Abstract
Cryogen spray cooling (CSC) could be applied clinically for the laser therapy of Ota’s nevus, a dermal hyperplastic pigmented disease with a morbidity rate of 0.1–0.6% in the Asian population. An accurate, efficient, complete simulation system that considers the entire spray cooling process, including cryogen flow in the tube nozzle, spray dynamics and internal phase change heat transfer (cold injury) in skin tissue, was established to determine suitable cryogen and cooling parameters. The optimum spray distances for R134a, R404A and R32 were determined to be 66.0, 43.1 and 22.5 mm, respectively. The corresponding maximum surface heat fluxes were 363.5, 459.9, and 603.6 kW∙m−2, respectively. The maximum surface heat flux of R32 with small spray distance was 1.66 times as large as that of R134a, indicating the potentially good cooling performance and precise targeted cooling of R32 during the laser therapy of Ota’s nevus. The cooling durations that caused cold injury of skin tissue were 2.3, 1.4, and 1.1 s for R134a, R404A, and R32, respectively. The interval between CSC and laser irradiation was optimized to 90–162 ms for R134a, R404A and R32, in consideration of the cooling effect, depth, uniformity, and risk of cold injury.
Highlights
Ota’s nevus is a dermal disorder of hyperpigmentation with a morbidity rate of 0.1%–0.6% in the Asian population [1]
Differing between traditional engineering materials, the macro-scale heat transfer and the micro-scale cellular-level biophysical events must be considered simultaneously when solving the heat and mass transfer dynamics to evaluate the cold injury of skin tissue under Cryogen spray cooling (CSC), and this task can be executed with the multi-scale heat transfer model developed by Li et al [31]
The spray fields with R134a, R404A and R32 were compared in this study
Summary
Ota’s nevus is a dermal disorder of hyperpigmentation with a morbidity rate of 0.1%–0.6% in the Asian population [1]. The spray distances for R134a and R404A spray cooling have been optimized for the laser therapy of PWS [18], the spray distance and other parameters should be re-optimized for Ota’s nevus due to the large difference in tissue composition and structure. CSC is a complicated process that merges the gas–liquid flow in the capillary tube, the atomization, evaporation and breakup of spray droplets, the boiling heat transfer on the skin surface, and the phase-change heat transfer (cold injury) inside bio-tissue. An accurate, efficient, complete simulation system that considers the entire process of cryogen spray was constructed by considering the processes of gas–liquid flow through a nozzle tube, the atomization, evaporation and breakup of spray droplets, and phase-change heat transfer (cold injury) inside bio-tissue, using well established mathematical models for the literature. CSC and laser irradiation was determined by comprehensively considering cooling performance, cooling depth, cooling uniformity and risk of cold injury to provide theoretical guidance for the safe and precise clinical application of CSC in the laser treatment of Ota’s nevus
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