Onset Of Detonation Research Articles

Stability of spinning detonation waves

The present paper reports the results of a study of the stability of spinning detonation near the detonation limits. The detonation velocity as well as the structure are observed for long distances of propagation. Stable mixtures (with regular transverse wave pattern) and unstable mixtures (with irregular transverse wave pattern) are investigated. It is found that the local velocity fluctuates as the spinning detonation propagates and the fluctuations increase when past the first onset of spinning detonation towards the limits (failure). For stable mixtures of C2H2+2.5O2 with high argon dilution, the fluctuations are less than for unstable mixtures of C2H2+5N2O and CH4+2O2. Limits are indicated by abrupt failure of the detonation after some distance of propagation. Long smoked foils (∼3m) indicate that the spinning structure is not constant, but varies periodically as the detonation propagates. Past the first onset of spinning detonation, the fluctuation in the spin structure occurs more frequently and failure is evidenced by the disappearance of transverse waves trajectory on the smoked foil. For stable mixtures, the spinning detonation structure is more stable with a constant pitch and a well defined straight trajectory of the “spin head”. However for unstable mixtures, the spinning detonation is more unstable with varying spin pitch and a “wavy” trajectory of the “spin head”. Also extraneous modes appear periodically superimposed on the spin mode. Subjecting the detonation to a finite perturbation narrows the range in which spinning detonation can be observed. In general the detonation suffers a disruption both in the structure and velocity but recovers after some distance downstream of the obstacle. However for the very unstable mixture of CH4+2O2, it is found that past the limits, abrupt reinitiation occurs downstream of the obstacle similar to the phenomenon of the onset of detonation as in DDT.

An experimental investigation of super knock combustion mode using a one-dimensional constant volume bomb

Super knock induced by pre-ignition in highly boosted spark ignition engines can cause very high peak pressure, which may lead to severe engine damage. Although it is difficult to investigate the mechanism of super-knock due to its inherent randomness, the very high peak pressure implies that super knock may relate to detonation. In this study, a tube-like one-dimensional constant volume bomb, which simplifies the geometry of a real engine's combustion chamber near top dead center, was used to better understand the fundamental phenomenon underlying super knock. H2/O2 mixture was used to maintain reaction intensity even at lower pressure than that in real highly boosted engines. Simultaneous high speed shadowgraphy and pressure measurement were conducted to study the effects of initial pressure and temperature on combustion mode and flame propagation. By comparing the frequencies of super knock pressure oscillation in the boosted engine and after-detonation pressure wave in the constant volume bomb, a relation can be found between the super knock and detonation. The experimental results also show that the detonation tendency of H2/O2 mixture in the constant volume bomb increases with increasing initial pressure but decreases with increasing initial temperature, indicating that the mixture density i.e. energy density plays an important role in detonation onset.

Critical deflagration waves leading to detonation onset under different boundary conditions**Project supported by the National Natural Science Foundation of China (Grant No. 51206182).

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.

An experimental study on the onset processes of detonation waves downstream of a perforated plate

Detonation waves propagating steadily in a rectangular tube were destroyed upon transmission through a perforated plate placed perpendicular to the direction of propagation. Subsequent re-initiation of the detonation waves downstream are investigated. Two explosive mixtures of C2H2+2.5O2+70%Ar (stable detonation with regular cellular pattern) and H2+N2O (unstable detonation with irregular cellular structure) are studied to illustrate the difference in the re-initiation processes. Simultaneous schlieren photography and smoked foil record permit the different features of the onset of detonation in the stable and unstable mixtures to be observed and analyzed in detail. It is found that two types of re-initiation processes are present. In the stable mixture, the onset of detonation is via multiple localized explosions distributed in the reaction zone, which gradually couple the reaction zones with the precursor shock to develop the original cellular structures. Whereas in the unstable mixtures, local perturbations usually at the boundary layer region near the wall lead to one or two strong local explosions forming overdriven detonation wave that decay to C–J detonation subsequently.

