Detonation Initiation Process Research Articles

Numerical study of reflector configuration design on shock wave focusing detonation initiation process

Numerical simulation research was carried out to study the effect of the parabolic cavity shape on detonation initiation process for shock wave focusing engine with kerosene as fuel. The results showed that the peak pressure in the cavity was decreased significantly as the depth of the cavity was enhanced. An umbrella-shaped aerodynamic structure which is composed by the expansion of the fan and two vortices was generated under the K–H instability effect when the supersonic jet entered into the cavity. When the inlet jet temperature was at ambient temperature (300 K) or set as 500 K, nearly no ignition was generated for all the three cavity configurations. As the jet temperature was increased to 600 K, the flame can be formed and stay in the cavity for all of these three cavities. However, the coupling between the flame propagation and the reflected shock wave is not close that detonation was not generated as well. As the inlet jet temperature was further increased to 700 K, the hot spot at the bottom of the cavity could be coupled with the reflected shock wave and detonation wave was successfully formed for all of these three cavity configurations.

Effects of micrometer-scale cavities on the shock-to-detonation transition in a heterogeneous LX-17 energetic material

Cavities and other fracture structures within energetic materials may have significant impact on their performance. The mechanism on how hot spots induced by cavity collapse affect the detonation initiation process is still not fully understood. In this work, two-dimensional simulations are conducted for heterogeneous LX-17 energetic material containing array-distributed cavities to investigate the detonation initiation process induced by the impaction of the incident shock wave (ISW), and the impacts of cavity size and volume fraction on the shock-to-detonation transition (SDT) are also evaluated. First, we fix the cavity radius to be 40 μm and the cavity volume fraction to be 12.57%, and compare the detonation initiation processes for neat and heterogeneous LX-17 energetic materials. The results indicate that cavities within LX-17 can accelerate the detonation initiation, i.e., shortening the initiation distance and time. Then, the flow characteristics and incident shock wave evolutions during the cavity collapse process are analyzed. The results show that the interaction between the cavity and the incident shock wave results in the local hot spots and causes LX-17 reactant to auto-ignite, so as to accelerate the shock-to-detonation transition. Finally, the influence of the cavity size and volume fraction on the detonation initiation process is assessed. It is found that as the cavity volume fraction increases, the detonation initiation distance and time increase and even become larger than the results predicted of the neat case, i.e., the acceleration effect of cavities on the detonation initiation weakens and the cavities even inhibits the shock-to-detonation transition. When the cavity volume fraction is fixed, it is found cases of small-size cavity predict longer initiation distance and time than cases of large-size cavity. The analysis indicates that increasing cavity volume fraction corresponds to smaller density of LX-17 reactant, and the hot spot duration time is shorter for cases of small-size cavity than cases of large-size cavity. Therefore, the detonation initiation distance and time increase as the cavity volume fraction increases and the cavity size decreases.

Numerical study on the deflagration to detonation transition promoted by transverse jet

To reveal the mechanism of deflagration to detonation transition promoted by transverse jet, the flame acceleration and detonation initiation process of H2/Air under transverse jet was numerically investigated. According to the flame propagation characteristics and the main factors that promote flame acceleration in different periods, the process of deflagration to detonation transition was divided into four stages: initial flame development stage, turbulence-flame interaction stage, shock-flame interaction stage, detonation initiation stage. The flame acceleration process of each stage was analyzed in detail. The results indicated that the curling and stretching effect of the transverse jet and vortexes on the flame can effectively increase the flame velocity. The flame would constantly produce compression waves during acceleration. The transverse wave formed by the superposition of the compression waves would be reflected multiple times between the walls and act on the flame front, which is critical for flame acceleration. In addition, the positive feedback between the leading shock wave and the flame was also important for the continuous increase of flame velocity.

