Theoretical and experimental analysis of droplet diameter, temperature, and evaporation rate evolution in cryogenic sprays

Cryogenic sprays are used for cooling human skin during selected laser treatments in dermatology. In order to optimize their cooling efficiency, a detailed characterization and understanding of cryogen spray formation is required. Various instruments and procedures are used to obtain mean size (D), velocity (V), and temperature (T) of tetrafluoroethane spray droplets from straight-tube nozzles. A single-droplet evaporation model is used to predict droplet diameter and temperature as a function of distance from the nozzle, D(z) and T(z), from the values of D, V, and T at the nozzle exit, i.e., D0, V0, and T0. In the model, it is assumed that D and V decrease in accordance with the D2-law, and due to drag force, respectively. To compute T(z), the instantaneous D and V are incorporated into a phase-change heat transfer balance, which includes a heat convection term. The predicted evolutions of T(z) and D(z) are in reasonable agreement with experimental data.