Numerical Simulation of Flame-Vortex Interactions in Natural and Synthetic Gas Mixtures
Weiler, Justin D.
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The interactions between laminar premixed flames and counter-rotating vortex pairs in natural and synthetic gas mixtures have been computationally investigated through the use of Direct Numerical Simulations and parallel processing. Using a computational model for premixed combustion, laminar flames are simulated for single- and two-component fuel mixtures of methane, carbon monoxide, and hydrogen. These laminar flames are forced to interact with superimposed laminar vortex pairs, which mimic the effects of a pulsed, two-dimensional slot-injection. The premixed flames are parameterized by their unstretched laminar flame speed, heat release, and flame thickness. The simulated vortices are of a fixed size (relative to the flame thickness) and are parameterized, solely, by their rotational velocity (relative to the flame speed). Strain rate and surface curvature measurements are made along the stretched flame surfaces to study the effects of additive syngas species (CO and H2) on lean methane-air flames. For flames that share the same unstretched laminar flame speed, heat release, and flame thickness, it is observed that the effects of carbon monoxide on methane-air mixtures are essentially negigible while the effects of hydrogen are quite substantial. The dynamics of stretched CH4/Air and CH4/CO/Air flames are nearly identical to one another for interactions with both strong and weak vortices. However, the CH4/H2/Air flames demonstrate a remarkable tendency toward surface area growth. Over comparable interaction periods, the flame surface area produced during interactions with CH4/H2/Air flames was found to be more than double that of the pure CH4/Air flames. Despite several obvious differences, all of the interactions revealed the same basic phenomena, including vortex breakdown and flame pinch-off (i.e. pocket formation). In general, the strain rate and surface curvature magnitudes were found to be lower for the CH4/H2/Air flames, and comparable between CH4/Air and CH4/CO/Air flames. Rates of flame stretching are not explicitely determined, but are, instead, addressed through observation of their individual components. Two different models are used to determine local displacement speed values. A discrepancy between practical and theoretical definitions of the displacement speed is evident based on the instantaneous results for CH4/Air and CH4/H2/Air flames interacting with weak and strong vortices.