Numerical investigation of the effect of reactive gas jets on the flame acceleration and DDT process

Jet obstacles can quickly induce the deflagration to detonation transition (DDT) process and reduce the thrust loss of an engine. However, there are few studies on the use of combustible premixed gases as the reactive jet obstacle. Based on the OpenFOAM platform, a detailed numerical investigation of the flame acceleration and DDT process is carried out with different initial jet velocities and the number of jet obstacles. The results show that, although the stronger flame generated by the reactive jet obstacles will reduce the virtual blockage ratio to a greater extent, the turbulence and combustion heat release effect generated by them still significantly promote the flame acceleration. In terms of flame acceleration, initially, increasing the initial jet velocity has no obvious effect on the flame acceleration. Later, turbulence and combustion heat release begin to dominate the flame acceleration and significantly promote the flame acceleration. Since the jets in this study adopt the same combustible premixed gases, increasing the number of reactive jet obstacles downstream does not improve the upstream flame acceleration. In DDT, increasing the initial velocity of the reactive jet can improve the detonation initiation, but the effect of the obstacle number on DDT is greatly affected by the initial jet velocity. Specifically, at a low initial jet velocity, more jet obstacles can significantly promote the detonation initiation, while at a high initial jet velocity, the enhanced turbulence caused by the jet is the dominant factor for the flame acceleration. Therefore, compared with the number of jet obstacles, the detonation initiation process of the premixed gases is more sensitive to the change in the initial jet velocity.

Effects of staggered opposed hot jets on the initiation and propagation of gaseous detonation in a supersonic combustible inflow

In this paper, the detonation initiation mechanism of a supersonic combustible mixture triggered by a staggered opposing combined hot jets was performed. Two-dimensional reactive Navier–Stokes equations with a one-step Arrhenius chemistry model were solved using a structured adaptive mesh refinement framework. The results show that a high temperature and pressure region triggers a rapid detonation initiation after the jet-induced bow shock focusing. Further analysis showed that there is a large baroclinic torque behind the local detonation wave induced by the staggered hot jet, which leads to a large Richtmyer–Meshkov instability at the end of the unburned jet, and the generated periodic shedding vortex structure thereby enhances the diffusion effect in the unburned region. However, the released heat cannot support the propagation of the detonation wave. In addition, different jet intensity distribution schemes and jet spacing will change the ignition point position. It is worth noting that the distance of detonation initiation can be significantly shortened by reducing the front jet intensity while maintaining the total jet energy. Increasing the jet spacing will significantly slow down the detonation initiation process.

Flame acceleration and onset of detonation in inhomogeneous mixture of hydrogen-air in an obstructed channel

Flame acceleration (FA) and deflagration-to-detonation transition (DDT) in combustible premixed gases is a complex process involving the flow and combustion phenomena. Practically, the combustible premixed gases are often inhomogeneous. The numerical study aims to explore the effects of different solid obstacle distribution patterns on the FA and DDT process in the inhomogeneous concentration field. Results show that the shape of detonation wave is arc-shaped due to the inhomogeneous hydrogen concentration, and the detonation initiation process can be classified into three types: i) detonation induced by the coupling of the flame surface and the high-pressure zone; ii) detonation induced by the interaction between flame surface and reflected shock wave; iii) detonation induced by the interaction between the oblique shock wave or Mach stem and obstacles in the flow field. Also, this study finds that the flame evolutions have experienced four velocity augmentation regions before the detonation initiation. Further, the rich hydrogen content in the aperture and the higher unilateral blockage ratio can both promote flame acceleration, and further shorten the run-up time of the DDT.

Experimental investigation on spray and detonation initiation characteristics of premixed/non-premixed RDE

In rotating detonation engines (RDE) supplied with liquid fuel, stability of the detonation wave considerably depends on the quality of mixing in the gas–liquid flow field. In this study, RDE and linearized RDE, using aviation kerosene (RP-3) and oxygen-enriched air with 29% oxygen content as propellants, are proposed to analyze the characteristics of cold-state spray and detonation initiation. The result is more evident in non-premixed RDE, where the recirculation zone in the combustor affects the movement of droplets and concentration distribution of kerosene. As the combustor pressure increases, speed of the airflow entering the combustor decreases and the aerodynamic shearing effect is weakened, which is not conducive to droplet breakup. The increase in fuel flow has a significant effect on non-premixed injection; the Sauter mean diameter (SMD) decreases from 90.1 to 21.5 μm; the detonation delay time is prolonged with increased equivalence ratio. The detonation with better performance is existed in the non-premixed RDE because of the concentration gradient of the wavefront mixture avoiding the flashback. However, in the premixed RDE fueled by atomizing nozzles, the effect of the flow changes was not prominent. During the detonation initiation process, the droplets tend to be burnt in the deflagration, and nearly 200 ms are required to achieve a stable detonation.

