Studies of premixed and non-premixed hydrogen flames

The hydrogen oxidation chemistry constitutes the foundation of the kinetics of all carbon- and hydrogen-containing fuels. The validation of rate constants of hydrogen-related reactions can be complicated by uncertainties associated with experimental data caused by the high reactivity and diffusivity of hydrogen. In the present investigation accurate experimental data on flame propagation and extinction were determined for premixed and non-premixed hydrogen flames at pressures between p = 1 and 7 atm. The experiments were designed to sensitize the three-body H + O2 + M → HO2 + M reaction, whose rate is subject to notable uncertainty. This was achieved by increasing the pressure and by adding to the reactants H2O and CO2 whose collision efficiencies are high compared to other species. In the present study, directly measured flame properties were compared against computed ones, in order to eliminate uncertainties associated with extrapolations, as is the case for laminar flame speeds. The measured extinction strain rates exhibit both a positive and negative dependence on pressure with and without weighting with the density, and this non-monotonic behavior is caused by the competition between the H + O2 → O + OH and H + O2 + M → HO2 + M reactions as well as HO2 kinetic pathways as pressure increases. The various kinetic models considered in this investigation did not reproduce equally well the non-premixed flame extinction data with added H2O. On the other hand, the predicted extinction strain rates were consistent between the various models in the case of added CO2. Finally, it was shown that the formulation of binary diffusion coefficient pairs including H–N2 and H2–N2 has a first order effect on the prediction of extinction strain rates of non-premixed H2 flames.