Mechanism Of Detonation Propagation Research Articles

Effects of velocity shear layer on detonation propagation in a supersonic expanding combustor

This study investigates the mechanism of detonation propagation in a stoichiometric hydrogen–oxygen mixture with non-uniform flow velocity entering an expanding combustor. For simulation of the detonation propagation, the Navier–Stokes equations with a one-step two-species chemistry model are solved by employing the hybrid sixth-order weighted essentially non-oscillatory centered difference scheme. The self-sustaining mechanism of detonation propagation in an expanding combustor under the action of non-uniform supersonic flow with a velocity shear layer is revealed. The results show that under the influence of velocity shear layer, two different unburned jets are produced behind the detonation front. These jets are induced by the velocity shear layers and the Prandtl–Meyer expansion fan. The two jets interact and mix gradually. The interaction between the mixed unburned jets and highly unstable shear layers creates large-scale vortices that intensify the turbulent mixing of the unburned jets. Meanwhile, the baroclinic mechanism generates numerous vortices on the boundary of the unburned jet. These vortices promote the mixing of the burned and unburned gases, which eventually leads to the rapid consumption of the unburned pockets. The heat released due to the burning of the unreacted pockets behind the detonation wave supports a self-sustaining propagation of the detonation wave. When the velocity difference among the shear layers increases, the surface fluctuation of the detonation wave increases.

Numerical investigation on the rotating detonation critical mode for a methane–air mixture in an annular tube using reactive Navier–Stokes equations

Previous experimental studies show that gaseous detonation propagation in an annular tube can be divided into three categories, i.e., stable mode, critical mode and unstable mode. However, the mechanism of detonation propagation with the critical mode is not well understood due to insufficient experimental data. In this paper, detonation propagation with the critical mode in the annular tube was numerically studied for the methane–air mixture based on reactive Navier–Stokes equations, in which the convective term was integrated by the fifth-order weighted essentially non-oscillatory. The numerical results show that the trajectories of the triple points of the shock wave front were drawn to obtain cell structure with “petal” pattern, which is agree with the experimental results. The wall curvature plays an important role for this phenomenon. The outer wall is concave, which can continuously compress the detonation wave and make it stronger. The inner wall is convex, which can produce an isentropic expansion and decouples the reaction front from the shock wave. The transverse shock wave emanating from the triple point moving from the outer wall reignites detonation near the inner wall, which implies that compression of the outer wall plays a key role in the rotating detonation propagation. As the transverse shock wave moves away from the inner wall, the detonation wave near it gradually weakens. Because of the back and forth reflection of the transverse shock wave between the inner and outer walls, the rotating detonation wave can be maintained and its cellular structure presents “petal” pattern. Meanwhile, the averaged detonation velocity at the outer wall is higher than that at the inner wall.

Propagation mechanism of gaseous detonations in annular channels with spiral for acetylene-oxygen mixtures

The propagation mechanism of detonations in the annular channel with a spiral is experimentally studied for an acetylene and oxygen mixture with different levels of argon dilution. Detonation velocity is measured through the placement of regularly spaced photodiodes along the test tube. A high-speed camera is also used to supplement the photodiodes when the luminosity decreases. Smoked foils of long length are inserted into the annular channel to register the cellular structures. The results indicate that the effect of the spiral on detonation propagation highly depends on the intrinsic instability of the detonation. With a decrease in the initial pressure, the propagation changes from spiral-induced multi-head mode to spiral-induced single-head mode and then spiral-induced shock-flame complex mode; then, the propagation fails as the limit is approached. Discontinuities in normalized velocity are found for mixtures with high argon dilution, which indicates the transition from single-head spin to shock-flame complex, as observed from the smoked foils. Such a phenomenon is not observed in cases with an unstable mixture of C2H2 + 2.5O2 or in cases with large roughness. This is because of the different mechanisms stable and unstable detonations and the corresponding response to the boundary change. The critical width of the annular channel normalized by cell size for the discontinuity is found to slightly larger than 1/π, which corresponds to the lowest quenching limit in smooth tubes, particularly for stable detonations. The critical diameter normalized by cell size for the discontinuity in the circular tube is almost twice the critical width in the annular channel, equal to the geometrical scaling based on front curvature theory.

