Effects of fuel premixing ratio on supersonic flame propagation and its mode

In order to examine the impact of varying proportions of premixed fuel in relation to the total fuel injected into the combustion chamber on the ignition process， flame propagation characteristics， and combustion mode within the combustion chamber of a super-combustion ramjet engine， RANS numerical method was employed to conduct a numerical investigation of an axisymmetric scramjet engine combustion chamber featuring concave cavities and a central conical rod configuration. Under all operating conditions with a total fuel ratio of 0.6 in the combustion chamber， distinct combustion dynamics are observed depending on the premixed fuel ratio. When the equivalence ratio of the premixed fuel is 0.0 or 0.1 and the remaining fuel is injected centrally， a large amount of heat is released after ignition of the centrally injected fuel， leading to thermal choking. Simultaneously， the flame， under the influence of the incoming airflow， was observed to undergo local extinguishing and re-ignition， resulting in flame oscillation. During this phase， premixed and diffusion flames were found to dominate different regions of the chamber， respectively. As the premixed fuel ratio increased， the oscillation phenomenon of the combustion flame was gradually mitigated by the effect of the premixed fuel in the incoming airflow. The oscillation range of the flame front was observed to narrow and eventually anchor to the leeward side of the central cone. The study indicates that the premixed fuel in the incoming airflow exerts an inhibitory effect on the combustion flame oscillation within the chamber configuration. With the gradual increase of the fuel premixing ratio， the flame propagation speed in the combustion chamber is observed to accelerate， and the distribution of premixed combustion zones， as indicated by the combustion index SFI， becomes more extensive.