Droplet impingement dynamics: effect of surface temperature during boiling andnon-boiling conditions

Abstract

This study investigates the hydrodynamic characteristics of droplet impingement on heatedsurfaces and compares the effect of surface temperature when using water and ananofluid on a polished and nanostructured surface. Results are obtained foran impact Reynolds number and Weber number of approximately 1700 and 25,respectively. Three discs are used: polished silicon, nanostructured porous silicon andgold-coated polished silicon. Seven surface temperatures, including single-phase(non-boiling) and two-phase (boiling) conditions, are included. Droplet impact velocity,transient spreading diameter and dynamic contact angle are measured. Results ofwater and a water-based single-wall carbon-nanotube nanofluid impinging on apolished silicon surface are compared to determine the effects of nanoparticles onimpinging dynamics. The nanofluid results in larger spreading velocities, largerspreading diameters and an increase in early-stage dynamic contact angle. Results ofwater impinging on both polished silicon and nanostructured silicon show that thenanostructured surface enhances the heat transfer for evaporative cooling at lower surfacetemperatures, which is indicated by a shorter evaporation time. Using a nanofluid or ananostructured surface can reduce the total evaporation time up to 20% and 37%,respectively. Experimental data are compared with models that predict dynamic contactangle and non-dimensional maximum spreading diameter. Results show that themolecular-kinetic theory’s dynamic contact angle model agrees well with currentexperimental data for later times, but over-predicts at early times. Predictions of maximumspreading diameter based on surface energy analyses indicate that these modelsover-predict unless empirical coefficients are adjusted to fit the test conditions. This is aconsequence of underestimates of the dissipative energy for the conditions studied.