Asymptotic Analysis of Ignition in Nonpremixed Counterflowing Hydrogen versus Heated Air

ABSTRACT

Following the recent computational result that the ignition of counterflowing hydrogen versus heated air possesses three distinct ignition limits similar to those of the homogeneous explosion limits of hydrogen/air mixtures, a large activation energy asymptotic analysis with reduced mechanisms has been conducted to provide an analytical description of this phenomenon. Starting with an appropriate eight-step mechanism for hydrogen oxidation, for each limit the relevant reduced mechanism is derived and the asymptotic structure analyzed. Results show that while the first and third limits are strongly affected by transport, the second limit is dominated by chemistry. In fact, the controlling chemistry for the second ignition limit is identical to that of the second explosion limit of the homogeneous mixture. The predicted ignition limits agree reasonably well with the numerical results obtained with full mechanism.

Key Words:

• Hydrogen combustion
• nonpremixed ignition
• explosion limits