Detonation Limits Research Articles

Detonability limits in thin annular channels

In this paper, detonability limits in two-dimensional annular channels are investigated. Since the channel heights are small in comparison to the tube diameter, curvature effects can be neglected and the annular channels can be considered to be essentially two-dimensional. Mixtures that are highly diluted with argon are used since previous investigations seem to indicate that detonations in such mixtures are “stable” in that cellular instabilities play minor roles on the propagation of the detonation. For stable detonations where the ZND structure is valid, boundary layer effects can be modeled as a flow divergence term in the conservation of mass equation following the pioneering work of Fay [J.A. Fay, Phys. Fluids 2(3) (1959) 283–289]. Expansion due to flow divergence in the reaction zone results in a velocity deficit. There exists a maximum deficit when an eigenvalue detonation velocity can no longer be found, which can be taken as the onset of the detonability limits. Experimentally, it was found that unlike “unstable” detonations, the detonability limits for “stable” detonations are well-defined. No unstable near-limit phenomena (e.g., galloping detonations) was observed. Good agreement is found between the theoretical predictions and the experimentally obtained velocity deficits and limits in the two channel heights of 2.2 and 6.9 mm for hydrogen–oxygen and acetylene–oxygen mixtures diluted with over 50% argon. It may be concluded that at least for these special mixtures where the detonation is “stable,” the failure mechanism is due to flow divergence caused by the negative displacement thickness of the boundary layer behind the leading shock front of the detonation wave.

Effect of chemically active additives on the detonation wave velocity and the detonation limits in lean mixtures

On the basis of the Zel’dovich-von Neumann-Doering detonation theory with allowance for heat losses, as well as the theory of chain processes through, the example of the oxidation of hydrogen-rich mixtures in the presence of an inhibitor, it is shown that taking into account inhibition reactions and the trimolecular chain termination has the consequence that there are not only heat losses but also “chemical” losses. It is demonstrated that the presence of only “chemical” losses alone can ensure that there is a concentration limit of detonation and that the combustion wave near the limit is supersonic. Theoretical estimates agree qualitatively with the experimental data on the inhibition of a developed detonation wave in H2-air mixtures with additives of a propane-butane mixture (0.5–4%) at a pressure of 1 atm.

Total flammable mass and volume within a vapor cloud produced by a continuous fuel-gas or volatile liquid-fuel release

The top-hat jet/plume model has recently been employed to obtain simple closed-form expressions for the mass of fuel in the flammable region of a vapor “cloud” produced by an axisymmetric (round) continuous-turbulent jet having positive or negative buoyancy [1]. The fuel release may be a gas or a volatile liquid. In this paper, the top-hat analysis is extended to obtain closed-form approximate expressions for the total mass (fuel + entrained air) and volume of the flammable region of a release cloud produced by either a round or a plane (two-dimensional) buoyant jet. These expressions lead to predicted average fuel concentrations in the flammable regions of the release clouds which, when compared with the stoichiometric concentration, serve as indicators of the potential severity of release cloud explosions. For a fixed release mass, the combustion overpressure following ignition of a hydrogen/air cloud is anticipated to be significantly lower than that due to ignition of a hydrocarbon/air cloud. The predicted average hydrogen concentration within the flammable region of the release cloud is below the lower detonability limit. The facility with which the expressions can be used for predictions of combustion overpressures is illustrated for propane releases and deflagrations in a closed compartment.

Experimental Investigations on Detonability Limits of Pulse Detonation Rocket Engine

Experimental Investigations on Detonability Limits of Pulse Detonation Rocket Engine

Observations of the cellular structure of fuel–air detonations

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

On the Hydrodynamic Thickness of Cellular Detonations

The characterization of the detonation dynamic parameters (detonability limits, direct initiation energy, critical tube diameter, etc.) requires a characteristic length scale for the detonation wave in the direction of propagation. However, most detonations are unstable, their reaction zones are turbulent, and their structure departs significantly from the idealized one-dimensional Zel'dovich-Von Neumann-Doring model. It is argued that the most suitable length scale to characterize a turbulent detonation wave is the location of the sonic surface, which separates the statistically stationary flow of the reaction zone structure from the unsteady expansions behind the wave. Previous real and numerical experiments are reviewed in order to determine the relation between the global location of the mean sonic surface and the chemical, mechanical, and thermodynamic relaxation processes occurring in the detonation wave structure. Based on the experimental evidence, we postulate that the structure of turbulent detonations can be modeled in the one-dimensional Zel'dovich-Neumann-Doring framework, with the turbulence effects as source terms in the momentum and energy equations. These source terms involve the relaxation rates for the mechanical fluctuations, thermal fluctuations and the chemical exothermicity towards equilibrium. In the framework of the idealized one-dimensional structure with source terms, the sonic surface location is governed by the balance between the competing source terms satisfying the generalized Chapman-Jouguet criterion. We recommend that future work in detonation research should be focused at: 1) acquiring a large experimental database for the mean detonation properties (detonation velocity, location of sonic surface and mean reaction zone profiles); 2) the development of the appropriate source terms involving the turbulent fluctuations in the averaged equations of motion.

