Experimental investigation on the Leidenfrost phenomenon of droplet impact on heated silicon carbide surfaces

Abstract

Due to its superiority in suppressing hydrogen generation under severe accident conditions, silicon carbide (SiC) has been regarded as one of the promising candidates among the diverse accident tolerant fuel (ATF) claddings. Droplet impact experiments are conducted on preheated CVD-SiC surfaces with different roughness and the polished sintered-SiC and stainless-steel surface. The effects of surface roughness, contact angle and thermal properties on the impact behavior and cooling efficiency are discussed. The experiments are carried out in the range of 10 < We < 120 and Tsurf < 460 °C. The observed droplet impact phenomena on all the surfaces are categorized into five regimes, i.e. deposition, rebound with secondary atomization, breakup with secondary atomization, rebound and breakup. Deposition corresponds to nucleate boiling. Rebound and breakup are the hydrodynamic phenomena of film boiling. The rest two correspond to transition boiling. Droplet breakup can be induced thermally or mechanically in the transition boiling. Surface roughness can enhance droplet breakup both in the film and transition boiling. The small contact angle also promotes breakup. The thermal property of surfaces has little effect on the critical Weber number for droplet breakup, but affect the Leidenfrost point temperature (LPT) and CHF temperature significantly. The rupture of liquid film induced by roughness reduces the stagnant pressure beneath the droplet and lower the LPT. The droplet spreading dynamics are analyzed quantitatively, and developed an empirical model for droplet maximum spreading factor. It is found that the LPT model based on homogeneous nucleation mechanism coincides well with present experimental results.