Effect of spurt duration on the heat transfer dynamics during cryogen spray cooling

Guillermo Aguilar,J Stuart Nelson,Guo-Xiang Wang

doi:10.1088/0031-9155/48/14/309

Abstract

Although cryogen spray cooling (CSC) is used to minimize the risk of epidermal damage during laser dermatologic surgery, optimization of the current cooling approach is needed to permit the safe use of higher light doses, which should improve the therapeutic outcome in many patients. The objective of this study was to measure the effect of spurt duration (Δt) on the heat transfer dynamics during CSC using a model skin phantom. A fast-response temperature sensor was constructed to record the changes in surface temperature during CSC. Temperature measurements as a function of Δt at two nozzle-to-skin distances (z = 50 and 20 mm) were performed. The average surface heat fluxes (q) and heat transfer coefficients (h) for each Δt were computed using an inverse heat conduction problem algorithm. It was observed that q undergoes a marked dynamic variation during the entire Δt, with a maximum heat flux (qc) occurring early in the spurt (5–15 ms), followed by a quick decrease. The estimated qc vary from 450 to 600 kW m−2, corresponding to h maxima of 10 and 17–22 kW m−2 K−1 for z = 50 and 20 mm, respectively. For z = 50 mm, spurts longer than 40 ms do not increase the total heat removal (Q) within the first 200 ms. However, for z = 20 mm, Δt longer than 100 ms are required to achieve the same Q. It is shown that the heat transfer dynamics and the time it takes to reach qc during CSC can be understood through classic boiling theory as a transition from transient to nucleate boiling. Based on the results of this model skin phantom, it is shown that spurts longer than 40 ms have a negligible impact on both q and Q within clinically relevant cooling times (10–100 ms).

Highlights

Cryogen spray cooling (CSC) is used in conjunction with various dermatologic laser surgeries (Nelson et al 1995, 2000, Waldorf et al 1997, Chang et al 1998, Kelly et al 1999, Verkruysse et al 2000)
The surface temperature starts to slowly drop once the cryogen droplets impinge on the surface, but it decreases faster once the temperature drops below 10 ◦C
If t is shorter than 20 ms, the surface temperature does not drop below Tb

Summary

Introduction

Cryogen spray cooling (CSC) is used in conjunction with various dermatologic laser surgeries (Nelson et al 1995, 2000, Waldorf et al 1997, Chang et al 1998, Kelly et al 1999, Verkruysse et al 2000). The purpose of CSC is to cool selectively the epidermis to prevent excessive epidermal heating after laser irradiation, while deeper targeted chromophores are thermally photocoagulated. During CSC, the skin surface is sprayed with tetrafluoroethane (R-134a) for a short period of time (10–100 ms). R-134a in liquid state is released from a pressurized container through a valve–nozzle system and atomized into small droplets (3–20 μm) (Aguilar et al 2000, 2001b). The low liquid temperature (−26 to −60 ◦C) (Aguilar et al 2000, 2001b) of the impinging cryogen droplets can effectively cool the skin surface

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