Observations of the cellular structure of fuel–air detonations

Detonation cell widths, which provide a measure of detonability of a mixture, were measured for hydrocarbon–air and hydrogen–air–diluent mixtures. Results were obtained from a 0.43-m-diameter, 13.1-m-long heated detonation tube with an initial pressure of 101 kPa and an initial temperature between 25 and 100 °C. The cell widths of simple cyclic hydrocarbons are somewhat smaller than those of comparable straight-chain alkanes. Cyclic hydrocarbons tested generally had similar cell sizes despite differences in degree of bond saturation, bond strain energy, oxygen substitution, and chemical structure. There was a significant reduction in the cell width of octane, a straight-chain alkane, when it was mixed with small quantities of hexyl nitrate. The effect of a diluent, such as steam and carbon dioxide, on the cell width of a hydrogen–air mixture is shown over a wide range of mixture stoichiometries. The data illustrate the effects of initial temperature and pressure on the cell width when compared to previous studies. Not only is carbon dioxide more effective than steam at increasing the mixture cell width, but also its effectiveness increases relative to that of steam with increasing concentrations. The detonability limits, which are dependent on the facility geometry and type of initiator used in this study, were measured for fuel-lean and fuel-rich hydrogen–air mixtures and stoichiometric hydrogen–air mixtures diluted with steam. The detonability limits are nominally at the flammability limits for hydrogen–air mixtures. The subcellular structure within a fuel-lean hydrogen–air detonation cell was recorded using a sooted foil. The uniform fine structure of the self-sustained transverse wave and the irregular structure of the overdriven lead shock wave are shown at the triple point path that marks the boundary between detonation cells.     

Direct observations of reaction zone structure in propagating detonations

We report experimental observations of the reaction zone structure of self-sustaining, cellular detonations propagating near the Chapman-Jouguet state in hydrogen-oxygen-argon/nitrogen mixtures. Two-dimensional cross sections perpendicular to the propagation direction were imaged using the technique of planar laser induced fluorescence (PLIF) and, in some cases, compared to simultaneously acquired schlieren images. Images are obtained which clearly show the nature of the disturbances in an intermediate chemical species (OH) created by the variations in the strength of the leading shock front associated with the transverse wave instability of a propagating detonation. The images are compared to 2-D, unsteady simulations with a reduced model of the chemical reaction processes in the hydrogen-oxygen-argon system. We interpret the experimental and numerical images using simple models of the detonation front structure based on the “weak” version of the flow near the triple point or intersection of three shock waves, two of which make up the shock front and the third corresponding to the wave propagating transversely to the front. Both the unsteady simulations and the triple point calculations are consistent with the creation of keystone-shaped regions of low reactivity behind the incident shock near the end of the oscillation cycle within the “cell.”     

Flame propagation regimes and critical conditions for flame acceleration and detonation transition for hydrogen-air mixtures at cryogenic temperatures

A series of more than 100 experiments with hydrogen-air mixtures have been performed at cryogenic temperatures from 90 to 130 K and ambient pressure. A wide range of hydrogen concentrations from 8 to 60%H2 in a shock tube of 5-m long and 54 mm id was tested. Flame propagation regimes were investigated for all hydrogen compositions at three different blockage ratios 0, 30% and 60% as a function of initial temperature. Piezoelectric pressure sensors and InGaAs photo-diodes have been applied to monitor the flame and shock propagation velocity of the combustion process. More than 150 experiments at ambient pressure and temperature were conducted as the reference data for cryogenic experiments. The critical expansion ratio σ1 for an effective flame acceleration to the speed of sound was experimentally found at cryogenic temperatures. The detonability criteria for smooth and obstructed channels were used to evaluate the detonation cell sizes at cryogenic temperatures as well. The main peculiarities of cryogenic combustion with respect to the safety assessment were that the maximum combustion pressure was several times higher and the run-up-distance to detonation was two times shorter compared to ambient temperature independent of lower chemical reactivity at cryogenic conditions.     