Exploring the controlling mechanisms for gradient evolution in unsteady detonation flows

Proper characterization of the transient interaction between shock front motion and chemical energy release is the key for further understanding of unsteady detonation dynamics. We have shown that the gradients could be the adequate intermediate quantities to reflect the transient interaction between shock front motion and chemical energy release through the Lagrangian particle analysis of the simulated direct detonation initiation (DDI) process in H2–O2–Ar mixtures. Specifically, the “shock change equations” are verified to describe the direct relation between the shock front motion and the gradients immediately behind the shock. Moreover, given the time derivatives of shock speed, the gradient evolution in the induction zone can be reproduced by the gradient evolution equations that are deduced from the Euler equations, no matter if the shock front undergoes rapid deceleration or acceleration. While in the reaction zone where the heat release is significant, it is demonstrated that the evolutions of velocity, pressure, and their gradients can be described by the Zel'dovich–von Neumann–Döring model with a constant wave speed that is below the Chapman–Jouguet speed, no matter if the shock front undergoes rapid deceleration or acceleration. These two distinct controlling mechanisms are verified in both planar and spherical DDI processes, showing their general applicability. This work suggests a new perspective, in terms of gradient evolution, for further understanding the unsteady detonation dynamics.

Numerical study on detonation initiation process in the chamber with characteristic structures of the dual-mode scramjet combustor

In order to improve the performance of traditional ramjet, a ramjet based on pulse detonation (PD-Ramjet) instead of isobaric combustion was proposed. Several special-shaped detonation chambers which retained the characteristic structures of the dual-mode scramjet combustor were designed. Characteristic structures included the expansion channel and the cavity. The filling process and detonation initiation process of the stoichiometric hydrogen/air mixture under the incoming flow condition of the sub-combustion mode were studied in the detonation chambers with characteristic structures by two-dimensional numerical simulation method, and the influences of the characteristic structures on the filling process and detonation initiation process were analyzed. The simulation and analysis results indicated that the hydrogen concentration at both sides of the chamber near the outlet was low due to the structure of the expansion channel, and the pressure and velocity of the detonation wave decreased gradually after the expansion channel. The cavity had a significant influence on the filling process, which resulted in the uneven hydrogen concentration downstream the cavity, especially near the outlet of the chamber. The intensity of the detonation wave attenuated to a certain extent by reason of the cavity, where the peak pressure first decayed due to the sudden expansion of the flow path and then rose owing to the reflection at the cavity aft wall.

Effect of solid obstacle distribution on flame acceleration and DDT in obstructed channels filled with hydrogen-air mixture

Ensuring hydrogen safety has become of great significance nowadays, whose leakage can possible result in the deflagration to detonation (DDT). The numerical study aims to explore the effect of solid obstacle distribution on the DDT in a homogeneous hydrogen-air mixture. Results show that the detonation initiation process can be classified into two types: i) local spherical detonation caused by the coupling of flame surface and high-pressure region; ii) detonation triggered by the interaction between the upper wall and multiple compression waves in front of the flame. Also, this study finds that the flame acceleration (FA) experiences two periodic “acceleration-deceleration” processes before the detonation initiation, and the initiation distance and time are the shortest when the obstacles are symmetrically distributed. Further, the higher the unilateral blockage ratio, the more unfavorable DDT occurs. The present results highlight the effect of different solid obstacle distribution patterns on the FA and DDT process.

Detonation initiation by shock focusing at elevated pressure conditions in a pulse detonation combustor

This work contains experimental investigations on the correlation of the detonation initiation process via a shock-focusing device with various initial pressures and mass flow rates. A pulse detonation combustor is operated with stoichiometric hydrogen--air--oxygen mixtures in single cycle operation. A rotationally symmetric shock-focusing geometry evokes the onset of a detonation by the focusing of the reflected leading shock wave, while a blockage plate at the rear end of the test rig is applied to induce an elevated initial pressure. The results show that the reactivity has a major influence on the success rate of detonation initiation. However, measurements with different blockage plates suggest that the mass flow rate has to be considered as well when predicting the success rate. Three main statements can be drawn from the results. (1) An increase in the mean flow velocity induces higher velocity fluctuations which result in a stronger leading shock ahead of the accelerating deflagration front. (2) An increase in the initial static pressure reduces the critical shock strength that must be exceeded to ensure successful detonation initiation by shock focusing. (3) Since the initial pressure is directly linked to the mass flow rate, these contrary trends can cancel each other out, which could be observed for 40% vol. of oxygen in the oxidizer. High-speed images were taken, which confirm that the detonation is initiated in the center of the converging--diverging nozzle due to focusing of the leading shock.

