Transient cooling performance of R134a flash-evaporation spray enhanced by cold air jet: A novel heat transfer enhancement method in laser dermatology. (15th December 2022)

• Record Type: Journal Article
• Authors: Li, Bufa; Wang, Junfeng; Tian, Jiameng; Chen, Bin; Xu, Haojie; Jia, Hao
• Is Part Of: International journal of heat and mass transfer. Volume 199(2022)
• Publication Date: 2022-12-15
• Subjects: Cryogen spray cooling -- Nevus of Ota -- Laser therapy -- Jet impinging cooling; Heat -- Transmission -- Periodicals; Mass transfer -- Periodicals; Chaleur -- Transmission -- Périodiques; Transfert de masse -- Périodiques; Electronic journals; 621.4022
• DOI: 10.1016/j.ijheatmasstransfer.2022.123468
• Languages: English
• ISSNs: 0017-9310

Abstract

Highlights: The cold air jet was used to enhance the transient cooling performance of R134a flash-evaporation spray. The heat transfer enhancement mechanism was analyzed. A non-dimensional correlation of normalized Nusselt number and corresponding time was summarized. The low temperature air jet and small impact angle produced preferable heat transfer enhancement. The liquid film residence time ( τ ) decreases from 301 to 234 ms. Abstract: Nevus of Ota is a dermal hyperplastic pigmented disease with a morbidity rate of 0.1–0.6% in the Asian population. Laser surgery with the assistance of cryogen spray cooling (CSC) is a potential clinical treatment for nevus of Ota. However, for Asians with high melanin content, it is necessary to enhance cooling capability without the increment of refrigerant amount. In this study, a R134a flash-evaporation cryogen spray cooling coupled with cold air jet (CSC-CAJ) was developed to improve the cooling capacity. Epoxy resin was employed as a skin phantom to investigate the heat transfer characteristics of CSC-CAJ with different air jet parameters. Air jet parameters of CSC-CAJ are recommended to be Ta = -7 ℃, Va = 3 m 3 /h Za = 10 mm and Φa = 30°. Compared to CSC, CSC-CAJ could reduce the minimum surface temperature from -39 to -42.1 ℃, and the average value of convective heat transfer coefficient ( h ) of CSC-CAJ could increase from 8 to 12.9 W/(m 2 ·k) or 61.3% for the recommended parameters. The liquid film residence time ( τ ) decreases from 301 to 234 ms or 23.3%. In addition, based on the visualization observation and surface heat transfer measurement, the frost formation phenomenon on the surface and the heat transfer enhancement mechanisms are well understood. Four modes, including Leidonfrost mode, no runoff mode, runoff mode and evaporation mode, are proposed to describe the four stages of CSC-CAJ. A non-dimensional correlation of normalized Nusselt number ( Nu *) and time ( t *) is summarized. The primary heat transfer enhancement mechanism of CSC-CAJ is found to be the increase in cryogen evaporation rate and the ratio of heat removal from the phantom surface to phase change latent heat.