Computational Realization of Popping Impinging Sprays of Hypergolic Bipropellants by a Eulerian-Lagrangian Approach

This work adopts a Eulerian-Lagrangian approach to numerically simulate the spray impingement of MMH (Monomethyl hydrazine)/NTO (nitrogen tetroxide), which are prevalent rocket engine bipropellants for deep space missions and satellite orbital maneuvers. The emphasis of the work is to computationally realize the popping phenomenon and to study its parametric dependence on liquid and gas-phase reaction rates. The liquid-phase reaction of MMH/NTO is realized based on the extended spray equation, incorporating the additional independent variable, propellant mass fraction, to account for the mixing of droplets. The spray popping can be computationally reproduced over wide ranges of Damk\"ohler numbers for both liquid- and gas-phase reactions. Furthermore, the computational results have been validated through qualitative comparison with experimental images and quantitative comparison with experimental frequencies. The present results verify our hypothesis that the heat release from the liquid-phase reaction enhances the evaporation of MMH and NTO so that the intense gas-phase reaction zone around the spray impingement point periodically separates the MMH and NTO impinging sprays to cause the popping phenomenon. Furthermore, it was found that the popping phenomenon can be suppressed by reducing the Damk\"ohler numbers of liquid-phase reaction and therefore to suppress the evaporation of the propellants. This work is believed to provide valuable understanding for avoiding the off-design popping phenomenon that may reduce combustion efficiency and increase the risk of combustion instability in rocket engines.