Radial Heat Transfer Dynamics during Cryogen Spray Cooling

Cryogen spray cooling (CSC) is a heat extraction process that protects the epidermal layer during skin laser surgery of port wine stain (PWS) birthmarks and other specific dermatoses. The objective of the present work is to investigate temporal and radial variations on the heat transferred at the surface of a skin model during CSC. A fast-response thermal sensor is used to measure temperature across the radius of the sprayed surface of a skin model. These interior measurements along with an inverse heat conduction algorithm are used to determine the heat transferred at the surface. Results show that radial and temporal variations of the boundary conditions have a strong influence on the homogeneity of heat extraction from skin. However, there are subregions of uniform cooling. It is also observed that the surface heat flux undergoes a marked dynamic variation, with a maximum heat flux occurring at the center of the sprayed surface early in the spurt followed by a quick decrease. The study shows that external conditions must be taken into account and ideally controlled to guarantee uniform protection during CSC. INTRODUCTION Cooling and heating processes are essential to laser surgery for treating specific dermatologic vascular lesions, such as port wine stain (PWS) birthmarks. PWS is a congenital and progressive vascular malformation of the dermis that occurs in approximately 0.7% of children. To remove these birthmarks, laser energy is irradiated at appropriate wavelengths inducing permanent ll correspondence to this author; email: [email protected] 1 thermal damage to PWS blood vessels. However, laser energy is also absorbed by epidermal melanin causing localized heating therein. As a consequence, complications such as hypertrophic scarring and skin dyspigmentation may occur. Epidermal protection is provided by means of cryogen spray cooling (CSC), which is a spatially selective heat transfer technique that spurts liquid cryogen onto the skin surface. The cryogen evaporates extracting heat from the epidermis thereby increasing the fluence threshold for epidermal damage [1]. The heat extraction from skin during CSC is a function of many fundamental spray parameters that vary in time and space within the spray cone, such as average droplet diameter and velocity, mass flow rate, temperature and spray density [2, 3]. Therefore, the heat extraction process during CSC-assisted skin laser surgery is non uniform [4]. Uniform heat extraction within the sprayed area is highly desirable—to provide homogenous epidermal protection within the laser beam diameter—but not practical, and, despite surface variations, most numerical studies on CSC-assisted skin laser surgery for PWS to date assumed constant heat transfer conditions at the skin surface. In the present study, surface thermal variations during CSC are experimentally and numerically investigated. A fast-response temperature sensor is used to measure temporal and radial temperature changes along the radius of the sprayed surface of a skin model during CSC. Interior temperature measurements along with an inverse heat conduction problem algorithm are then used to calculate temperatures and heat fluxes at the surface of the skin model. Copyright © 2004 by ASME Plexiglas Scotch tape Aluminum foil