Comparative investigation on the spray characteristics and heat transfer dynamics of pulsed spray cooling with volatile cryogens

Cryogen spray cooling with R134a has been widely used in laser dermatology such as port wine stain to prevent unspecific thermal injury due to the absorption of laser energy by melanin in the epidermis. However, R134a spray cooling has shown poor efficacy for darkly-pigmented human skin. This paper presents an experimental study on the spray characteristics and heat transfer dynamics of pulsed spray cooling with three kinds of volatile cryogens (R134a, R407C and R404A), the latter two of which have potential to improve the cooling effect due to their lower saturation temperature. The spray patterns reveal that R404A generates the narrowest spray width and the smallest cooling spot on the cooling surface, which facilitates the accurate control of the cooling area. The R404A spray produces the smallest droplet diameter and the largest velocity, whereas the opposite applies to the R134a spray. The droplet temperature measurements show that R404A spray has the lowest temperature of spray, while the R134a spray has the highest. However, the non-dimensional temperature in the decrease period shows a similar pattern, independent of the cryogens. For the common spray distance in laser surgery (30 mm), R404A spray leads to the lowest surface temperature and highest heat flux, and also removes the most heat from the cooling surface, indicating that R404A has the strongest cooling capacity. Furthermore, R404A spray has the least liquid film resistance time on the cooling surface, implying that it provides the best selectively spatial cooling effect. The results of spray and heat transfer all reveal that R404A possesses significant superiority with regard to pulsed spray cooling for epidermis protection, with the potential of substituting the current R134a in the application of laser dermatology, especially in the case of darkly-pigmented human skin.