Flame temperatures in non-premixed flames

In preliminary dimensioning of cryogenic LRE injectors, temperatures near the injectors lips must be estimated beforehand to assess potential thermo-structural problems. The usual assumption is that the maximum temperature is the adiabatic flame temperature of the premixed reactants at injection conditions, even if LRE injectors operate in the diffusion flame regime. This paper shows that the actual "adiabatic flame temperature" in such cases may be higher than that of a premixed flame at the same operating conditions. To demonstrate it, an axial-symmetric unsteady simulation of a supercritical coaxial LOx/CH4 injector, with a finite-thickness LOX post-tip, was carried out using a single step reaction mechanism for its kinetics (2O2+CH4→CO2+2H2O). The post-tip create a recirculation zone where reactants diffuse and burn after been preheated by the hot recirculation products. The characteristic residence time inside this zone is typically much longer than the chemical kinetics time and thermo-chemical equilibrium may be assumed. The premixed adiabatic flame temperature was calculated, with the NASA CEA400 (equilibrium) code, at the same injection conditions of the diffusion flame simulated with FLUENT software. The equilibrium code uses ideal gas properties, and these were also utilized for the axially symmetric CFD simulation. Results indicate that in the recirculation region the diffusion flame oscillates periodically and reaches peak temperatures close to 6800 K, while the premixed-flame temperature predicted by the NASA equilibrium code, with the same products, reaches about 5000 K. The reason stays in the fact that the reactants are preheated, and that the Cp of the products behave differently with the temperature. Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc.