Cylindrical Detonation Research Articles

Research on the initiation and wavefront evolution of diverging cylindrical detonations of acetylene and oxygen mixtures

The present study focuses on fundamental research about the transition from a planar detonation to a cylindrical detonation and the propagation of the diverging cylindrical detonation. We aim to figure out the mechanism of the transition and propagation of the diverging cylindrical detonation by analyzing the cellular pattern and critical initial pressure. The findings highlight that the successful detonation in a cylindrical chamber via transition through a straight channel is predominantly influenced by diffraction at the corners and the successive continuous reflections between the front and rear walls. Depending on the initial pressure, the initiation modes exhibit characteristics of subcritical, critical, and supercritical three-stage processes. Sustained propagation of cylindrical detonations necessitates increasing the number of cells to match the growth rate in the front region. Experimental investigations reveal two distinct modes of cell number increase: mild and violent. In the case of the former, cell number increase predominantly occurs on a scale of two to three times the characteristic cell size of the Chapman-Jouguet detonation. In contrast, a decaying Mach stem undergoes twisting and evolves into local kinks, leading to the development of new triple-wave points. The latter mode typically occurs near the limit, where cell increase primarily arises from randomly occurring local explosions, and operates on a scale of ten times the characteristic cell size or the chamber diameter. In addition, a numerical study of two-dimensional fundamental problems abstracted from the experiment is conducted to help interpret experimental results and reveal more about the physics of the problem.

Real Gas Effect on Motion of Self-gravitating Cylindrical Detonation Waves

Abstract: The effect of real gas on the adiabatic propagation of strong cylindrical imploding detonation waves in the nonhomogeneous medium having power varying density distribution. Ignoring the effect of overtaking disturbances the CCW solution has been obtained for the problem at the detonation waves are initially in Chapman-Jouguet state. The analytical expressions for the detonation velocity just behind the front along with other flow variables are derived. The expressions for the freely propagating velocity of detonation front are derived. Using the jump condition across the strong detonation front, the numerical values of the pressure and density across the front have been computed with the help of the software MATLAB. It is observed that the variation of flow parameters with convergence of detonation front is depend upon the change in Alfven Mach number, the parameters of realness of the gas and initial density distribution. Finally, it is found that change in parameter of realness of the medium plays an important role on the post flow variables. The effect of change in Alfven Mach number and density parameter is also discussed through graphs. The outcome of the present study is compared with the results for the case of ideal reacting gas.

Excitation of Cylindrical Detonation by a Decaying Shock Wave

Excitation of Cylindrical Detonation by a Decaying Shock Wave

Dynamic fracture of metal rods under overdriven effect of cylindrical detonation collision

AbstractThe collision of detonation waves is widely used in weapon design due to its ability to efficiently achieve an ultra‐high pressure region. This work discusses the dynamic fracture of metal rods under the collision of cylindrical detonations. In the experiments, the detonation wave collision is generated by detonating an explosive charge at its end position along two parallel initiation lines, which are detonated by a plane wave generator. The recovered metal rods were broken around their centers due to the superposition of detonation waves. A blue oxidation zone appeared at the fracture surfaces of steel rods in the contact zone under the charge, indicating the local existence of high temperatures. A finite element model was used to simulate and describe the dynamic fracture of metal rods caused by detonation wave collision. The three‐shock theory was used to examine the detonation wave collision. When the incidence angle was between 0° and 40°, the reflected shock wave pressure was approximately 2.1 times the CJ pressure. At an incidence angle of 44.84°, the ratio of Mach and CJ pressure reached a maximum of 3.1. The concentration degree of detonation pressure was developed to evaluate the superposition influence on the dynamic fracture of metal rods. The microstructure of fracture surfaces revealed that metal rods preferred shear fracture under high pressure concentration. With a low degree of pressure concentration, a brittle fracture may emerge. This research serves as a guide for designing and evaluating multimode warheads.

