Macrokinetics of decomposition of loose packed RDX in shock and detonation waves

The boundary retardation method was used to study the decomposition of loose packed density RDX behind the front of shock waves with amplitudes ranges from values critical for reaction initiation to those typical of detonation. The threshold pressure P* and temperature T* at which the transition from a relatively slow decomposition in hotspots to fast bulk conversion occurs. It was demonstrated that this transition is associated with a change in the macrokinetics of the conversion of HE material behind the shock front. At pressures below P*, an analogy with the kinetic regularities of the thermal decomposition of RDX at normal pressure is observed. At P > P*, the conversion occurs in the regime of an adiabatic thermal explosion at a very high rate and has no analogues in chemical kinetics. The normal detonation regime is realized at P > P*. It was shown that the macrokinetic characteristics of normal detonation cannot be described within the framework of traditional approaches, with the help of the Arrhenius law or the concept of deflagration propagating from hotspots.     

High-pressure ethylene oxidation behind reflected shock waves

Oxidation of ethylene/air mixtures has been investigated behind reflected shock waves in a shock tube of 76 mm in diameter. Experiments were performed within the temperature range of 1060–1520 K, pressures of 5.9–16.5 atm, and stoichiometries of  = 0.5, 1.0, and 2.0. Emissions of OH (308.9 nm), CH (431.5 nm) and C₂ (516.5 nm) molecules, pressures and ion current records were implemented to measure ignition times of the mixture along the centreline of the tube and in the boundary layer. Empirical correlations for ethylene ignition times have been deduced from the experimental data. Auto-ignition modes (strong, transient and weak) and ignition limits of the mixtures were identified comparing velocities of reflected shock wave and reaction front at different locations from the reflecting wall. Extensive database for validations of high-temperature ethylene reaction mechanism and numerical methods for reaction flow simulations has been obtained from experimental observations.     

Front shock behavior of stable curved detonation waves in rectangular-cross-section curved channels

The propagation of curved detonation waves of gaseous explosives stabilized in rectangular-cross-section curved channels is investigated. Three types of stoichiometric test gases, C₂H₄ + 3O₂, 2H₂ + O₂, and 2C₂H₂ + 5O₂ + 7Ar, are evaluated. The ratio of the inner radius of the curved channel (r_i) to the normal detonation cell width (λ) is an important factor in stabilizing curved detonation waves. The lower boundary of stabilization is around r_i/λ = 23, regardless of the test gas. The stabilized curved detonation waves eventually attain a specific curved shape as they propagate through the curved channels. The specific curved shapes of stabilized curved detonation waves are approximately formulated, and the normal detonation velocity (D_n)?curvature (κ) relations are evaluated. The D_n nondimensionalized by the Chapman–Jouguet (CJ) detonation velocity (D_CJ) is a function of the κ nondimensionalized by λ. The D_n/D_CJ?λκ relation does not depend on the type of test gas. The propagation behavior of the stabilized curved detonation waves is controlled by the D_n/D_CJ?λκ relation. Due to this propagation characteristic, the fully-developed, stabilized curved detonation waves propagate through the curved channels while maintaining a specific curved shape with a constant angular velocity. Self-similarity is seen in the front shock shapes of the stabilized curved detonation waves with the same r_i/λ, regardless of the curved channel and test gas.     

Propagation of shock and detonation waves in channels with U-shaped bends of limiting curvature

Systematic experimental and theoretical studies of the propagation of shock and detonation waves in cylindrical tubes and planar channels with two U-shaped bends of limiting curvature were performed. It was demonstrated that U-shaped bends substantially facilitate detonation initiation in gases. The minimum shock wave velocity required to initiate the detonation of a stoichiometric propane-air mixture under normal conditions in a near-critical diameter tube with two U-shaped bends of limiting curvature was found to be ～800 m/s.     