双点激光点火直接起爆的数值模拟

针对双点激光点火直接起爆过程中爆轰波的形成、发展和传播问题,采用高精度数值模拟方法求解带化学反应的二维欧拉方程组,研究了不同环境压力情况对流场结构与波系变化的影响.结果表明,环境压力会影响激波强度与爆轰波的传播速度,是决定双点激光点火形成的火核在碰撞过程中能否实现爆轰并维持爆轰波传播的重要因素,利用双激光点相互作用形成...     

Numerical simulation of detonation wave propagation through a rigid permeable barrier

The results of calculation of the detonation propagation in a porous medium for hydrogen-air mixture are presented. The porous medium was specified explicitly and consisted of sets of individual obstacles in the form of solid walls or the sets of finite-size plates. Various modes of detonation propagation depending on obstacle parameters are obtained: propagation in a cellular mode, stationary propagation with destruction of the cellular structure of the detonation front, propagation of a monotonically attenuating detonation wave with destruction of the cellular structure of the front. The possibility of reducing the detonation propagation velocity by replacing solid plates with finite-size ones was shown. The effect of the geometrical parameters of the plates and the step of it installation on the degree of detonation attenuation was estimated. It was determined that an increase in the number of plates leads to a stronger attenuation of the detonation.     

Velocity fluctuation near the detonation limits

In this study, the velocity fluctuation near the detonation limits is investigated experimentally. Five explosive mixtures in five different diameter tubes were used and the choice of the mixtures included those considered as “stable” with regular cellular pattern and “unstable” with highly irregular cellular pattern. Photodiodes spaced at regular intervals along the tube were used to measure the detonation velocity. Piezoelectric transducers were also used to record the pressure profiles. Smoked foils were used to register the cellular detonation structure. Away from the limits, the detonation is found to propagate at a steady velocity throughout the length of the tube and the fluctuations of the local velocity are generally small. For stable mixtures with high argon dilution, the onset of the detonation limits is indicated by an abrupt drop in the detonation velocity to about 0.4V_CJ after a short distance of travel. The detonation may continue to propagate at this low velocity before decaying eventually to a deflagration wave. For deflagrations the optical detector sometimes failed to register a signal due to low luminosity of the front. In unstable mixtures, galloping detonations are observed only in small diameter tubes (e.g., D = 12.7, 3.2 and 1.5 mm). A large number of fairly reproducible cycles of galloping detonations can be observed in very small diameter tubes. In large diameter tubes (e.g., D = 31.7 and 50.8 mm), no galloping detonations are observed in all stable and unstable mixtures. For stable mixtures, no galloping detonations are observed even in small diameter tubes of D = 3.2 and 1.5 mm. Smoked foils records show that the cellular detonation structure changes from multi-headed to single-headed spin as the limit is approached. In a galloping detonation cycle, a decay from multi-headed to single-headed detonation is observed. However, the cellular structure vanishes for further decay of the galloping detonation to the low velocity phase of the galloping cycle. Although galloping detonations could be considered to define the boundary for detonation limits, this definition lacks generality since galloping detonations are not always observed in all mixtures and in all tube diameters. Thus the onset of single-headed spin is perhaps the most appropriate criterion of the detonation limits in tubes.     

Numerical study of acoustic coupling in spinning detonation propagating in a circular tube

