Comparison between excited hydroxyl radical and blue radiation from hydrogen rocket combustion

This paper compares the two dominant types of radiation from hydrogen flames under rocket-combustion conditions: ultraviolet and blue radiation. The physical sources of both types are scrutinized and basic modeling strategies are presented. Data were collected from four different combustors with pressures ranging up to 80 bar. The ultraviolet and blue radiations are discussed with regard to their spectral content between 290 and 430 nm; (self-) absorption; and total, spatial, and dynamic radiance. Both types of radiation are found to be strongly dependent on the ground-state hydroxyl-radical concentration, and the regions of emissivity are similar. Pressure is shown to have a higher impact on the emissivity of the blue radiation. Radiation from the hydroxyl radical is strongly influenced by self-absorption, thereby limiting the transparency of the flame. Recommendations for the interpretation of both types of flame radiation are given.