# 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  
Now we consider the mutual effect of hydrogen and heptane in the process of foamed emulsion combustion. The experimental analysis [19], as well as the calculations, show that the heptane addition into the foam bubbled with stoichiometric hydrogen-oxygen mixture leads to the flame deceleration and its further quenching at a certain critical concentration of heptane (Figure 4). In the case of the lean hydrogen-oxygen mixture used as a foaming gas, the combustion limit extends while the dependence of the average flame speed on the heptane concentration demonstrates the ambiguous effect of heptane on the combustion (Figure 4). At low heptane content, the combustion mode occurs to be intensified compared with a combustion mode in the pure hydrogen microfoam. The inhibition effect is also observed but at relatively high heptane content. According to the obtained results (Figure 4), there are two basic scenarios of flame evolution in a hydrogen/heptane blend. The first one concerns the case where the hydrogen content is high, and hydrogen-oxygen combustion is the primary process (2H\({}_{2}\) + O\({}_{2}\) mixture). The second concerns the case with an excess of oxygen (0.5H\({}_{2}\) + O\({}_{2}\) and 0.6H\({}_{2}\) + O\({}_{2}\) mixtures) when the heptane oxidation kinetics proceeds along with hydrogen oxidation. In the first case, hydrogen oxidation proceeds much faster than heptane oxidation. So, this case can be considered as hydrogen combustion in the presence of a small additive of heptane. The addition of heptane, in turn, causes the termination of the chain-branching process via the reduction of concentrations of active radicals H, O, and OH via reactions such as  
Figure 4: Averaged flame speed dependence on the heptane content in the initial emulsion at various hydrogen content in the foaming gas mixture. Experimental data are taken from [19]. Error bars are estimated on the base of three independent tests with the same foam. \[\mathrm{C_{7}H_{16}+H} \Leftrightarrow \mathrm{C_{7}H_{15}+H_{2}}\] \[\mathrm{C_{7}H_{16}+OH} \Leftrightarrow \mathrm{C_{7}H_{15}+H_{2}O}\]  
The formation of alkyl radicals, such as \(\mathrm{C_{7}H_{15}}\), via the above-mentioned reactions and reactions such as  
\[\mathrm{C_{7}H_{16}+O_{2}\Leftrightarrow C_{7}H_{15}+HO_{2}}\]  
also provides the termination of the chain-branching process [34; 45]. As a result, the heptane addition leads to the deceleration of the hydrogen oxidation process and the corresponding change in the overall flame dynamics. As a result, the flame is quenched, and the quenching takes place at a relatively low concentration of heptane in the emulsion (\(\sim\)3-5 vol.%). When speaking about the chemical pathways defining either intensification or termination of combustion kinetics, it is of paramount importance to understand in which reactions free radicals are involved since the birth and termination of such radicals define the particular pathway via which the reaction proceeds. Nowadays, most of the combustion features can be analyzed with the use of contemporary representations of the chemical kinetics of combustion. Thus, for example, the chemical addition of OH radicals into the combustion zone [46] is known to intensify the combustion since the chain-branching pathway is intensified. In the opposite case, excluding OH radicals from the reaction zone via chemical reactions with added active agents [47] defines the termination of the chain-branching process, and the combustion is inhibited. That behavior is similar to the intrinsic chain path termination in the combustion process [48]. Further, it is precisely this mechanism that is involved in hydrogen combustion inhibition via OH recombination when interacting with heptane molecules (\(\mathrm{C_{7}H_{16}}\)). At low hydrogen content and corresponding oxygen excess in the foaming gas, heptane oxidation proceeds along with hydrogen oxidation reactions.