Flame stabilization in high pressure LOx/GH2 and GCH4 combustion

Planar laser induced fluorescence (PLIF) of OH is used to examine flame stabilization in high pressure cryogenic flames formed by injecting a central jet of low speed liquid oxygen surrounded by a high speed gaseous stream of hydrogen or methane. In the LO_x/GH₂ experiments injection conditions are transcritical as the chamber pressure is above critical but the temperature is below critical . In the LO_x/GCH₄ experiments the chamber pressure and LO_x injection temperature are below critical , . Hydrogen or methane are injected at room temperature LIF images delineate the flame edge in the injector nearfield. The two flames are stabilized in the vicinity of the liquid oxygen injector lip but the anchor point is found to lie closer to the lip in the LO_x/GH₂ case and its displacement from shot to shot is of a smaller amplitude than that corresponding to the LO_x/GCH₄ flame. Interpretation of these data is based on a previous analysis which indicates that stabilization is essentially controlled by a dimensionless group formed by comparing the lip thickness to the flame edge thickness Ψ = h_s/δ_f. It is found that Ψ slightly exceeds unity in the LO_x/GH₂ case essentially fulfilling the stability condition while Ψ < 1 in the LO_x/GCH₄ case. In this last situation the flame is thicker than the characteristic thickness h_s and it is therefore sensitive to the high speed methane stream. Anchoring is imperfect and the flame edge moves with the turbulent eddies shed from the lip. Global stabilization is achieved dynamically but the reactive layer is not well established and the large amplitude motion of the edge is a symptom of a possible lift-off. Theoretical estimates indicate that LO_x/GCH₄ flame stabilization requires a thicker lip size than the LO_x/GH₂ propellant couple.