the theoretical module of spray height and effective flux proposed hereinto, an equation of spray height and effective flux on the surface was deduced, thus effectively separating outlet pressure from effective flux, two influencing factors that are not easily varied independently, and being enabled to analyze the effects of spray height, outlet pressure and effective flux on heat transfer characteristics respectively. The results showed that inlet pressure had very small effect on heat transfer, while the effective flux had significant effect and the heat transfer could be maximized at certain value of effective flux. Meanwhile, using ultra-thin aluminum as analog heat source and combining with infrared radiation thermography method, this study visualized the heat transfer of spray cooling under three different nozzles. It was found that compared with common solid nozzle, the nozzle able to produce droplets on rotating orbit of shearing force had stronger heat transfer. However, in term of temperature gradient distribution on the heat sink surface, the nozzle unable to produce droplets having shearing rotating orbit could reach better effect. As the input power of the film heat source increased, the heat transfer coefficient of spray cooling decreased and tended to decrease significantly under high mass flux and nozzle pressure. As to heat transfer enhancement of spray cooling, an experimental study was made focusing on the effects of the microgrooves capillary structures of heat sink surfaces and the spray positions relative to the nozzle and the heat sink surface on the heat transfer characteristics. It was found that the capacity of heat removal on microgrooves capillary heat sink surface was50%stronger than on smooth surface, and that the ability was maximized when the normal direction and axial direction of the rectangular microgrooves capillary surface was parallel to the ground. Meanwhile, with the help of high speed camera with a micro-len and a maximum speed of100000fps, this study visualized the dynamics of single micro-bubble in the inner microgrooves capillary surface under the impact of high speed droplets. The results showed that from the phase of the thin liquid film to the phase of dry-up period, the bubbles on the microgrooves capillary surface turned from non-existent to be existent as the heat flux density increased on the heat sink surface; the nucleation site density of producing bubbles increased; when it was partially dried up, the bubbles in thin fluid film area grew faster and thus enhancing the heat transfer in partial area; in addition, the life cycle of micro-bubble was found to be only about lms, thus strengthening the agitation within the thin fluid film and enhancing the heat transfer. Besides, it was observed that in the microgrooves capillary, only single bubble growth module existed. Combining with the growth module of single bubble in microgrooves, and analyzing the experimental data using MATLAB and VERSON software, this study concluded that the life cycle of micro-bubble in microgrooves capillary and breakup equivalent diameter did not decrease simply as the heat flux density increased. Instead, they were fluctuating repetitively. In general, the bubble life cycle and breakup equivalent diameter decreased as the heat flux increased.At last, analyses were made on the experimental data obtained by using ultra-thin aluminum as analog heat source and FC-72as working fluid. It was found that the heat transfer coefficient h on heat sink surface in single-phase non-boiling region was correlated with mass flux, outlet pressure and the input power on heat sink surface. By fitting and modifying the heat transfer correlation, an equation of heat transfer correlation with wider application scope in non-boiling region was proposed.