新型喷嘴R404a闪蒸瞬态喷雾冷却传热特性

Heat transfer dynamics of R404a flashing pulsed spray cooling using expansion-chamber nozzle

制冷剂闪蒸瞬态喷雾冷却在激光治疗血管性皮肤病手术中得到广泛应用,主要用于激光照射前选择性地冷却表皮。本文搭建了喷雾冷却实验台,设计不同长径比膨胀腔新型透明喷嘴,应用薄膜热电偶瞬态温度快速测量技术和杜哈梅尔定理计算方法对比分析了膨胀型喷嘴和传统直管型喷嘴应用低沸点的新型制冷剂R404a的闪蒸瞬态喷雾冷却表面动态传热特性。应用高速摄像仪和采用背光法对新型喷嘴膨胀腔内部制冷剂流动形态进行观察,发现制冷剂液体在膨胀腔内首先发生一次闪蒸破碎,剧烈相变产生大量气泡,可导致液滴温度在喷嘴内部大幅降低,而直管型喷嘴制冷剂只在喷嘴喷出后才形成闪蒸喷雾并降温。因此,膨胀腔型喷嘴比直管喷嘴具有更佳的冷却效果,能够产生更低的冷却温度,更高的热通量、传热系数和换热量。膨胀腔长径比是影响此类喷嘴喷雾冷却效果的关键因素,在长径比1:2时其冷却能力最强、有效冷却时间最长。

Cryogen spray cooling(CSC) with R134 a has been successfully implemented to cool selectively the epidermis prior to laser irradiation in the laser dermatology such as port wine stain(PWS). To enhance the cooling capacity of CSC, this paper conducts an experimental study on the heat transfer dynamics of pulsed flashing spray cooling, using the lower boiling point cryogen R404(-46.50℃ at 1 atm) compared with R134a(-26.07℃ at 1 atm). New kind of transparent expansion-chamber nozzles with different aspect ratio of chamber length to chamber diameter are designed. A fast-response film thermocouple is used to measure the surface temperature and then the Duhamel's theorem is employed to calculate the heat flux and heat transfer coefficient. High speed camera with the backlit illumination method is implemented to record internal flow within the expansion chamber. The high speed camera results find that the liquid forms the first flashing breakup within the expansion chamber and then the second flashing spray outside the nozzle exit. Violent phase change and large bubbles could be obviously observed within the expansion chamber, which should cause much lower droplets temperature at the nozzle exit comparing with the traditional straight-tube nozzle which has been used in the clinical surgery. Therefore, the expansion-chamber nozzles could produce the lower surface temperature, the higher heat flux and higher heat transfer coefficient, and thus the larger total heat removal, comparing with the straight-tube nozzle. Further, the chamber aspect ratio has significant effect on the heat transfer dynamics. It's found that the nozzle with the chamber aspect ratio of 1:2 generates the best cooling capacity and the longest cooling time.