Experimental study on the intrinsic instabilities of spherically expanding CH4/H2/CO2/O2 flames

•It is easier for TD instability at smaller flame thickness to split flame cracks.•Hydrogen addition alters cellularity more remarkably at lower oxygen fractions.•Oxygen enrichment hardly changes cellular structures at higher hydrogen fractions.•Pec exhibits an opposite dependence on oxygen enrichment than on hydrogen addition.•A formula between the critical flame radius and mixture composition is proposed. To study the instabilities of spherically expanding CH4/H2/CO2/O2 flames with an equivalence ratio of 0.80, isochoric combustion experiments were carried out in a 36 L constant volume bomb at an initial temperature (298±2 K) and atmospheric pressure (100 kPa). The crack length Lcrack, average cell area Scell, critical flame radius Rc and critical Peclet number Pec of CH4/H2/CO2/O2 flames were evaluated. The results show that the evolution of Lcrack at low hydrogen fractions is slightly affected by oxygen enrichment but significantly affected at high hydrogen fractions. Thermodiffusive instability dominates the evolution of Scell at low hydrogen fractions, but hydrodynamic instability dominates at high hydrogen fractions. At lower oxygen fractions, hydrogen addition results in more significant enhancement of the thermodiffusive and hydrodynamic instabilities. As the oxygen fraction increases, the maximum size of the thermodiffusive cells decreases, and the thermodiffusive instability weakens due to the increasing effective Lewis number, but the flame thickness decreases. This indicates that the weak thermodiffusive instability is amplified by the small flame thickness. With increasing oxygen fraction, the unstable region of the peninsula by the linear stability theory narrows in the Pe–n plane but broadens in the R–n plane. Therefore, the unstable peninsula in the R–n plane is consistent with the trend of crack growth observed in the experiments. Additionally, the experimental critical flame radius Rc,1 is closer to Rc calculated by the linear stability theory than Rc,2, where Rc,1 and Rc,2 represent the cross-cracking of the thermodiffusive cells and the onset of the hydrodynamic cells, respectively. Pecl exhibits a linear dependence on the Markstein number Ma. Moreover, a power law correlation of the experimental Rc with respect to oxygen enrichment and hydrogen addition is proposed.