Effect of obstacle thickness on the propagation mechanisms of a detonation wave

Effects of obstacle thickness on the mechanism of detonation propagation in 2H2–O2 mixture are investigated experimentally in a square channel with a cross-section of 300 × 300 mm and 6 m long. Two typical obstacles with the thicknesses of 20, 60 and 80 mm are considered, i.e., a square orifice and a perforated plate. Eight equally spaced pressure transducers are used to record the time-of-arrival of the combustion wave, from which the average velocity can be determined. The smoked foil is employed to record the detonation cellular structures. Three various propagation regimes are seen: (1) below the critical pressure, a steady detonation wave is not formed before the obstacles, and then the detonation wave is produced by the mechanism of DDT; (2) near the critical condition, the steady detonation wave is decayed firstly across the obstacles, afterwards, the re-initiation mechanism is observed by the shock-wall interaction; (3) well within the limits, the perturbations induced by the obstacles nearly have no effect on the detonation propagation. Moreover, two different ignition mechanisms are observed after the obstacles, i.e., symmetric and asymmetric ignition. The critical condition of detonation propagation can be quantified as DH/λ > 1 where DH is the hydraulic diameter and λ is the cell size.

Velocity behavior downstream of perforated plates with large blockage ratio for unstable and stable detonations

Due to the excellent thermal propulsion performances of detonation, it has been applied for the purpose of aerospace propulsion devices, e.g., pulse detonation engines (PDEs), rotating detonation engines (RDEs), and oblique detonation wave engines (ODWEs). However, it remains challenging for developing those new-concept propulsion devices, mainly because it is a formidable task to establish a steady and self-sustained detonation in the hypersonic flow and combustible mixture. One of the fundamental problems is to understand the interaction of diffractions and the detonation waves within the engines. In this study, perforated plates with various blockage ratios are seated at the beginning of the detonation propagation to explore the diffractions that generated from the plates on the detonation propagation mechanism. Two explosive mixtures of C2H2 + 5N2O and C2H2 + 2.5O2 + 70% Ar are studied to illustrate the difference in propagation velocity behavior of detonation after it suffers from large-scale diffractions. The results show that diffractions have less effect on the propagation of highly unstable mixture, and the effect becomes obvious as BR increases to 0.962; this phenomenon occurs because detonation has an irregular cellular pattern and sub-structures are characterized by highly unstable detonation, in which the detonation instabilities are amplified by the large perturbations of obstacles, leading to an augment in forming more cellular cells of detonation in its front that cover the weakening effects from diffractions. In contrast, the diffractions significantly affect the stable mixture, manifested by a remarkable increase of the critical pressure for a self-sustained detonation downstream of the obstacle with the augment of blockage ratio; this phenomenon can be attributed to the diffractions being distributed along the curvature over the detonation surface, thereby causing more excessive curvature of the entire detonation front, which in turn exacerbates the failure of detonation.

Visualization of detonation propagation in a round tube equipped with repeating orifice plates

Self-luminous, high-speed photography was used to visualize fast-flame and detonation propagation through a transparent round tube equipped with repeating orifice plates, in stoichiometric hydrogen-oxygen mixtures at initial pressures up to 60 kPa. Experiments were conducted in a 1.55 m, 7.6 cm inner-diameter plastic tube filled with equally spaced 5.33 cm and 3.81 cm orifice plates (50% and 75% area blockage ratio, respectively). The unprecedented visualization of quasi-detonation propagation in a round tube was used to identify the propagation mechanisms. For both sets of orifice plates, fast-flames were observed below a critical initial pressure. Fast-flame propagation involved the interaction of an uncoupled shock wave and flame with the orifice plates. Detonation propagation involved repeated detonation failure and initiation along the channel length; the limits measured in the 50% and 75% blockage ratio (BR) orifice plates were 7 kPa and 40 kPa, respectively. The orifice diameter-to-detonation cell size ratio (d/λ) corresponding to these limits are 1.4 and 14, respectively. It is proposed that the significant variance in the d/λ at the two limits is attributable to the difference in the detonation propagation mechanism. For the 50% BR orifice plates, near the limit, detonation initiation occurred on the tube wall between orifice plates following reflection of the lead shock wave. Whereas, for the 75% BR orifice plates, detonation initiation at the tube wall was not possible for initial pressures up to 40 kPa. This is the result of a weaker shock wave at the time of reflection due primarily to the larger distance from the orifice edge to the tube wall. Steady propagation of a curved detonation wave was observed for the 50% BR orifice plates for an initial pressure of 50 kPa (d/λ = 25), or greater; a similar propagation was not observed in the 75% BR orifice plates at initial pressures up to 60 kPa (d/λ = 27). Numerical simulations carried out using a single-step reaction model demonstrated the key processes involved in detonation initiation at the tube wall and the orifice plate but could not predict quantitatively the critical initial pressure required for detonation propagation measured in the experiments.

