Detonation Of Mixtures Research Articles

On the origin of the double cellular structure of the detonation in gaseous nitromethane and its mixtures with oxygen

An experimental study of the detonation in gaseous nitromethane (NM) and nitromethane-oxygen mixtures has exhibited unambiguously the existence of a double cellular structure in the range of equivalence ratio \(\phi\) from 1.3 to 1.75 (NM). Calculations of the reaction zone of the detonation in the same range of equivalence ratio, using a detailed chemical scheme in the ZND model, demonstrate that the chemical energy is released in two main successive distinct exothermic reaction steps characterized by their own induction length which justifies the existence of a two levels detonation cellular structure. This result strengthens the idea that the cellular detonation structure finds its origin in instabilities amplified by delayed local high energy release rate inside the reaction zone.

Accurate Spatial Resolution Estimates for Reactive Supersonic Flow with Detailed Chemistry

Ar obust method is developed and used to provide rational estimates of reaction zone thicknesses in onedimensional steady gas-phase detonations in mixtures of inviscid ideal reacting gases whose chemistry is described by detailed kinetics of the interactions of N molecular species constituted from L atomic elements. The conservation principles are cast as a set of algebraic relations giving pressure, temperature, density, velocity, and L species mass fractions as functions of the remaining N‐L species mass fractions. These are used to recast the N‐L species evolution equations as a self-contained system of nonlinear ordinary differential equations of the form dYi/dx = fi (Y1 ,..., YN‐L). These equations are numerically integrated from a shock to an equilibrium end state. The eigenvalues of the Jacobian of fi are calculated at every point in space, and their reciprocals give local estimates of all length scales. Application of the method to the standard problem of a stoichiometric Chapman‐Jouguet hydrogen‐air detonation in a mixture with ambient pressure of 1 atm and temperature of 298 K reveals that the finest length scale is on the order of 10 −5 cm; this is orders of magnitude smaller than both the induction zone length, 10 −2 cm, and the overall reaction zone length, 10 0 cm. To achieve numerical stability and convergence of the solution at a rate consistent with the order of accuracy of the numerical method as the spatial grid is refined, it is shown that one must employ a grid with a finer spatial discretization than the smallest physical length scale. It is shown that published results of detonation structures predicted by models with detailed kinetics are typically underresolved by one to five orders of magnitude.

Numerical modelling of liquid-fuelled detonations in tubes

A numerical model for liquid-fuelled detonation is developed. An Eulerian–Lagrangian formulation with two-way coupling between the gas and droplet phases is used to numerically simulate detonation of JP-10 fuel droplets in O2 in a 1.5 m tube. The results of spatial and temporal resolution studies are presented. Then, the consequence of various choices of droplet drag and convective-enhancement sub-models is investigated. It is found that the detailed effects of such models are secondary to those of chemical energy release of the detonation. Detonation structure is shown to vary with initial droplet size and with the amount of initial fuel vapour present. For the range of small droplets considered, the self-propagating detonation velocity depends only minimally on such parameters. Small deficits in propagation velocity from the gaseous Chapman–Jouguet (C–J) value appear to be due to increasing inhomogeneity of the fuel–oxidant mixture as droplet size increases. Such results are in general agreement with the limited experimental data available. Also comparable to experimentally observed trends, results show that the existence of some initial fuel vapour increases the ease of detonability of liquid-fuelled mixtures. More generally, smaller droplet sizes and higher levels of heating and prevaporization are shown to enhance quick transition to a sustained self-propagating detonation.

