Influence of H2 on the response of lean premixed CH4 flames to high strained flows

A combined experimental and numerical investigation on the effects of H2 addition to lean-premixed CH4 flames in highly strained counterflow fields (with strain rates up to 8000 s 1 ) using preheated flows indicate significant enhancement of lean flammability limits and extinction strain rates for relatively small amounts of H 2 addition. Numerical modeling of the counterflow opposed jet configuration used in this study indicated extinction strain rates which were within 5% of experimentally measured values for equivalence ratios ranging from 0.75 to less than 0.4. Both experimental and numerical results indicate that increasing H 2 in the fuel significantly increases flame speeds and thus extinction strain rates. Furthermore, increasing H 2 decreases the dependency of extinction equivalence ratio on the strain rate of the flow. For all of the mixtures investigated, extinction temperatures depend primarily on equivalence ratio and not fuel composition for the range of H2 content studied, which suggests that extinction can be correlated to flame temperature and O2 concentration. Nonetheless, H2 addition greatly increases the maximum allowable strain rate before extinction temperatures are reached. Inspection of the model-predicted species profiles suggest that the enhancement of CH4 burning rates with H2 addition is driven by early H2 breakdown increasing radical production rates early in the flame zone to enhance CH4 ignition under conditions where otherwise CH4 combustion might be prone to undergo extinction. © 2003 The Combustion Institute. All rights reserved.