Detonability of simple and representative components of pyrolysis products of kerosene: pulsed detonation engine application

The use of liquid fuels such as kerosene is of interest for the Pulse Detonation Engine (PDE). In this context, a representative gaseous mixture of the lighter products resulting from the decomposition of a kerosene of type JP-10 was studied. The detonability limits of simple components (hydrogen, ethylene, propylene) and mixtures of these components were tested in a 50 mm diameter and 2.5 m long detonation tube. This dimension is compatible with the applications of the aircraft industry and more particularly the PDE. The influences of the nitrogen dilution, geometry of the DDT device (Schelkin spiral), ignition energy and initial pressure were investigated.

Effect of Chemically Active Additives on the Detonation Wave Velocity and Detonation Limit in Rich Mixtures

Effect of Chemically Active Additives on the Detonation Wave Velocity and Detonation Limit in Rich Mixtures

Effects of hydraulic resistance and heat losses on detonability and flammability limits

This paper presents an analysis of a one-dimensional combustion model capable of describing both deflagrations and detonations. Incorporating volumetric terms to account for hydraulic and thermal losses, the quenching diameters, below which each type of combustion wave cannot propagate, are calculated. The main conclusion is that, as expected, detonations have larger quenching diameters than deflagrations for sufficiently high activation energies. However, the opposite result is found for relatively low activation energies.

Detonation propagation limits in homogeneous and heterogeneous systems

Detonation propagation limits of concentration, initial pressure and tube diameter in homogeneous fuel/air mixtures through the deflagration-to-detonation transition were experimentally figured out. The detonation quenching diameter of a narrow channel in fuel/oxygen mixtures was applied to determine the testing condition for detonation arresters. The destruction techniques for chlorofluorocarbons by gaseous detonation or deflagration were developed. Propagation limit of decomposing detonation of acetylene and ozone was experimentally determined. The structure and propagation mechanism of the soot layer detonation were clarified. Propagation limits of the film detonation were experimentally investigated and the relationship among the limit of initial pressure, tube diameter, and oxygen concentration was evaluated. Initiation energy for solid(fuel)/liquid(oxygen/nitrogen) system was predicted.

Propagation and quenching of detonation waves in particle laden mixtures

Propagation and quenching of curved detonation waves in particle laden mixtures were investigated analytically and numerically using the square wave model and quasi-steady state assumption, respectively. An analytical expression describing the combined effects of heat and momentum losses because of the interaction between the particle and gas phases, as well as the effect of wave curvature on detonation velocity, was obtained. Detonation quenching and multiple detonation regimes were examined. Numerical simulation of the detonation wave structure was also made. The results showed that particle heat loss, momentum loss and wave curvature considerably reduce the detonation speed and cause detonation quenching. It is also shown that, for fixed particle volume fractions, smaller particles cause a larger heat loss from the gas phase and result in a lower detonation velocity and narrow detonation limit.

Helium dilution for ram accelerator operation

Experimental detonation characteristics (detonability limits and detonation velocities) of methane-oxygen-helium mixtures at initial pressures of up to 5 MPa were measured in a 90 mm caliber, 11.7 m long (130 calibers) smooth-bore detonation tube. Stoichiometric as well as fuel-rich mixtures were investigated in order to provide performance data for ram accelerator applications. The experimental results were used to adjust and validate calculations of the Chapman-Jouguet detonation characteristics by means of a thermochemical code based on various real gas equations of state. On the basis of the detonation velocity data, a series of intermolecular parameters involved in the thermochemical code was determined. Furthermore, the investigation enabled to determine of the upper detonable area of these dense ternary mixtures under the present experimental conditions.

Effects of momentum and heat losses on the multiplicity of detonation regimes

Employing Zeldovich’s (1940) quasi-one-dimensional formulation the multiplicity of detonation regimes occasionally observed in obstacle-laden systems is explored. The paper is an extension of the previously studied adiabatic version of the problem where, in addition to the well-known sub-CJ quasi-detonation, the low-speed supersonic as well as subsonic detonation regimes were identified. It is shown that the hysteresic loop associated with non-uniqueness of detonation regimes may be located entirely within the supersonic domain, the situation often encountered in experiments. By adopting a one-step bimolecular kinetics the well known dependency of the transition on the initial pressure is explained. The incorporation of heat losses, apart from bringing up detonability limits, strongly affects the low-speed regimes. The latter are found to occur only in the systems where the Reynolds analogy is strongly violated (rough tubes, porous media), and do not arise in smooth-walled tubes. The disparity between detonability and flammability limits is discussed.

