Experimental evidence of the multiplicity of stationary solutions in ultra-lean hydrogen flames

The morphology and propagation speed of premixed flames are mainly controlled by the fuel-air concentration and by the environmental conditions (temperature, pressure and geometry). Changes in these parameters induce large variations of the flame shape and speed compared to the theoretical planar construct. As shown in recent numerical simulations, very lean hydrogen flames in narrow confined geometries can form either circular or double-cell isolated flames propagating at different velocities under the same controlling parameters. In this work, we show numerical and experimental evidences of this multiplicity of flame solutions and, using the equivalence ratio φ as the controlling parameter, we determine the critical values of φ that define the flame characteristics in a bifurcation diagram.