Numerical Investigation of Combustion Mode Transition and Classification in Hypergolic Propellant Impinging Jets

Hydrazine (N2H4)/nitrogen tetroxide (NTO) gaseous impinging jets are numerically investigated to clarify combustion modes in hypergolic propellant systems. The compressible Navier – Stokes equations are solved with a detailed chemical-kinetic mechanism for N2H4-NTO mixtures. Two distinct combustion modes and their transition are identified depending on the injection temperature: the separate mode and the mix mode. In the separate mode, the jets are separated by a high-temperature non-premixed oxidation flame, while a lower-temperature N2H4 decomposition flame appears downstream of the fuel jet. In contrast, no separating non-premixed flame is formed in the mix mode. The jets mix prior to breakup, and premixed flames develop downstream because the ignition delay time exceeds the jet breakup time. As a result, autoignition occurs in a distributed manner over a broad downstream region. The jet breakup time and the ignition delay time of N2H4-NTO mixtures are identified as critical factors governing the combustion mode. Based on these findings, a classification criterion is proposed using two parameters – the jet breakup time and the ignition delay time – to predict whether hypergolic propellant impinging jets operate in the separate or mix mode.