Spinning detonations propagating in a circular tube were numerically investigated with a two-step reaction model by Korobeinikov et al. The time evolutions of the simulation results were utilized to reveal the propagation behavior of single-headed spinning detonation. Three distinct propagation modes, steady, unstable, and pulsating modes, are observed in a circular tube. The track angles on a wall were numerically reproduced with various initial pressures and diameters, and the simulated track angles of steady and unstable modes showed good agreement with those of the previous reports. In the case of steady mode, transverse detonation always couples with an acoustic wave at the contact surface of burned and unburned gas and maintains stable rotation without changing the detonation front structure. The detonation velocity maintains almost a CJ value. We analyze the effect of acoustic coupling in the radial direction using the acoustic theory and the extent of Mach leg. Acoustic theory states that in the radial direction transverse wave and Mach leg can rotate in the circumferential direction when Mach number of unburned gas behind the incident shock wave in the transverse detonation attached coordinate is larger than 1.841. Unstable mode shows periodical change in the shock front structure and repeats decoupling and coupling with transverse detonation and acoustic wave. Spinning detonation maintains its propagation with periodic generation of sub-transverse detonation (new reaction front at transverse wave). Corresponding to its cycle, whisker is periodically generated, and complex Mach interaction periodically appears at shock front. Its velocity history shows the fluctuation whose behavior agrees well with that of rapid fluctuation mode by Lee et al. In the case of pulsating mode, as acoustic coupling between transverse detonation and acoustic wave is not satisfied, shock structure of spinning detonation is disturbed, which causes failure of spinning detonation.     

Detonability of natural gas–air mixtures

Direct initiation experiments were carried out in a 105 cm diameter tube to study detonation properties and evaluate the detonability limits for mixtures of natural gas (NG) with air. The natural gas was primarily methane with 1.5–1.7% of ethane. A stoichiometric methane–oxygen mixture contained in a large plastic bag was used as a detonation initiator. Self-supporting detonations with velocities and pressures close to theoretical CJ values were observed in NG–air mixtures containing from 5.3% to 15.6% of NG at atmospheric pressure. These detonability limits are wider than previously measured in smaller channels, and close to the flammability limits. Detonation cell patterns recorded near the limits vary from large cells of the size of the tube to spiral traces of spin detonations. Away from the limits, detonation cell sizes decrease to about 20 cm for 10% NG, and are consistent with existing data for methane–air mixtures obtained in smaller channels. Observed cell patterns are very irregular, and contain secondary cell structures inside primary cells and fine structures inside spin traces.     

Deflagration to detonation transition in aluminum dust-air mixture under weak ignition condition

Deflagration to detonation transition (DDT) in flake aluminum dust-air mixture was studied in a 199 mm inner diameter and 29.6 m long horizontal tube. 40 sets of dust dispersion system were used to disperse flake aluminum into the experimental tube. An electric spark of 40 J was used to ignite the aluminum-air mixture. Self-sustained detonation was observed and the characteristics of deflagration to detonation transition process were studied. Contributed to the transverse wave and cellular structure of detonation wave front in aluminum-air mixture, the propagation velocity of detonation wave ranges from 1480 m/s to 1820 m/s and the maximum overpressure oscillates between 40 and 102 bar. Single head spinning detonation wave in aluminum dust-air mixture was observed and the cell size was evaluated.     

Experimental study on the detonation propagation behaviors through a small-bore orifice plate in hydrogen-air mixtures

In this study, the regimes of detonation transmission through a single orifice plate were investigated systematically in a 6-m length and 90-mm inner diameter round tube. A series of experiments on the detonation propagation mechanisms in hydrogen-air mixtures were performed. A single obstacle with different orifice size (d) from 10 to 60 mm was adopted to study the effects of the induced perturbations on the detonation propagation. Here, the thickness of orifice plate (δ) was fixed at 10.33 mm. Detonation velocity was determined from the time-of-arrival (TOA) of the detonation wave recorded by eight high-speed piezoelectric pressure transducers (PCB102B06). Detonation cellular size was obtained by the smoked foil technique. The characteristic of detonation velocity evolution were quantitatively analyzed after it passes through a single obstacle, and particular attention was paid to the cases for which the blockage ratio (BR) is greater than 0.9, i.e., the cases of small hole diameter of d < 25 mm. The experimental results showed that, in a smooth tube, only super-critical condition and sub-critical condition can be observed. After the orifice plate is introduced into the tube, critical condition occurs. The detonation re-initiation with distinct cellular structures was experimentally observed. Of note is that when the blockage ratio (BR) values in the range of 0.802–0.96, it was easier to detonate at the fuel-lean side. Finally, the critical condition for detonation propagation through an orifice plate was quantified as d/λ > 1 where λ is the detonation cell size.     

