Experimental and Numerical Investigation of Flame Acceleration in an Obstructed Channel

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Johansen, Craig
Combustion , Compressible Flow
The purpose of this study is to experimentally and numerically investigate flame acceleration in an obstructed channel. The motivation for this research is for the development of Pulse Detonation Engines (PDEs), which are unsteady propulsion devices that utilize the detonative mode of combustion. A literature survey on flame acceleration in the context of PDEs is presented, which covers a wide range of combustion regimes including laminar combustion, turbulent combustion, and finally detonation. An overview of current numerical modeling strategies is also presented along with a selection of recent numerical studies focused on flame acceleration in obstructed channels. Experimentally, the effect of obstacle blockage ratio on flame acceleration was investigated in a modular channel. The channel had a square cross-section and obstacles were mounted onto the top and bottom surfaces. Schlieren images were used to study the flame shape and the centerline flame velocity. A novel visualization technique has been developed to study the unburned gas flow ahead of the flame front. Flame propagation at speeds above the speed of sound in the reactants was also studied as compression waves formed in the unburned gas. It was found that shock reflection from obstacle surfaces and subsequent flame interaction dominates flame acceleration at these higher flame speeds. The unburned gas flow field ahead of the flame front was simulated using Large Eddy Simulation (LES) and was compared to the visualization technique developed experimentally. The detailed unsteady calculation was used to further study the development of recirculation zones behind the obstacle surfaces and the generation of turbulence in the shear layers. The unburned gas flow field was investigated to give insight into the speed and shape of the flame as it propagates into these regions. Flame propagation was modeled using a flame surface density combustion model and simulations showed flame interactions with the turbulent flow field and how three-dimensional vortical structures augmented the flame shape and increase total area.
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