<title>Influence of cryogen spray cooling parameters on the heat extraction rate from a sprayed surface</title>

Abstract

Cryogen spray cooling is used to prevent epidermal thermal damage during port-wine stain laser therapy, despite the limited understanding of the fluid dynamics, thermodynamics, and heat transfer characteristics of cryogen sprays. In recent studies, it has been suggested that the heat flux through human skin could be increased by changing physical parameters such as nozzle-to-skin distance, nozzle diameter, and/or by depositing cryogen in sequential spurts. These changes affect spray parameters such as droplet diameter, velocity, and spray temperature. Therefore, in order to optimize new nozzle designs, it is necessary to explore the influence that these fundamental spray parameters have on heat extraction.In this paper, various valve/nozzle configurations were characterized. A Phase Doppler Particle Analyzer was used to determine the average diameter, velocity, and droplet concentration of various cryogen sprays. The mass flux delivered by each valve/nozzle configuration was also measured, along with the average spray temperature. A custom-made device consisting of an insulated metallic disk was used to measure the heat extracted by different sprays. The results showed that there are significant differences in the heat extracted by the different valve/nozzle configurations. These variations are proportionally influenced by mass fluxes. Strong correlations were also observed between average droplet velocities and heat extraction. These findings indicate that mass flux has a dominant effect on heat extraction from human skin during cryogen spray cooling. It is also apparent that kinetic and thermal energies are other parameters to be considered when optimizing heat extraction.

Effects of mass flow rate and droplet velocity on surface heat flux during cryogen spray cooling

“…Our results complement our previous hypothesis (Karapetian 2002) that larger ṁ also results in more liquid cryogen deposition on the surface, which indirectly produces higher lateral fluid velocity in the liquid cryogen layer and thus increased q max . Higher ṁ also lowers the cryogen liquid layer temperature by quickly replenishing it with incoming droplets that are colder than the cryogen boiling temperature due to evaporative cooling during flight (Aguilar et al 2001a).…”

“…It may be hypothesized that the difference is due to the larger and faster droplets produced by nozzles with D N = 1.4 mm, which penetrate deeper into the liquid cryogen layer formed on the skin surface during CSC. In a subsequent study (Karapetian et al 2002) using various nozzle geometries, no perceptible correlations of d and T to the total heat removed (Q) and maximum heat flux (q max ) were noted. Furthermore, contrary to results reported by Pikkula et al (2001), who suggested that a variation in ṁ has only a modest impact on q, our studies showed that ṁ strongly correlates with Q and q max (Karapetian et al 2002), where a three-fold increase in ṁ led to an identical increase in Q and an 11-fold increase in q max .…”

“…In a subsequent study (Karapetian et al 2002) using various nozzle geometries, no perceptible correlations of d and T to the total heat removed (Q) and maximum heat flux (q max ) were noted. Furthermore, contrary to results reported by Pikkula et al (2001), who suggested that a variation in ṁ has only a modest impact on q, our studies showed that ṁ strongly correlates with Q and q max (Karapetian et al 2002), where a three-fold increase in ṁ led to an identical increase in Q and an 11-fold increase in q max . Similar trends in the effect of ṁ on steady state q have been observed in studies of water sprays cooling a hot plate (Ciofalo et al 1999, Schmidt andBoye 2001).…”

Radial and temporal variations in surface heat transfer during cryogen spray cooling

“…Its purpose is to protect the epidermis from excessive heating induced by unintended melanin absorption of laser light (Nelson et al 1995). 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 , Karapetian 2002, that vary in time and space within the spray cone. Therefore, heat extraction from human skin during CSC-assisted laser surgery is non-uniform (Franco et al 2004a(Franco et al , 2004b.…”