Formation and evolution of two-dimensional cellular detonations

This paper reports the results of numerical simulations of cellular detonations generated by using numerical noise as a source of initial fluctuations imposed on a strong planar shock propagating through the reactive medium. The calculations show that a plane detonation wave moving at Chapman-Jouguet (CJ) velocity is unstable to transverse perturbations with wavelength greater than one or two half-reaction-zone lengths. The numerical noise affects the initial cell formation process, but it has no influence on the cell size and regularity of the structures developed. Increasing the activation energy results in more irregular structures characterized by stronger triple points, larger variations of the local shock velocity inside the detonation cell, and higher frequency of appearance and disappearance of triple points. These features of the systems with irregular cellular structures can account for the experimental observation that such systems are less affected by boundary conditions. For the two-dimensional detonation, the average reaction zone is larger and maximum reaction rate is lower than in the one-dimensional case. This means that the formation of detonation cells reduces the maximum entropy production in the reaction zone, and slows down the approach of the system to the equilibrium state. This effect is shown to increase with activation energy due to larger unreacted gas pockets, and deeper penetration of the pockets into the region of mostly burned material.     

Detonation cellular structure in NO2/N2O4-fuel gaseous mixtures

Experimental and numerical studies of the detonation in NO₂-N₂O₄/fuel (H₂, CH₄, and C₂H₆) gaseous mixtures show that for equivalence ratio Φ>0.8-1, (1) the detonation has a double cellular structure, the ratio between the cell size of each net being at least one order of magnitude; (2) inside the detonation reaction zone the chemical energy is released in two successive exothermic steps. Their chemical induction lengths, defined between the leading shock front and each local maximum heat release rate associated with each step, differ by at least one order of magnitude. The chemical reaction NO₂ + H → NO + OH is mainly responsible for the first exothermic step (fast kinetics), NO being the oxidizer on the second one (slow kinetics). Existence of correlations between calculated induction lengths and corresponding cell sizes strengthen the assumption that the cellular structure originates from local strong gradients of chemical heat release inside the detonation reaction zone.     

Propagation process of H2/air rotating detonation wave and influence factors in plane-radial structure

The rotating detonation wave (RDW) propagation processes and influence factors are simulated in the plane-radial structure. The effects of inner radii of curvature, domain widths and stagnation pressures on propagation mode are studied. The RDW is initiated, and two kinds of propagation mode are obtained and analyzed. The flow field structure, parameters variation and influence factors on unstable propagation mode are explored in depth, and the geometrical and injection conditions of the unstable propagation are obtained. Results indicate that the decoupling and re-initiation occur repeatedly during the unstable propagation mode of the RDW, and the angular velocities of leading shock wave vary accordingly. When the domain width remains constant, the range of stagnation-pressure under unstable propagation mode increases as the inner radius increases. But the RDW propagates steadily when the inner radius increases to a certain value (Larger than 40 mm in this study). The effect of curvature radius and initial pressure ahead of detonation wave on the unstable propagation mode in this calculation model is similar to that in a curved channel. When r_i +0.464p_a > 80.932 or r_i ≥ 40 mm, the detonation wave can propagate steadily in the annular domain. When the curvature radius remains constant, the stagnation-pressure range of the unstable propagation mode decreases as the domain width increases.     

The flame propagation characteristics and detonation parameters of ammonia/oxygen in a large-scale horizontal tube: As a carbon-free fuel and hydrogen-energy carrier