Effects of double-jet positions on detonation initiation characteristics

In order to initiate the detonation waves efficiently, the ignition method of two jets was proposed and utilized to initiate the mixture of H2/Air in a detonation chamber. The numerical simulation based on H2/Air detailed chemical reaction kinetics mechanism was carried out to study the influence of the two jets arrangement positions on detonation initiation. The reliability of the numerical simulation was verified by the calculation results of CEA and the experimental results. The simulation results indicated that the distance from the first jet to left end of the detonation chamber (L1) and the distance between the two jets (L2) showed significant influence on the detonation initiation process and the distribution of the reflected shock wave. The reflected wave formed by the reflection between the detonation waves from the jet tubes and the wall contributed to the propagation of the flame and the generation of the plane detonation waves. The OH concentration curve and the temperature curve were coupled. The formation and the consumption of OH was one of the main factors which controlling the detonation waves and the temperature behind the detonation waves. When L1 was greater than 10 mm, the velocity of the detonation waves decreased as L1 increased. Furthermore, when L1 and L2 both equaled to 10 mm, the velocity of the detonation wave was maximized.

Numerical investigation on detonation initiation using toroidal shock wave focusing

Shock wave focusing was considered to be a potential alternative method to initiate detonation wave in detonation-based propulsion system. A two-dimensional numerical investigation was carried out for hydrogen-air mixtures. Toroidal shock wave formation and detonation initiation by shock wave focusing were analyzed. Besides, considering the flame accelerating zone width and length are potential geometric factors affecting detonation initiation. Effects of different structural parameters on jet intensity and shock wave focusing detonation initiation process were compared. The numerical results indicated that flame acceleration is the critical factor determining toroidal shock wave formation. One possible mechanism of rapid detonation initiation is that shock waves produced by two explosions can compress premixed gas rapidly, and accelerate the deflagration to detonation transition. In addition, with the increase of flame acceleration zone width, two explosion pressures decrease, as a result, the detonation initiation time and distance increase. When the flame accelerating zone width is 1 mm, the time and distance are 380 μs and 28.85 mm, which are the shortest. With the increase of flame acceleration zone length, two explosion pressures increase, the detonation initiation time and distance decrease. When the flame acceleration zone length is 140 mm, the shortest initiation time and distance are 30 mm and 382 μs respectively.

Effect of actuating voltage and discharge gap on plasma assisted detonation initiation process

The influence of actuating voltage and discharge gap on plasma assisted detonation initiation by alternating current dielectric barrier discharge was studied in detail. A loose coupling method was used to simulate the detonation initiation process of a hydrogen–oxygen mixture in a detonation tube under different actuating voltage amplitudes and discharge gap sizes. Both the discharge products and the detonation forming process assisted by the plasma were analyzed. It was found that the patterns of the temporal and spatial distributions of discharge products in one cycle keep unchanged as changing the two discharge operating parameters. However, the adoption of a higher actuating voltage leads to a higher active species concentration within the discharge zone, and atom H is the most sensitive to the variations of the actuating voltage amplitude among the given species. Adopting a larger discharge gap results in a lower concentration of the active species, and all species have the same sensitivity to the variations of the gap. With respect to the reaction flow of the detonation tube, the corresponding deflagration to detonation transition (DDT) time and distance become slightly longer when a higher actuating voltage is chosen. The acceleration effect of plasma is more prominent with a smaller discharge gap, and the benefit builds gradually throughout the DDT process. Generally, these two control parameters have little effect on the amplitude of the flow field parameters, and they do not alter the combustion degree within the reaction zone.

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.

Effect of actuating frequency on plasma assisted detonation initiation**Project supported by the Open Project of Science and Technology on Scramjet Laboratory, China (Grant No. CG-2014-05-118) and the National Natural Science Foundation of China (Grant No. 91441123).

