Measurement and theoretical prediction on the surface emittance of carbonized layer of silica/phenolic ablative material

As a typical fiber reinforced resin-based ablative material, silica/phenolic composite has important applications in the thermal protection of high-speed aircrafts. At high temperature, it can form a carbonized layer, whose surface emittance is directly related to its capacity of radiative heat dissipation. In this work, carbonization experiments of the silica/phenolic composite and pure phenolic resin were designed to obtain their corresponding carbonized samples. The spectral emittances in the wavelength range from 2.5 to 25 μm of the carbonized samples were measured by a Fourier transform infrared spectroscopy with a blackbody furnace and a sample heater at temperatures from 200 to 500 °C. The results showed that the spectral emittance of the carbonized ablative material is close to that of the carbonized phenolic resin. The spectral emittances of the two carbonized samples increase with temperature and almost do not change with wavelength, indicating that the carbonized samples have the nature of a grey body. A prediction model of the total emittance was constructed based on the grey body assumption and a geometric model extracted from structure characterization of the carbonized ablative material. The predicted total emittances and those obtained from the integration of the measured spectral results are all higher than 0.8, and the deviations are below 10%. Theoretical predictions of the model show that the total emittance of the carbonized ablative material increases with the increasing porosity (ϕ) and total emittance (εs) of the fiber bundle covered with carbonized phenolic resin. Both the experimental and theoretical results reveal that the high total emittance of the carbonized silica/phenolic ablative material can be mainly attributed to the high emittance of the carbonized phenolic resin.