Experimental study on the intrinsic instabilities of spherically expanding CH4/H-2/CO2/O-2 flames

To study the instabilities of spherically expanding CH4/H-2/CO2/O-2 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 L-crack, average cell area S-cell, critical flame radius R-c and critical Peclet number Pe(c) of CH4/H-2/CO2/O-2 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 S-cell 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 R-c,R-1 is closer to Rc calculated by the linear stability theory than R-c,R-2, where R-c,R-1 and R-c,(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.

Automated summary

Isochoric combustion experiments were conducted to study the instabilities of CH4/H-2/CO2/O-2 flames with an equivalence ratio of 0.80. The results revealed that oxygen enrichment had a slight effect on the crack length evolution at low hydrogen fractions but a significant effect at high hydrogen fractions. Thermodiffusive instability dominated the average cell area evolution at low hydrogen fractions, while hydrodynamic instability dominated at high hydrogen fractions. The addition of hydrogen resulted in a more significant enhancement of both thermodiffusive and hydrodynamic instabilities at lower oxygen fractions. The unstable region in the R-n plane was found to be consistent with the trend of crack growth observed in the experiments.