Direct detonation initiation in hydrogen/air mixture: effects of compositional gradient and hotspot condition

Two-dimensional simulations are conducted to investigate the direct initiation of cylindrical detonation in hydrogen/air mixtures with detailed chemistry. The effects of hotspot condition and mixture composition gradient on detonation initiation are studied. Different hotspot pressures and compositions are first considered in the uniform mixture. It is found that detonation initiation fails for low hotspot pressures and the critical regime dominates with high hotspot pressures. Detonation is directly initiated from the reactive hotspot, whilst it is ignited somewhere beyond the non-reactive hotspots. Two cell diverging patterns (i.e. abrupt and gradual) are identified and the detailed mechanisms are analysed. Moreover, cell coalescence occurs if many irregular cells are generated initially, which promotes the local cell growth. We also consider non-uniform detonable mixtures. The results show that the initiated detonation experiences self-sustaining propagation, highly unstable propagation and extinction in mixtures with a linearly decreasing equivalence ratio along the radial direction, i.e. 1 → 0.9, 1 → 0.5 and 1 → 0. Moreover, the hydrodynamic structure analysis shows that, for the self-sustaining detonations, the hydrodynamic thickness increases at the overdriven stage, decreases as the cells are generated and eventually becomes almost constant at the cell diverging stage, within which the sonic plane shows a ‘sawtooth’ pattern. However, in the detonation extinction cases, the hydrodynamic thickness continuously increases, and no ‘sawtooth’ sonic plane can be observed.

REALIZATION OF CONTINIOUSLY ROTATING DETONATION FOR SYNGAS-AIR MIXTURES

Numerical simulation of continuously rotating detonations (CRD) of stoichiometric two-fuel mixture with air has been carried out for the cylindrical annular detonation chamber (DC) of the rocket-type engine. Thesyngas (1 - α)óO + αH2, a binary mixture of hydrogen H2 and carbon monoxide CO, is taken as a fuel. The global flow structure in the DC and the detailed structure of the transverse detonation wave (TDW) front in thecontinuously rotating regime have been studied. Integral characteristics of the detonation process - the distribution of average values of static and total pressure along the length of the DC - and the value of specific impulse on the DC exit have been obtained. The regions of existence of stable CRD regime in coordinates “stagnation pressure pm - stagnation temperature Tm” in the injection manifold (receiver) have been determined. The minimal values of the DC length and radius for CRD for some regions at the pm-Tm plane have been found.

Numerical study of continuously rotating detonation in two-fuel gaseous mixture

Numerical simulation of continuously rotating detonations of stoichiometric two-fuel mixture with air has been carried out for the cylindrical annular detonation chamber (DC) of the rocket-type engine. The syngas (1-α)СO+αH2, a binary mixture of hydrogen H2 and carbon monoxide CO, is taken. We studied the global flow structure in DC, and the detailed structure of the transverse wave (TW) front in the continuous rotating regime. Integral characteristics of the detonation process − the distribution of average values of static and total pressure along the length of the DC, and the value of specific impulse have been obtained. The region of existence of stable continuous detonation regime in coordinates of the stagnation pressure - temperature in injection manifold (receiver) and the geometric limit of stable TW have been determined.

Formation of converging cylindrical detonation front

Purpose. To develop a laser method for initiating a converging cylindrical front of a detonation wave and a method for calculating the kinematic parameters of the cylindrical shell walls, accelerated by the pressure of the detonation products of an external explosive charge. Methodology. An experimental technology for the manufacture of a photosensitive explosive composite and an experimental technique for igniting the surface of its layer with an extended laser beam without the use of a fiber-optic cable are used. The results of simulation modeling the Monte Carlo method were used to study the effect of illumination on the process of ignition of explosives by laser pulsed radiation. For the selected type of photosensitive explosive composite, its explosive and optical characteristics, the distance from the surface of the explosive charge to the lens scattering the laser beam, and taking into account the total area of the expanded beam, the regularities of the distribution of the radiation energy density over the vertical and horizontal sections of the laser beam were studied. Findings. The analysis of the scientific and technical level of methods of shock-wave processing of materials in the region of ultrahigh pressures from the point of view of the fundamental value of the cumulation of energy in the waves of a converging cylindrical detonation and shock front is carried out. Physicomathematical modeling was carried out and the regularities of pressure increase in the wave front were established in the process of approaching the shell walls to the axis. The scientific results of modeling converging cylindrical shells under the influence of the pressure of the explosion products have been analyzed. A method for laser initiation of a converging cylindrical front of a detonation wave has been developed, and a method for calculating the kinematic parameters of the converging walls of a cylindrical shell has been proposed. Originality. A technique has been developed for determining the energy characteristics of an expanded laser beam, calculating the laser radiation energy required to initiate detonation simultaneously on the entire lateral cylindrical surface of a photosensitive explosive composite. The idea of technical implementation of the cumulation of converging cylindrical detonation and shock waves was developed further. A technique has been developed for the numerical determination of the change in the internal average compression rate of the shell during the movement of its walls towards the axis for various ratios of its external radius to the wall thickness and taking into account the increase in pressure in the converging detonation front. Practical value. For the first time, a method for laser initiation of a converging cylindrical front of a detonation wave was developed and a device was tested that forms a converging cylindrical front of a detonation wave and a corresponding shock front in the material under study by the impact of a metal shell converging to the axis. The core of the device is a laser explosive initiation system that uses light-sensitive explosive composites to initiate an explosive charge.

