HYDROGEN–OXYGEN INDUCTION TIMES ABOVE CROSSOVER TEMPERATURES

Abstract

Ignition in hydrogen–oxygen systems above crossover temperatures and under various conditions of pressure and composition is addressed computationally and by asymptotic methods. Different descriptions of the detailed chemistry are evaluated through comparison of computed and measured ignition times, and a balance between accuracy and simplicity is struck in selecting rate parameters to be used in investigating reduced chemistry. Through numerical calculations for isobaric, homogeneous, and adiabatic hydrogen–air mixtures it is shown that the detailed chemistry can be reduced to only six elementary steps for determining induction times over the range of conditions addressed. From these six steps, an analytical expression for ignition time is derived which agrees well with computational results concerning dependence on pressure, temperature, and composition. It is shown that O and OH maintain steady states during ignition for stoichiometric and fuel-rich mixtures, whereas H maintains, a steady state for sufficiently fuel-lean conditions. Simple asymptotic formulas for the induction times are derived for these two limits that demonstrate the limiting effect of O₂ under rich conditions and of H₂ under lean conditions. These formulas are combined in an approximate way to obtain an expression for the ignition time that can be used for all equivalence ratios.

Keywords:

• hydrogen-oxygen ignition times
• crossover temperatures
• short mechanism
• analytical induction time
• asymptotic analysis

The work of ALS was supported by the Fifth Framework Programme of the European Commission under the Energy, Environment and Sustainable Development Contract No. EVG1-CT-2001-00042 EXPRO and by the Spanish MCYT under project No DPI2001-4603-E; the work of the other authors was supported by the National Science Foundation through Grant No. CTS 0129562.

Notes

^aUnits are mol, s, cm³, kJ, and K.

^bChaperon efficiencies are 2.5 for H₂, 12.0 for H₂O, 1.9 for CO, 3.8 for CO₂, and 1.0 for all other species.

^cChaperon efficiencies are 2.5 for H₂, 16.3 for H₂O, 1.9 for CO, 3.8 for CO₂, and 1.0 for all other species.

^dChaperon efficiencies are 0.5 for Ar, 0.3 for O₂, 12.0 for H₂O, 0.75 for CO, 2.0 for CO₂, 3.0 for C₂H₆, and 1.0 for all other species; falloff by the Troe formulation with F_c  = exp(−T/345 K) + exp (−345 K/T) (Gilbert et al., 1983; Troe, 2001).

^eChaperon efficiencies are 1.0 for all species.

^fChaperon efficiencies are 0.7 for Ar, 2.0 for H₂, 6.0 for H₂O, 1.5 for CO, 2.0 for CO₂, 2.0 for CH₄, 3.0 for C₂H₆, and 1.0 for all other species; falloff by the Troe formulation with F_c  = 0.265 exp (−T/94 K + 0.735 exp(−T/1756 K)  + exp (−5182 K/T) (Petersen and Hanson, 1999).

¹See http://www.mae.ucsd.edu/combustion/cermech.