As a carbon-free fuel and a hydrogen-energy carrier, ammonia is a potential candidate for future energy utilization. Therefore, in order to promote the application of ammonia in detonation engines and to evaluate the safety of ammonia related industrial process, DDT experiments for ammonia/oxygen mixtures with different ERs were carried out in a large-scale horizontal tube. Moreover, pressure transducers and self-developed temperature sensors were applied to record the overpressure and the instantaneous flame temperature during DDT process. The results show that the DDT process in ammonia/oxygen mixtures contains four stages: Slow propagation stage, Flame and pressure wave acceleration stage, Fast propagation and detonation wave formation stage, Detonation wave self-sustained propagation stage. For stoichiometric ammonia/oxygen mixtures, flame front and the leading shock wave propagate one after another with different velocity, until they closely coupled and propagated together with one steady velocity. At the same time, it is found that an interesting retonation wave propagates backward. The peak overpressure, detonation velocity, and flame temperature of the self-sustained detonation are 2 MPa, 2000 m/s and 3500 K, respectively. With the ER increased from 0.6 to 1.6, the detonation velocities and peak overpressures ranged from 2310 m/s to 2480 m/s and 25.6 bar–28.7 bar, respectively. In addition, the detonation parameters of ammonia were compared with those of methane and hydrogen to evaluate the detonation performance and destructiveness of ammonia.     

Propagation of hydrogen–oxygen flames in Hele-Shaw cells

Flame propagation in Hele-Shaw cells with a micro-sized gap was experimentally investigated. The evolution of flame front morphology was recorded via Schlieren photographs as the hydrogen-oxygen (H₂–O₂) mixture was ignited at ambient temperature and pressure. By varying gap size, two different regimes of flame propagation are identified: 1) the non-accelerating flame in narrow gaps; 2) the self-accelerating flame in relatively wide gaps. For the former, the initial flame front is globally circular, and subsequently evolves into branches separated from the surface, exhibiting dendritic-growth and fingering shapes. In the latter regimes, the flame front exhibits a cellular structure and accelerates nearly sonic speed due to hydrodynamic instabilities. It is found that the flame acceleration depends non-monotonically on the gap size due to the competing mechanisms of viscosity friction and heat loss through the walls. The effect of equivalence ratio on the non-accelerating flame is studied to identify the mechanism controlling the local extinction flame.     

The evolution and cellular structure of a detonation subsequent to a head-on interaction with a shock wave

This paper analyzes the results of a head-on collision between a detonation and a planar shock wave. The evolution of the detonation cellular structure subsequent to the frontal collision was examined through smoked foil experiments. It is shown that a large reduction in cell size is observed following the frontal collision, and that the detonation cell widths are correlated well with the chemical kinetic calculations from the ZND model. From chemical kinetic calculations, the density increase caused by shock compression appears to be the main factor leading to the significant reduction in cell size. It was found that depending on the initial conditions, the transition to the final cellular pattern can be either smooth or spotty. This phenomenon appears to be equivalent to Oppenheim's strong and mild reflected shock ignition experiments. The difference between these two transitions is, however, more related to the stability of the incident detonation and the strength of the perturbation generated by the incident shock.     

Effect of orifice plate on the transmission mechanism of a detonation wave in hydrogen-oxygen mixtures

In this paper, a square orifice plate with 60 mm thick and the blockage ratio (BR) of 0.889 is employed to systematically explore the transmission regime of a steady detonation wave in hydrogen-oxygen mixtures. The influence of hydrogen mole fraction is also considered. The average velocity of combustion wave can be determined by evenly mounting eight high-speed pressure sensors on the tube wall, and the detonation cellular patterns can be also registered by the soot foil technique. The experimental results indicate that for the condition of smooth tube, the hydrogen concentration limits range of detonation successful propagation is 37.5%–73.68%. Two propagation modes can be obtained, i.e., the regimes of fast flame and steady detonation. The hydrogen concentration limits range is narrowed to 42.53%–69.51% in the tube with a square orifice plate. Three propagation regimes are observed: (1) near the low limit, a steady detonation wave can be produced before the obstacle, and the phenomenon of detonation decay is seen across the square orifice plate because of the influence of diffraction resulting in the mechanism of detonation failure. The failed detonation wave is not re-ignited because of the lower hydrogen concentration; (2) as the hydrogen mole fraction is increased to 42.53%, the mechanism of detonation re-ignition can be seen after the detonation decay. Well within the limits, the same detonation re-initiation phenomenon also can be observed; (3) as the hydrogen concentration is further enhanced to 69.7% beyond the upper limit, a stable detonation wave is not produced prior to the orifice plate, and the combustion wave front maintain the mode of fast flame until the end of the channel. Finally, it can be found that the detonation wave can successfully survive from the diffraction only when the effective diameter (d_eff) is at least greater than one cell size (λ).     

