Bistability analysis of opposed jet combustor under cold and reacting flow conditions

Abstract

Opposed Jet Combustor (OJC) is a widely used layout to study and measure flame properties. The fundamental assumption of the counterflow configuration is that the flow is steady and quasi-one-dimensional. In principle, OJC stagnation planes are generated when two streams of opposing follow impinge against the other. The characteristic of such formed stagnation zone is a function of momentum balancing. But a detailed analysis shows that the flow regime developed in OJC is much more complex. It is seen that for the same set of boundary conditions, the stagnation plane can exist at more than one axial location, thereby showcasing the phenomenon of bistability. In this work, a conical opposed jet combustor producing a flat nozzle exit velocity profile was employed to study this phenomenon of bistability under isothermal and reacting flow conditions. The flow field impingement region was analyzed in search of instabilities and asymmetries. Phase Doppler particle analyzer, camera, and thermocouple measurements were employed to map this asymmetric behavior. The result showcases that the axisymmetric cold flow exhibits three flow regimes, i.e., (1) single symmetric, (2) steady-state multiplicity, and (3) time-dependent unsteady state. More importantly, the bistability has been conclusively witnessed even during combustion experiments. This bistable branching observed during combustion was different from that of cold flow. With the reacting flow, the bistable planes showcase different degrees of stability, with only one plane being stable. The results observed here showcase the need to consider the possibility of bistability even during combustion experiments.