Dynamics of cryogen deposition relative to heat extraction rate during cryogen spray cooling

Abstract

Goal is to investigate how delivery nozzle design influences the cooling rate of cryogen spray as used in skin laser treatments. Cryogen was sprayed through nozzles that consist of metal tubes with either a narrow or wide diameter and two different lengths. Fast-flashlamp photography showed that the wide nozzles, in particular the long wide one, produced a cryogen jet (very small spray cone angle) rather than a spray '(cone angles of about 1 5°orhigher) and appeared to atomize the cryogen less finely than the narrow nozzles. We measured the cooling rate by spraying some cryogen on an epoxy-block with thermocouples embedded. The heat extraction rate of the wide nozzles was higher than that of the narrow nozzles. The results suggest that finely atomized droplets produced by the narrow nozzles do not have enough kinetic energy to break through a layer of liquid cryogen accumulated on the object, which may act as a thermal barrier and, thus, slow down heat extraction. Presumably, larger droplets or non-broken jets ensure a more violent impact on this layer and therefore ensure an enhanced thermal contact. The margin of error for the heat extraction estimate is analyzed when using the epoxy-block. We introduce a complementary method for estimating heat extraction rate ofcryogen sprays.

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

“…heat flux) was also observed in a study on surface cooling with a water spray [14]. This may be due to the ability of faster droplets of the same size to penetrate deeper into the liquid cryogen layer that forms on the skin surface during CSC [6]. It may also be attributable to the effect of the axial velocity transferring to a radial velocity of the liquid cryogen moving over the surface, thus enhancing the convective heat transfer coefficient.…”

“…In this study, the metallic disk method was used because the high thermal conductivity of silver ensures that the measurements are highly influenced by the surface effects of heat removal making this method very sensitive to changes in heat flux [6]. The disk used is thin (.17 mm) relative to its diameter (3.2 mm) and changes in temperature occur quickly, which makes this method very useful during short timed dynamic experiments.…”

“…The disk used is thin (.17 mm) relative to its diameter (3.2 mm) and changes in temperature occur quickly, which makes this method very useful during short timed dynamic experiments. Other substrates used to measure surface heat extraction include an epoxy block [6,10,11] or insulated copper rod [6,12]. The epoxy block has similar thermal properties to human skin and, therefore, provides an excellent medium to estimate the temperature profiles within skin.…”

“…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.…”

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

“…There are only a few investigations that address lateral variations in heat transfer during CSC. Verkruysse et al (1999) investigated Table 1. Thermal properties of epidermis (Duck 1990) and sensor components (Incropera and Dewitt 1996, http://www.matweb.com ≈0.5 × 10 −7 ≈1.3 × 10 −7 ≈8.4 × 10 −5 ≈1.2 × 10 −7…”