Autoignition and combustion wave propagation in a spatial reactivity gradient environment constructed using the shock-converging method

Abstract Studies on autoignition and combustion wave propagation in a spatial reactivity gradient environment constructed using the shock-converging method were conducted to obtain a new understanding of combustion modes and their mutual transition mechanism. Autoignition induced by the converging cylindrical shock wave and weak detonation can be observed by experiment and numerical simulation. Research indicates that for an appropriate spatial reactivity gradient, the velocity of the weak detonation wave can decrease and adjust to the local sound speed. This leads to the transformation of the weak detonation into the self-propagating Chapman-Jouguet (CJ) detonation. The speed of the spontaneous reaction wave characterizing the end of the chemical reaction is never less than the local sound speed; hence, there is no overdriven phenomenon throughout the weak-to-CJ detonation transition. This conforms to the fact that the overdriven phenomenon is not a necessary condition to initiate the detonation, and the combustion has higher thermal efficiency compared with isochoric combustion. Further research indicates that CJ detonation propagating in the preheated reactant can also transform into weak detonation for an appropriate spatial reactivity gradient. The initial CJ detonation degrades into a shock that decays and finally disappears. The wave-eliminating effect of the CJ-to-weak detonation transition can ensure the flow field quality and thermal efficiency of the combustion. In summary, performing a controllable weak detonation between isochoric combustion and CJ detonation in an engine has significant practical importance for improving combustion thermal efficiency and knock suppression.

A study on detonation-diffraction reflection point distances in H2/O2, C2H2/O2, and C2H4/O2 systems

Recently, Kawasaki and Kasahara (2019) reported that reflection point distance, which is a detonation characteristic length relevant to the diffraction process, is a useful measure; i.e., the critical condition for detonation diffraction can be universally expressed in terms of the diffraction point distance, independent of mixture stability. However, their findings were limited to their experimental conditions only. In this study, we performed high-speed visualization of processes of cylindrical (line-symmetric) detonation diffraction around a 90-degree corner for two series of experiments to obtained reflection point distances, lr, as a novel characteristic length, and examined critical conditions of reinitiation expressed in terms of the reflection point distance. In the first experimental series, stoichiometric C2H2/O2 mixtures with 50% Ar dilution were employed, and the channel width lc was varied to 5, 10, 15, and 20 mm to investigate the influences of the boundary condition of the flow field. In the second experimental series, H2/O2, C2H2/O2, or C2H4/O2 mixtures with different equivalence ratios were employed to investigate influences of the reaction systems. Our results confirmed that the channel width does not affect the reflection point distance or the critical condition. The critical condition was also independent of fuel species and equivalence ratio, and can be uniquely expressed as lr / lc = 4.0 ± 0.6 in terms of the reflection point distance.

The formation of a detonation wave with multipoint initiation

The results of experimental investigations of the formation of detonation waves with multipoint initiation for the case of a planar and converging cylindrical wave are presented. A special initiation system was used, where the main element was modeled for manufacturing on a three dimensional printer. A solution was found to form a smooth detonation wave for the planar case. Studies of a converging cylindrical detonation wave have shown the need to solve some fundamental issues related to the nonstationarity of such a wave.

