Evolution of surface droplets and flow patterns on C/SiC during thermal ablation

Abstract

Degradation of high temperature ceramics caused by thermal ablation is a great challenge for maintaining structural integrity and stability under service conditions. In order to advance the understanding of the thermal ablation mechanisms of C/SiC composites, we adopt here the optical observation system to visualize the surface evolution of C/SiC in wind-tunnel test, and demonstrate experimentally the various flow characteristics of liquid SiO2 generated on the sample surface at different temperatures. Three different flow patterns (droplet flow, stream flow, and liquid film flow) corresponding to 1600 ℃, 1750 ℃, and 1950 ℃ were observed on the sample surface. Numerical simulation shows that the radius of the droplets plays a critical role in determining the flow patterns, as evidenced by the experiments. Droplets with large radius (≥0.5 mm) deform severely and cause “tailing”; droplets with small radius deform little under shear. We also observe that the large SiO2 droplet will continuously break into small droplets and eventually the droplets will oscillate with a certain radius. With the ablation rate increasing, a large number of droplets merge with each other to form a liquid film, and periodic K-H (Kelvin-Helmholtz) instability occurs at the gas-liquid interface.