MILD OXIDATION REGIMES AND MULTIPLE CRITICALITY IN NONPREMIXED HYDROGEN-AIR COUNTERFLOW

This study investigates experimentally and computationally the existen ce of mild oxidation regimes and multiple ignition and extinction stat es in a system of nonpremixed, counterflowing hydrogen against heated air. Spontaneous Raman spectroscopy measurements of the water concentr ation show that up to three stable stationary states can be achieved f or identical boundary conditions. Computationally, up to five steady-s tate solutions can be found, although only three are likely to be stab le. This multiplicity is the result of combined thermokinetic and tran sport effects on the behavior of critical ignition and extinction stat es. To understand these effects, the system response was simulated usi ng detailed kinetics and transport properties, and S-curve sensitivity was employed to identify the dominant chemistry near the critical sta tes and to simplify the kinetic mechanism. The response to changes in the fuel concentration and system pressure was investigated experiment ally by measuring the air temperatures corresponding to ignition and e xtinction, for fuel concentrations in the range of 6-38% H-2 in N-2 by volume, and pressures between 0.3 and 8 atm, at a constant pressure-w eighted strain rate of 300 s(-1). The experimental results were found to agree well with the computational results. The experimental triple- solution multiplicity disappears for fuel concentrations in excess of similar to 25% or below similar to 7% H-2 in N-2, and was only found i n the pressure range between similar to 1.5 and 7 atm, at 9% H-2 in N- 2 and a pressure-weighted strain rate of 300 s(-1). In addition, the r esponse to changes in the strain rate was studied computationally, for strain rates between 10 and 40,000 s(-1) and for air boundary tempera tures ranging between 950 and 1100 K. The same features of up to five steady-state multiplicities and up to two ignition and extinction stat es can be obtained by changing the flow strain rate. In the strain rat e space, the computational quintuple-solution multiplicity extends fro m similar to 100-10,000 s(-1), at 9% H-2 in N-2 and 4 atm. (C) 1998 by The Combustion Institute.