What Differences Does Large Eddy Simulation Find among Traditional, High-Temperature, and Moderate or Intense Low Oxygen Dilution Combustion Processes of a CH_4/H_2 Jet Flame in Hot Oxidizer Coflow?

摘要

Large eddy simulation (LES) of a CH4/H-2 diffusion jet flame in hot coflow (JHC) is undertaken to find distinct behaviors of moderate or intense low oxygen dilution (MILD) combustion (MILDC), high temperature combustion (HTC), and traditional combustion (TC). These three JHC flames are realized by using different coflow temperatures and oxygen mass fractions: (1) 1300 K and 9% for MILDC; (2) 1300 K and 30% for HTC; (3) 600 K and 30% for TC. The modeling of LES combining the eddy dissipation concept (EDC) with a global four-step reaction mechanism is validated by the JHC measurements of Dally et al. (Proc. Combust. Inst. 2002, 29, 1147-1154). The instantaneous and time-averaged velocities, temperatures, and species concentrations such as carbon monoxide (CO) are presented and compared for the three cases. It is demonstrated that the JHC flames of MILDC and HTC both develop from autoignition nearly immediately downstream of the nozzle exit, while the lift-off JHC flame of TC evolves from an induced-ignition with a significant delay. Manifestly, combustion reactions proceed gently in the MILDC case and highly aggressively in the TC case. In both MILDC and HTC cases, stable combustion ensues in the very near field. While most heat releases around the stoichiometric location and transfers away slowly, combustion species, for example, CO, diffuse more rapidly across the jet flow. The JHC flame for TC behaves completely differently. With a wobbling flame base, large-scale flame oscillations enhance crosswise turbulent mixing and heat transfer. Consequently, high temperatures and high CO concentrations concurrently emerge across the central region in the mid field. Besides, local extinction and reignition appear to occur frequently in the TC and do not happen in the HTC and MILDC.