Deflagration-to-detonation transition in nitromethane mist/aluminum dust/air mixtures

Deflagration-to-detonation transitions (DDTs) in nitromethane mist/air mixture clouds, flaked aluminum dust/air mixture clouds, and nitromethane mist/flaked aluminum dust/air mixture clouds have been studied in an experimental tube of inner diameter 199 mm and length 29.6 m. The mist/dust/air multiphase mixtures were generated by injecting liquid/dust samples into the experimental tube. The droplet dimensions of the nitromethane mist cloud were analyzed, calculated, and measured. The multiphase fuel/air mixture clouds were ignited by means of an electric spark. The characteristics of different stages during the DDT process in the clouds of the various mixtures in the experimental tube have been studied and analyzed. A self-sustained quasi-detonation wave formed in the nitromethane mist/air mixture clouds, while a self-sustained detonation wave formed in the flaked aluminum dust/air mixture clouds and in the nitromethane mist/flaked aluminum dust/air mixture clouds. The detonation parameters of the clouds of the different mixtures have been studied and compared.     

High speed turbulent deflagrations and transition to detonation in H2air mixtures

An experimental investigation on flame acceleration and transition to detonation in H₂air mixtures has been carried out in a tube which had a 5 cm cross-sectional diameter and was 11 m long. Obstacles in the form of a spiral coil (6 mm diameter tubing, pitch 5 cm, blockage ratio BR = 0.44) and repeated orifice plates spaced 5 cm apart with blockage ratios of BR = 0.44 and 0.6 were used. The obstacle section was 3 m long. The compositional range of H₂ in air extended from 10 to 45%, the initial pressure of the experiment was 1 atm, and the mixture was at room temperature. The results indicate that steady-state flame (or detonation) speeds are attained over a flame travel of 10–40 tube diameters. For H₂ ? 13% maximum flame speeds are subsonic, typically below 200 m/s. A sharp transition occurs at about 13% H₂ when the flame speed reaches supersonic values. A second transition to the so-called quasi-detonation regime occurs near the stoichiometric composition when the flame speed reaches a critical value of the order of 800 m/s. The maximum value of the averaged pressure is found to be between the normal C-J detonation pressure and the constant volume explosion value. Of particular interest is the observation that at a critical composition of about 17% H₂ transition to normal C-J detonation occurs when the flame exits into the smooth obstacle-free portion of the tube. For compositions below 17% H₂, the high speed turbulent deflagration is observed to decay in this portion of the tube. The detonation cell size for 17% H₂ is about 150 mm and corresponds closely to the value of πD that has been proposed to designate the onset of single-head spinning detonation, in this case for the 5 cm diameter tube used. This supports the limit criterion, namely, that for confined detonations in tubes, the onset of single-head spin gives the limiting composition for stable propagation of a detonation wave.     

Experimental study on transmission of an overdriven detonation wave from propane/oxygen to propane/air

Two sets of experiments were performed to achieve a strong overdriven state in a weaker mixture by propagating an overdriven detonation wave via a deflagration-to-detonation transition (DDT) process. First, preliminary experiments with a propane/oxygen mixture were used to evaluate the attenuation of the overdriven detonation wave in the DDT process. Next, experiments were performed wherein a propane/oxygen mixture was separated from a propane/air mixture by a thin diaphragm to observe the transmission of an overdriven detonation wave. Based on the characteristic relations, a simple wave intersection model was used to calculate the state of the transmitted detonation wave. The results showed that a rarefaction effect must be included to ensure that there is no overestimate of the post-transmission wave properties when the incident detonation wave is overdriven. The strength of the incident overdriven detonation wave plays an important role in the wave transmission process. The experimental results showed that a transmitted overdriven detonation wave occurs instantaneously with a strong incident overdriven detonation wave. The near-CJ state of the incident wave leads to a transmitted shock wave, and then the transition to the overdriven detonation wave occurs downstream. The attenuation process for the overdriven detonation wave decaying to a near-CJ state occurs in all tests. After the attenuation process, an unstable detonation wave was observed in most tests. This may be attributed to the increase in the cell width in the attenuation process that exceeds the detonability cell width limit.     

