Effect of CO on the propagation of natural gas flames with heat flux burner

Abstract

The demand to improve the sustainability of liquid rocket engines inspired the substitution of conventional fuels by liquified natural gas and the usage of the thermal energy of the exhaust gas by recirculation into the combustor. This leads to the question of how the mixing of exhaust gases with the fuel affects the combustion of natural gas flames. Therefore, we investigated by simulations and experimentally using a heat flux burner, the influence of the addition of CO on the propagation speed, shape, and stability of adiabatic CH₄/O₂/CO₂ flames. We analyzed CO contents of 10%, 20%, and 30% and varied the equivalence ratio, according to the typical operating conditions of liquid rocket engines, between 1.0 and 1.4. Under stoichiometric conditions, the adiabatic laminar burning velocity measures 39 cm/s at 10% CO content, 34 cm/s at 20%, and 29.5 cm/s at 30%. Due to hydrodynamic instabilities, the flames form cellular structures which increase the total flame front area and, thus, their propagation speed. On the other hand, the addition of CO dampens these instabilities and, consequently, decreases the adiabatic flame propagation speed. CO affects the flames the strongest for stochiometric conditions but weakens for more rich mixtures. For high equivalence ratios the flame thickens, the density ratio of the burned and unburned gases decreases, and the flame cells disappear. The presented results reveal the important role of CO on the propagation and the morphology of natural gas flames, especially for close to stoichiometric mixtures. This study offers new insights for optimizing combustion in liquid rocket engines, particularly through the addition of CO to enhance flame propagation and stability.