Detonation Limits of Air Mixtures with Two-Component Gaseous Fuels

The problem of detonation limits for ternary mixtures of air with a two-component gaseous fuel is considered for a detonation region represented using the Le Chatelier rule. Examples are given of incorrect treatment of conditions for detonation suppression in hydrogen–air mixtures by the addition of hydrocarbons ignoring the overall composition of the mixture. It is suggested that the range of explosion hazard of lean hydrogen–air mixtures is extended by the addition of small amounts of hydrocarbon gases. Key words: detonation, detonation limits, multicomponent fuel mixtures, suppression and promotion of detonation.

Decomposing detonation and deflagration properties of ozone/oxygen mixtures

In this study, the decomposing detonation and deflagration properties of ozone/oxygen mixtures of up to 20 vol.% of ozone in oxygen under high pressure of up to 1.0 MPa in a tube were experimentally investigated. The mixtures were ignited by an electric spark at the end of the tube. Flame propagation properties such as flame velocity and pressure were measured with thermocouples and piezo electric transducers mounted along the tube. Slow and constant flame propagation profiles were obtained. We also investigated the quenching ability of a wire gauze as well as the concentration limit for flame propagation. However, in spite of slow flame propagation velocity and easy flame quenching properties under these experimental conditions, direct initiation of detonation by the driver detonation of the stoichiometric oxy-hydrogen mixture was easily achieved at much lower concentrations than the limit of deflagration. The observed detonation properties, such as wave velocity and pressure, agreed fairly well with C–J calculated values. The detonation velocity (900–1200 m/s) and the pressure ratio to initial pressures (5–9.5) were not affected by the initial pressure of the mixtures. Near the detonation limit, typical spinning detonations with oscillatory pressure waves were observed.

Experiments on spinning detonations with detailed analysis of the shock structure

Spinning detonations are characteristic of detonation limit phenomena in round tubes. In this work we study experimentally the structure of the transverse wave of single-headed spinning detonations. The flow field is experimentally analysed and an original approach enables us to calculate the overall shock structure. The calculations and experimental results indicate that the actual structure of the spinning detonation tries to match closely to the condition where the state parameters (pressure and temperature) reach their maximum values. This condition corresponds to a spinning head where the Mach stem is normal to the incoming flow and could be readily used as boundary condition by further investigators to determine the structure of spinning detonations.

Effect of chemically active additives on the velocity of detonation waves and on the limit of gaseous detonation

A one-dimensional model of gaseous detonation waves with heat energy losses was used to explain the narrowing of detonation limits in the presence of small amounts of efficient inhibitors by the example of hydrogen oxidation, and it was found that chemical energy losses due to chain termination should be taken into account along with heat and friction energy losses.

Detonation of a mixture of RDX particles partially filling a cylindrical channel

Using a mathematical model of a two-phase, two-velocity medium, detonation in an annular layer of a suspension of volatile secondary explosives adjacent to the wall of a cylindrical channel is numerically investigated. The dynamics of formation and the special features of the structures of a two-dimensional reaction zone of a detonation wave in a gas mixture of RDX particles are discussed. A detonation regime with a vortex structure of the reaction zone is obtained in calculations for the first time. The geometrical limits of detonation in a channel are determined.

Obturator and detonation experiments in the subdetonative ram accelerator

An experimental investigation has been undertaken to improve understanding of the role of the obturator and detonations in the subdetonative ram accelerator starting process. Ram accelerator start experiments were conducted with various obturator geometries to determine the obturator dynamics and assess its effect on the outcome of a start attempt. The obturator rapidly decelerates upon entrance and then moves backwards. Reversal of direction occurs more rapidly after propellant ignition, for less massive obturators, and solid geometries. Perforated geometries and decreasing obturator mass are less conducive to igniting a given propellant, as evidenced by the flowfield and start attempt outcome data presented. Wave unstarts were observed to occur with and without detonations, indicating more than one mechanism responsible for this type of start failure. Piston-initiated detonation experiments were conducted by firing the obturators without the ram accelerator projectile. The piston experiments identified the detonation limits for a wide range of propellants, but were found to not always be indicative of the upper Mach number at which a ram accelerator can be successfully started. In some instances a successful start or wave fall-off would occur at Mach numbers above which a piston alone detonated the propellant. Thus, the projectile can play a mitigating role in detonation initiation and use of piston detonation limits to quantitatively define a detonation wave unstart limit was not realized.

Low-velocity detonation limits of gaseous mixtures

Low-velocity detonation regimes in acetylene-oxygen mixtures are studied experimentally. Data are obtained on the kinematics of detonation waves and on the boundaries of the high-velocity, galloping, and low-velocity detonation regions. In lean mixtures, the lower pressure limit for the detonation regimes in narrow channels is found to be an order of magnitude lower than assumed previously under the assumption that the limiting detonation is always spin detonation. The major structural characteristics of low-velocity detonations are calculated. The boundary between low-velocity and galloping detonation is found to correspond almost exactly to equality between the induction time in a particle and the time for it to move from shock wave to flame. Nearly harmonic oscillations of the flame are observed with periods that are related to the longitudinal size of low-velocity detonation waves.