Flameholding in Non-premixed Supersonic Flows

1 Introduction A balance between the flame propagation speed and the fluid velocity must be maintained to maintain the flame in a chemically reacting flow. Since the fluid velocity exceeds the flame speed in supersonic combustion applications, the flameholding issue is solved by generation of some sort of recirculation region which ensures sufficient residence time so that the processes involved-fuel-air mixing, ignition and chemical reactions propagation – can take place to completion. These processes are determined by local conditions of gas composition, temperature and velocity and are substantially different in non-premixed cases, such as encountered in most practical applications, than in premixed cases which are easier to analyze and predict. A substantial database of flame stability exists for premixed gases (Ozawa, 1971, Huelmantell et al, 1957, Ogorodnikov et al, 2001) from which stability limits for the rich and lean flames have been obtained for a number of fuel/air systems. The stability limit is usually cast in terms of a flameholding boundary on an equivalence ratio vs. a stability parameter plane. The stability parameter depends, in general, on the flow velocity, temperature, size and shape of the flameholder and has received various formulations in different studies, from empirical formulations to expressions that reflect global Damkhöler numbers. In the case of non-premixed gases the determination of stability limits is less straightforward, mostly due to the non-homogeneity of the parameters in the recirculation region behind the flameholder. It is difficult to estimate the spatial species concentration and temperature distribution in the recirculation regions of these flows due to the presence of large gradients and the complex, three-dimensional, flow structure. These difficulties are compounded by the uncertainty in the shape of the recirculation region, which depends on the amount of heat release which, in turn, is dictated by the local mixing and combustion efficiencies. The following discussion is focused on the characteristics of the flowfield in the region of a recirculation region with implications on the flameholding analysis and modeling. A simple recirculation region is the rearward facing step shown schematically in Figure 1. The rearward facing step has been among the early solutions for flameholding in supersonic flows and continues to exist in combination with other geometrical configurations in most currently proposed flameholders. Among the advantages offered by the rearward facing step is (i) the good separation between the increased pressure due to combustion in its base and the upstream incoming flow and, …