Experimental study of detonation propagation in a square tube filled with orifice plates

DDT experiments were conducted in a 6000 mm long square cross-section (112 mm × 112 mm) tube with various obstacle configurations with hydrogen-air mixtures and ethylene-air mixtures at ambient pressure (101 kPa) and room temperature (298 K). Square orifice plates with inner side 86.8 mm and 70.8 mm (BR = 0.4 and 0.6) and round orifice plates with inner diameter 80.0 mm (BR = 0.6) were used to assemble the obstacle configurations. The plates were installed at 1, 2 and 3 times the tube inner side. Soot foils were placed between the two orifice plates at the end of the tube for S=3D, where S is the obstacle spacing and D is the tube inner side. The DDT limits were determined based on the flame velocity above the isobaric sound speed of the burnt products. The results show that at the DDT limits, the criterion deff/λ≈1 is not pervasive, i.e., deff/λ decreases with the obstacle spacing increase, in which deff and λ are the effective diameter of the orifice and the detonation cell size. Within the limits, the measured velocity for BR = 0.6 square orifice plates is higher than that for round orifice plates. On the other hand, no obvious difference in the limits can be observed for the BR = 0.6 obstacle configurations. Soot foils provide insights into the detonation propagation mechanism in the orifice plate section. It is shown that hot spots formed via the interactions between the decoupled shock wave and the tube wall can be responsible for the re-initiation of detonation. In addition, overdriven detonations induced by shock focus at the corners, followed by a band of fine cells. For less sensitive mixture and smaller orifice, the re-initiation distance is longer. Near the limits, no cellular structure can be observed, indicating longer cycle period for detonation re-initiation. This also accounts for the significant velocity fluctuation for larger spacing (S=2D and S=3D) when the limits are approached.

Experimental study of DDT in hydrogen-methane-air mixtures in a tube filled with square orifice plates

Abstract Deflagration to detonation (DDT) experiments with stoichiometric hydrogen-methane-air mixtures at ambient pressure (1 atm) and temperature (293 K) were carried out in a 112 × 112 mm by 6 m long tube with various obstacle configurations. The hydrogen fraction in fuel mixtures ranged from 0 to 1. Two types square orifice plates with opening sides of 86.8 mm and 70.8 mm (blockage ratios of 0.4 and 0.6), spaced at 1, 2, 3 and 4 times the tube height were employed. Flame signals were detected by photodiodes spaced evenly, from which the average velocity could determined. The detonation cell size was obtained using soot foil technique. CJ detonations can still propagate in less sensitive mixtures with larger detonation cell sizes. At the DDT limits, the ratio of the height of the orifice plate (d) and the detonation cell size (λ) increases with the increase of the blockage ratio and the decrease of obstacle spacing. This indicate that in a square tube with square orifice plates, the detonation initiation process as well as the detonation propagation mechanism govern the DDT limits.

The effects of large scale perturbation-generating obstacles on the propagation of detonation filled with methane–oxygen mixture

Orifice plates with large blockage ratios (BR=0.691, 0.826, 0.923 and 0.962) are placed in the entrance of the detonation transmission to investigate the artificially large-scale perturbations on the detonation propagation mechanism. Obstacles create large perturbations, thereby generating flow instability to affect the detonation transmission. In this study, stoichiometric methane–oxygen is examined because it has a wide application in the industry; therefore, its safety concerns must be addressed. Furthermore, the methane–oxygen mixture is characterized by a highly irregular cellular detonation front, and it is the representative of those potentially combustible mixtures used in practical aerospace application. Velocity behaviors after the detonation passes through the obstacles with different BR near the critical condition are analyzed; particular attention is paid to the cases for which BR is larger than 0.9 (i.e., 0.923 and 0.962), and the results are also compared with relatively smaller BR cases (BR=0.691 and 0.826). The effect of the diffraction, shock reflection and detonation instability on the propagation of detonation is examined. Finally, the analysis of critical length scale between the cell size (λ/d) and BR when the detonation fails or succeeds as it passes the obstacles is also performed, this work aims to explore the mechanism of critical perturbation scale on the detonation propagation.

On the propagation mechanism of a detonation wave in a round tube with orifice plates

This study deals with the investigation of the detonation propagation mechanism in a circular tube with orifice plates. Experiments were performed with hydrogen air in a 10-cm-inner-diameter tube with the second half of the tube filled with equally spaced orifice plates. A self-sustained Chapman–Jouguet (CJ) detonation wave was initiated in the smooth first half of the tube and transmitted into the orifice-plate-laden second half of the tube. The details of the propagation were obtained using the soot-foil technique. Two types of foils were used between obstacles, a wall-foil placed on the tube wall, and a flat-foil (sooted on both sides) placed horizontally across the diameter of the tube. When placed after the first orifice plate, the flat foil shows symmetric detonation wave diffraction and failure, while the wall foil shows re-initiation via multiple local hot spots created when the decoupled shock wave interacts with the tube wall. At the end of the tube, where the detonation propagated at an average velocity much lower than the theoretical CJ value, the detonation propagation is much more asymmetric with only a few hot spots on the tube wall leading to local detonation initiation. Consecutive foils also show that the detonation structure changes after each obstacle interaction. For a mixture near the detonation propagation limit, detonation re-initiation occurs at a single wall hot spot producing a patch of small detonation cells. The local overdriven detonation wave is short lived, but is sufficient to keep the global explosion front propagating. Results associated with the effect of orifice plate blockage and spacing on the detonation propagation mechanism are also presented.

