Detonation Failure Research Articles

Pulse Detonation Engine Characterization and Control Using Tunable Diode-Laser Sensors

One of the phenomena limiting the performance of pulse detonation engines (PDEs) is detonation failure due to pulse-to-pulse interference. To better understand and control such interferences, two novel laser diagnostic techniques, based on absorption spectroscopy, have been developed and then used to demonstrate effective realtime control. The e rst technique utilizes a tunable diode-laser (TDL) sensor to measure H 2O temperature and concentration in the tube tail end at the Naval Postgraduate School’ s (NPS) PDE facility and in the tube head end at Stanford University’ s (SU) PDE facility. This sensor, capable of measuring temperatures from 300 to 1300 K at 3.33 kHz, reveals the temporal history of temperature for multipulse engines. In its application to the NPS facility, the sensor shows a distinct change in temperature proe le when the engine pulserate is changed from 5 Hz, where successful detonations are achieved, to 7 Hz, where interference produces undesirable e ame holding and subsequent dee agrations on some pulses. We observed that the geometry evaluated possessed excess recirculation at the higher pulse rates resulting in e ame holding at or near the point of injection. In its application to the SU PDE, this sensor reveals a temperature proe le characteristic of detonation failure that could be used in future control schemes. The second diagnostic technique developed is used to monitor fuel and is employed in an active, real-time control scheme. For this sensor, we monitor the C 2H4 (ethylene) concentration at the tail end of the NPS PDE initiator tube, which is operating at 20 Hz. When fuel is detected at the tail end, the sensor sends a signal to e re the ignitor. Compared to e xed-timing ignitor actuation, this control promotes more consistent detonation initiation and reduces mise re events. These two new laser diagnostic techniques provide useful tools for studying pulse-to-pulse interference and lay the groundwork for future, more advanced TDL-based PDE control strategies.

A numerical study on the detonation behaviour of double reactive cassettes by impacts of projectiles with different nose shapes

The detonation behaviours of two layers of reactive cassettes by the impact of cylindrical steel projectiles with different nose shapes were numerically simulated at 1800 m s −1 based on the forest fire explosive reaction rate model. When a projectile with a flat-ended nose impacted the target, a run distance-induced detonation took place at the bulk of the first cassette. In the case of a cone-ended projectile impact, however, a back plate-assisted detonation took place via the superposition of incident and reflective shock waves at the region near the back plate of the first cassette. Since such a mechanism was not observable in the conventional critical initiation criteria (CIC) test, the previously developed CIC models were suggested to be inadequate to predict the detonation sensitivity of a reactive cassette. When a hemispherically nosed projectile impacted the target, none of the detonation mechanisms were triggered, resulting in the failure of detonation. The detonation of the second reactive cassette was not directly influenced by the impact shock of the projectile, but it was achieved only when the first cassette was successfully detonated thereby transferring sufficient energy to the second one.

The failure mechanism of gaseous detonations: experiments in porous wall tubes

To clarify the important role of the transverse wave structure in real detonations, we conducted experiments in porous wall tubes, as to attenuate the detonation’s transverse waves. Flow visualization and measurements of the attenuation and failure processes for three typical unstable mixtures (oxy-acetylene, oxy-methane, and oxy-propane mixtures), characterized by their highly irregular frontal structure, revealed the ongoing competition between transverse wave elimination at the porous wall and re-amplification of triple points within the reaction zone. At critical conditions, when these two effects balance, a unique failure limit is found, namely d∗ ≈ 4λ. Below this limit, the detonations fail. These experiments thus illustrate that transverse wave interactions are essential in the ignition and propagation mechanism for such unstable detonations. In comparison, experiments with argon-diluted detonations displaying a regular cellular structure with weaker transverse waves indicate that their transverse waves do not play a significant role in their propagation mechanism. The failure of these stable detonations is due to the global curvature mechanism caused by the mass divergence into the porous wall, leading to the slow distribution of frontal curvature. The results obtained in diluted and undiluted mixtures are also compared with a model taking into account the mass divergence at the permeable tube walls. Very good agreement is found for the argon-diluted mixtures, while the agreement for the unstable mixtures is very poor. This further indicates that the weak transverse wave structure in argon-diluted detonations, unlike the undiluted ones, does not significantly contribute to the ignition mechanism. Hence these argon-diluted detonations can be well approximated by a ZND reaction zone structure.

