Measurement of heat flux and heat transfer coefficient during continuous cryogen spray cooling for laser dermatologic surgery

Cryogen spray cooling (CSC) has been used for selective epidermal cooling of human skin during laser therapy of patients with port wine stain (PWS) birthmarks. Unfortunately, current commercial CSC devices do not provide optimal cooling selectivity and, therefore, provide insufficient epidermal protection for some PWS patients. To assist in the development of improved atomizing nozzle designs, a reliable method to quantify the CSC heat flux is needed. We introduce a novel method to determine the heat flux (q_s) and heat transfer coefficient (h) at the surface of a sprayed object, based on measurements of steady-state temperature gradients along a thin copper rod during continuous cryogen spraying. For an atomizing nozzle of inner diameter d _N = 0.7 mm, we found that q_s varies from 15 to 130 W/cm² and h increases nonlinearly from 15000 to 35000 W/m².K in the explored range of surface temperatures (T_s , from -32 to -7°C). Values of q_s obtained with a wider diameter nozzle (d_N = 1.4 mm) are approximately twice as large than those of the narrow nozzle. The corresponding values of h are significantly higher (32000-40000 W/m².K) and almost independent of T_s within the same temperature range. When combined with fast flashlamp photography (FFLP) of spray shapes and sprayed surfaces, the results demonstrate that the liquid cryogen layer, as deposited by finely atomized sprays from narrower nozzles, can significantly impair q_s. In contrast, the higher-momentum impact of coarser sprays from wider nozzles reduces the thickness of the liquid layer in the impact area and/or enhances convection within it, yielding a larger q_s