Sequential cryogen spraying for heat flux control at the skin surface

Abstract

Heat transfer rate at the skin-air interface is of critical importance for the benefits of cryogen spray cooling in combination with laser therapy of shallow subsurface skin lesions, such as port-wine stain birthmarks. With some cryogen spray devices, a layer of liquid cryogen builds up on the skin surface during the spurt, which may impair heat transfer across the skin surface due to relatively low thermal conductivity and potentially higher temperature of the liquid cryogen layer as compared to the spray droplets. While the mass flux of cryogen delivery can be adjusted by varying the atomizing nozzle geometry, this may strongly affect other spray properties, such as lateral spread (cone), droplet size, velocity, and temperature distribution. We present here first experiments with sequential cryogen spraying, which may enable accurate mass flux control through variation of spray duty cycle, while minimally affecting other spray characteristics. The observed increase of cooling rate and efficiency at moderate duty cycle levels supports the above described hypothesis of "isolating" liquid layer, and demonstrates a novel approach to optimization of cryogen spray devices for individual laser dermatological applications.

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

“…This method, however, lacks the sensitivity to monitor small changes in heat flux due to the low thermal conductivity of the epoxy, the poor interface between the * For convenience Figures 4 & 5 present the maximum heat flux (q max ), which is the dependent parameter, on the shared horizontal axis. thermocouple and epoxy, and other factors [8]. The insulated copper rod method can be used effectively only for steady-state measurements of heat flux and heat transfer coefficient.…”

“…A custom-made device similar to those reported previously [5,8] was designed and built to measure the maximum heat flux (q max ), and the total heat removed (Q total ) by a 100 ms spurt for all valve/nozzle configurations used in this study. The device consists of a silver disk (3.18 mm diameter, 0.17 mm thickness), thermally insulated by epoxy (k T =.218 W/m-˚K).…”

“…In recent studies, it has been shown that heat flux through the skin can be increased by changing parameters such as nozzle-to-skin distance [5] and nozzle diameter [3,6,7] and/or by depositing cryogen in sequential spurts [8]. These changes influence the fundamental spray parameters such as mass flux, average droplet diameter and velocity, droplet concentration and temperature, which in turn affect the heat flux at the skin surface.…”

“…To measure the heat extracted, we used a technique that involves a metallic disk embedded in an insulating epoxy [5,8]. The mass flux for each configuration was computed by measuring the mass of cryogen expelled during timed sprays.…”

Numerical Prediction of the Intracellular ICE Formation Zone during Cryosurgery on a Nodular Basal Cell Carcinoma Using Liquid Nitrogen Spray

“…This was later confirmed also by Franco et al [163] in terms of the uniformity in the protection these sprays could provide when used in conjunction with laser irradiation. Nevertheless, the reader must not forget that non-uniformities in the heat transfer coefficient over the entire spray area exist and depend on the following parameters: geometric parameters such as nozzle diameter-length [164], nozzle-to-skin distance [165] and nozzle angle [162], cryogen spurt duration [166][167][168][169][170][171][172][173], ambient pressure [174][175][176], humidity and frost formation effects [177][178][179][180], skin indentation effects [181,182], and the dynamics of cryogen spray deposition, such as cryogenic spray shape, the droplet velocity and diameter [183]. 3.…”