Three-Dimensional Simulation of Deflagration-to-Detonation Transition with a Detailed Chemical Reaction Model

A three-dimensional simulation of the deflagration-to-detonation transition (DDT) in a H2/O2 mixture in a rectangular tube is performed under adiabatic and isothermal wall boundary conditions. In the isothermal wall boundary case, a local explosion triggering the onset of detonation occurs near the center of the tube behind the incident shock wave, which agrees qualitatively with the two-dimensional simulation in our earlier study. In contrast, in the adiabatic case, auto-ignition is observed near the corner of the tube before the local explosion occurs on each wall behind the flame, which is accelerated by the high-temperature condition (preheated zone) caused by the generation of compression waves. The flame overtakes the incident shock and propagates toward the upper and lower walls. In some experimental studies, the local explosion occurred near the wall. Therefore, the present adiabatic case received special attention. Moreover, these phenomena are discussed in detail in terms of the flame acceleration, preheated zone, and x–t diagram.

Influence of hydrodynamic instabilities on the propagation mechanism of fast flames

The present work investigates the structure of fast supersonic turbulent flames typically observed as precursors to the onset of detonation. These high speed deflagrations are obtained after the interaction of a detonation wave with cylindrical obstacles. Two mixtures having the same propensity for local hot spot formation were considered, namely hydrogen–oxygen and methane-oxygen. It was shown that the methane mixture sustained turbulent fast flames, while the hydrogen mixture did not. Detailed high speed visualizations of nearly two-dimensional flow fields permitted to identify the key mechanism involved. It was found that the shock reflections in methane permitted to drive strong jets behind periodically formed Mach shocks on the front of the deflagration. These provided local enhancement of the reactivity through mixing, conducive to new hot spot formation. The hot spot formation forward sustained the shock reflection processes conducive to further jetting action. The subsequent organization of the front in stronger fewer modes eventually lead to the transition to detonation. In the hydrogen system, for similar thermo-chemical parameters, the absence of these jets did not permit to establish such fast flames. This jetting slip line instability in shock reflections (and lack thereof in hydrogen) was correlated with the value of the isentropic exponent and its control of Mach shock jetting (Mach & Radulescu, PROCI, 2011).

The Onset of Detonation Behind Shock Waves of Moderate Intensity in Gas Phase

The shock-to-detonation transition (SDT) in gaseous n-heptane/oxygen/argon mixtures has been experimentally studied, using a shock tube, at low initial pressure (2–4 kPa) for a better understanding of the deflagration-to-detonation transition process. The detonation is generated by a precursory shock wave (PSW), with a Mach number smaller than that of the self-sustained detonation. Pressure (P2) and temperature (T2) behind incident shock waves have been accurately determined from the PSW velocity. The transition occurs in the measurement zone located between 3.20 m and 3.65 m from the shock tube diaphragm. The auto-ignition of mixture behind PSW is immediately followed by the onset of a combustion wave, which propagates at near Chapman–Jouguet (CJ) detonation velocity in the mixture carried at P2, T2 conditions. Consequently, the pressure peak can reach 350 times the initial pressure during the transition. The combustion wave merges with the PSW to form an overdriven detonation propagating in the initial mixture at velocity that progressively decreases to a value close to CJ value, of the order of 2 000 m.s−1. The experimental particles heating times are compared with the computed ignition delay times, τI,2, by using a detailed kinetic model of n-heptane oxidation. For stoichiometric n-heptane/oxygen mixtures diluted by 50% Ar, the experimental delay times are compatible with those computed. In this case, τi,2 is governed by the so-called “high temperature mechanism” (T2 > 1000 K) and varies exponentially with temperature. For undiluted mixtures, the particle heating times is much shorter than τi, 2. Then, the chemistry is governed by the “low temperature mechanism” (T2 < 950 K). Between 750 K and 950 K, τi, 2 decreases or small changes with temperature decrease but is sensitive to pressure. Turbulence in boundary layer could also promote the SDT.

Three-Dimensional Numerical Simulations of Spherical Flame Evolutions in Shock and Reshock Accelerated Flows

