Ray Tracing and Spectral Modelling of Excited Hydroxyl Radiation from Cryogenic Flames in Rocket Combustion Chambers

Sammanfattning: A visualisation procedure was developed which predicts excited hydroxyl (OH') radiation from the Computational Fluid Dynamics (CFD) solutions of cryogenic hydrogen-oxygen rocket flames. The model of backward ray tracing through inhomogeneous media with a continuously changing refractive index was implemented. It obtains the optical paths of light rays that originate in the rocket chamber, pass through the window and enter a simulated camera. Through the use of spectral modelling, the emission and absorption spectra eλ and κλ are simulated on the ray path from information about temperature, pressure and concentration of constituent species at relevant points. By solving a radiative transfer equation with the integration of emission and absorption spectra along the ray line-by-line, a spectral radiance is calculated, multiplied with the spectral filter transmittance and then integrated into total radiance. The values of total radiances at the window edge are visualised as a simulated 2D image. Such images are comparable with the OH' measurement images. The modelling of refraction effects results in up to 20 % of total radiance range absolute difference compared to line-of-sight integration. The implementation of accurate self-absorption corrects significant over-prediction, which occurs if the flame is assumed to be optically thin. Modelling of refraction results in images with recognisable areas where the effect of a liquid oxygen (LOx) jet core can be observed, as the light is significantly refracted. The algorithm is parallelised and thus ready for use on big computational clusters. It uses partial pre-computation of spectra to reduce computational effort.