Liquid film instability analysis in a hybrid rocket engine

Instabilities in the liquid layer of liquefying hybrid rocket fuels have been investigated. Film stability has been modeled within the context of Kelvin-Helmholtz instability. The existing dispersion relations for a stratified gas-liquid flow in an horizontal rectangular channel have been deduced and applied to the present purpose. Specific attention is given to the introduction of viscosity and flat plate boundaries inside the base inviscid unbounded model. Theoretical evaluations have been compared to experimental data. An ambientcondition test campaign has been carried out at DLR Lampoldshausen (Germany), testing paraffin based fuels and gaseous oxygen. A two dimensional chamber with windows on both sides has been equipped with high speed video imaging setup. Owing to the need of a method for video post-processing, initially an existing MATLAB® code has been adjusted for wavelength estimation by point-click over the acquired images. Ignition transients have required a specific statistical analysis based on autoregressive models. At a later stage, it has been decided to avoid the full user intervention and make the procedure completely automatic. Bibliographic research has suggested the idea to find periodic patterns by decomposing the scalar field of flame luminosity via POD. A new code performing principal component analysis through the NIPALS algorithm has been developed. To the authors knowledge, this has been the first attempt to apply POD to luminosity data in the hybrid rocket context of paraffin based fuels. Power spectral density analysis over the results of the decomposition has provided an insight of the dominant wavelengths. The observed long waves have been retrieved. Experimental results partially match with theory.