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

Cryogen spray cooling (CSC) with R134a 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.