Multi-species time-history measurements during n-hexadecane oxidation behind reflected shock waves

Species concentration time-histories were measured during oxidation for the large, normal-alkane, diesel-surrogate component n-hexadecane. Measurements were performed behind reflected shock waves in an aerosol shock tube, which allowed for high fuel loading without pre-test heating and possible decomposition and oxidation. Experiments were conducted using near-stoichiometric mixtures of n-hexadecane and 4% oxygen in argon at temperatures of 1165–1352 K and pressures near 2 atm. Concentration time-histories were recorded for five species: C₂H₄, CH₄, OH, CO₂, and H₂O. Methane was monitored using DFG laser absorption near 3.4 μm; OH was monitored using UV laser absorption at 306.5 nm; C₂H₄ was monitored using a CO₂ gas laser at 10.5 μm; and CO₂ and H₂O were monitored using tunable DFB diode laser absorption at 2.7 and 2.5 μm, respectively. These time-histories provide critically needed kinetic targets to test and refine large reaction mechanisms. Comparisons were made with the predictions of two diesel-surrogate reaction mechanisms (Westbrook et al. [1]; Ranzi et al. [9]) that include n-hexadecane, and areas of needed improvement in the mechanisms were identified. Comparisons of the intermediate product yields of ethylene for n-hexadecane with those found for other smaller n-alkanes, show that an n-hexadecane mechanism derived from a simple hierarchical extrapolation from a smaller n-alkane mechanism does not properly simulate the experimental measurements.     

Role of shock waves in initiating the detonation of pentaerythritol tetranitrate by a pulsed electron beam

The results of measurements of the velocity of shock waves generated in pressed pentaerythritol tetranitrate samples by a pulsed electron beam (0.25 MeV, 15 J/cm², and 20 ns) and of the velocity of expansion of the explosion products into vacuum are presented. It was demonstrated that, during the interaction of the electron beam with pentaerythritol tetranitrate, it experiences decomposition accompanied by a pressure rise high enough to produce a shock-wave initiation of the sample.     

Formation of nitrogen oxides in detonation waves

Based on detailed kinetic calculations and experimental data, it is demonstrated that the emission of nitrogen oxides from detonation burner units (DBUs) is significantly lower than that from powerful conventional burners with similar characteristics. Under certain conditions, realized largely in DBUs with rotating detonation, the main component of the nitrogen oxides may turn out to be N₂O.     

Thermal oxidation of detonation nanodiamond

In this work the results of investigation of detonation ultradispersed diamond (UDD) powder by means of thermogravimetric analysis (TGA) and Raman scattering (RS) are presented. Using the TGA method the temperature regions corresponding to the oxidation of different carbon fractions included in the composition of UDD powder were determined. In particular, it was established that heat treatment in the air at a temperature not exceeding 550°C leads to the oxidation and removal of nondiamond carbon, while the diamond part of the UDD remains unchanged. The form of the diamond RS band in the spectra of the UDD powder oxidized at 550°C shows good agreement with the model of phonon confinement. Based on the comparison of the results of experimental data and theoretical calculations for the RS band forms the size of the UDD crystal grains was defined as 4–5 nm.     

Numerical resolution of pulsating detonation waves

The canonical problem of the one-dimensional, pulsating, overdriven detonation wave has been studied for over 30 years, not only for its phenomenological relation to the evolution of multidimensional detonation instabilities, but also to provide a robust, reactive, high-speed flowfield with which to test numerical schemes. The present study examines this flowfield using high-order, essentially non-oscillatory schemes, systematically varying the level of resolution of the reaction zone, the size and retention of information in the computational domain, the initial conditions, and the order of the scheme. It is found that there can be profound differences in peak pressures as well as in the period of oscillation, not only for cases in which the reaction front is under-resolved, but for cases in which the computation is corrupted due to a too-small computational domain. Methods for estimating the required size of the computational domain to reduce costs while avoiding erroneous solutions are proposed and tested.     

Unstable combustion induced by oblique shock waves at the non-attaching condition of the oblique detonation wave

