Deflagration to detonation transition in weakly confined conditions for a type of potentially novel green primary explosive:Al/Fe_(2)O_(3)/RDX hybrid nanocomposites

The properties of the combustion and deflagration to detonation transition(DDT)of Al/Fe_(2)O_(3)/RDX hybrid nanocomposites,a type of potentially novel lead-free primary explosives,were tested in weakly confined conditions,and the interaction of Al/Fe_(2)O_(3)nanothermite and RDX in the DDT process was studied in detail.Results show that the amount of the Al/Fe_(2)O_(3)nanothermite has a great effect on the DDT properties of Al/Fe_(2)O_(3)/RDX nanocomposites.The addition of Al/Fe_(2)O_(3)nanothermite to RDX apparently improves the firing properties of RDX.A small amount of Al/Fe_(2)O_(3)nanothermite greatly increases the initial combustion velocity of Al/Fe_(2)O_(3)/RDX nanocomposites,accelerating their DDT process.When the contents of Al/Fe_(2)O_(3)nanothermite are less than 20 wt%,the DDT mechanisms of Al/Fe_(2)O_(3)/RDX nanocomposites follow the distinct abrupt mode,and are consistent with that of RDX,though their DDT processes are different.The RDX added into the Al/Fe_(2)O_(3)nanothermite increases the latter's peak combustion velocity and makes it generate the DDT when the RDX content is at least 10 wt%.RDX plays a key role in the shock compressive combustion,the formation and the properties of the DDT in the flame propagation of nanocomposites.Compared with RDX,the fast DDT of Al/Fe_(2)O_(3)/RDX nanocomposites could be obtained by adjusting the chemical constituents of nanocomposites.

Theoretical analysis on deflagration-to-detonation transition

The study on deflagration-to-detonation transition （DDT） is very important because this mechanism has relevance to safety issues in industries, where combustible premixed gases are in general use. However, the quantitative prediction of DDT is one of the major unsolved problems in combustion and detonation theory to date. In this paper, the DDT process is studied theoretically and the critical condition is given by a concise theoretical expression. The results show that a deflagration wave propagating with about 60% Chapman-Jouguet （C J） detonation velocity is a critical condition. This velocity is the maximum propagating velocity of a deflagration wave and almost equal to the sound speed of combustion products. When this critical condition is reached, a CJ detonation is triggered immediately. This is the quantitative criteria of the DDT process.

Effects of heat loss and viscosity friction at walls on flame acceleration and deflagration to detonation transition

The coupled effect of wall heat loss and viscosity friction on flame propagation and deflagration to detonation transition(DDT) in micro-scale channel is investigated by high-resolution numerical simulations.The results show that when the heat loss at walls is considered, the oscillating flame presents a reciprocating motion of the flame front.The channel width and Boit number are varied to understand the effect of heat loss on the oscillating flame and DDT.It is found that the oscillating propagation is determined by the competition between wall heat loss and viscous friction.The flame retreat is led by the adverse pressure gradient caused by thermal contraction, while it is inhibited by the viscous effects of wall friction and flame boundary layer.The adverse pressure gradient formed in front of a flame, caused by the heat loss and thermal contraction, is the main reason for the flame retreat.Furthermore, the oscillating flame can develop to a detonation due to the pressure rise by thermal expansion and wall friction.The transition to detonation depends non-monotonically on the channel width.

Critical deflagration waves leading to detonation onset under different boundary conditions

High-speed turbulent critical deflagration waves before detonation onset in H2–air mixture propagated into a square cross section channel, which was assembled of optional rigid rough, rigid smooth, or flexible walls. The corresponding propagation characteristic and the influence of the wall boundaries on the propagation were investigated via high-speed shadowgraph and a high-frequency pressure sampling system. As a comprehensive supplement to the different walls effect investigation, the effect of porous absorbing walls on the detonation propagation was also investigated via smoke foils and the high-frequency pressure sampling system. Results are as follows. In the critical deflagration stage, the leading shock and the closely following turbulent flame front travel at a speed of nearly half the CJ detonation velocity. In the preheated zone, a zonary flame arises from the overlapping part of the boundary layer and the pressure waves, and then merges into the mainstream flame. Among these wall boundary conditions, the rigid rough wall plays a most positive role in the formation of the zonary flame and thus accelerates the transition of the deflagration to detonation（DDT）, which is due to the boost of the boundary layer growth and the pressure wave reflection. Even though the flexible wall is not conducive to the pressure wave reflection, it brings out a faster boundary layer growth, which plays a more significant role in the zonary flame formation. Additionally, the porous absorbing wall absorbs the transverse wave and yields detonation decay and velocity deficit. After the absorbing wall, below some low initial pressure conditions, no re-initiation occurs and the deflagration propagates in critical deflagration for a relatively long distance.

