Correlations between spray properties and heat transfer dynamics during cryogen spray cooling

Cryogen spray cooling (CSC) has been used along with pulsed lasers for nearly a decade to irreversibly photocoagulate a variety of vascular lesions. However, the fundamental mechanisms that take place at the skin surface are still incompletely understood. In this work, we built a fast-response temperature sensor with the objective to determine the time in which liquid cryogen remains on the skin surface during and after a short spurt of cryogen - the residence time (t_r). Measurements are conducted systematically at various distances from the nozzle (z) and various spurt durations (Δt) for two nozzles that produce completely different spray characteristics. It was found that for each nozzle, there is a critical spray distance (z_c) where an abrupt increase in t_r occurs and another critical distance (z_max) that can be related to maximal cryogen deposition. Furthermore, using experimentally measured average droplet diameter (d) and velocity (v) for sprays produced by each nozzle, we defined a spray characteristic time τ(z) as the ratio of d to v. This parameter allows us to represent our experimental data on a single curve for each nozzle, by plotting the dimensionless residence time (t_r/τ) as a function of the product of dimensionless spurt duration (Δt/τ) and a dimensionless factor (m/m_max). The factor m/m_max is envisioned as an effective mass deposition which, due to the evaporation of cryogen droplets in-flight and the spray-surface interactions, is a strong function of z. These results represent a step towards a more complete understanding and quantification of the physics involved in CSC.