Transition to detonation in a hydrogen-air mixtures due to shock wave focusing in the 90 - deg corner

This work presents experimental observation of the ignition modes due to shock wave focusing in a 90 - deg corner in a mixture of 20%–55% H₂ in air with the main purpose of recognizing critical conditions for transition to detonation. The results showed three ignition modes, first ‘weak’ ignition followed by deflagration with ignition delay time higher than ∼1 μs, second ‘strong’ with instantaneous transition to detonation, and third with deflagrative ignition and delayed transition to detonation. The transition was observed only when specific shock wave velocity was reached. The transition velocity for stoichiometric mixture was approx. 715 m/s corresponding to M = 1.75 and 71% of speed of sound in products. For leaner or richer mixtures, the transition velocity increased approaching the speed of sound in products at approx. 18% and 58% H₂.     

Experimental evaluation of aluminum powder fuel in a hydrogen/oxygen detonation tube

Solid powder detonation technology was investigated in this study. Several problems related to the controllable and uniform injection of solid powder fuels were examined. This study demonstrated the feasibility of solid powder detonation technology by designing fluidization devices and evaluating the solid powder supply system. A stable supply of powder was achieved, and ignition tests were conducted using Al/H₂/O₂ and H₂/O₂ fuels. The maximum detonation wave velocity obtained using Al/H₂/O₂ was 2950 m/s, which was 40% higher than that obtained using H₂/O₂. We observed a mixed two-phase double-front detonation structure, with hydrogen/oxygen being detonated in the first front and aluminum powder/oxygen being detonated in the second wave. At a relatively low equivalence ratio, the aluminum powder/oxygen wave surface pressure was higher, whereas the hydrogen/oxygen wave surface pressure was higher at a higher equivalence.     

Electrochemical hydrogen and electricity production by using anodes made of commercial aluminum

Two commercial aluminum alloys have been studied as anodes for simultaneous electrochemical hydrogen and electricity production, as compared with pure aluminum. Both are very active in corrosive hydrogen production and functional as anodes in aluminum-air batteries. Both produced approximately 3 times more hydrogen than pure aluminum while their electrochemical characteristics were substantially preserved with only small modifications with respect to pure aluminum. Devices employing commercial aluminum anodes can be constructed and may be used for producing both hydrogen and electricity at the expense of aluminum.     

Mitigation of H2/air gaseous detonation via utilization of PAN-based carbon fibre felt

This paper first studied the mitigation of the hydrogen/air mixture detonation wave by tuning the tube inner wall with the absorbing material of polyacrylonitrile (PAN)-based carbon fibre felt. The experimental tests were performed in a single-trial circular cross-section tube filled with premixed hydrogen and air detonative mixture. The pressure values and flame front propagation were measured by means of pressure transducers and photodiodes respectively. The attenuation regimes of detonation wave in walled tubes with different thicknesses and layouts of absorption material were compared. The PAN-based carbon fibre felt makes a significant attenuation on the detonation propagation. The decoupling of leading shock wave and flame front can be observed under the effect of this absorbing layer. The ultimate strength close to the tube end and propagation velocity of the combustion wave decrease with the increase of felt thickness. When the interval layout felt is adopted, the spacing distance has almost no impact on the attenuation effectiveness. When the sectional layout is adopted, the effectiveness of detonation mitigation is however improved for a higher proportion of the absorbing material.     

负温度梯度对预混气体反应波传播模态的影响

基于一维数值模拟研究了负温度梯度对氢气/空气混合气反应波传播模态的影响,重点论述燃烧模态的转变机理.研究表明:当反应波沿着负温度梯度传播时,可以观测到超声速爆燃向爆震、爆震向亚声速爆燃的转变;其中,超声速爆燃传播过程由顺序自燃控制;当其传播速度低于当地Chapman-Jouguet爆震速度时,会发生超声速爆燃向爆震的转变.此外,在低温区域也观测到了爆震向亚声速爆燃的转变.Zel’dovich(ZND)结构和RankineHugoniot曲线表明,诱导激波后混合气反应活性对维持爆震传播具有重要作用.     

