HYPOBARIC PRESSURE EFFECTS ON R-134a COOLING SPRAYS FOR LASER DERMATOLOGY

R-134a sprays have been widely used as a method to protect the epidermis during laser therapies of hyper-vascular lesions, such as port wine stains, due to their high heat flux and precise control of cooling duration. Recent studies have shown that the use of vacuum cups during laser treatment can physically dilate the vasculature of interest and improve therapeutic outcome. However, the effects of these vacuum pressures on the cooling sprays remains unknown. In this paper, characteristics of steady-state and transient R-134a sprays, measured with a Phase Doppler Particle Analyzer (PDPA) at different pressures are presented. Ambient pressure changes from 0 to -50 kPa result in significant droplet size reductions while simultaneously increasing droplet velocities. The combined results is a slight increase in inertial Weber numbers for most conditions and a decrease in liquid volume flux. These changes appear to occur mainly during primary and secondary atomization of the spray at distances within beyond 30 mm from the nozzle tip. Hypobaric pressures appear to have little effect on droplet evaporation in flight beyond 30mm. The resulting implications for transient heat transfer in a skin phantom model is also discussed.