Deflagration and detonation induced by shock wave focusing at different Mach numbers

Shock wave focusing is an effective way to create a hot spot or a high-pressure and hightemperature region at a certain place,showing its unique usage in detonation initiation,which is beneficial for the development of detonation-based engines.The flame propagation behavior after the autoignition induced by shock wave focusing is crucial to the formation and self-sustaining of the detonation wave.In this study,wedge reflectors with two different angles(60°and 90°)and a planar reflector are employed,and the Mach number of incident shock waves ranging from 2.0 to 2.8 is utilized to trigger different flame propagation modes.Dynamic pressure transducers and the high-speed schlieren imaging system are both employed to investigate the shock-shock collision and ignition procedure.The results reveal a total of four flame propagation modes:deflagration,DDT(Deflagration-to-Detonation Transition),unsteady detonation,and direct detonation.The detonation wave formed in the DDT and unsteady detonation mode is only approximately 75%-85%of the Chapman-Jouguet(C-J)speed;meanwhile,the directly induced detonation wave speed is close to the C-J speed.Transverse waves,which are strong evidence for the existence of detonation waves,are discovered in experiments.The usage of wedge reflectors significantly reduces the initial pressure difference ratio needed for direct detonation ignition.We also provide a practical method for differentiating between detonation and deflagration modes,which involves contrasting the speed of the reflected shock wave with the speed of the theoretically nonreactive reflected shock wave.These findings should serve as a reference for the detonation initiation technique in advanced detonation propulsion engines.

Ultrafast one-step synthesis of N and Ti^3＋ codoped TiO2 nanosheets via energetic material deflagration

An energetic-material （NAN3） deflagration method for preparing N- and Ti3＋-codoped TiO2 nanosheets （NT-TiO2） was developed. In this method, N radicals filled the crystal lattice, and Na clusters captured partial O from TiO2. The deflagration process was fast and facile and can be completed within 〈 I s after ignition. The obtained NT-TiO2 exhibited rough surfaces with nanopits and nanoholes. The doping concentration can be regulated by controlling the NaN3 addition. The NT-TiO2 samples showed significant enhancements in the visible-light absorption and photoelectric response. The simultaneously produced N radicals and Na clusters from NaN3 deflagration served as N sources and reduction agents, respectively. Additionally, the high deflagration temperature/ pressure improved the reactivity of N radicals and Na dusters. Thus, the present NaN3 deflagration method was demonstrated as an ultrafast and effective approach to fabricate NT-TiO2 with a visible-light response. The proposed NaN3 deflagration method allows the ultrafast synthesis of new functional materials via the efficient deflagration of energetic materials.

Detonation initiation developing from the Richtmyer-Meshkov instability

Detonation initiation resulting from the Richtmyer-Meshkov instability is investigated numerically in the configuration of the shock/spark-induced-deflagration interaction in a combustive gas mixture. Two-dimensional multi-species Navier-Stokes equations implemented with the detailed chemical reaction model are solved with the dispersion-controlled dissipative scheme. Numerical results show that the spark can create a blast wave and ignite deflagrations. Then, the deflagration waves are enhanced due to the Richtmyer-Meshkov instability, which provides detonation initiations with local environment conditions. By examining the deflagration fronts, two kinds of the initiation mechanisms are identified. One is referred to as the deflagration front acceleration with the help of the weak shock wave, occurring on the convex surfaces, and the other is the hot spot explosion deriving from the deflagration front focusing, occurring on the concave surfaces.