Critical Conditions for Plane-to-Cylindrical Detonation Wave Transformation

The critical conditions for the transformation of a plane detonation wave into a cylindrical detonation wave are experimentally found. For mixtures of oxygen with hydrogen or methane, the geometric conditions are determined for detonation initiation in a cylindrical gap at various diameters of the orifice through which the plane detonation enters the gap. To identify features of observed explosive processes, the results of two-dimensional numerical modeling are analyzed.

Laboratory Explosive System for Cylindrical Compression

One method of studying materials or plasma under pressure pulse loading is axisymmetric compression using a convergent cylindrical detonation wave. Such waves are often generated by multipoint initiation and have a number of specific features that may affect the properties of the test objects. To solve specific problems, it is proposed to use a laboratory explosive system based on a converging cylindrical detonation wave with 12–48 initiation points. The TNT equivalent of the charge is less than 1 kg. The main method of research is visualization using a domestic high-speed Nanogeit camera with a nanosecond time resolution. The structure of the converging detonation wave is shown, and its velocity along the radius is determined. It is shown that for a charge of limited thickness, the curvature of the detonation wave front for various explosives depends only on the distance to the initiation point.

Propagation and failure mechanism of cylindrical detonation in free space

Cylindrical detonations propagating in free space characterized by different activation energies were computationally studied. It is found that unstable detonations with the 2-D cellular structure have more velocity deficit than those without the cellular structure computed with the 1-D simulation. The weakening is due to lengthening of the detonation structure and the unreacted pocket behind the cylindrical front, while propagation sustenance depends strongly on the re-amplification and regeneration of transverse shocks and triple points. For low activation energies, cellular detonation can be initiated in free space through the subcritical initiation path due to absence of unreacted pockets, and the propagation is not very sensitive to the attenuation of transverse waves and triple points. However, for high activation energy the unreacted pocket aggravates initiation such that even a cellular detonation first established is prone to quench due to the lack of re-amplification of the transverse wave and the triple point. When considering confinement, it is demonstrated that a detonation that quenches in free space can be reinitiated in confined space.

Formation features of cylindrical detonation wave at multipoint initiation

The experimental results showing the formation features of a cylindrical detonation wave with a multipoint initiation are presented in this work. One of the main features of the process is the presence of the “nodes” on the detonation wave, i.e. points of convergence of detonation waves from the neighbor points of initiation. The other feature is the presence of the “bundles” which are exeunt to the detonation products. Nodes are the points with high energy level at detonation front. They may cause hydrodynamic instabilities during the compression of a metal liner. Parietal flows in the cell structure of detonation products may be reason for formation of “bundles”. It should be noted that the detonation wave is always convex between the neighbor “nodes”, although in the initial moment the triple-wave Mach configuration is forming and it should lead to aligning of cylindrical wave. The experiments with the use of high explosives with different densities (from 0.95 to 1.65 g/cm3) were carried out on the laboratory installation. The characteristic features of the detonation wave formed by the multipoint initiation can be seen both in solid and in liquid high explosives. Different design features (interlayers made of different materials etc.) did not lead to disappearing of “bundles” and smoothing of “nodes” at the detonation wave.

Deflagration-to-detonation transition in spiral channels

The deflagration-to-detonation transition in hydrogen–air mixtures that fill spiral channels has been studied. A spiral channel has been produced in a cylindrical detonation tube with a twisted ribbon inside. The gas mixture has been ignited by means of a spark gap switch. The predetonation distance versus the twisted ribbon configuration and molar ratio between the gas mixture components has been determined. A pulling force exerted by the detonation tube after a single event of hydrogen–air mixture burnout has been found for four configurations of the twisted ribbon. Conditions under which the use of a spiral tube can be more effective (increase the pulling force) have been formulated.

