Experimental and Theoretical Study on the Thermal Characteristics of Flashing Spray Cooling UsingR404a
Cryogen spray cooling (CSC) has been used in the laser dermatology to eliminate or minimize laser-induced irreversible thermal injuries to the epidermis. However, nonspecific thermal injuries will occur even with very low exposure due to insufficient CSC-induced heat removal from skin epidermis by the currently used R134a for darkly pigmented human skins. In this work, thermal characteristic of R404a flashing spray is investigated since R404a with lower boiling point has potential of larger cooling efficiency than R134a. The experimental temperature results find that the global temperature profile of R404a presents the contractive shape, rather different from the expansion shape of R134a. R404a spray has much smaller spray thermal width and spray cold width than R134a spray, which means it has better spatial selectivity for the accurate control of cooling area in laser dermatology. A simple droplet evaporation model was employed to predict single droplet temperature evolution along the axial distance. The results of the evaporation model find that the predicted temperature evolution is highly sensitive to the initial droplet parameters. The temperature prediction with larger initial diameter and velocity agrees better with the experimental dada in the spray center region, while it presumably reflects the temperature variation of droplets at the spray periphery with smaller initial diameter and velocity.