# Unsteady Combustion of the Heptane-in-Water Emulsion Foamed with Hydrogen-Oxygen Mixture
## 3 Results and Discussion
### Mutual Effect of Hydrogen and Heptane in the Process of Foamed Emulsion Combustion
In such conditions, the flame dynamics are determined by the combustion of the heptane-hydrogen blend from the very beginning. Here, one can distinguish two stages in the process of flame propagation through the heptane-based emulsion foamed with the lean hydrogen-oxygen mixture. At the first stage, the flame propagates at a relatively low speed (the visible flame speed is not higher than \(\sim\)100 m/s, Figure 3a). As one can see from Figure 2, the combustion temperature at this early stage is rather low, less than 2000 K. Herewith, while the flame front is not so fast, the heptane has time to be evaporated from the surface of the emulsion droplets, and there is a substantial accumulation of heptane vapor in the pre-flame zone (see blue line on Figure 5b). The hydrogen-heptane blend occurs to be less reactive than the pure hydrogen-oxygen mixture (compare solid and dashed profiles in Figure 5), and the flame front in the case of hydrogen-heptane blend occurs to be widened compared with the case of hydrogen foam. Nevertheless, the flame accelerates via hydrodynamic mechanism discussed in Section 3.1. In the process of flame acceleration the temperature of the flame front increases, as is seen from the data presented in Figure 2a. Herewith, there is a certain steepening of the temperature profile (compare solid and dashed lines in Figure 6a) that indicates that the flame thickness decreases, and the hydrogen oxidation kinetics achieves an increasingly dominant role. At a sufficiently high speed of flame propagation, the heptane has less time to be evaporated from the surface of the droplets in the pre-flame zone, and there is no significant accumulation of heptane vapor in the pre-flame zone (Figure 6b). At the same time, one can see (Figures 2b and 6a) that the concentration of \(\mathrm{CO_{2}}\) is extremely low behind the flame front. This indicates that there is no heptane combustion inside the flame front, and heptane burns down much later in the combustion products area. The quenching of the flame in the case of the lean hydrogen-oxygen mixture takes place only at relatively high heptane content (Figure 4). Herewith, the quenching takes place at the early stage of slow flame propagation. In that case, a significant excess of heptane is observed in the pre-flame zone. Excessive heptane starts to play the role of inhibitor being involved in the reactions of chain termination mentioned above. Summarizing the data presented above, one can distinguish three basic modes of the mutual effect of hydrogen and heptane in the process of foamed emulsion combustion. First of all, there is a mode where the heptane inhibits the hydrogen combustion, and this effect is pronounced in the case of a stoichiometric hydrogen-oxygen mixture. High-temperature combustion determines an intense heat flux from the flame front. That causes heptane evaporation, so heptane takes part in the reaction process on the scales of the flame front. Herewith, the combustion of the stoichiometric hydrogen-oxygen mixture can be quenched even at relatively low concentrations of heptane in the emulsion. The second mode is observed in the emulsions foamed with the lean hydrogen-oxygen mixtures. At an early stage of flame propagation, even at relatively low combustion temperature, the heptane has time to evaporate and accumulate in the pre-flame zone. Here, heptane also takes part in the oxidation reaction together with the hydrogen. In that case, the combustion of the hydrogen-heptane blend is observed. Each component can act as an inhibitor for the other (in different temperature ranges [49]), but both are engaged in the oxidation process. The third mode is realized at higher speeds of flame propagation. In that case, the heptane has no time to evaporate in the pre-flame zone. Thus, almost pure hydrogen combustion proceeds on the scales of the flame front, defining intense burning, flame acceleration, and even transition to detonation [19], as in the case of pure hydrogen-based microfoam [20].