To realize a quick initiation of detonation in insensitive fuel-air mixtures in the combustion chamber of a PDE operating in the air-breathing mode, the authors have proposed a new detonation initiator using a circular disk as a "reflecting board" near the exit of a predetonator tube. When a fuel-oxygen mixture fills the predetonator tube as a driver gas, the mixture change and the abrupt area change occur simultaneously at the exit. This paper describes the promoting effect of the reflecting board on the detonation transition through the mixture change. The combustible mixtures in the combustion chamber are stoichiometric hydrogen-oxygen mixtures diluted with nitrogen. Main results obtained in this study are in the followings: The detonation wave maintains the propagation velocity in the fuel-oxygen mixture right after the abrupt area change, this results in the increase of the distance at which the head of a rarefaction wave reaches the axis of the predetonator and the incident planar detonation wave disappears. Because the cell size right after the abrupt area change is near the size of the fuel-oxygen mixture, the cylindrical detonation wave survives the axial expansion even when the reflecting board separation is less than the propagation limit of the cylindrical detonation wave propagating in the diluted mixture.
The combustion and thrust characteristics of a ram accelerator in a case where the projectile speed was close to the Chapman-Jouguet (CJ) detonation speed were experimentally investigated. Methane-air mixtures with different equivalence ratios were used in the experiments. In the case of the fuel-lean condition, positive net thrust was not obtained although the combustion region was kept behind the projectile. That is to say, the combustion near the projectile was extremely weak. In the case of the equivalence ratio of unity, a flamelet attached to the projectile shoulder was observed. The flamelet generated the secondary shock wave. The secondary shock wave induced the ignition of the mixture, and the combustion occurred near the tube wall, thus enhancing the combustion of the nearby projectile. The results indicate that the appropriate configuration of the flame can drastically enhance the thrust.
A thermochemical model was newly proposed in the present study. The proposed model was developed to maintain an accuracy for detonation simulations with the detailed chemical reaction model under thermally perfect gas condition and to reduce computational load. The proposed model consists of three gas components, which are premixed detonable gas, burnt gas and inert gas. For chemical reactions, it is considered that premixed detonable gas reacts to burnt gas under the one-step irreversible chemical reactions. The thermodynamic data for premixed detonable gas and inert gas follow the enthalpy changes of the mixture as thermally perfect gas, and it is assumed that the burnt gas is isentropically expanded. In the present study, single-cycle and multiple-cycle operations of one-dimensional and two-dimensional single-tube Pulse Detonation Engine (PDE) were investigated with the proposed model. For one-dimensional simulations of single-tube, single-cycle PDE, advantages of the proposed model compared with some other models were shown using pressure and temperature profiles at the closed and open ends of the tube. Furthermore, two-dimensional simulations of single-tube, multiple-cycle PDE were carried out to investigate the spatial distributions and histories of the thermodynamic properties with the proposed and the detailed models. The spatial distributions and histories of the thermodynamic properties of the proposed model were in good agreement with those of the detailed model. The proposed model dramatically reduced the CPU time required for simulation to about 9 % for one-dimensional analysis and 14 % for two-dimensional analysis, compared with those of the detailed chemical reaction model.