Spectral Emissivity Measurement of Supersonic Nozzles for Radiative Cooling Performance Evaluation

In this study, emissivity was employed as the primary metric for evaluating the radiative-cooling performance of a supersonic nozzle under flight-like conditions. A supersonic nozzle was fabricated by PBF, after which combustion (hot-fire) tests and emissivity measurements were carried out. These data enabled quantification and visualization of radiant energy in the 8–14 µm-wavelength band during combustion. The hot-fire tests revealed a clear cap-shock pattern, confirming that the nozzle flow was fully developed. Emissivity measurements showed that the additively manufactured surface—subsequently treated by the AMS 5662 heat-treatment process—followed the angular-emission trends reported in previous studies. The surface exhibited a high roughness (Ra ≈ 30 µm) and an emissivity of 0.85 in the 8–14 µm band; a temperature-dependent emissivity fitting function was accordingly derived. By coupling the combustion test results with the emissivity data, the actual temperature distribution along the nozzle surface and the corresponding radiant energy in the 8–14 µm band were quantitatively reconstructed and visualized. The maximum emissive power in this band reached 2214 W m−2, representing at least 16.61% of the total black-body radiation at 700 K.