Transient cooling performance of R134a flash-evaporation spray enhanced by cold air jet: A novel heat transfer enhancement method in laser dermatology

Abstract

Nevus of Ota is a dermal hyperplastic pigmented disease with a morbidity rate of 0.1–0.6% in the Asian population. Laser surgery with the assistance of cryogen spray cooling (CSC) is a potential clinical treatment for nevus of Ota. However, for Asians with high melanin content, it is necessary to enhance cooling capability without the increment of refrigerant amount. In this study, a R134a flash-evaporation cryogen spray cooling coupled with cold air jet (CSC-CAJ) was developed to improve the cooling capacity. Epoxy resin was employed as a skin phantom to investigate the heat transfer characteristics of CSC-CAJ with different air jet parameters. Air jet parameters of CSC-CAJ are recommended to be Ta = -7 ℃, Va = 3 m3/h Za = 10 mm and Φa = 30°. Compared to CSC, CSC-CAJ could reduce the minimum surface temperature from -39 to -42.1 ℃, and the average value of convective heat transfer coefficient (h) of CSC-CAJ could increase from 8 to 12.9 W/(m2·k) or 61.3% for the recommended parameters. The liquid film residence time (τ) decreases from 301 to 234 ms or 23.3%. In addition, based on the visualization observation and surface heat transfer measurement, the frost formation phenomenon on the surface and the heat transfer enhancement mechanisms are well understood. Four modes, including Leidonfrost mode, no runoff mode, runoff mode and evaporation mode, are proposed to describe the four stages of CSC-CAJ. A non-dimensional correlation of normalized Nusselt number (Nu*) and time (t*) is summarized. The primary heat transfer enhancement mechanism of CSC-CAJ is found to be the increase in cryogen evaporation rate and the ratio of heat removal from the phantom surface to phase change latent heat.