Coupling effect of hypobaric pressure and spray distance on heat transfer dynamics of R134a pulsed flashing spray cooling

Cryogen spray cooling (CSC) with R134a has been commonly used in the laser dermatology. Lots of strategies have been proposed to improve the cooling capacity of CSC, among which the hypobaric pressure method (lowering local pressure on the targeted skin) is of great potential. This paper experimentally investigated the coupling effect of hypobaric pressure and spray distance on the surface heat transfer dynamics of R134a pulsed flashing spray cooling. The absolute pressure within the vacuum chamber (i.e. spray back pressure) could be adjusted from 100kPa to 0.1kPa, and the spray distance was from 10mm to 50mm. It's found that lowering the pressure enhanced the spray angle and droplet evaporation rate. A transitional pressure of 10kPa was found, under which the spray pattern became much more homogeneous, while droplet diameter and velocity decreased much faster with the reduction of the pressure. At the rather short distance (10 mm), lowering pressure always benefited enhancing the cooling capacity, which produced 2.6 times maximum heat flux with 0.1kPa than that with the atmospheric pressure (from 247 kW/m² to 641 kW/m²). For other longer spray distances (> 10 mm), nevertheless, lowering the pressure slightly helped to increase the maximum heat flux with the pressure larger than the transitional pressure, while further lowering the pressure caused the rapid decrease of the maximum heat flux. The hypobaric pressure should be appropriately chosen in conjunction with spray distance to ensure higher cooling capacity for the CSC. The suitable spray distances coupling with the pressures were recommended.