Dynamic response and crack propagation of pre-flawed square tube under internal hydrogen-oxygen detonation

With a validated fluid-structure-fracture coupling approach, this paper studied the dynamic response and crack propagation of pre-flawed square tube under internal hydrogen-oxygen detonation. Fracture of tube was judged by a bivariate failure criterion derived from the underlying failure mechanism at high strain rate conditions. A programed burn approach based on the CJ theory was applied to simulate gaseous detonation. The coupling between detonation wave and tube was realized by penalty contact algorithm with an improved contact stiffness calculation formula. It was demonstrated that the peak pressure at tube edge is 29% higher than that at the middle of tube face. The dominant crack driving force comes from the specific vibration and deformation modes of square tube, where the deformed round section of tube corresponds to the maximum stress wave that travels behind the flexural waves on the tube. Above mechanism makes the backward cracks branch or turn before the forward cracks and the speeds of front and back branch cracks comparable to each other, which is opposite or different from the cases of round tubes. The crack behaviors with different initial flaw locations and detonation pressures were summarized and identified in detail. The forward crack speed can be up to 900 m/s, while the backward crack speeds are generally 65%–85% of above and the branch cracks run at about 100 m/s. In addition, the crack speed has a certain increase immediately after crack branching or turning. Among the three initial flaw location cases, the tube with initial flaw at the middle of face is most resistant to crack propagation under internal detonations.     

Sensitization of pentane-oxygen mixtures to DDT via cool flame oxidation

The effect of cool flame partial oxidation on the detonation sensitivity of a hydrocarbon fuel was investigated experimentally. The detonation sensitivity was quantified by measuring the run-up distance required for a deflagration to transit to a detonation wave (DDT) in a rough tube. Fuel rich pentane-oxygen mixtures at sub-atmospheric initial pressures were studied. Subsequent to the injection of the mixture into a heated detonation tube, the mixture underwent cool flame oxidation after a well-controlled delay time, dependent on the temperature of the tube. Typical delays ranged from 0.5 to 2 s (depending on temperature) and were reproducible to within one hundred milliseconds. This delay permitted the mixture in the detonation tube to be spark-ignited at various stages of the cool flame process using an igniter driven by a delay generator. The results show that increasing mixture temperature from room temperature to values below the cool flame region (below 250°C) resulted in an increase in run-up distance. However, as the mixture began to undergo cool flame oxidization, a significant reduction in the run-up distance was obtained (as large as 50%). The sensitization effect was found to occur only at the initial stage of the cool flame oxidation reaction. If the mixture was ignited at times long after the onset of cool flame, the mixture was found to be desensitized and the run-up distance increased. The sensitizing effect of the cool flame partial oxidation may be attributed to the presence of peroxides and free radicals associated with the initial cool flame process. However, these radical species are consumed as the cool flame reaction proceeds and the mixture becomes insensitive again.     

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.     

The effect of a shock wave on the ignition behavior of aluminum particles in a shock tube

The effect of a shock wave on the ignition behavior of 5 μm aluminum (Al) particles was studied in a series of experiments by means of a horizontal shock tube with an inner diameter of 70 mm. To isolate the shock effect from other effects, the experiments were conducted in an inert argon (Ar) atmosphere in addition to a few control experiments in air. The use of Ar as driven gas also helps to produce strong shocks. Every aluminum particle is initially covered with a layer of amorphous aluminum oxide (Al₂O₃). The Al₂O₃ passivates the particle, thus playing a key role in the ignition and combustion mechanisms of an Al particle. The experiments showed a strong emission of light originating from the particles immediately after the shock wave has passed them. Spectral analysis revealed strong AlO bands even in experiments in which the volatilization temperature of Al₂O₃ was not exceeded. The emission spectrum of the flame permits the determination of a grey-body temperature. The existence of AlO molecules and the analysis of samples taken after an experiment give a strong evidence of the influence of a shock wave on the ignition and reaction mechanism of Al particle combustion.     