The instability of oblique shock wave (OSW) induced combustion is examined for a wedge with a flow turning angle greater than the maximum attach angle of the oblique detonation wave (ODW), where archival results rarely exist for this case in previous literatures. Numerical simulations were carried out for wedges of different length scales to account for the ratio of the chemical and fluid dynamic time scales. The results reveal three different regimes of combustion. (1) No ignition or decoupled combustion was observed if a fluid dynamic time is shorter than a chemical time behind an OSW. (2) Oscillatory combustion was observed behind an OSW if a fluid dynamic time is longer than a chemical time behind an OSW and the fluid dynamic time is shorter than the chemical time behind a normal shock wave (NSW) at the same Mach number. (3) Detached bow shock-induced combustion (or detached overdriven detonation wave) was observed if a fluid dynamic time is longer than a chemical time behind a NSW. Since no ignition or decoupled combustion occurs as a very slow reaction and the detached wave occurs as an infinitely fast reaction, the finite rate chemistry is considered to be the key for the oscillating combustion induced by an OSW over a wedge of a finite length with a flow turning angle greater than the maximum attach angle for an ODW. Since this case has not been previously reported, grid independency was tested intensively to account for the interaction between the shock and reaction waves and to determine the critical time scale where the oscillating combustion can be observed.     

Effect of thermal pressure in converging detonation waves

Propagation of converging detonation waves in various explosives is studied using the equation of state, which considers both the thermal and elastic pressures. It is seen that the rate of increase of thermal pressure is higher than that of the elastic pressure during convergence. The present equation of state is better since it also gives the variation of temperature, whereas the polytropic form of the equation of state is independent of temperature. It is seen that the total detonation pressure is slightly greater than the elastic pressure. Results are compared with those reported elsewhere.     

Oblique detonation waves stabilized in rectangular-cross-section bent tubes

Oblique detonation waves, which are generated by a fundamental detonation phenomenon occurring in bent tubes, may be applied to fuel combustion in high-efficiency engines such as a pulse detonation engine (PDE) and a rotating detonation engine (RDE). The present study has experimentally demonstrated that steady-state oblique detonation waves propagated stably through rectangular-cross-section bent tubes by visualizing these waves using a high-speed camera and the shadowgraph method. The oblique detonation waves were stabilized under the conditions of high initial pressure and a large curvature radius of the inside wall of the rectangular-cross-section bent tube. The geometrical shapes of the stabilized oblique detonation waves were calculated, and the results of the calculation were in good agreement with those of our experiment. Moreover, it was experimentally shown that the critical condition under which steady-state oblique detonation waves can stably propagate through the rectangular-cross-section bent tubes was the curvature radius of the inside wall of the rectangular-cross-section bent tube equivalent to 14–40 times the cell width.     

Formation of detonation waves in channels and interaction of the waves with penetrable screens

Two-dimensional channel flows with shock waves resulting from the detonation of a combustible gas mixture are considered. Conditions for detonation and the parameters of the shock waves are determined. The feasibility of reducing the shock wave intensity and loads on the structure by mounting a set of mesh screens in the channel is investigated. The numerical computation of detonation initiation in an air-hydrogen mixture and subsequent passage of shock waves through the mesh screens is carried out. Basic quantitative characteristics of shock wave reduction depending on the mesh screen penetrability and mutual arrangement of variously penetrable screens are obtained.     

Generation of magnetic field by detonation waves

The generation of a magnetic field by a system of detonation waves in a condensed explosive is reported. The convergence of the detonation waves, which exhibit a high conductivity in the chemical reaction zone, increases the magnetic field at the axis of the system. The fact of magnetic field generation is demonstrated experimentally. Features of the new method of magnetic cumulation are discussed. A simple compression model that qualitatively agrees with experimental data is proposed.     

Gravitational shock waves

This paper reports on and summarizes some recent progress on gravitational shocks, i.e., discontinuities in the Riemann curvature tensor. It is shown how the constraint equations play a crucial rôle in determining the nature and propagation of the shocks. Existence results are stated and are illustrated by some examples from numerical relativity.     

Brane-induced-gravity shock waves

We construct exact gravitational field solutions for a relativistic particle localized on a tensional brane in brane-induced gravity. They are a generalization of gravitational shock waves in 4D de Sitter space. We provide the metrics for both the normal branch and the self-inflating branch Dvali-Gabadadze-Porrati brane worlds, and compare them to the 4D Einstein gravity solution and to the case when gravity resides only in the 5D bulk, without any brane-localized curvature terms. At short distances the wave profile looks the same as in four dimensions. The corrections appear only far from the source, where they differ from the long distance corrections in 4D de Sitter space. We also discover a new nonperturbative channel for energy emission into the bulk from the self-inflating [corrected] branch, when gravity is modified at the de Sitter radius.