Dynamic Structure of Gaseous Detonation

Abstract

In contrast to the laminar one-dimensional ZDN model, the three-dimensional cellular structure of homogeneous gaseous detonation has been firmly established. The structure consists of an ensemble of interacting transverse shock waves sustained by the energy release from chemical reactions. The frequencies of the transverse fluctuations spread over a wide spectrum, however a dominant wavelength (cell size) can usually be identified from the triple point trajectories inscribed on a smoked foil. Regularity of the smoked foil pattern reflects the frequency (or wavelength) spectrum of the transverse wave oscillation. The recent results have demonstrated that the correlations between the dynamic detonation parameters and the dominant cell size alone are inadequate. The cell regularity, as characterized by a stability parameter, must necessarily also play an important role. Recent experiments, aswell as numerical simulations, have confirmed the essential role of transverse waves on the propagation mechanism. Damping of the transverse waves of an established C-J detonation by acoustic absorbing walls leads to decoupling of the reaction zone from the leading shock. Absorbing walls also suppress flame acceleration and transition to detonation even in the presence of obstacles. The role played by transverse shocks is credited to vorticity generation via initial interactions (Mach reflections and shear layers) and to the baroclinic mechanism of pressure and density gradient field interactions. The continuous spectrum of burning rates suggests no clear distinction can be made regarding the deflagration and the detonation mode of combustion. Shock waves due to pressure fluctuations are an integral part of the compressible turbulence in addition to velocity fluctuations from eddy motion. In terms of mechanism, there appears to be no sharp distinction between turbulent deflagration and detonation. However, the detonation is a unique self-sustained spatio temporal structure that is independent of boundary conditions. It is on this basis that one can define a detonation wave. Interpreting the detonation as an ordered structure in a highly non-equilibrium medium, it may be regarded as localized states of non-linear fields. The recent development in non-linear field theory may offer an interesting approach to further understanding of the fundamental physics involved. It appears that current experimental diagnostic techniques and numerical computation capabilities can in general give more detailed information on the detonation structure than can be utilized and interpreted adequately. It is suggested that future directions should aim at the choice of a novel global length scale (e.g. hydrodynamic thickness) other than the cell size to characterize the wave thickness. Together with an appropriate stability parameter that measures the spectrum of transverse fluctuations (or cell regularity), it is proposed that the data for the dynamic parameter be re-examined to achieve a more appropriate correlation.