Fluidic Flame Stabilization in a Planar Combustor Using a Transverse Slot Jet

Abstract

T HE stabilization of a premixed flame in a high-speed internal environment has received a considerable amount of interest from many researchers, for example, [1–3]. Flame stabilization behind a bluff body represents the most common strategy for flame anchoring [4–8]. Bluff bodies and rearward-facing step flows stabilize the flame by introducing a low-velocity recirculation zone containing combustion products that act as a continuous ignition source. Although these flame-holding devices provide an environment suitable for flame holding, a drag penalty is incurred. An alternative fluidic-based approach using a transverse slot jet to generate a “virtual bluff body” would reduce the thrust penalties through the removal of form drag while producing a flowfield with flame-holding potential. Figure 1 shows two schematics for combustors employing bluff body and fluidic methods for flame holding. The dashed box in the figure represents the control volume that will be used to demonstrate that form drag imposes a penalty. A momentum balance in the streamwise direction for the two situations, employing the assumption of negligible viscous shear, indicates that the drag force on the bluff body FD will result in a loss in either streamwise momentum or an increased pressure drop across the burner. For the fluidic case, a balancewill be maintained between pressure drop and an increase in streamwise momentum. A simple one-dimensional analysis of the systems, shown in Fig. 1, using a drag coefficient based on the inlet flow properties was conducted to determine the additional total pressure losses due to flame-holder drag. The calculation employs constant specific heats, models the fluid as air, and employs conservation of mass, momentum, the ideal gas equation, Mach number definition, and stagnation relations. Figure 2 shows the additional total pressure loss due to form drag on the flame holder as a function of inlet Mach number and drag coefficient. In addition to thrust penalty reduction, the fluidic flame holder allows active control of the recirculation zone size, providing dynamic control of the stabilization characteristics that will allow a broader operating envelope and improved off-design performance. Dynamic control would allow for optimization of the tradeoff between combustion efficiency and flame stability. The cost of the fluidic actuation is considered in terms of the required mass flow rate to achieve the necessary recirculation zone. For the present experiments, the fluidic flow ratewas nominally 7% of themain flow rate. The objective of the current note is to document the operating characteristics of a fluidic flame holder consisting of a planar transverse jet issuing into a channel flow. The influence of the test chamber initial conditions on the scaling of the induced recirculation zonewill be shown. It will also be shown that the jet equivalence ratio can be used to manipulate the rich and lean blowout limits.