The three-dimensional spherical flame evolutions induced by incident and reflected shock waves in a rectangular shock tube are numerically simulated using the compressible reactive Navier–Stokes equations with a single-step Arrhenius chemical reaction. Four cases, including variable parameters of flame size and number, shock wave strength, and mixture reactivity, are considered in order to investigate the effects of these parameters on the flame evolutions and detonation onsets. The three-dimensional visualized results and the time-dependent integral and statistical results for the shock–flame interactions are obtained. The morphology of the flame evolutions for all cases studied shows the severe distortion, expansion, and corrugation of flame disturbed by shock waves, especially by a reflected shock wave. The unstable flame produces the three-dimensional reactive shock bifurcation (RSB) structure, as the flame number, the shock wave strength, or the mixture reactivity increase. Further, detonations can occur in a later stage of flame evolution at the different three-dimensional spatial locations for different cases through the shock–detonation–transition (SDT) mechanism. Unlike the three-dimensional spatial dissimilarities of flame patterns and detonation initiation locations among cases studied, the time-dependent integral and statistical properties of flame developments prior to detonation onset show the more similar behaviors for all cases. During the passage of shock and reshock waves, the spherical flames are compressed and distorted, resulting in the vorticity deposition within flame and the well-mixing between the burned and unburned gases. Physical process dominates the compression phases of the flame evolution. Following the passage of the shock and reshock waves, the well-mixing facilitates the chemical heat release and expands (accelerates) the distorted flame. The chemical process becomes more prominent, especially under the reshock condition. In the flame expansion phases, the baroclinic effect is weakened, and the vortex stretching effect is enhanced with the development of flame. In addition, the well-mixing by the passages of shock waves promotes the chemical reaction of the flame, which in turn burns out the mixing zone of flame and therefore inhibits the mixing.

Experimental Studies of Rod Impact on Bare/Uncovered PBX 9501 Explosive

We present an experimental investigation of the dynamics of rod impact on bare or uncovered PBX 9501 explosive. By weight, PBX 9501 contains 95% octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine, commonly known as HMX. The plastic binder is Estane combined with a nitro-plasticizer made of bis(2,2-dinitropropyl)acetal and bis(2,2-dinitropropyl)formal. Experiments were conducted using our single stage gas-gun, allowing us to get very repeatable impact velocities. A stainless steel rod attached to the front of the projectile impacted the explosive target. Impact occurred while the projectile was still in the barrel, allowing for accurate centering of the rod on the target. Seven channels of Photonic Doppler Velocimetry (PDV) were used to measure shock waveprofiles after transmission through the explosive. Velocimetry measurement points were located at different radii from the center of the rod, allowing us to capture the 2-D characteristics of the flow. Also, multiple sets or sequences of experiments were performed. Each set of experiments used a constant rod diameter, impact velocity and PDV probe locations: the thing that was varied from experiment to experiment was the thickness of the PBX 9501 explosive sample. In this way, we built up a detailed experimental record of the impact and reaction dynamics. All experiments discussed used a rod impact velocity near 0.744km/s resulting in an impact stress of ~ 4.5GPa. One series of experiments with 17mm diameter rods resulted in initiation with onset of detonation between 6 and 8mm into the explosive. Another series of experiments with 8.5mm diameter rods resulted in failure to detonate, reactions being quenched by rarefaction waves emanating from the edge of the rod.

Shock initiation of the tri-amino-tri-nitro-benzene based explosive PBX 9502 cooled to −55 °C

We report a series of shock initiation experiments on PBX 9502 cooled to −55 °C. PBX 9502 consists of 95% dry aminated tri-amino-tri-nitro-benzene (TATB) and 5% poly-chloro-trifluoro-ethylene5 (Kel-F 800) binder. PBX 9502 samples were shock initiated by projectile impact from a two stage gas gun. Buildup to detonation was measured with 10 or more particle velocity gauges embedded at different depths in the sample. Three shock wave trackers measured the position of the shock front with time. Particle velocity vs. time wave-profiles and coordinates for onset of detonation were obtained as a function of the impact stress or pressure. PBX 9502 sample temperatures were monitored using type-E thermocouples, two inside the sample and two on the sample surface. Additional thermocouples were mounted on other parts of the cooling apparatus. Wave profiles from embedded gauges are qualitatively similar to those observed at 23 °C. However, at −55 °C, PBX 9502 is much less sensitive than at 23 °C. For example, at an inpact stress of 15.4 GPa, the distance to detonation at −55 °C is 7.8 mm. At 23 °C, the distance is 4.3 mm.

On some conditions for detonation initiation downstream of a perforated plate

An experimental study is presented of the influence of detonation wave parameters and detonation product composition upstream of a perforated plate on the onset of detonation downstream. Experiments were performed in detonation tube 106 mm in diameter, separated into two sections by a perforated plate combined with a diaphragm. The tube was equipped with pressure sensors and a semi-cylindrical smoked foil. Hydrogen–air mixtures with different hydrogen concentrations were used upstream and downstream of the perforated plate. It is shown for mixtures containing 25 and 34 % of hydrogen in air that the onset of detonation downstream depends on detonation parameters upstream of the perforated plate. An increase in the initial pressure upstream of the plate leads to detonation initiation immediately downstream. The variation of mixture composition upstream of a perforated plate does not affect on detonation initiation downstream under the present experimental conditions.