Transverse wave generation mechanism in rotating detonation

Detonation engines are expected to be included in a number of aerospace thrusters in the future. Several types of detonation engines are currently under examination, including the rotating detonation engine (RDE). Although the RDE has been explored experimentally, its rotating detonation propagation mechanism is not well understood. This paper clarifies the detonation mechanism and dynamics of the RDE by 2D and 3D simulation using compressible Euler equations with a full chemical reaction mechanism of H2/O2 and H2/Air, especially from the triple-point and transverse detonation points of view. A total variation diminishing (TVD) scheme is used for the mixture of H2/Air, and an advection upwind splitting method difference vector (AUSMDV) scheme is used for the mixture of H2/O2. The use of an AUSMDV scheme provides a much clearer detonation structure than does the TVD scheme. We focus on the complex interaction mechanism of the detonation front and burned mixture gases. We found out that at this interaction point, an unreacted gas pocket appears and ignites periodically to generate transverse waves at the detonation front and maintain detonation propagation.

Distinctive features of galloping detonation in a supersonic combustible-mixture flow under an inert gas layer

The problem of detonation initiation in a supersonic flow of a stoichiometric propane-air mixture occupying partially or completely the cross-section of a plane channel is considered. The initiation in the flow is produced by a step or a wall completely cutting off the flow. The study is conducted within the framework of one-stage combustion kinetics. A numerical method based on the Godunov scheme is employed. The critical conditions for detonation formation are determined in terms of the oncoming flow velocity. A previously unknown mechanism of detonation propagation is found; it is related with the presence of the combustible mixture in the wall layer under an inert gas layer. It is due to the formation of a complicated wave structure of the flow characterized by the penetration of a shock wave formed in the inert gas layer into a combustible mixture layer ahead of the detonation wave with the result that the latter layer is heated and ignited. The process as a whole is periodic in nature, as distinct from the conventional cellular detonation in a homogeneous fluid. Many problems arise in connection with the use of detonation in engines and other power plants. The most important among them are detonation excitation and stabilization in combustion chambers. The detonation initiation within a layer under conditions of unbounded space and a fluid at rest was experimentally investigated in [1]. In the case of a combustion chamber bounded in the transverse direction, some new effects accompanying the detonation might be expected.

Influence of the initial gas pressure on the detonation limits and parameters of low-density secondary explosives in an inert porous medium

This paper reports the results of an experimental study of the detonation of low-density (3–40 mg/cm3) secondary high explosives (HEs) in evacuated and gas-filled inert porous media. A convective-jet mechanism of detonation propagation was found to occur under all conditions studied. The effects of the type of HE and initial gas pressure on the critical density of the HE and detonation parameters were elucidated. It is shown that, in the presence of air, detonation can occur at a lower volume-averaged density of the HE than in the case of the evacuated media and there are two limits (minimal and maximal) for the initial gas pressure. As the volume-averaged density HE decreases, the limits approach each other and, for a certain critical density of the HE, detonation exists only at one value of the initial pressure.

Detonation and Physicochemical Properties of Cannon Powders

Detonation and physicochemical properties of several cannon powders were studied. Dependences of the critical diameter and detonation velocity of heptachannel pyroxylin powders on the powder element diameter were determined. The mechanism of detonation propagation over a charge was studied for various positions of the powder elements relative to one another and to the shock‐wave front.

Propagation mechanism of detonation : three-dimensional phenomena

Propagation mechanism of detonation : three-dimensional phenomena

A Molecular Dynamics Study of an Exothrmically Reacting System; The Effects of Finite Lifetime Activated Complex on the Structure of the Detonation Wave Front

The detonation wave propagation phenomena in an exothermically reacting hard disk system are studied using the molecular dynamics method. The model reaction is \(A{\rightleftarrows}A^{*}{\rightleftarrows}B\) type exothermic isomerization reaction, where A * corresponds to an activated complex. The detonation propagation mechanism and the relaxation processes at the wave front are compared with those of the similar reaction system with no activated complex, \(A{\rightleftarrows}B\). The existence of the activated complex A * does not change the essentials of the propagation mechanism but changes the structure of the wave front.