THE GENERALIZED RIEMANN PROBLEM FOR A SCALAR NONCONVEX COMBUSTION MODEL-THE PERTURBATION ON INITIAL BINDING ENERGY

In this article, we study the generalized Riemann problem for a scalar non- convex Chapman-Jouguet combustion model in a neighborhood of the origin （t 〉 0） on the （x, t） plane. We focus our attention to the perturbation on initial binding energy. The solutions are obtained constructively under the entropy conditions. It can be found that the solutions are essentially different from the corresponding Riemann solutions for some cases. Especially, two important phenomena are observed： the transition from detonation to deflagration followed by a shock, which appears in the numerical simulations [7, 27]; the transition from deflagration to detonation （DDT）, which is one of the core problems in gas dynamic combustion.

Riemann problem of a Chapman-Jouguet combustion model for pressure-gradient system

In this paper,the Riemann problem of a Chapman-Jouguet combustion model for the pressure-gradient equations is considered.By analyzing in phase space,existence and uniqueness of the solution to the Riemann problem are obtained constructively under the global entropy conditions.

Generalized Riemann problem for gas dynamic combustion

The generalized Riemann problem for gas dynamic combustion in a neighborhood of the origin t 0 in the （x, t） plane is considered. Under the modified entropy conditions, the unique solutions are constructed, which are the limits of the selfsimilar Zeldovich-von Neumann-Dring （ZND） combustion model. The results show that, for some cases, there are intrinsical differences between the structures of the perturbed Riemann solutions and the corresponding Riemann solutions. Especially, a strong detonation in the corresponding Riemann solution may be transformed into a weak deflagration coalescing with the pre-compression shock wave after perturbation. Moreover, in some cases, although no combustion wave exists in the corresponding Riemann solution, the combustion wave may occur after perturbation, which shows the instability of the unburnt gases.

High explosive unexploded ordnance neutralization - Tallboy air bomb case study

On October 13,2020,on the fairway connecting the Polish Baltic ports ofSwinoujscie and Szczecin,an underwater hazardous object of historical origin in the form of the British deep penetration bomb Tallboy dropped during the bombing of the German cruiser"Lützow"in April 1945 was neutralized successfully.It is believed to be the first underwater action concerning this type of bomb,which has previously been neutralized on land in Europe(Germany,one confirmed case).The preparation of the operation,on an unprecedented scale at national,European and global level,took one year and included a series of projects related to clearing the space around the bomb from other identified UXO objects,international consultations,historical analyses,determination of the risk to residents and critical infrastructure in the event of an explosion of the bomb containing approximately 2400 kg of the TORPEX explosive(with an increased force equivalent to almost 3600 kg of TNT).The object was neutralized on spot at the depth of 12 m,near a ferry crossing,by specialists from the 41st Navy EOD Team from the 12th MCM Squadron(8th Coastal Defence Flotilla),using the Low Order Deflagration technique(underwater deflagration method).In the case discussed,there was an accumulation of unfavourable conditions which practically excluded the use of blow-in-situ explosive methods(BIP),as well as the extraction of the object and its transport to a military ground.After a partial deflagration of the explosive,the explosive was detonated(DDT).Estimates indicate that the deflagration level reached between 55 and 60%,which significantly reduced the strength and effects of the underwater explosion.

The Generalized Riemann Problem for Chaplygin Gas with Combustion

We study the generalized Riemann problem of the Chapman-Jouguet model for an ideal combustible Chaplygin gas. By analyzing the wave curves in the phase plane, we construct uniquely the solution of the generalized Riemann problem under the global entropy conditions. We find that although there is no combustion wave of the corresponding Riemann solution, the combustion wave may occur after perturbation which reveals the instability of the unburnt gas.

Study on the Detonation Danger of Solid Propellants

A measurement system to study shock initiation behavior of solid propellants was established experimentally. By using this system, the study on shock initiation to the recovered solid propellants with micro damage was performed, especially on the deflagration to denonation transition (DDT) process of solid propellants under both the strong and weak conditions of restriction. The experimental results show that there is a fully compression region in DDT process..

Wave Interactions for Chaplygin Gas with Combustion

The elementary wave interactions for the Chapman-Jouguet model with combustion are investigated. We obtain the unique solution of the initial value problem under the global entropy conditions. We analyze the elementary wave interactions in the phase plane and construct uniquely the solution of this initial value problem. It is found that the combustion wave solution of the corresponding Riemann may be extinguished after perturbation which shows that the unburnt gas is unstable.

Effect of actuating frequency on plasma assisted detonation initiation

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.

