Study On Heat Transfer Performance, Enhancement, And Surface Temperature Non-uniformity In Spray Cooling

With the rapid development of the miniaturization and integration of electronic components, the heat flux of the electronic components is increasingly growing, and then the heat dissipation problems have greatly restricted the development of high power electronic compoents. However, Spray cooling is a promising candidate to provide high rate heat flux removal of electronic devices due to its high heat flux capacity, small demand of working fluid, and so on. This paper focuses on the heat transfer characteristics of the spray cooling, the influencing factors and formation mechanism of the surface temperature non-uniformity, and the effect of the enhanced surface and additives on the heat transfer characteristics of the spray cooling.First, the spray cooling heat transfer characteristics and its major influencing factors were studied by experimental method, and then the heat transfers in single-phase and two-phase region were correlated. The findings indicate that the spray height and spray flow rate have the largest effect in spray cooling single-phase region heat transfer, which was mainly affected by the changing of the spray characteristics parameter;the effect of inlet temperature on the spray cooling heat transfer in single-phase region is relatively small, which was mainly affected by the changing of the thermal properties of the working fluid. The system pressure enhanced the spray cooling heat transfer by decreasing the working fluid saturation temperature, and then the working fluid can work in two-phase region in low surface temperature. Based on the analysis of the single-phase and two-phase heat transfer characteristics, the heat transfers in single-phase and two-phase region were correlated successfully, The heat transfer in single-phase region was mainly associated with the Weber number, Reynolds number, Prandtl number, and dimensionless temperature. And the heat transfer in two-phase region was mainly associated with the Weber number. Reynolds number, Prandtl number, and Jacques number.Combined the modified model and the experimental analysis of the phenomenon, the the effect of spray height, spray flow rate, heat flux, different nozzles,system pressure and gravity conditions on the spray cooling surface temperature non-uniformity were analyzed. The results show that:the experimental results and calculative results agree well, the error is less than 10%. The spray height, spray flow rate and nozzle type could affect significantly the surface temperature non-uniformity, which was mainly because that the above factors changed the spray characteristics parameters, then the spray cooling heat transfer was affected, thus the surface temperature non-uniformity changed. The surface temperature non-uniformity increases with the increase of the heat flux, when the surface temperature reaches saturation temperature, the working fluid enters two-phase region, then a large number of surface boiling bubbles occur in the heating surface, the bubble absorbed heat evenly in the growth process. Therefore, the growth trend decreases. In low system pressure, the working fluid can work in the two-phase region in low temperature, then the surface temperature non-uniformity will superior to that in the normal system pressure. As for the effect of the gravity conditions on the surface temperature non-uniformity, the difference of the spray cooling surface temperature non-uniformity between the gravity condition and microgravity condition is small in the single-phase region, while in the two-phase region, the surface temperature non-uniformity difference increases, the spray cooling surface temperature non-uniformity under microgravity was far greater than that under normal gravity conditions, which was mainly due to that the boiling bubble on the heating surface under microgravity conditions was difficult to escape.In order to study the formation mechanism of the surface temperature non-uniformity, the fluid film thickness, fluid film velocity, and surface temperature distribution in different droplet velocity non-uniformity, SMD distribution non-uniformity, droplet number distribution non-uniformity, and different heating power were investigated, then the surface temperature non-uniformity in single-phase and two-phase region were correlated. The results showed that:the fluid film thickness, velocity and surface temperature distribution vary with the variation of the spray characteristics parameters non-uniformity. With the decrease of the spray parameters non-uniformity, the fluid film thickness on the heating surface becomes more uniform, the fluid film velocity increases, thus the surface temperature non-uniformity decreases. The highest surface temperature point is in the heating surface center, and the lowest point appears in the boundary between the film and non-film coverage area. The effect of the droplet number distribution non-uniformity on surface temperature non-uniformity is largest, followed by the droplet velocity distribution non-uniformity, and the effect of droplet SMD distribution non-uniformity is min. Finally, the surface temperature non-uniformity in single-phase and two-phase region was correlated with error of 20%. The effects of enhanced surface and the working fluid containing NaC1, Na2SO4, 2BH and 1-Octanol were studied, and the spray cooling heat transfer enhancement method by using high alcohols surfactant was proposed. Straight rib structure could increase effective heat transfer area, and then the spray cooling capability was strengthened. But the heat heat transfer capability enhancement is less than 12%. The heat transfer could be improved by using the salts additives and the high alcohols surfactants. However, this effect does not increase monotonically with the increase of the concentration. While the concentration increases to a certain extent, the spray cooling heat transfer declines and the surface temperature non-uniformity also increases. The salts additives (NaC1 and Na2SO4) solution could cause the maximum heat transfer capability improvement for 29%and 31%, respectively. But the salts additive has no practical application value because that the salts additive could block the nozzles and corrode the experimental system. The high alcohols surfactants (1-Octanol and 2EH) could cause the maximum heat transfer capability improvement for 28%and 36%, respectively. And the high alcohols surfactants have a broad application prospects.