Behavior of preheated premixed flames at rich conditions

Over the past decade, a series of studies has demonstrated non-catalytic hydrogen production via partial oxidation of fuel-rich mixtures that lie well beyond the conventional rich flammability limits. In all cases, heat recirculation from the post-reaction zone to the unburned mixture leads to significant increases of combustion temperatures and mass burning flux. Although the feasibility of self-sustained combustion at extremely rich conditions is well established, fundamental questions on this combustion regime have not been addressed. As a first step, the present study relates the inherently non-adiabatic operation of heat-recirculating combustors to adiabatic combustion with highly preheated fuel/air mixtures. Standard numerical and experimental techniques are used to illuminate fundamental aspects of rich combustion for a broad range of inlet temperatures (T-in = 200-1000 K) and equivalence ratios (phi = 1.33-2.7). Numerical results show conventional flame structures at equivalence ratios in excess of phi = 2.0 for CH4/air combustion, which is corroborated by experiments with a flat flame burner and highly preheated fuel/air mixtures. A detailed numerical analysis reveals that adiabatic flame propagation at rich preheated conditions is governed by characteristic temperatures T-star within the flame structure. These temperatures are shown to be primarily dependent on the laminar burning flux, whereas the equivalence ratio plays a subordinate role. (c) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.