Investigation of cellularization characteristics of hydrogen-methane-ethanol expanding spherical flame at elevated pressures

Abstract

The incorporation of ethanol into hydrocarbon fuels for use is attracting increasing interest and it is necessary to investigate its inherent flame instability for better application in combustion units. The instability of hydrogen-methane-ethanol spherically expanding flame has been investigated at an initial temperature of 400 K, initial pressures of (2–4 bar), ethanol fraction of (20%, 50%, 80%), and equivalence ratios (Ф) 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. As the ethanol ratio increased, the hydrodynamic effect was enhanced. The thermal-diffusion effect was discovered to stabilize the flame surface under all conditions, as judged jointly by the effective Lewis number and the critical Lewis number. The critical conditions (critical radius and Peclet number) at the onset of unstability were evaluated, and it was found that the flames were more prone to flame instability at higher pressures. The critical Peclet number increased with the increase in the equivalence ratio when the ethanol ratio was 20%, and showed the opposite trend when the ethanol ratio was 50% and 80%. In addition, as the ethanol ratio increases, the stability of the lean mixtures flame increases, while the rich mixtures flame suffers from early onset of instability. The theoretical and experimental results were consistent, with some differences at Ф = 1.4. An empirical correlation formula for the critical Peclet number ($P{e}_c$) and Markstein number (M_b) was further proposed ($P{e}_c$= 18.03M_b+214.78). Finally, the Karlovitz number was used to study the instability behavior of the flame. The critical Karlovitz number (Ka_c) decreased with increasing M_b and the tendency of the flame to suffer from instability diminished, and the following correlation was obtained $K{a}_c=0.05635\times e^{-0.13852M_b}$. Furthermore, the flame was more unstable in rich mixtures, this was consistent with the conclusion of instability derived from the critical radius.