Transient surface temperatures upon the impact of a single droplet onto a heated surface in the film evaporation regime

Abstract

Single droplet impingement on a heated surface is a fundamental problem of basic interest in understanding complex phenomena in spray cooling and other applications involving heat transfer to impinging droplets. The surface temperature experiences rapid temporal gradients upon droplet impact. These temperature transients are strongly dependent on the hydrodynamic and heat transfer regimes. Experimental results are presented for the transient surface temperature in the film evaporation heat transfer regime for water drop diameters of 2.1 and 3.5 mm at drop Reynolds numbers up to 34,000 and Weber number up to 5540 are reported for surface temperatures of 60 to 100 ∘C. High-speed video was used to capture the dynamics of the impact. Intrinsic thermocouples were manufactured to measure surface temperature with sufficient temporal resolution. It was found that the hydrodynamics of the impact were not significantly affected by the surface temperature. Heat removal was found to be more effective at lower impact velocities. The maximum temperature drop occurred at the maximum spreading diameter. An analytical model was developed to successfully predict the transient Nusslet number of the droplet motion on the surface for both free-falling and air-propelled droplets.