Correlations between Spray Properties and Heat Transfer Dynamics during Cryogen Spray Cooling

INTRODUCTION Cryogen spray cooling (CSC) has been used along with pulsed lasers for nearly a decade to irreversibly photocoagulate a variety of vascular lesions. However, the fundamental mechanisms that take place at the skin surface are still incompletely understood. In this work, we built a fast-response temperature sensor with the objective to determine the time in which liquid cryogen remains on the skin surface during and after a short spurt of cryogen—the residence time (tr). Measurements are conducted systematically at various distances from the nozzle (z) and various spurt durations (∆t) for two nozzles that produce completely different spray characteristics. It was found that for each nozzle, there is a critical spray distance (zc) where an abrupt increase in tr occurs and another critical distance (zmax) that can be related to maximal cryogen deposition. Furthermore, using experimentally measured average droplet diameter (d) and velocity (v) for sprays produced by each nozzle, we defined a spray characteristic time τ(z) as the ratio of d to v. This parameter allows us to represent our experimental data on a single curve for each nozzle, by plotting the dimensionless residence time (tr/τ) as a function of the product of dimensionless spurt duration (∆t/τ) and a dimensionless factor (m/mmax). The factor m/mmax is envisioned as an effective mass deposition which, due to the evaporation of cryogen droplets in-flight and the spray-surface interactions, is a strong function of z. These results represent a step towards a more complete understanding and quantification of the physics involved in CSC. Laser surgery is effectively used to irreversibly photocoagulate port wine stain (PWS)[1,2] birthmarks and other vascular lesions such as telangiectasias, hemangiomas[3], and rhytides. However, epidermal melanin acts as a nonspecific target, absorbing a significant portion of laser light and thus, limiting the dose that can be safely delivered to the underlying targets. Moreover, if laser light absorption within the epidermis is excessive, it can lead to permanent damage such as scarring and dyspigmentation. To avoid this, cryogen spray cooling (CSC) was introduced as a mean to cool the skin prior to laser irradiation [1, 2]. On a different realm, there have been a significant amount of studies on water sprays, from which empirical correlations have been developed to predict various spray characteristics. For instance, Elkobt [4] amongst many others, proposed correlations to estimate the Sauter mean diameter (SMD) of droplets produced by plain orifice atomizers (similar to those used on the cryogenic sprays produced by commercial devices). Unfortunately, none of these correlations accounted for liquid evaporation, which is likely significant during cryogenic sprays. Studies of evaporating liquids, such as fuels have also been carried out [5][6], but almost none of them dealing with the interaction with solid surfaces. Finally, only a few studies on cryogenic sprays have been reported, In particular, Ingebo [7][8][9] worked with two-fluid type nozzles, where a high velocity gas flow was used to atomize the liquid cryogen. He studied the effect of the gas temperature, gas properties, and vaporization on the average spray droplet diameter. However, the nozzles he used were different kinds of atomization devices than those used for PWS treatment.