UC Irvine

The objective of the present work is to correlate the time-dependent 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. Copyright © 2007 by ASME.