One-dimensional plate impact experiments on the cyclotetramethylene tetranitramine (HMX) based explosive EDC32

Eight one-dimensional plate impact experiments have been performed to study both the Shock to Detonation Transition and Hugoniot state in the cyclotetramethylene tetranitramine (HMX) based explosive EDC32. The experiments covered shock pressures ranging from 0.59 to 7.5 GPa with sustained shocks, double shocks, and short pulse shocks. Experiments were instrumented with embedded magnetic particle velocity gauges. Results include; (1) wave profiles of particle velocity vs. time vs. depth in the explosive, (2) time-distance coordinates for onset of detonation vs. initial shock pressure (aka the Pop-plot), (3) a reactants Hugoniot, and (4) measurement of the Hugoniot Elastic Limit of 0.22.GPa.

Near limit behavior of the detonation velocity

The behavior of the detonation velocity near the limits is investigated. Circular tubes of diameters 65, 44 and 13mm are used. To simulate a quasi two-dimensional rectangular geometry thin annular channels are also used. The annular channels are formed by a 1.5m long insert of a smaller diameter tube into the larger outer diameter detonation tube. Premixed mixtures of C2H2+2.5O2+70%Ar, CH4+2O2 and C2H2+5N2O+50%Ar are used in the present study. The high argon dilution stoichiometric C2H2+2.5O2 mixture has a regular cell size and piecewise laminar reaction zone and thus referred to as “stable”. The other two mixtures give highly irregular cell pattern and a turbulent reaction zone and are hence, referred to as “unstable” mixtures. Pressure transducers and optical fibers spaced 10cm apart along the tube are used for pressure and velocity measurements. Cell size of the three mixtures studied is also determined using smoked foils in both the circular tubes and annular channels. The ratio d/λ (representing the number of cells across the tube diameter) is found to be an appropriate sensitivity parameter to characterize the mixture. The present results indicate that well within the limit, the detonation velocity is generally a few percent below the theoretical Chapman–Jouguet (CJ) value. As the limit is approached, the velocity decreases rapidly to a minimum value before the detonation fails. The narrow range of values of d/λ of the mixture where the velocity drops rapidly is found to correspond to the range of values for the onset of single headed spinning detonations. Thus we may conclude that the onset of single headed spin can be used as a criterion for defining the limits. Spinning detonations are also observed near the limits in annular channels.

Experimental and numerical investigation of DDT in hydrogen–Air behind a single obstacle

Abstract Two-dimensional numerical simulations of deflagration-to-detonation transition (DDT) in hydrogen–air mixtures are presented and compared with experiments. The investigated geometry was a 3 m long square channel. One end was closed and had a single obstacle placed 1 m from the end, and the other end was open to the atmosphere. The mixture was ignited at the closed end. Experiments and simulations showed that DDT occurred within 1 m behind the obstacle. The onset of detonation followed a series of local explosions occurring far behind the leading edge of the flame in a layer of unburned reactants between the flame and the walls. A local explosion was also seen in the experiments, and the pressure records indicated that there may have been more. Furthermore, local explosions were observed in the experiments and simulations which did not detonate. The explosions should have sufficient strength and should explode in a layer of sufficient height to result in a detonation. The numerical resolution was 0.5 mm per square cell, and further details of the combustion model used are provided in the paper.

Fluid mechanics of pulse detonation thrusters

The advantages of constant volume combustion cycle as compared to constant pressure combustion in terms of thermodynamic efficiency have focused the researches of advanced propulsion on detonation engines. The paper gives coverage of efforts undertaken during past decades in adjusting detonations for propulsion applications, and highlights new challenges in studying fluid flow dynamics relevant to onset of detonation.

Some observations on the initiation and onset of detonation

The results of experimental studies during which transition to detonation events occurred are presented. These observations and their interpretation are then discussed, and the conditions for the onset of detonation are described, with particular attention paid to the nature of the phenomena of deflagration-to-detonation transition. The resulting implications for predicting detonation evolution using computational fluid dynamic methods in practical applications are also discussed.

