No abstract

The authors examine the effects of inhomogeneity in the equivalence ratio on detonation propagation by using a set of two-dimensional numerical simulations of the detailed reaction chemistry of an H2/air mixture. A random field of fluctuations but with statistical characteristics is introduced, and several combinations of the root mean square (RMS) and characteristic length scales of the fluctuations are considered to investigate the evolutions of the cellular structure, speed of detonation, and shock pressure under these setups. The results indicate that an increase in the RMS enlarged the cell formed by the original triple points as well as the characteristic length scale to promote the transition from a single cellular pattern to a double cellular pattern. The large cell of the double cellular pattern was formed by triple points generated from local explosion, and the decoupling or curvature of the detonation wave within an extremely lean region was important for this process. Moreover, sustainable detonation propagation under these configurations benefited from the strong transverse detonation generated by the local explosion as well as the propagation of these original triple points along the stoichiometric region, where their collisions reinitiated detonation in the extremely lean region. The instantaneous and average speeds of detonation were calculated. The former followed the trend of evolution of the normalized potential instantaneous energy release, whereas the latter decreased with an increase in . However, the value of had a non-monotonic influence that can be attributed to two factors.

The main objective of the present work is to investigate the end-gas autoignition and detonation development in a confined space with the presence of wall heat loss by two-dimensional numerical simulations with a hydrogen/air mixture. The effects of turbulence–heat loss interactions, initial temperature, equivalence ratio, and wall temperature on end-gas combustion modes are analyzed in detail. The results show that with the presence of wall heat loss, end-gas autoignition takes place in the hot core regions away from the walls, and the autoignition fronts touching the wall can lead to a much larger wall heat flux than that induced by main flame–wall interactions. In the base cases, increasing the turbulence intensity promotes the end-gas autoignition mode transition from thermal explosion-detonation to thermal explosion-deflagration and finally to no-autoignition, whereas detonation takes place in all cases regardless of the turbulence intensity after the initial temperature or equivalence ratio is raised. However, in these cases with a low equivalence ratio, the detonation propagation is unstable, which can be easily decoupled spontaneously after it encounters the cold flow. It is further found that for the cases with unstable detonation propagation, the burned mass fraction (BMF) dominates the knock intensity, whereas for the cases with stable detonation propagation, the maximum pressure in a chamber will extremely depend on the local and instantaneous interactions between the pressure/shock waves, but the effect of BMF becomes minor.

The deflagration to detonation transition (DDT) remains one of the most interesting and mysterious physical phenomena in the combustion of energetic materials, which contains substantial complicated and nonlinear characteristics. In the present work, the effect of the chemical reactivity of different fuels and diluent gases on the end-gas autoignition and detonation development in a confined space was investigated. Five fuels (hydrogen, methane, iso-octane, n-heptane, and PRF50) and three diluent gases (argon, nitrogen, and carbon dioxide) were used to change the chemical reactivity. The results showed that both the chemical reactivity and shock wave had a significant influence on the end-gas autoignition and detonation development. For mixtures with different diluent gases, it was observed that the transition thresholds (denoted by critical oxygen fraction) increased in the order of argon, nitrogen, and carbon dioxide. Different detonation modes with varying shock compressions were observed under different diluents for n-heptane. Although the flame propagation of different fuels differs at 21% oxygen fraction, end-gas autoignition and detonation development processes can still be observed in all kinds of fuels when the oxygen fraction was elevated to a certain value. The transition thresholds increased in the order of hydrogen, n-heptane, PRF50, iso-octane, and methane. Further analysis revealed that the fuel with a shorter ignition delay usually required a lower flame tip velocity, accomplished with a delayed occurrence of detonation. In addition, the transition threshold was determined by the chemical reactivity and flame speed.