Abstract: Aiming at elucidating the detailed ignition and flame characteristics of gelled hypergolic bipropellants utilizing an impinging injector, an experiment is conducted to investigate the combustion processes of gelled Monomethylhydrazine/Nitrogen Tetroxide(MMH/NTO) in a square combustion chamber with a single impinging injector. Unlike-impinging injector, including 75°, 90°, and 105° impinging angles, and 90° impinging angle triplet impinging injector, including Fuel-Oxidizer-Fuel (F-O-F) and Oxidizer-Fuel-Oxidizer(O-F-O), are used in the experiment. Schlieren technology and a high-speed camera are combined to investigate the spray and combustion processes firstly, then natural flame images are obtained by a high-speed camera directly. The ignition distance, axial flame speed, flame angle, and induction distance are obtained by image processing technology, and then the effect of injector type and jet velocity of fuel are discussed. The results show that the combustion of gelled MMH/NTO occurrs after liquid sheets breakup into ligaments, and the gas diffusion rate increases with the increasing of fuel jet velocity. The ignition distance of gelled MMH/NTO is the shortest when using the 105° unlike-impinging injector. The axial flame speed increases with the increasing of fuel jet velocity, and it's quicker when the impinging angle is 90°. During the steady combustion, the flame angle increases when the jet velocity of fuel increases, but the induction distance decreases conversely. The flame angle is the maximum when using the 90° unlike-impinging injector, and induction distance is the shortest if using the 105° unlike-impinging injector.

Key words: monomethylhydrazine, nitrogen tetroxide, gel, impinging injector, ignition, flame