The effect of fuel composition on the non-premixed flame structure of LNG/LOx mixtures at supercritical pressure

Methane and Liquefied Natural Gas (LNG) have been recently considered as propellants, together with cryogenic oxygen (LOx), for Liquid Rocket Engines (LRE) applications, having shown some advantages over the commonly used propellants. In this framework the aim of the present study is to understand how the composition of LNG, with respect to the pure methane, can influence non-premixed combustion at typical LRE supercritical operating conditions. In the present work, the flame structures of LNG/LOx mixtures at supercritical pressures are investigated by means of a general fluid formulation for the unsteady laminar flamelet equations. Real fluid effects, which are commonly encountered in LRE thrust chambers because of high pressure pressure conditions, are taken into account by means of a comprehensive three-parameter cubic equation of state. Flame structures are analyzed at elevated pressures ranging from near-critical up to largely supercritical, for pure methane and various representative mixtures models of LNG. The LNG/LOx steady state flame structure characteristics are found to show close similarities to those of the pure methane. On the other hand pollutants species, such as early stage soot precursors (i.e. acetylene), are found to be strongly dependent from LNG com- position. Sooting tendencies of LNG is further investigated with a more detailed chemical mechanism in order to shed light on Polycyclic Aromatic Hydrocarbons (PAH) presence at supercritical pressures. A single representative mixture for LNG composition is analyzed showing significantly larger production of soot precursors, such as benzene and naphthalene.