Spin detonation of fuel-air mixtures in a cylindrical combustor

with a gap of width δ = 0.1, 0.2, 0.3, or 1.0 cm. At the same time, acetylene was delivered into the chamber through a spray nozzle supplied with in-pair counterflow channels with the total cross-sectional area S δ f = 2 cm 2 , situated at a distance L f = 0.1 cm downstream of the air-supply slit, and inclined at an angle of 45i . The channels are uniformly distributed over the chamber circumference. The gases were delivered from a separate receivers with volumes V rA = 79.8 l for air and V rf = 13.3 l for acetylene through electrically driven fast valves. Detonation products flew out directly into the atmosphere. The duration of the process was set within the time range τ d  (0.3‐0.55) s by a control system. The flow rate of the components varied within the limits G A0 = 5.3‐2.12 kg s ‐1 and G f0 = 0.3‐0.21 kg s ‐1 . The fuel-excess factor was Φ = 0.44‐1.37 (here, the subscripts A, f, and 0 denote air, acetylene, and the initial state, respectively). The detonation was initiated by an electrical detonator with an explosive mass of 0.5 g. The entire process was photographed through the longitudinal windows of the detonation chamber on photographic film by a photochronograph with a falling drum [9]. In order to illuminate the wave structure and detonation products, a small acetylene jet was injected into the chamber beginning oppositely to the corresponding window. Using illuminated trajectories, we determined the axial component of the flow velocity v = k v pf . Here, k = 37.8 is the image-diminution factor, α is the inclination angle of a trajectory with respect the horizontal line, and v pf = 50 m s ‐1 is the speed of photographic-film motion. Pressure-sensor signals from the gas receivers, collectors, and from the detonation chamber were registered by a computer system.

Two-step chemical-kinetic descriptions for hydrocarbon–oxygen-diluent ignition and detonation applications

Two-step chemical-kinetic mechanisms for acetylene, ethylene, propane, and JP-10 detonations in oxygen-diluent mixtures are described for use in detonation studies. Conditions addressed cover post-shock temperatures between 1300 and 2500 K, pressures between 1 and 100 bar, and equivalence ratios from 0.5 to 2.0. These mechanisms were developed through reduction from a detailed mechanism consisting of 193 reactions among 41 species that has been tested extensively against shock tube ignition times, counter flow flame-structure measurements, and flame-speed data from literature. Ignition times and temperature histories from the simplified descriptions are in good agreement with those from the detailed mechanism. For all of these fuels, the two-step mechanism consists of a fuel-consumption step that produces radicals, CO, and H 2O and releases most of the heat, followed by a CO-oxidation, radical-recombination step, which releases the remainder of the heat thus approaching equilibrium. For acetylene and ethylene, the first step proceeds at a rate, which is the sum of the rates of an initiation step and the H + O 2 branching step. For propane and JP-10, due to the fundamentally different chemical-kinetic processes involved, this step has been assigned a rate that is proportional to a characteristic fuel-consumption rate. The second step proceeds at the rate of the elementary H + OH recombination reaction. More complex descriptions involving skeletal mechanisms are available for acetylene, ethylene, and propane. Encouraging results have been obtained through applications of these simplified mechanisms in detonation studies.

Numerical simulation of detonations in mixtures of gases and solid particles

This article examines the structure and stability of detonations in mixtures of gases and solid particles via direct numerical simulation. Cases with both reactive and inert particles are considered. First, the two-phase flow model is presented and the assumptions that it is based upon are discussed. Steady-wave structures admitted by the model are subsequently analysed. Next, the algorithm employed for the numerical simulations is described. It is a recently developed high-order shock-capturing algorithm for compressible two-phase flows. The accuracy of the algorithm has been verified through a series of code validation and numerical convergence tests, some of which are included in this article. Subsequently, numerical results for both one-dimensional and two-dimensional detonations are presented and discussed. These results show that the mass, momentum and energy transfers between the two phases result in detonation structures that are substantially different from those observed in the corresponding purely gaseous flows. The effect of certain important parameters, such as particle reactivity, particle volume fraction, and particle diameter, are examined in detail. The numerical results predict that increased particle reactivity suppresses the flow instabilities and increases the detonation velocities. It is further predicted that sufficiently high particle volume fractions can cause detonation quenching regardless of particle reactivity.

