Heat transfer characteristics during pulsed spray cooling with R404A at different spray distances and back pressures

In order to protect the epidermis from unwanted thermal damage more effectively in laser dermatology for treating darkly pigmented human skin, a novel method is presented here that combines the hypobaric pressure-modulatable technique and a new cryogen of R404A with a far lower boiling temperature than R134a. The influence of spray distance (5-50 mm) and spray back pressure (1-100 kPa) on heat transfer dynamics is systematically investigated. Lowering the pressure always reduces the minimum surface temperature (T_min) over 50 °C, and significantly enhances the maximum heat flux (q_max) by 1.9 times for short spray distances of 5 mm. For longer distances over 20 mm, T_min can be first reduced, and q_max undergoes an initial increase and a subsequential decrease as the pressure is above 20 kPa. Further lowering pressure below 20 kPa makes T_min increase inversely, and causes a sharp reduction of q_max. To obtain low T_min and high q_max, an appropriate spray distance and pressure should be employed.