Heat transfer dynamics of transient liquid CO₂ spray cooling for replacement of conventional R134a spray cooling in laser dermatology

To protect the epidermis from thermal injury, selective cooling is essential during laser surgery. Among many cooling techniques, cryogen spray cooling is widely applied because it can provide powerful cooling with tens of milliseconds. However, conventional R134a spray often fails to provide sufficient cooling for the darkly pigmented skins, meanwhile, R134a is not friendly to the environment due to high GWP of 1300. Liquid CO₂ spray might be an ideal solution to these problems with advantages of very low GWP of 1 and high latent heat. In this study, liquid CO₂ was used to form a low-temperature spray for rapid cooling. The spray pattern presented an explosive atomization characterized by bowl-like shape, large spray width and angle at nozzle exit due to Joule-Thompson effect. The effects of spurt duration, spray height and container pressure on surface temperature, heat flux and heat transfer coefficient were fully investigated. Results showed that Liquid CO₂ spray provided much higher cooling capacity with a maximum heat flux twice of that of R134a spray cooling. Decreasing spray height and increasing spurt duration both enhanced the heat flux and lower surface temperature in the range of 20∼40 mm and 40∼100 ms. Elevating the container pressure improved the cooling capacity at spray periphery while had little effect at spray center. Finally, three dimensionless correlations were derived to express the dynamic heat transfer of CO₂ spray cooling.