The propagation mechanism of detonation wave in a round tube filled with larger blockage ratio orifice plates

In this study, the detonation propagation mechanisms for the stoichiometric hydrogen-oxygen mixture are explored systematically in a circular tube with 6-m in length and an inner diameter of 90-mm. The continuous orifice plates with BR = 0.93 are adopted to investigate the characteristics of detonation diffraction, failure and initiation. High-speed piezoelectric pressure transducers are used to obtain the average velocity, and the smoked foil technique is adopted to record the detonation cellular patterns. The results indicate that three various propagation regimes can be observed, i.e., steady detonation, quasi-detonation and fast flame. In the smooth tube, only the steady detonation and fast flame modes are seen. When the initial pressure is greater than the critical value, the detonation can propagate at about the theoretical CJ velocity. Near the critical pressure, a sudden velocity drop is observed. Of note is that the single-headed spin and double-headed detonation cannot occur because of the limitation of the aspect ratio. In the tube filled with obstacles, the averaged wave velocity is decayed severely. Only the mechanisms of the quasi-detonation and fast flame can be seen. In the quasi-detonation mode, the critical value of d/λ is greater than 7.36, which is far larger than 1. Two different detonation ignition regimes produced by the shock reflection from the wall are observed, i.e., the initiation positions occur in the vicinity of the tube wall and the surface of the orifice plate.     

Numerical simulation of laser-induced detonation in mixture of hydrogen with suspended metal particles

Simulation of interaction of laser pulse with gas–particle mixtures plays an important role in environmental and engineering applications. The injection of metal particles with low evaporation temperature and ionization potential causes optical breakdown on individual particle, and leads to drop of detonation minimum pulse energy in the mixture. The mathematical models and numerical methodology for computer modelling of optical breakdown and laser-induced detonation in gas–particle mixture are developed. Sub-models of optical breakdown on metal particle include heating of particle to boiling temperature, formation of vapour aureole around the particle, ionization of vapour aureole and development of electron avalanche, appearance of micro-plasma spots and their expansion, propagation of shock wave in the volume occupied by the particle. Laser-induced detonation in the mixture of hydrogen with flake aluminium particles is simulated based on Eulerian approach, and minimum pure energy of detonation is calculated for different parameters of laser pulse, mass fractions of particles and compositions of gas mixture.     

Experimental investigation of deflagration to detonation transition in hydrocarbon-air gaseous mixtures

The paper presents the results of investigation of deflagration to detonation transition in gas mixtures with exothermic chemical reaction using the experimental method of nonintrusive diagnostics of the process. Schlieren photochronography in the optical sections in different places of the tube is performed using the laser as a source of light. Experimental results of visualization of the transition process in hydrocarbon-air gas mixtures show several different flow patterns: (1) The detonation wave originates in the flame zone. (2) The detonation wave originates between the flame zone and primary shock wave. (3) The secondary combustion zone originates between primary shock and the flame and causes the detonation. (4) Spontaneous flame occurs that leads to the combustion to detonation transition. The influence of the flame zone on the originating strong detonation wave is noticed.     

Mechanism of indirect initiation of detonation

Based on the first-order Arrhenius kinetics of chemical reaction and hydrodynamics, we proposed a mechanism to interpret the physical process of detonation onset. In the proposed mechanism, all the movements of chemical mixture are described by the characteristic waves of hyperbolic system. Each wave in different manner contributes to the transition from deflagration to detonation. The triggered detonation is the result of interaction of the multiple waves, more accurately, is a direct result of the re-ignition in the gaseous explosive in the unreacted zone by the reaction-released energy that is transferred in the form of the characteristic wave. This mechanism provides a complete and theoretic explanation to “explosion in the explosion” observed in experiments. It associated with the traditional ignition theory may be used to build up the criterion for deflagration-to-detonation transition (DDT). The mechanism is further verified by our numerical solutions to the mathematic model.     

