Differential diffusion effects in counter-flow premixed hydrogen-enriched methane and propane flames

Abstract

The effects of differential diffusion and stretch sensitivity on propagation and stabilization of lean premixed hydrogen-enriched methane-air and propane-air flames are studied in a turbulent counter-flow apparatus. In these experiments, the unstretched laminar flame speed is kept constant through decreasing the mixture equivalence ratio, in order to minimize the effects of chemistry and highlight the effects of differential diffusion during hydrogen-enrichment. Bulk flow properties are also kept constant between laminar and turbulent flames. High-speed particle image velocimetry (PIV) is applied to quantify the flow velocity field using oil droplet seeding, enabling simultaneous flame position and velocity measurements. Data processing tools are developed through this study to quantify instantaneous local measurements of flame position, flame curvature, and apparent turbulent flame velocity within the imaged plane. Probability density functions (PDF) of instantaneous flame position show that, in hydrogen-enriched methane-air flames (effective Lewis number < 1), differential diffusion increases the turbulent burning rates throughout the whole hydrogen-enrichment range. However, in hydrogen-enriched propane-air flames, these effects are only observed at hydrogen content above 60% (by volume), where effective Lewis number falls below unity. PDFs of flame position also illustrated that the effects of differential diffusion become significant when the effective Lewis number < 0.8. In contrast, PDFs of turbulent flame velocities only showed a slight increase in local instantaneous velocities with increasing hydrogen content. Furthermore, it was illustrated that differential diffusion affects the flame front topology by increasing instantaneous flamelet curvature at below unity Lewis numbers, consistent with flame stability theory.