Investigation of cellular characteristics of hydrogen-ethanol flame at elevated temperatures and pressures

Abstract

The blending of hydrogen into ethanol for use is attracting increasing interest and it is necessary to study its laminar combustion characteristics for better application in combustion units. The instability of hydrogen-ethanol spherically expanding flame has been investigated at initial temperatures of (370 K, 400 K, and 450 K), initial pressures of (2–4 bar), and equivalence ratios of (0.7–1.4) using the constant volume combustion chamber (CVCC). High-speed schlieren technology was used to record flame propagation images. The effects of hydrodynamic and thermal-diffusion effect on the inherent instability of the flame were investigated. The thermal-diffusion effect was found to stabilize the flame surface under all conditions, as judged jointly by the effective Lewis number and the critical Lewis number. The main contribution to flame instability therefore came from hydrodynamic effects, and this was conclusively verified by looking at the trends in the global growth rates of the various instability parameters. The effect of changes in initial pressure and initial temperature on the critical conditions for flame instability (critical radius and Peclet number) was analyzed using linear stability theory. It was found that flames were more prone to cell instability at higher pressures and temperatures. In addition, as the equivalence ratio increased, the monotonic variation of the critical Peclet number and the non-monotonic variation of the flame thickness caused a trend of decreasing and then increasing critical radius. It was noteworthy that the most unstable state for all conditions studied was at about Ф=1.2. The comparison between theoretical calculations and experimental results was generally consistent, with only some differences at Ф = 1.4. Moreover, empirical correlation formulas for the critical Peclet number (Pec) and Markstein number (Ma) were proposed.