Spray and Cooling Dynamics of Cryogen Sprays Impinging on a Human Skin Model

The objective of the present work is to correlate the timedependent flow characteristics of cryogen sprays to the induced thermal dynamics at the surface of a human skin model. First, a numerical analysis to evaluate our skin model is carried out. Next, diameter and axial velocity of droplets impinging onto the skin model are measured. Diameter, velocity and surface temperature are acquired simultaneously at the center of the spray cone close to and at the skin model surface, respectively. Spurt durations of 10, 30 and 50 ms are investigated. Finally, measurements are used to compute spray number, mass and kinetic energy fluxes and surface heat flux. Numerical modeling shows that, subject to the same heat flux, the thermal response of our model and human skin is qualitatively similar but the total temperature drop in skin is about 50% less than that of the model. A simple transformation can be used to map the temperature response of the model to that of skin. Experimental measurements show that during the initial spray transient, fast and small droplets (respect to steady state values) induce large temperature drops and the highest heat flux because the temperature difference between liquid and substrate is the largest; during the spray steady state, surface temperature remains at its lowest value; during the final transient, droplets are fast and small again, although their impact on the surface heat transfer is negligible due to decreasing mass and kinetic energy fluxes and reduced temperature differences between liquid and substrate. ∗Address all correspondence to this author; email: [email protected] INTRODUCTION Cutaneous laser surgery assisted by cryogen spray cooling has proven essential in cutaneous and cosmetic laser surgery. A short cryogen spurt precools the epidermis to avoid excessive, unintended injury therein during laser irradiation from excessive heating induced by melanin absorption of laser light [1]. Heat extraction from skin during CSC is a function of many fundamental spray parameters, such as average droplet diameter and velocity, mass flow rate, temperature and spray density among others [2], that vary in time and space within the spray cone. The objectives of the present study are the following: (i) to evaluate an epoxy block substrate as a thermal model of human skin; (ii) to determine the flow characteristics of cryogen sprays in transient state impinging onto the human skin model; (iii) to correlate the spray characteristics to the surface heat transferred from the human skin model. EXPERIMENTAL AND NUMERICAL METHODS First, a numerical analysis to evaluate an epoxy block substrate as skin model is carried out. Next, an epoxy substrate with an embedded thin foil sensor is used to measure surface temperatures, and a phase doppler particle analyzer (PDPA) is used to measure the diameter and axial velocity of cryogen droplets impinging onto the skin model with thermal sensor. PDPA and surface temperature measurements are acquired simultaneously at the center of the spray cone 32.5 and 35 mm downstream from the nozzle tip, respectively. Spurt durations of 10, 30 and 50 ms are investigated. Finally, the surface heat flux is computed using the solution of a direct heat conduction problem, and spray num1 Copyright c © 2007 by ASME epidermis epoxy k (W/m K) 0.21 0.14 ρ (kg/m3) 120