Visualization of Cryogenic Liquid Nitrogen Jet using Raman Scattering

Abstract

An investigation on injection characteristics simulating real rocket combustion chamber conditions has been carried on through this research. Liquid nitrogen was used for simulating cryogenic rocket propellants, and injection experiments were conducted inside a high pressure chamber to form various pressure conditions. To overcome the limitation of short visualization window and to have quantified result, laser induced Raman scattering technique has been applied to the investigation of the cryogenic flow. An emission of Raman scattering and a proper combination of filters have been verified prior to the main experiments. The filter set was formed to block the incident beam wavelength with an amount of 10-16. Breakdowninducing laser power was also measured and the power was limited not to form a plasma. The main experiments were conducted with varying ambient pressure conditions from 20 bar to 50 bar, which is from subcritical to supercritical condition of nitrogen. Raman scattering signal images from the nitrogen jet have been depicted by ICCD camera, and the images went through a series of reduction process consist of background subtraction, Gaussian beam profile effect normalization, and beam attenuation effect compensation. The refined data was quantified into a density map, and analyzed from the various viewpoint. The density maps showed the distribution of nitrogen jet after the injection in absolute numbers. Moreover, with focusing on axially 0.5d position, the trend of injection density change with the ambient pressure increase has been observed, which had increased under the subcritical conditions but had converged under the supercritical conditions. The analysis on normalized properties also has been conducted. Axially normalized data showed the trend of faster dissipation along the axis when the ambient pressure increases under the supercritical conditions. Radially normalized data was used for the jet similarity study, and it showed larger deviation under the subcritical conditions. Ramanographs were drawn for the ease of understanding density distribution in two dimension, and compared with unfiltered scattering signal image, which showed the advantage of applying Raman scattering technique.