Effects of hydrogen addition on laminar and turbulent premixed methane and iso-octane–air flames

Abstract

The effect of adding hydrogen to methane and iso-octane–air premixtures has been investigated under both laminar and turbulent conditions. Five percent by mass of hydrogen was added the to base fuel. Measurements were performed at 5 bar and the equivalence ratio was varied from the lean ignition limit to either the rich ignition or sooting/buoyancy limit. The laminar burning velocity, u l, increased for fuel lean and stoichiometric methane/hydrogen mixtures compared with pure methane. A doubling in u l was observed at the lean limit. There was no increase in u l for ϕ ⩾ 1.2. Laminar hydrogen/iso-octane mixtures burned faster than those of pure iso-octane–air over the whole range of equivalence ratios investigated. For both methane and iso-octane, addition of hydrogen resulted in earlier onset of laminar flame instabilities and a reduction in the Markstein length (where measurement was possible). In the case of turbulent combustion, hydrogen addition (for both fuels), resulted in an approximate doubling of turbulent burning velocity at the lean limit. As the equivalence ratio was increased, the effect of adding hydrogen reduced until, at the rich ignition (methane) or sooting/buoyant (iso-octane) limit, no change in turbulent burning velocity was measured. The increased laminar burning velocity was primarily responsible for the increased turbulent burn rate with hydrogen addition. For lean iso-octane–air mixtures hydrogen addition resulted in further enhancement of the turbulent burning velocity over and above that expected on the basis of the laminar burning velocity. This was associated with a reduction of the flames sensitivity to strain rate, a result of the molecular mobility of H 2.