Lewis number effects on lean premixed combustion characteristics of multi-component fuel blends

Variation in natural gas composition, alongside the potential for H 2 enrichment, creates the potential for significant changes to premixed flame behaviour. To strengthen fundamental understanding of lean multi-component alternative fuel blends, an outwardly propagating spherical flame was employed to measure the flame speeds and Markstein lengths of C 1 -C 4 hydrocarbons, alongside precisely mixed blends of CH 4 /C 2 H 6 , CH 4 /C 3 H 8 and CH 4 /H 2 . Theoretical relationships between Markstein length and Lewis Number are explored alongside effective Lewis number formulations. Under lean conditions, equal volumetric additions of H 2 and C 3 H 8 (30% vol.) to CH 4 resulted in similar augmentation of burning velocity, however, opposite susceptibility to preferential diffusional instability was noted. At a fixed equivalence ratio of 0.65, limited changes in composition provide a marked change in the premixed flame response with the addition of C 2 H 6 and C 3 H 8 to CH 4 . For lean CH 4 /H 2 mixtures, a diffusional based Lewis Number formulation yielded a favourable correlation, whilst a heat-release model resulted in better agreement for lean CH 4 /C 3 H 8 blends. Modelling work suggests that measured enhancement of lean CH 4 flames upon H 2 or C 3 H 8 is strongly correlated to changes in volumetric heat release rates and production of H radicals. Furthermore, a systematic analysis of the flame speed enhancement effects (thermal, kinetic, diffusive) of H 2 and C 3 H 8 addition to methane was undertaken. Augmented flame propagation of CH 4 /H 2 and CH 4 /C 3 H 8 was demonstrated to be principally an Arrhenius effect, predominantly through reduction of associated activation energy. Finally, plausible short-term variations in composition with hydrogen-enriched multicomponent natural gas flames were investigated experimentally and numerically. At the leanest conditions, small variations in CH 4 :C 3 H 8 content at a fixed H 2 fraction resulted in discernible changes in stretch related behaviour, a reflection of the thermo-diffusive behaviour of each fuel's response. (c) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.