中心锥体结构脉冲爆震火箭发动机初步实验

为了改善采用液态燃料的脉冲爆震火箭发动机内部燃料的雾化以及燃料混合物的掺混状况,采用了一种中心锥体结构.该结构发动机不采用Shchelkin螺旋增爆装置,而采用中心锥体结构、二级供应方式.采用航空煤油为燃料、压缩氧气为氧化剂、压缩氮气为隔离气体,在该结构脉冲爆震火箭发动机上获得了充分发展的爆震波并且能够在多循环条件下稳定工作.实验结果表明,该结构可以大大缩短DDT(deflagra-tion to detonation transition)距离,在实验条件下爆燃向爆震转变距离约为管径的5倍.较之同一管径采用Shchelkin螺旋增爆装置的脉冲爆震火箭发动机,该结构发动机的爆燃向爆震转变距离缩短了57.5%.

On supersonic combustion

Some basic concepts and features of supersonic combustion are explained from the view point of macroscopic aerodynamics. Two kinds of interpretations of supersonic combustion are proposed. The difference between supersonic combustion and subsonic combustion is discussed, and the mechanism of supersonic combustion propagation and the limitation of heat addition in supersonic flow are pointed out. The results of the calculation of deflagration in supersonic flow show that the entropy increment and the total pressure loss of the combustion products may decrease with the increase of combustion velocity. It is also demonstrated that the oblique detonation wave angle may not be controlled by the wedge angle under weak underdriven solution conditions and be determined only by combustion velocity. Therefore, the weak underdriven solution may become self-sustaining oblique detonation waves with a constant wave angle.

Multicomponent flow modeling

We present multicomponent flow models derived from the kinetic theory of gases and investigate the symmetric hyperbolic-parabolic structure of the resulting system of partial differential equations.We address the Cauchy problem for smooth solutions as well as the existence of deflagration waves,also termed anchored waves.We further discuss related models which have a similar hyperbolic-parabolic structure,notably the SaintVenant system with a temperature equation as well as the equations governing chemical equilibrium flows.We next investigate multicomponent ionized and magnetized flow models with anisotropic transport fluxes which have a different mathematical structure.We finally discuss numerical algorithms specifically devoted to complex chemistry flows,in particular the evaluation of multicomponent transport properties,as well as the impact of multicomponent transport.

Numerical Research on Effect of Sudden Cross-Section Expansion on Detonation Initiation

Numerical simulation has been performed to investigate the DDT(Deflagration-to-Detonation Transition)mechanism by solving fully compressible reactive flow for hydrogen/air mixtures in tube with sudden cross-section expansion.The results reveal the acceleration action of abrupt cross-section on DDT,which is validated by comparing the run up distance and time with corresponding long annular tube and single tube.Detailed discussion of flow field variations finds that the DDT process in cross-section abrupt tube can be divided into three stages(flame acceleration,transition to detonation,and detonation propagation stages respectively)according to different flame modes.Particularly,it is found that formation of vortex could accelerate DDT by promoting turbulent mixing of hot products and cold reactants.Further comparative analysis on DDT characteristics of cross-section abrupt tube with different annular gap lengths shows that different mechanisms dominate in the single tube zone.The conclusions in present study support the cross-section abrupt tube as a means to enhance DDT and provide an alternative potential in practical pre-detonation initiator and pulse detonation engine applications.

Investigation on C_(2)H_(4)-Air combustion mode in a non-premixed rotating detonation combustor

Based on the working characteristics of the rotating detonation combustor,the combustion mode of C_(2)H_(4)-Air under non-premixed injection conditions is experimentally studied in this paper.By changing the equivalence ratio,we observed the acoustic deflagration mode,fast deflagration mode,stable detonation mode,and weak detonation mode in the combustor.The velocity and pressure of the shock wave increase gradually as the equivalence ratio increases from 0.6 to 1.8.The stable detonation region appears near the stoichiometric ratio and the velocity of the detonation wave is relatively stable.When the equivalence ratio of the mixture is larger than 1.32,the stable detonation wave will suddenly extinguish,forming a weak detonation mode until the end of the combustor operation.The combustion mode of weak detonation is greatly affected by the fuel injection pressure ratio,and the release rate of energy is the main reason for the formation of deflagration mode or detonation mode.