Seismic Effect Produced by Detonation of Air-Fuel Mixture in a Borehole

Using laboratory, field, and numerical experiments, it is shown that detonation of air-fuel mixture in borehole located in dense rocks is an effective method for generating the transverse seismic wave. Different operation regimes are investigated, and the seismic efficiency is determined for the method in question. The scheme reliably ensuring the repetitive detonation of mixture under various initial pressures in a borehole is proposed.

Critical tube measurements at elevated initial mixture temperatures

Experimental results obtained on the transmission of a planar detonation wave from a cylindrical tube into an unconfined volume are reported. Experiments are performed using a 9.1 mm long tube connected to an 88.9 cm inner diameter receiver vessel. Both the detonation tube and receiver vessel can be heated. Tests are performed using 10, 20, and 27.3 cm tubes. The minimum, or critical, hydrogen concentration in air that results in detonation transmission into the receiver vessel is measured at an initial pressure of 1 atm and initial temperatures of 300, 500, and 650 K. The value of the ratio of the tube diameter, d, and the critical mixture detonation cell size, u c , is found to be between 18 and 24. The data show no correlation between the value of d/ u c and the initial mixture temperature and there is no observable effect of temperature on the regularity of the soot foil line spacing. The uncertainty in the average cell size measurement of - 25% is not sufficient to explain the significant departure from the classical empirical correlation d/ u c =13. It is proposed that the d/ u c =13 is neither a unique nor adequate correlation for describing detonation diffraction in mixtures considered to have a regular or irregular detonation cellular structure.

Dynamics of planar gaseous detonations near Chapman-Jouguet conditions for small heat release

A description of the dynamics of planar detonations is developed for detonations in mixtures having heat release small compared with the thermal enthalpy, under conditions that include small overdrive, extending to Chapman-Jouguet. This is a preliminary step in studying cellular structures at small overdrive by a nonlinear analysis of weakly unstable detonations. The formulation exhibits a bifurcation from stable to unstable propagation that occurs when the magnitude of the heat release, the temperature sensitivity of the heat-release rate or the specific-heat ratio becomes too large (the number of internal molecular degrees of freedom becomes too small) or the degree of overdrive becomes too small. The results complement those of a previous analysis that applied for large overdrive. An integral-equation formulation is suggested that embodies both limits, with a universal bifurcation parameter.

Ethylene Ignition and Detonation Chemistry, Part 1: Detailed Modeling and Experimental Comparison

A numerical investigation is reported on the chemical kinetics of C 2 H 4 ignition and detonation in oxygen-diluent mixtures. Conditions addressed cover initial (postshock) temperatures between 1000 and 2500 K, pressures between 0.5 and 100 bar and equivalence ratios between 0.5 and 2. Attention is paid to histories of species concentrations and temperature, to autoignition times, and to subsequent heat release that is relevant to detonation structure. A detailed mechanism is proposed consisting of 148 reversible elementary reactions among 34 chemical species. This mechanism is shown to provide good agreement with most ignition times measured in shock tubes and with burning-velocity measurements available in the literature. The results provide a basis for developing simplified descriptions that can be used in multidimensional detonation studies for applications to propulsion devices.

The flowfield and performance of pulse detonation engines

Numerical simulations of an idealized pulse detonation engine consisting of a tube closed at one end and open at the other are presented in this paper. Uncertainties noted previously in the specification of open-boundary conditions in one-dimensional simulations are resolved by conducting multidimensional simulations and extracting the needed information. Further confidence in the simulations is gained by comparing some of the computed flow variables with recent experimental data. The results from the simulations are then used to elucidate the detailed features of the flowfield and the role of key parameters that determine the overall propulsive performance. Specific values for the performance measures of a pulse detonation engine operating on a stoichiometric ethylene/oxygen mixture are calculated from the simulations to be 2100 N s/m 3 for the impulse (independent of system size), 163–165 s for the mixture-based specific impulse and 725–740 s for the fuel-based specific impulse. The computed performance measures are found to be in good agreement with most experimental data and an explanation is provided where differences are observed. Results from various simulations are used to arrive at a general expression that can be used to estimate the impulse from various detonable mixtures.

