Studying The Heat Transfer Performance In The Non-boiling Regime Of Spray Cooling

Developmental Trend of future electronic systems is smaller size, higher integration value, high heat flux and how to remove the so amount of heat flux is a great challenge for this domain. Spray cooling is an advanced control cooling technology increasing interest for electronic cooling and other high flux heat removal technique, which is characterized by small fluid inventory, high heat transfer coefficient and no hysteresis of boiling.However, the current studies which contradict each other because of the experimental condition mainly focused on boiling regime and the conclusion was only apporiate for high temperature. The non-boiling regime's spray cooling performances, as an important part of the spray cooling process, has been investigated by very few researchers.In view of this, the primary purpose of this paper is to develop comprehensive design tools vital to the implementation of spray cooling heat transfer performance using semi-solid swirl nozzles and solid pressure-atomizing nozzles with a large ranged mass flux.The main experimental studies:1. Spray characteristics:The spray parameters, mainly the droplet diameter, velocity and the spray pattern, were measured by a Dantec PDPA (Phase Doppler Particle Analyzer).It showed that distribution of the streamwise mean velocity and Sauter Mean Diameter (SMD) of droplet were not uniform and this tendencies were more obvious to semi-solid swir pressure-atomizing nozzles.There was a stagnation zone at the center of the heated surface which may be attributed to the effect of the recirculating zone resulting from the strong rotational flow. In the recirculating zone the SMD and mean velocity are minimum, while at outer side of the spray zone, they are comparatively large.2. Studying the spray characterics, surface roughness influencing the heat transfer performance of the spray cooling in the non-boiling regime.The results showed that droplet velocity, droplet diameter, droplet number flux and mass flux all affected the heat transfer. Combining theoretical analysis with experimental results, we concluded that:1) heat flux was increased with the increasing of droplet velocity and droplet number flux; 2) Flim evaporation is very important to heat transfer in non-boiling regime of spray cooling. As test surface temperature increased, film evaporation increased as well and heat transfer performance enhanced; 3) The surface temperature unevenly distributed in the non-boiling regimen using semi-soild swir nozzle.4) Compared with smooth wall, the rough wall has better heat transfer performance and cooling efficiency in non-boiling region.Besides those, the effect of spray characteristics on heat transfer was studied thoroughly and the conclusion that mass flux was the main factor to affect the heat transfer was proposed.3. Experiments were performed to study the effects of spray inclination angle (the angle between the normal of the square test surface and the axis of symmetry of the spray), the mass flux as well as the surface temperature on the heat transfer performance in non-boiling regime by using water sprays. Experiments revealed that there is an optimal orifice-to-surface distance where heat transfer performance is best. And this occurred when the major axis of the elliptical spray impact area is just intersecting the square test surface at inclined sprays. Knowing the inclination angle and spray cone angle as well as the test size, the optimum orifice-to-surface distance can be easily determined. Too small orifice-to-surface distances would result in only a small fraction of the test surface impacted by the spray, while too large a distance causes a substantial fraction of the spray liquid falling wastefully outside the test surface. Both extremes made heat transfer performance decrease. The heat transfer performance and the cooling efficiencies increased with increasing inclination angle from 0°to 49°. Despite the increased heat transfer, it takes longer time at larger inclination angles for the test surface to reach a steady state between power increments.4. Experiments were conducted to study the effects of enhanced surfaces on heat transfer performance during water spray cooling in non-boiling regime. The surface enhancement is straight fins. The structures were machined on the top surface of heated copper blocks with a cross-sectional area of 10 mm×10 mm. The spray was performed using solid pressure-atomizing nozzles with a mass flux of 44-53 Kg/ (m2·s). It is found that the heat transfer is obviously enhanced for straight fin surfaces relative to the flat surface. However, the increment decreases as fin height increases. For flat surface and enhanced surfaces with a fin height of 0.1 mm and 0.2 mm, as mass flux increases, heat flux increases as well. However, for finned surface with a height of 0.4 mm, heat flux is not sensitive to coolant mass flux. Changed film thickness and the form of water/surface interaction due to enhanced surface structure (different fin height) are the main reasons for changing of local heat transfer coefficient. Straight fin surface not only increased the area of heat transfer, but also provided a driving force for liquid spreading and enhanced heat transfer. The optimum heat transfer performance in the experiments is the enhanced surface with fins in 0.2mm height, and the surface with fins in 0.1mm and 0.4mm height next.5. Studying the heat flux corrlection in the non-boiling regime.The former datas was normalized in terms of non-dimensional groups.It showed that the main dimensional number influencing the heat transfer were Re Number,Weber Number and non-dimensional temperatureξ. Furthermore, Generalized correlations were developed for local Nusselt number as a function of the spray Reynolds number (116.2