An experimental investigation on MMH/NTO impinging jets flame characteristics at high chamber pressure

Monomethyl hydrazine (MMH) and nitrogen tetroxide (NTO) propellant is the most prevalent combination in hypergolic liquid rocket engines for space propulsion scenarios. However, limited work has been reported on its combustion characteristics, especially at high chamber pressure. This work, for the first time, experimentally investigated the flame characteristics of MMH/NTO impinging jets at chamber pressure p_c up to 5.1 MPa and at different flow rate Q and mixture ratio O/F conditions. High speed OH* chemiluminescence imaging technique resolved the structure, location, and direction of the flame, as well as the local reactivity of this liquid and gas phase combustion event. Results show that the flame is a cone shaped one, and its dimensional parameters depend on the test conditions. Two regions representing liquid phase reaction and main flame are distinctly observed, respectively at the impingement point and a certain distance downstream. In addition, the conventional flame direction estimation approach based on combined momentum direction deviates from the current hot test measurements, and a new correlation for flame direction prediction is proposed by correcting the O/F dependence. Finally, the maximum OH* intensity at the impingement point I_{max, imp} indicates that chamber pressure and flow rate weakly affect liquid phase reactivity, but more fuel injection favors it. However, in the main flame region, the maximum OH* intensity I_{max, main} quickly rises with increasing p_c, and both Q and O/F non-monotonically affect I_{max, main} due to the competition between local mixing and chemical reaction. This work is believed to be of significant merit for injector and chamber design for future high performance engines.