Simulation of hydrogen deflagration and detonation in a BWR reactor building

A systematic study was carried out to investigate the hydrogen behaviour in a BWR reactor building during a severe accident. BWR core contains a large amount of Zircaloy and the containment is relatively small. Because containment leakage cannot be totally excluded, hydrogen can build up in the reactor building, where the atmosphere is normal air. The objective of the work was to investigate, whether hydrogen can form flammable and detonable mixtures in the reactor building, evaluate the possibility of onset of detonation and assess the pressure loads under detonation conditions. The safety concern is, whether the hydrogen in the reactor building can detonate and whether the external detonation can jeopardize the containment integrity. The analysis indicated that the possibility of flame acceleration and deflagration-to-detonation transition (DDT) in the reactor building could not be ruled out in case of a 20 mm 2 leakage from the containment. The detonation analyses indicated that maximum pressure spike of about 7 MPa was observed in the reactor building room selected for the analysis.

Detonation wave initiation of ram accelerator propellants

The current ram accelerator operations have shown that data on the ability of the propellants to detonate are required. Previous studies examined the efficacy of initiation techniques based on piston impact. The purpose of the present work is to analyze the effects of detonation wave transmission from a detonating mixture into a low sensitivity mixture. One-dimensional modeling based on the analysis of pressure vs particle velocity for the mixtures is used to interpret experimental data. Furthermore, calculations based on chemical kinetics (CHEMKIN code) are provided. Experimental data together with the modeling of the detonation transmission provide some new insight into the limiting conditions necessary to establish a Chapman-Jouguet (CJ) wave in a detonable mixture.

Effect of the Location of the Detonation Initiation Point and the Position of the Air–Fuel Cloud on Explosion‐Field Parameters

The effect of the location of the initiation point on explosion‐field parameters is studied numerically using as an example the detonation of a stoichiometric mixture of propane with air. The shape of the cloud of the mixture (toroid) and the ratio of its dimensions in calculations are typical of the volumes of air–fuel mixtures formed in accidents. The effect of the initiation point location within the cloud cross section was studied with variation in the position of the lower edge of the cloud above the underlying surface.

Kinetics of Chemical Reactions During Detonation of Mixtures of Organic Substances with Nitric Acid

The kinetics and mechanism of chemical reactions in detonation waves propagating in mixtures of nitric acid with nitroglycol, ethylene glycol dinitrate, and acetic anhydride were studied within the framework of the Dremin—Trofimov theory of the detonation failure diameter. The state parameters in shock and detonation waves were calculated using the SGKR software package. It was shown that the decomposition of mixtures of nitric acid with organic substances in a detonation wave is a complex reaction which includes several stages. Various kinetic models are considered; effective values of the kinetic parameters are calculated for each model and for the entire process.

Detonation in heterogeneous mixtures of liquids and particles

The mechanisms of detonation propagation in heterogeneous systems comprising closely packed particles and a liquid explosive are not fully understood. Recent experimental work has suggested the presence of two distinct modes of detonation propagation. One mode is valid for small particles (which is the regime we will address in this paper) with another mode for large particles. In this work we model numerically the detail of the wave interactions between the detonating liquid and the solid particles. The generic system of interest in our work is nitromethane and aluminium but our methodology can be applied to other liquids and particles. We have exercised our numerical models on the experiments described above. Our models can now qualitatively explain the observed variation in critical diameter with particle size. We also report some initial discrepancies in our predictions of wave speeds in nominally one dimensional experiments which can be explained by detailed modelling. We find that the complex wave interaction in the flow behind the leading shock in the detonating system of liquid and particles is characterised by at least two sonic points. The first is the standard CJ point in the reacting liquid. The second is a sonic point with respect to the sound speed in the inert material. This leads to a steady state zone in the flow behind the leading shock which is much longer than the reaction zone in the liquid alone. The width of this region scales linearly with particle size. Since the width of the subsonic region strongly influences the failure diameter we believe that this property of the flow is the origin of the observed increase in failure diameter with particle size for small inert particles.

