Parametric effect investigation on surface heat transfer performances during cryogen spray cooling

Cryogen spray cooling (CSC) has been a widely used auxiliary tool in laser dermatology such as port wine stain to prevent unspecific thermal injury due to laser energy absorption by the melanin in the epidermis. The present paper presents an experimental research on the effect of ambient temperature, relative humidity, and initial substrate temperature on heat transfer performances during R134a spray cooling. Results demonstrated that the cooling capability of R134a spray cooling can be obtained with small ambient temperature (T_a = 10 °C) and relative humidity (RH = 25%). Further investigation of cooling mechanism was conducted by studying the temporal and radial heat transfer distributions with different spray distances and nozzles. The heat transfer distribution presented large non-uniformity along radial locations. Two uniform cooling sub-regions of 0 ≤ r < 2 mm and 6 mm ≤ r < 10 mm were found under the spray distance of 30 mm and nozzle with an inner diameter of 1.0 mm. The heat transfer barrier was produced due to indirect contact between cold droplets and the substrate surface caused by bubbles and heat transfer is weakened by the low thermal conductivity of these bubbles.