Influence of nozzle diameter on spray characteristics and surface heat transfer dynamics in cryogen spray cooling for dermatologic laser surgery

Cryogen spray cooling (CSC) is an effective method to minimize or even eliminate laser-induced irreversible injury to the epidermis during laser surgery of various cutaneous anomalies such as port wine stain (PWS). This paper conducts an experimental investigation of the atomization characteristics and surface heat transfer dynamics in pulsed cryogen spray cooling of epidermis by R-134a, focusing on the effect of the nozzle diameter. A Phase Doppler Particle Analyzer (PDPA) is used to measure the distributions of the diameter and velocity of the liquid droplets in the spray. The temperature of liquid droplets in spray is measured by a micro-thermocouple with a bead diameter of around 100 μm. A thin-film thermocouple of 2 μm thickness is deposited directly onto the epoxy resin substrate (which serves as the skin phantom) to monitor the variation of the surface temperature induced by the CSC. The dynamic variation of the surface heat flux and convective heat transfer coefficient in CSC can then be quantified from the measured temperatures using the Duhamel's theorem. Four nozzles with diameter ranging from 0.48 mm to 1.75 mm have been used and a systematic parametric study was conducted to illustrate the effect of nozzle diameter on spray characters and surface heat transfer dynamics. It is found that the nozzle with the smaller diameter produces finer liquid droplets and larger droplets velocity than that with the larger diameter. Furthermore, the temperature of droplets from the smaller diameter nozzle decreases faster with the spray distance. Analysis of surface heat transfer indicates that the size and velocity of liquid droplets in spray has a large effect on surface heat flux and heat transfer coefficient, with large droplets leading to high heat flux at the cooling surface. In addition, a criterion is proposed to evaluate the cooling efficiency of a given nozzle based on the variation of heat extraction from the cooling surface within the effective cooling time. These results can be used to guide the selection of nozzles during cryogen spray cooling for laser dermatology.