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 investigation of hydrogen–air deflagrations and detonations in semi-confined flat layers

This paper presents results of an experimental investigation on the deflagration and deflagration-to-detonation transition (DDT) in an obstructed (blockage ratio BR = 50%), semi-confined flat layer filled with uniform hydrogen–air mixtures. The effect of mixture reactivity depending on flat layer thickness and its width is studied to evaluate the critical conditions for sonic flame propagation and the possibility for detonation onset. The experiments were performed in a transparent, rectangular channel with a length of 2.5 m. The flat layer thickness was varied from 0.06 to 0.24 m and the experiments were performed for different channel widths of 0.3, 0.6 and 0.9 m. The experimental results show flame velocity vs. hydrogen concentration for different thicknesses and widths of the semi-confined flat layer. Three different flame propagation regimes were observed: slow subsonic flame (M << 1), sonic deflagration (M ∼ 1) and detonation (M >> 1). It is shown that flame acceleration (FA) to sonic speed is independent of the width of the flat layer. The critical expansion ratio for effective flame acceleration to sonic speed was found to be linearly dependent on the reciprocal layer thickness.

Deflagration-to-detonation transition in gases in tubes with cavities

The existence of a supersonic second combustion mode — detonation — discovered by Mallard and Le Chatelier and by Berthelot and Vieille in 1881 posed the question of mechanisms for transition from one mode to the other. In the period 1959–1969, experiments by Salamandra, Soloukhin, Oppenheim, and their coworkers provided insights into this complex phenomenon. Since then, among all the phenomena related to combustion processes, deflagration-to-detonation transition is, undoubtedly, the most intriguing one. Deflagration-to-detonation transition (DDT) in gases is connected with gas and vapor explosion safety issues. Knowing mechanisms of detonation onset control is of major importance for creating effective mitigation measures addressing two major goals: to prevent DDT in the case of mixture ignition, or to arrest the detonation wave in the case where it has been initiated. A new impetus to the increase in interest in deflagration-to-detonation transition processes was given by the recent development of pulse detonation devices. The probable application of these principles to creation of a new generation of engines put the problem of effectiveness of pulse detonating devices at the top of current research needs. The effectiveness of the pulse detonation cycle turned out to be the key factor characterizing the Pulse Detonation Engine (PDE), whose operation modes were shown to be closely related to periodical onset and degeneration of a detonation wave. Those unsteady-state regimes should be self-sustained to guarantee a reliable operation of devices using the detonation mode of burning fuels as a constitutive part of their working cycle. Thus deflagration-to-detonation transition processes are of major importance for the issue. Minimizing the predetonation length and ensuring stability of the onset of detonation enable one to increase the effectiveness of a PDE. The DDT turned out to be the key factor characterizing the PDE operating cycle. Thus, the problem of DDT control in gaseous fuel–air mixtures became very acute. This paper contains results of theoretical and experimental investigations of DDT processes in combustible gaseous mixtures. In particular, the paper investigates the effect of cavities incorporated in detonation tubes at the onset of detonation in gases. Extensive numerical modeling and simulations allowed studying the features of deflagration-to-detonation transition in gases in tubes incorporating cavities of a wider cross section. The presence of cavities substantially affects the combustion modes being established in the device and their dependence on the governing parameters of the problem. The influence of geometrical characteristics of the confinement and flow turbulization on the onset of detonation and the influence of temperature and fuel concentration in the unburned mixture are discussed. It was demonstrated both experimentally and theoretically that the presence of cavities of wider cross section in the ignition part of the tube promotes DDT and shortens the predetonation length. At the same time, cavities incorporated along the whole length or in the far-end section inhibit detonation and bring about the onset of low-velocity galloping detonation or galloping combustion modes. The presence of cavities in the ignition section turns an increase in the initial mixture temperature into a DDT-promoting factor instead of a DDT-inhibiting factor.

Flame acceleration and explosion safety applications

Accidental explosions of flammable gases and reactive gas mixtures remain a significant concern in process industries. The present paper reviews the basic mechanism of Flame Acceleration (FA), the results of recent studies on FA, and their application to explosion safety. A short overview of the physical phenomena involved in FA is followed by the description of FA and flame propagation regimes in smooth tubes, obstructed channels, unconfined and semi-confined mixtures with or without congestion. A framework is summarized that may be used to evaluate the potential for FA and the onset of detonation. Advances made over recent years in the understanding of FA are discussed. These advances include: new theoretical models that address various aspects of the phenomena involved in FA, high resolution numerical simulations and new detailed experimental studies. A combination of the results of these studies is shown to allow for a significant improvement of our ability to address practical problems. Areas where additional research is required to improve reliability of predictions of FA for industrial safety applications are also discussed.