Experimental characterization of the cell cycle for multicellular detonations

The detonation front’s unstable structure leads to an unsteady and three-dimensional (3D) phenomenon that renders the study of the cell cycle challenging. Traditionally, fundamental studies are carried out in narrow channels where the detonation behavior is very peculiar (quasi two-dimensional with velocity deficit). In this study, we propose a fully experimental approach to study the cell cycle in the case of multicellular detonations. The cell cycle is characterized through three techniques: systematic and statistical analysis of soot foil, planar laser-induced fluorescence on nitric oxide, and Rayleigh scattering. These techniques provide measurements for cell size, local induction length, and local shock speed, respectively. The work is carried out in the 2$H_2$-O${}_2$-3.76Ar and the 2$H_2$-O${}_2$-3.76N${}_2$ mixtures at 293 K, and 20 kPa and 25 kPa, respectively. These conditions ensure that the cell pattern is considered being between regular and weakly irregular, thus, a shot-to-shot reconstruction of the cell cycle is possible. The cell widths follow a normal distribution, from which a quantitative parameter (2$\sigma$/$\lambda$) is proposed to assess the cell regularity, experimentally. The evolution of the speed and the local induction length are reconstructed along the cell cycle. The results agree with the available data for narrow channels and constitute the first of their kind for 3D detonation (i.e., multicellular in the transverse dimension). Two methods are proposed to analyze the local induction length ${\delta }_i$ and compare it to the available literature (experimental and numerical studies). The technique can be applied to mixtures where the mean cell width is a meaningful parameter from highly regular to irregular mixtures.

Novelty and Significance statement

For the first time, combined soot-foils, NO-PLIF, and Rayleigh scattering measurements were used to reconstruct and characterize the cellular cycle of multicellular detonations using a 2$H_2$-O${}_2$-3.76Ar and 2$H_2$-O${}_2$-3.76N${}_2$ mixtures at 293 K, and 20 kPa and 25 kPa, respectively. This constitutes the first combined measurements of local induction length and local front speed in a multicellular configuration, where the number of cells in the section is large, i.e., the cell width ($\lambda$) is small compared to all dimensions of the rig. This study provides two methodologies to extract an experimental ${\Delta }_i$, which is a quantity that can be compared with the ZND theory. Such measurements were not achievable in multicellular conditions until now. The measurement of the ${\Delta }_i$ demonstrates that a fully experimental correlation between this experimental ${\Delta }_i$ and $\lambda$ can be obtained for the first time. These new results are important for the quantitative validation of chemical kinetic schemes for predictive simulations of detonations.