Effect of Ignition Energy and Hydrogen Addition on Laminar Flame Speed, Ignition Delay Time, and Flame Rising Time of Lean Methane/Air Mixtures

Abstract

A series of experiments were performed to investigate the effect of ignition energy (Eig) and hydrogen addition on the laminar burning velocity (Su0), ignition delay time (tdelay), and flame rising time (trising) of lean methane−air mixtures. The mixtures at three different equivalence ratios (ϕ) of 0.6, 0.7, and 0.8 with varying hydrogen volume fractions from 0 to 50% were centrally ignited in a constant volume combustion chamber by a pair of pin-to-pin electrodes at a spark gap of 2.0 mm. In situ ignition energy (Eig ∼2.4 mJ ÷ 58 mJ) was calculated by integration of the product of current and voltage between positive and negative electrodes. The result revealed that the Su0 value increases non-linearly with increasing hydrogen fraction at three equivalence ratios of 0.6, 0.7, and 0.8, by which the increasing slope of Su0 changes from gradual to drastic when the hydrogen fraction is greater than 20%. tdelay and trising decrease quickly with increasing hydrogen fraction; however, trising drops faster than tdelay at ϕ = 0.6 and 0.7, and the reverse is true at ϕ = 0.8. Furthermore, tdelay transition is observed when Eig > Eig,critical, by which tdelay drastically drops in the pre-transition and gradually decreases in the post-transition. These results may be relevant to spark ignition engines operated under lean-burn conditions.