Direct numerical simulation of bluff-body stabilized flames
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Direct numerical simulation (DNS) is used to study reacting and non-reacting bluff-body stabilized flows. Three fuel-to-air velocity ratios were considered. The results show that three types of flow fields can be identified. For the fuel-to-air velocity ratio of 0:84, the flow field is dominated by the reverse flow of the coflowing air. The jet does not penetrate through the reverse flow, and two stagnation points are formed on the centerline, where the jet and air stagnate. The downstream flow field is similar to a wake flow, and therefore this flow is referred to as a bluff-body dominant flow. For the fuel-to-air velocity ratio of 1:4, the flow field is neither dominated by the jet nor the coflowing air and is referred to as an intermittent flow. For the fuel-to-air velocity ratio of 2:8, the jet penetrates through the reverse flow and no stagnation point is formed. The flow field is similar to that of a pure jet, therefore this flow is referred to as a jet dominant flow. In all cases, the coflowing air converges towards the centerline and forms a recirculation zone that extends to approximately one bluff-body diameter downstream. For the reacting flow, the methane combustion is modeled via a global, 1-step kinetic mechanism. The main effects of combustion on the flow field are an increase in the length of the recirculation zone, and a decrease in the velocity fluctuations. Based on the three velocity ratios, three types of flames are produced. For the velocity ratio of 0:84, a short mushroom shaped flame is produced near the fuel stagnation point. For the velocity ratio of 1:4, a medium length flame is produced in the shear layer region between the fuel and air. The flame is anchored in the recirculation zone, and extends downstream of the bluff-body. For the velocity ratio of 2:8, a long flame is produced near the centerline and is also anchored in the recirculation zone. All three flames are stabilized between the coflowing air and the jet, where the strain-rates are low, and the mixture fraction is near the stoichiometric value for methane combustion.