Numerical simulation of classical and rotating detonation waves in methane mixtures

A numerical simulations of a two-dimensional multi-front irregular structure of the detonation wave (DW) in methane-based mixtures at normal initial condition have been conducted. The computations have been performed in a wide range of channel height. From the analysis of the flow structure and the number of primary transverse waves in channel, the dominant size of the detonation cell for studied mixtures has been determined. We have simulated the cellular front structure in stoichiometric methane–air and methane–oxygen mixtures, and in a rich (equivalence ratio φ = 1.5) methane-air mixture. Based on the fundamental studies of front structure of the classical propagating DW in methane mixtures, numerical simulation of continuous spin detonation of rich (ϕ = 1.2) methane-oxygen mixture has been carried out in the cylindrical detonation chamber (DC) of the rocket-type engine. We studied the global flow structure in DC, and the detailed structure of the front of the continuous rotating DW. Integral characteristics of the detonation process - the distribution of average values of static and total pressure along the length of the DC, and the value of specific impulse have been obtained. The geometric limit of stable existence of rotating DW has been determined.

Effect of Cellular Instability on the Initiation of Cylindrical Detonations**Supported by the National Natural Science Foundation of China under Grant Nos 91541206 and 91441131.

The direct initiation of detonations in one-dimensional (1D) and two-dimensional (2D) cylindrical geometries is investigated through numerical simulations. In comparison of 1D and 2D simulations, it is found that cellular instability has a negative effect on the 2D initiation and makes it more difficult to initiate a sustaining 2D cylindrical detonation. This effect associates closely with the activation energy. For the lower activation energy, the 2D initiation of cylindrical detonations can be achieved through a subcritical initiation way. With increasing the activation energy, the 2D cylindrical detonation has increased difficulty in its initiation due to the presence of unreacted pockets behind the detonation front and usually requires rather larger source energy.

The influence of charge geometry on the response of partially confined right circular stainless steel cylinders subjected to blast loading

This article presents an experimental and numerical investigation into the effect of charge geometry on the structural response of right circular cylinders. Thin-walled, seamless 304 stainless steel cylinders were subjected to internal blast loads that were generated by detonating bare cylindrical plastic explosive charges. Charge diameter, charge mass and aspect ratio were varied. Partial confinement was created by closing one end of the cylinder and leaving the other end free to vent to air. The axial impulse, maximum diametric deflection and axial shortening increased with increasing aspect ratio when the charge diameter was kept constant. When the charge mass was kept constant, the long charges caused larger diametric deflections than their mass equivalent shorter charges. The shorter charges transferred more axial impulse to the ballistic pendulum than their mass equivalent longer charges. The concept of effective charge mass (axial and lateral) was used to interpret the results and developed to provide an empirical equation relating deflection to the charge dimensions. Numerical simulations were performed which gave good agreement with the experimental data and predicted similar trends. The model revealed that the longer charges produced more diametric deflections than their mass equivalent shorter charges because a high pressure (above 50MPa) zone, that covered a larger area and had a longer duration, developed at the cylinder wall in the case of longer charges. The insights from the model are in good agreement with the simple concept of lateral effective charge mass developed herein. When compared to spherical charge detonations in similar steel cylinders, the cylindrical charge detonations produced pressure waves which are more directional, have lower peak pressures and dissipate more gradually. The cylinders exhibited a more pronounced secondary bulge at the clamped boundary for cylindrical charge detonations.

The role of multidimensional instabilities in direct initiation of gaseous detonations in free space

We numerically investigate the direct initiation of detonations driven by the propagation of a blast wave into a unconfined gaseous combustible mixture to study the role played by multidimensional instabilities in direct initiation of stable and unstable detonations. To this end, we first model the dynamics of unsteady propagation of detonation using the one-dimensional compressible Euler equations with a one-step chemical reaction model and cylindrical geometrical source terms. Subsequently, we use two-dimensional compressible Euler equations with just the chemical reaction source term to directly model cylindrical detonations. The one-dimensional results suggest that there are three regimes in the direct initiation for stable detonations, that the critical energy for mildly unstable detonations is not unique, and that highly unstable detonations are not self-sustainable. These phenomena agree well with one-dimensional theories and computations available in the literature. However, our two-dimensional results indicate that one-dimensional approaches are valid only for stable detonations. In mildly and highly unstable detonations, one-dimensional approaches break down because they cannot take the effects and interactions of multidimensional instabilities into account. In fact, instabilities generated in multidimensional settings yield the formation of strong transverse waves that, on one hand, increase the risk of failure of the detonation and, on the other hand, lead to the initiation of local over-driven detonations that enhance the overall self-sustainability of the global process. The competition between these two possible outcomes plays an important role in the direct initiation of detonations.