Coaxial Airflow Modulation for Cryogen Spray Cooling: Towards Heat Transfer Enhancement and Film Control in Laser Dermatology

Abstract

In clinical treatment, effective cooling is crucial for avoiding skin injury, particularly for patients with higher melanin concentrations, however still recent advancements are limited by the thermal barring of the deposited liquid film or asymmetric heat transfer. Expanding upon our previous research on coaxial air-cryogen spray cooling, this study investigates the effects of converging and diverging airflow configurations to enhance heat transfer, reduce film deposition, and improve cooling uniformity. The heat flux on skin-like epoxy surface was calculated via Duhamel methodology, utilizing fast-responding thin-film thermocouple measurements. Meanwhile, the film and spray dynamics/morphologies were captured via high-speed video recording that employing Mie-scattering method, and further predicted through large eddy simulation coupled with discrete phase and Eulerian wall film modelling. The analysis of spray patterns, thermal features, and deposited film behavior reveals significant improvements in heat transfer, droplet evaporation, and film dynamics by airflow. The coaxial diameter ratio ($d_o/d$) of 1.0 and 1.5 demonstrated more effective spray dispersion, significantly influenced film dynamics during both cryogen on- and off-duties by reducing frost formation and enhanced evaporation efficiency. The $d_o/d$ = 1.5 configuration achieved the best performance with higher average heat transfer coefficient and lower minimum surface temperature, offering superior heat dissipation, particularly at the spray periphery, though it had longer film resistance due to slower evaporation at the impingement point. The $d_o/d$ = 1.5 and 0.5 cases showed distinct cooling patterns, with the former providing better overall cooling and heat transfer, while the latter concentrated cryogen in the impingement core, resulting in thicker films and slower evaporation. Thus, the divergent and convergent airflow designs offer superior cooling performance at the impingement point and the core region, respectively. These findings offer valuable insights for optimizing air-cryogenic spray cooling systems for precise temperature control of the skin.