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 SiO 2 generated on 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 SiO 2 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.