Effects of concentration gradient on pulsating and cellular detonations

In the field of explosion accidents and the combustion chamber of propulsion systems, gaseous mixtures are more likely to be highly non-uniform and a detonation usually propagates in a non-homogeneous medium. In this paper, one-dimensional (1D) pulsating detonations propagating in non-homogeneous medium with concentration gradient are studied by direct numerical simulation with a detailed chemistry. It is found that a periodically pulsating detonation can adjust to adapt to the environment with concentration gradient. The superficial concentration gradient can improve survivability of 1D detonation under highly unstable case, widening detonability limit corresponding to equivalence ratio; while the steep concentration gradient will quickly extinguish the detonation. The extend of detonability limit led by the inhomogeneity is reinforced by the presence of transverse wave in 2D detonations. The 2D detonation in the gradient has better viability compared with the 1D due to the coupling interaction of leading shock and transverse wave with the gradient and the transportation of H₂ and H along the gradient. This study also demonstrates that the simulations of detonations in the gradient requires detailed chemical mechanism.     

Effect of obstacles on the detonation diffraction and subsequent re-initiation

The DCRFoam solver (density-based compressible solver) built on the OpenFOAM platform is used to simulate the reflection and diffraction processes that occur when detonation waves collide with various objects. Static stoichiometric hydrogen–oxygen mixtures diluted with 70% Ar are used to form stable detonation waves with large cells, with initial conditions of 6.67 kPa pressure and 298 K temperature. The diameters of the cylindrical obstacle range from 6 mm to 22 mm, with x = 230 mm, x = 244 mm, and x = 257 mm being the chosen position. Cylindrical, square, triangular, and inverted triangular obstacles are used, and the quenched detonation re-initiation processes behind them are investigated. In the detonation diffraction process, four triple points exist at the same time due to the effect of cylindrical obstacles of smaller diameters. The re-initiation distance of the detonation wave increases with the increase of cylindrical obstacle diameter. Both the Mach reflection angle and the decoupled angle decrease as the diameter increases. When the location of the cylindrical obstacles is changed, the detonation wave dashes into the obstacles with its different front structures, it is easier to realize the detonation re-initiation when the weak incident shock at the front of a detonation wave strikes the obstacles, and the re-initiation distance decreases by 17.1% when compared with the longest re-initiation distance. The detonation re-initiation distance is shortest under the action of cylindrical obstacles, however the quenched detonation cannot be re-initiated when the inverted triangle and square obstacles are used. The suppression effects of inverted triangle and square obstacles on detonation waves are more evident.     

Suppression of hydrogen–air detonation using porous materials in the channels of different cross section

Propagation of a detonation wave in a porous channel with different cross–section was experimentally studied. Experiments were performed in three rectangular channels with cross–sectional dimensions of 20 × 40 mm, 10 × 40 mm and 10 × 30 mm with two opposite walls covered with porous material to study the detonation suppression in stoichiometric hydrogen–air mixtures at atmospheric pressure. Detonation was initiated in 3000 mm long circular channel 20 mm in diameter. Porous material was covering 1/2 or 1/3 of the channel internal surface. Polyurethane foam with a number of pores per inch ranging from 10 to 80 was used for detonation attenuation. Piezoelectric pressure sensors were used to obtain the shock wave pressure. Detonation decay into the shock wave and the flame front was visualized using schlieren photography. Shock wave velocity was also calculated using high–speed schlieren image sequences. The strongest pressure attenuation was recorded in a 10 mm wide channel with a porous coating with largest pores (2.5 mm) covering 1/3 of the internal walls. The results indicate that even covering 1/3 of the internal surface of the channel leads to detonation decay and significant shock wave attenuation.