Experimental study of hydrogen production process with aluminum and water

This study reported a novel hydrogen production experimental set up, which utilizes the chemical reaction between aluminum and water to produce hydrogen. The developed experimental setup had an aluminum powder spraying subsystem integrated within the overall setup. The effectiveness of this hydrogen production experimental set up was improved using 149-μm aluminum powder, and nitrogen gas as the medium to facilitate the spraying of the aluminum powder. Furthermore, the study utilized sodium hydroxide as the reaction promoter. The various experimental conditions implemented during the testing process included changes in the water temperature and system inputs. The criteria used to evaluate the system performance were the hydrogen yield and hydrogen production rate. The tap water was able to achieve a full hydrogen yield due to its composition, however, the 50% increase in NaOH mass trial was able to achieve a higher yield of 97.15% and 95.44% for the 3g and 6g aluminum sample test respectively. Furthermore, seawater was found to achieve a yield of 58.8%, which can be considered a viable option for future testing. Furthermore, seawater's abundance also adds to its viability for future testing. Also, the study results showed that an increase in reaction temperature best facilitates a chemical reaction taking place. This was evident during the staring temperature of the water test for the 6g aluminum samples. For instance, the maximum hydrogen production rate for the 70 °C was 35.04 mL/s, while the smallest peak for hydrogen production rate was observed using the 40 °C as the starting temperature. The 40 °C test produced a maximum hydrogen production rate value of 27.99 mL/s.     

Shock-induced ignition with single step Arrhenius kinetics

Shock-initiated ignition is studied numerically for single step Arrhenius kinetics. This is relevant to hydrogen safety, because hydrogen detonates easily, and detonation in shocked mixture may occur in deflagration to detonation transition scenarios, due to shock reflections on obstacles subsequent to flame acceleration. Simulation of ignition behind a shock moving into combustible mixture is difficult because of the singular nature of the initial conditions. The solution method includes two components. First, space as an independent variable is replaced by the ratio space over time, and second, initial conditions at a small non-zero time are used, obtained in closed form from short time asymptotics. This way, the initial singularity is effectively removed and the early process is well resolved. This method was used to study how the leading shock strength, and the heat release, affects the shock-initiated ignition process. The Essentially Non-Oscillatory algorithm used captures ignition, hot spot growth, birth of a secondary shock and transition into a detonation. Results show that for weaker shock cases as well as for higher heat release the evolution is more rapid, that a secondary shock forms closer to the contact surface and quickly becomes a detonation wave.     

Acceleration of laminar hydrogen/oxygen flames in a tube and the possible onset of detonation

The possibility is analysed of a laminar flame accelerating along a cylindrical tube, closed at one end, and inducing a deflagration to detonation transition in a stoichiometric H₂/O₂ mixture. The pressure and temperature ratios at the ensuing shock wave increase, as do laminar burning velocities, while autoignition delay times decrease. Combined with appreciable elongation of the flame, these enhance the strength of the shock. The conditions necessary for delay times of 0.05, 0.1, 1.0 and 5.0 ms, at an unburned mixture critical Reynolds number of 2300, are computed for different tube diameters. Probable consequences of the different delay times and hot spot reactivity gradients, including detonation, are all considered. The probability of a purely laminar propagation leading to a detonation is marginal. Only when the initial temperature is raised to 375 K, do purely laminar detonations become possible in tubes of between about 0.5 and 1.35 mm diameter.     

Oblique detonation wave stability around a spherical projectile by a high time resolution optical observation

Spherical projectiles were launched into detonable mixtures over a wide range of projectile velocities from near to about 1.8 times the Chapman–Jouguet (C–J) velocity. Oblique detonation waves (ODWs) and shock-induced combustions (SICs) stabilized around the projectiles were visualized with high time and high spatial resolutions using the Schlieren technique and a high-speed camera with a 1-μs frame speed. Unsteady wave structures called Straw Hat type structures consisting of a SIC region followed by a C–J ODW were observed near stabilizing criticalities of a C–J ODW, and they were divided into two propagation types, depending on whether the C–J ODW could be stabilized [11], [12], [14]. In the present study, we suggested wave structures of the Straw Hat types based on our examination of dozens of continuous images. Triple points were observed at the intersection of a bow shock, a C–J ODW and a transverse detonation or shock wave when projectile velocities were slightly higher than C–J velocities. Onsets of local explosions in the SIC region for stabilizing the ODW in the Straw Hat type structures have been reported [14]. We observed this stabilizing mechanism by visualizing onsets of periodical local explosions and their transition to spherical detonation waves when the projectile velocity was much higher than the C–J velocity. We also determined stabilizing criticalities using a stoichiometric acetylene-oxygen mixture diluted with argon or krypton in 50% or 75% volumetric fractions, respectively. We found that the stabilizing criticalities did not depend only on the ratio of the projectile diameter and the cell size of the mixture.