Performance And Heat Transfer Characteristics Of Evaporative Spray Cooling With Ammonia

Spray cooling is an efficient cooling technique by using the pressure/air atomizing nozzle to force a liquid stream through a small orifice, which produce a great dispersion of fine droplets. These droplets then impact a heated surface and take away large amounts of heat relying on evaporate. Spray cooling offer several advantages, such as strong heat transfer performance, high heat flux, less coolant, uniform temperature distribution, and so on. Spray cooling has been widely considered as the most effectivecooling technology for high-powered laser and integrated electronic component, furthermore, Spray cooling gradually became indispensable to aerospace, laser technology and defense industry demaned high heat flux cooling cutting-edge technology fields, especially in high power laser array cooling condition, which not only satisfy the low cooling surface temperature but better temperature distribution uniformity. Heat transfer mechanism of spray cooling is an extremely complex process and it needs to investigate further. In the present study, mainly aiming for the high-power solid laser diode array cooling demand, experiments have been performed to investigate the heat transfer of spray cooling with using ammonia as coolantant. The pressure atomization nozzles array and the gas-assisted single atomization nozzle were adopted to spray on the 25 mm×12 mm test surface, respectively. The main researches include: The system of spray cooling consists of the gas circuit and the liquid circuit thin-film resistors heater and cartrigde heaters act as the heating source. Experiments were performed to investigate heat transfer characteristics and surface temperature distribution for pressure atomization nozzle array at spray height of 10 mm. The heat transfer characteristics of the spray cooling were investigated under different ammonia saturation pressures, the effects of saturated vapor pressure and inlet flow rate on heat transfer characteristics were studied in detail. Furthermore, experiments were performed to evaluate heat transfer characteristics on micro-channel surface at low flow rate, different saturated evaporation pressure and micro-channel size, compared with the smooth surface experiment. The characteristics of the air atomization nozzle were estimated by using correlative theory research and experimental research. Experiments were performed to investigate heat transfer characteristics by changing the liquid flow, and gas phase flow remain unchanged, compared with the pressure single atomization experiment, Completed the micro-channel improved heat transfer surface experiment.The main research results are as follows:①Spray cooling experimental system including gas and liquid circuit was constructed, using ammonia as coolantant and nitrogen as assisted gas. The thin film resistor and cartigde heater acted as the heater source to meet the experimental requirements of high heat flux level respectively.②When keeping the lower heating power and spray height were constant, the heat transfer meachnism was mainly dominated by forced convective, and high inlet flow rates of liquid ammonia resulted in low surface temperature and uniform temperature distribution. The dominant heat transfer mechanism for spray cooling gradually converted forced convective heat transfer to the boiling with increasing heat flux, with the inlet volumetric flow decreasing, resulting in decreasing the uniform temperature distribution of heat transfer surface .③Latent heat of ammonia decreased with the increasing saturation pressure, it was useful to promote the single-phase convection to convert to the nucleate boiling at the lower superheat temperature. The surface temperature and heat transfer coefficient increased with increasing the saturation pressure of ammonia, but the variation trend of the superheat reversed.④Heat transfer coefficient was enhanced while superheat was reduced, when a high saturated vapor pressure was applied in the cooling system. A noticeable transition from forced convection heat transfer to nucleate boiling heat transfer was observed under lower flow rate. The heat transfer coefficient was 51% higher than that obtained under constant pressure.⑤Micro-channel surface can significantly enhance the heat transfer coeffeicent compared with a smooth surface at the same conditions. Inlet flows of nozzle, saturated evaporative pressure and micro-structure sizes have great influence on the heat transfer characteristics.⑥Performance of spray cooling wth using the gas-assisted single atomization is obviously increased by the gas assisted atomization to enhance the droplets speed and impact force. The surface temperature reduced and the critical heat flux values increased with the liquid flow increasing, when gas phase flow remained constant. When the experimental conditions remain unchanged, the spray cooling efficiency of using gas-assisted single atomization was superior to using the pressure atomization. In single-phase forced convective heat transfer process, more nitrogen kept stay in the bottom of the microchannel by the capillary force to form a thick gas layer which increased the heat transfer resistance, and led to the performance of spray cooling on the micro structure surface was worse than the performance of spray cooling on a smooth surface.