Experimental investigation on the behavior and heat transfer of droplet impacting on heated surface

Abstract

Water droplet impact onto a heated hydrophilic sapphire, with surface temperature from 100 to 600 °C, was visualized with synchronized bottom- and side-view high-speed cameras. The liquid–solid contact modes are defined based on the morphology in the bottom-view images. Four types of dynamic contact patterns, i.e., mottled, finger, ring and dot contact patterns, are identified. Synthesizing the side- and bottom-view images, droplet impact behaviors are classified into four regimes: deposition, rebound after contact, breakup after contact and Leidenfrost. The ring contact pattern, appearing at $T_w$≈300°C, entraps generated vapor and results in upward jetting. The bottom-view images are processed and analyzed to quantify the liquid–solid contact area in each frame. Based on the solution of one-dimensional transient heat conduction within the contact area, heat transfer resulted from instantaneous liquid–solid contact was estimated. The total amount of heat transfer (Q) resulted from single droplet impact decreases with increasing surface temperature ($T_w$). However, on the Q-$T_w$ curves the inflection points are observed near 300°C, implying heat transfer enhancement accompanied by upward jetting.