Application of exergy analysis to the hydrodynamic theory of detonation in gases

The exergy analysis was applied to the phenomenon of detonation in gases. Separately were considered the component processes of the pressure shock and the chemical reaction of combustion. The considerations were illustrated by the examples of detonation of stoichiometric mixtures of dry air and hydrogen or carbon monoxide or methane. A measure of the thermodynamic imperfectness of process is the exergy loss in the process. It was found that exergy loss of the shock is larger than the exergy loss of the chemical reaction of combustion. A steady detonation characterized by the Chapman–Jouguet point occurs at the extremum of these losses. The exergy loss of combustion during steady deflagration, considered for comparison, is larger than the total exergy loss of detonation. It suggests that from thermodynamic viewpoint, the detonation is a better method of fuel utilization than the deflagration. Therefore, the consideration of any new industrial burner device based on the detonation process would be motivated.

A study on detonation safety of some hydrocarbon‐air mixtures

AbstractHazards caused by detonation of hydrocarbon‐air mixtures have been a matter of concern for a long time. In this paper, critical initiation energy of detonation of some hydrocarbon air mixtures have been measured in confined (rectangle shock tube tests) and unconfined (plastic bag tests) condition. Based on the concept of explosion length, a formula for estimating the critical initiation energy of detonation under unconfined condition is presented. Detonation safety analysis of the studied hydrocarbons is carried out using the experimental results.

The interaction of periodically generated cylindrical detonations in a simulated hypersonic flow

A new experimental method of establishing oblique detonation waves in stationary detonable mixtures was demonstrated. This method of simulating oblique detonations in hypersonic flow involved the periodic initiation of multiple cylindrical detonation fronts, which upon interaction quickly transitioned to an oblique detonation wave. Its axisymmetric analog was the point ignition of multiple spherical detonation waves forming a conical oblique detonation. The two-dimensional system was chosen here to focus on the interaction of the waves and the motion of the triple points, as well as to determine how rapidly the interacting fronts form a single oblique wave. A stoichiometric mixture of acetylene and oxygen was utilized at pressures between 30 and 50 kPa in these experiments. Ignition was accomplished by branching an established normal detonation wave into five waves propagating in small tubes and time delaying then with respect to one another to initiate cylindrical detonations in the test section. The simulated hypersonic flow speeds were 1.5, 2, and 3 times the Chapman-Jouguet (C-J) velocity. The corresponding initiation frequencies were 1.2×10 5 , 1.6×10 5 , and 2.4×10 5 Hz. The measured oblique wave angles at these three simulated speeds were in good agreement with the expected values based on the wave front kinematics. Interrogation of the wave front velocities indicated that the undisturbed regions propagate at near C-J velocities, whereas the triple point regions propagate at velocities up to 4% to 13% greater than the undisturbed regions, helping to smooth the combined wave front. These results suggest that the periodic strong ignition of a detonable mixture in a hypersonic flow can be a viable method of sustaining detonative combustion without the need of a “flame holder.”

Experimental studies of shock-induced ignition and transition to detonation in ethylene and propane mixtures

Results are presented of ignition delay measurements, over the temperature range 1073–2211 K, obtained by monitoring CH emission from ethylene and propane mixtures. The relative influence of argon and nitrogen as diluents is also investigated. Comparisons with other experimental data are made and the validity of chemical kinetic reaction mechanisms discussed. Spark schlieren photographs of the reflection and subsequent ignition processes are also presented, and the effects of varying the diluent and degree of dilution away from ideal conditions investigated. In some cases, acceleration of the reaction wave has been observed, with transition to detonation occurring a short time later.