Aiming at studying the influence of actuating frequency on plasma assisted detonation initiation by alternating current dielectric barrier discharge, a loosely coupled method is used to simulate the detonation initiation process of a hydrogenoxygen mixture in a detonation tube at different actuating frequencies. Both the discharge products and the detonation forming process which is assisted by the plasma are analyzed. It is found that the patterns of the temporal and spatial distributions of discharge products in one cycle are not changed by the actuating frequency. However, the concentration of every species decreases as the actuating frequency rises, and atom O is the most sensitive to this variation, which is related to the decrease of discharge power. With respect to the reaction flow of the detonation tube, the deflagration-todetonation transition (DDT) time and distance both increase as the actuating frequency rises, but the degree of effect on DDT development during flow field evolution is erratic. Generally, the actuating frequency affects none of the amplitude value of the pressure, temperature, species concentration of the flow field, and the combustion degree within the reaction zone.

Stable detonation characteristics of premixed C2H4/O2 gas in narrow gaps

The detonation initiation and propagation characteristics of premixed gas in a confined channel are greatly influenced by some external factors, such as the channel size and the initial pressure. The influences of different gap height and initial pressure of stoichiometric C2H4/O2 combustible premixed gas on the detonation characteristics were investigated using the self-made narrow gaps. The flame propagation processes were captured by the high-speed photography and the detonation trajectories were recorded by the soot-foil technology. The results show that the gap height is found to be proportional to $$P_{0}^{ - 1.493}$$ of the dividing lines, which are used to distinguish the three different detonation states. The detonation initiation process can be accelerated by either decreasing the gap height or increasing the initial pressure. Furthermore, the detonation velocity deficit is closely related to the gap height and the initial pressure, and the relation can be expressed as $$d_{\text{V}} \propto H^{ - 0.8} P_{0}^{ - 0.5}$$ .

副室からの噴射によるデトネーション開始過程

副室からの噴射によるデトネーション開始過程

Numerical study of nonequilibrium plasma assisted detonation initiation in detonation tube

Nonequilibrium plasma has shown great merits in ignition and combustion nowadays, which should be especially useful for hypersonic propulsion. A coaxial electrodes configuration was established to investigate the effect of alternating current (AC) dielectric barrier discharge nonequilibrium plasma on the detonation initiation process in a hydrogen-oxygen mixture. A discharge simulation-combustion simulation loosely coupled method was used to simulate plasma assisted detonation initiation. First, the dielectric barrier discharge in the hydrogen-oxygen mixture driven by an AC voltage was simulated, which takes 17 kinds of particles (including positively charged particles, negatively charged particles, and neutral particles) and 47 reactions into account. The temporal and spatial characteristics of the discharge products were obtained. Then, the discharge products were incorporated into the combustion model of a detonation combustor as the initial conditions for the later detonation initiation simulation. Results showed that the number density distributions of plasma species are different in space and time, and develop highly nonuniformly from high voltage electrode to grounded electrode at certain times. All the active species reach their highest concentration at approximately 0.6T (T denotes a discharge cycle). Compared with the no plasma case, the differences of flowfield shape mainly appear in the early stage of the deflagration to detonation transition process. None of the sub-processes (including the very slow combustion, deflagration, over-driven detonation, detonation decay, and propagation of a self-sustained stable detonation wave) have been removed by the plasma. After the formation of a C-J detonation wave, the whole flowfield remains unchanged. With the help of plasma, the deflagration to detonation transition (DDT) time and distance are reduced by about 11.6% and 12.9%, respectively, which should be attributed to the active particles effect of nonequilibrium plasma and the local turbulent enhancing effect by the spatial characteristics of discharge. In addition, as the duration of forming a shock wave in the combustor is shortened by approximately 8.1%, it can be inferred that the plasma accelerates the DDT process more significantly before the flow becomes supersonic.

Evolution of detonation formation initiated by a spatially distributed, transient energy source

Detonations usually form through either direct initiation or deflagration-to-detonation transition (DDT). In this work, a detonation initiation process is introduced that shows attributes from each of these two processes. Energy is deposited into a finite volume of fluid in an amount of time that is similar to the acoustic time scale of the heated fluid volume. Two-dimensional simulations of the reactive Euler equations are used to solve for the evolving detonation initiation process. The results show behaviour similar to both direct initiation and DDT. Localized reaction transients are shown to be intimately related to the appearance of a detonation. Thermomechanical concepts are used to provide physical interpretations of the computational results in terms of the interaction between compressibility phenomena on the acoustic time scale and localized, spatially resolved, chemical energy addition on a heat-addition time scale.