Early flame propagation of hydrogen enriched methane-air mixtures at quasi laminar conditions in a rapid compression expansion machine

The temporal evolution of the kinematic properties of hydrogen enriched, lean premixed methane-air flames was studied experimentally in a rapid compression expansion ma- chine (RCEM) during transient operation. Schlieren imaging was used to capture the flame front propagation, while the temporally evolving flow field was simultaneously acquired by particle image velocimetry (PIV). A statistical analysis based on probability density functions (PDFs) of non-dimensional formulations of the flame curvature, stretch rate and displacement speed was performed to study the effect of the onset of flame instabilities on the aerodynamic characteristics of the propagating flame kernel. It was found that initial perturbations stemming from the presence of the spark plug electrodes and from the gas expansion led to the formation of large scale cusps related to the hydrodynamic instability, which is manifested by a notable negative skewness of the curvature PDF, validating recent numerical findings. Under the influence of thermal-diffusive effects, small scale cellular structures develop along the flame front beyond a certain radius, with their appearance shifting to earlier times with increasing hydrogen content or for leaner mixtures. The increasing width of the normalized curvature and stretch rate PDFs is proposed as an indicator for the onset of this type of instability. The adopted methodology allows to track the self-acceleration of the propagating flame front effecting from hydrogen enrichment of methane. The findings of this study provide valuable physical insight and aid in the development of more accurate models, capable of capturing these complex phenomena. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.