Effect of elevated temperatures (550–860 K) on the laminar flame speeds of methane/hydrogen blends

•First measurements of flame speeds of CH4/H2 blends at temperatures above 650 K.•Three equivalence ratios, with hydrogen fuel fractions varying from 0.1 to 0.4 were studied.•Empirical correlations were constructed using data from this work and the literature.•Differences in flame speed predictions using various kinetic models were observed.•Flame speed sensitivity analyses allowed the identification of important reactions. Experimental determination of laminar flame speeds was performed for methane/hydrogen mixtures in a 79% Ar/21% O2 oxidizer (referred to as “airgon”) at 1 atm. The shock-tube flame speed method enabled the first measurements at temperatures above 650 K and was employed to study binary fuel blends with various hydrogen fuel fractions (xH2 = 0.1–0.4) at equivalence ratios of 0.8, 1.0, and 1.2 in the unburned-gas temperature range of 550–860 K. Results were compared against simulations conducted using the GRI-3.0, FFCM-2 and Stagni et al. models. Good agreement between all three models and measurements was observed at lean and stoichiometric conditions, while the three models and data diverge in the rich case. Equivalent fuel/air flame speeds were computed from fuel/airgon measurements by applying a mixture-dependent scaling ratio. The scaled air equivalent data were compared against literature data and showed good agreement. Through sensitivity analyses, it was found that reactions related to neat methane combustion are highly relevant at all conditions, while H2-specific reactions only become important with high hydrogen content (xH2 ≥ 0.4). Additionally, differences in sensitivity coefficients were identified across the models, providing useful information for potential refinement of reaction rates, and consequently, the development of models which better represent new experimental data.