The effect of argon dilution on the stability of acetylene/oxygen detonations

Experimental observations indicate that dilutions with large amounts of argon lead to stable detonations having a regular cellular structure with only weak transverse waves. In the present study, the stabilizing effect of argon dilution in acetylene/oxygen detonations is investigated numerically by detailed numerical simulations of one-dimensional time-dependent pulsating detonations with a realistic seven-step chemistry model. The results show that heavy argon dilution in the mixture leads to single-frequency small-amplitude regular oscillations of the shock front pressure. As the dilution is decreased, the detonations become unstable, characterized by larger amplitude oscillations. The stabilizing role of argon is further investigated by analyzing the reaction zone structure of the steady Zeldovich-Von Neumann-Döring detonation with varying degrees of argon dilution. For the same characteristic induction lengths, the dilution with argon leads to lower temperatures in the reaction zone and slower exothermic reaction rates, thus rendering the reaction zone structure less temperature sensitive and more stable to hydrodynamic fluctuations. The present unsteady numerical simulations also indicate that with argon dilution less than approximately70%, a one-dimensional time-dependent detonation cannot self-propagate. Below this limit, due to the low-velocity excursions of the pulsating leading shock, the reactions are quenched and detonation failure occurs. This fundamental limit reveals that a one-dimensional shock-induced ignition mechanism in an unstable detonation is insufficient to account for the ignition and propagation mechanism in multidimensional detonations. These observations are consistent with recent experiments performed in porous wall tubes where, as the transverse waves were eliminated, the detonation could not self-sustain in the one-dimensional limit. The experimental stability limit, at which the transverse waves begin to play the dominant role, also corresponding to the loss of regularity in the cellular pattern, agrees very well with the stability limit determined numerically in the present study.

One-dimensional overdriven detonations with branched-chain kinetics

The dynamics of time-dependent, planar propagation of gaseous detonations is addressed on the basis of a three-step chemistry model that describes branched-chain processes. Relevant nondimensional parameters are the ratio of the heat release to the thermal enthalpy at the Neumann state, the nondimensional activation energies for the initiation and branching steps, the ratio of the branching time to the initiation time and the ratio of the branching time to the recombination time. The limit of strong overdrive is considered, in which pressure remains constant downstream from the leading shock in the first approximation, and the ratio of specific heats γ is taken to be near unity. A two-term expansion in the strong overdrive factor is introduced, and an integral equation is derived describing the nonlinear dynamics and exhibiting a bifurcation parameter, the reciprocal of the product of (γ−1), the nondimensional heat release and the nondimensional branching activation energy, with an acoustic correction. A stability analysis shows that, depending on values of the parameters, either the mode of lowest frequency or a mode of higher frequency may be most unstable. Numerical integrations exhibit different conditions under which oscillations die, low-frequency oscillations prevail, high-frequency oscillations prevail, highly nonlinear oscillations persist, or detonation failure occurs. This type of parametric analysis is feasible because of the relative simplicity of the model, which still is more realistic than a one-step, Arrhenius chemical approximation. In particular, by addressing the limit of slow radical recombination compared with branching, explicit results are derived for the critical value of the bifurcation parameter, involving the ratio of the recombination time to the induction time. The results help to clarify the general nature of one-dimensional detonation instability and provide simplifications that can be employed in efficiently relating gaseous detonation behavior to the true underlying chemistry.

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.

Measurement of detonation failure diameter in thin layers of an explosive solution

The detonation failure diameter was determined for mixtures of dinitrotoluene in concentrated nitric acid with zero and negative oxygen balance in flat glass dishes. For mixtures with negative oxygen balance, detonation was detected in layers 0.10 and 0.12 mm thick, and detonation failure took place in layers 0.08 mm thick. The data obtained gives complete form to the dependence of the detonation failure diameter on composition for dinitrotoluene solutions in HNO3.

Laboratory study on channel effect in emulsion explosives

Abstract The precursor air shock wave (PAS), which propagates ahead of the detonation front in an air channel, precompresses and desensitizes the unreacted explosive charges. Under some conditions, the PAS causes detonation failure. This phenomenon is known as the channel effect. To investigate the mechanism of this effect in emulsion explosives, some experimental works have been carried out using a high-speed framing camera. The results of photographic observation demonstrated that the difference between PAS velocity and detonation velocity was the dominate factor for the channel effect. It is assumed that a decrease in the PAS velocity can prevent the channel effect.

Experimental study on the channel effect in emulsion explosives

Abstract The precursor air shock wave (PAS), which propagates ahead of the detonation front in an air channel, precompresses and desensitizes the unreacted explosive charges. Under some conditions, the PAS causes detonation failure. This phenomenon is known as the channel effect. To investigate the mechanism of the channel effect in emulsion explosives, some experimental work has been carried out using a high-speed framing camera. The results of photographic observation demonstrated that the difference between PAS velocity and detonation velocity was the primary factor for the channel effect. It is assumed that a decrease in the PAS velocity can prevent the channel effect. The ability of increased surface roughness of the inner wall to decrease the PAS velocity was tested. Some experiments were conducted to investigate the effects of surface roughness on the PAS velocity and the detonation propagation. The experimental results indicate that increased surface roughness can reduce the PAS velocity and prevent detonation failure.

Failure and reignition of one-dimensional detonations—The high activation energy limit

The structure of failed one-dimensional detonations is derived using a high activation energy analysis. On a time scale longer than the chemical time based upon the von Neumann temperature, high activation energy one-dimensional detonations break down into a weaker shock, a contact surface separating hot burned gases from a colder, unburned mixture, and an expansion wave. While this leading order solution is unaffected by chemistry, hence self-similar, the perturbation, accounting for the chemistry, which depends upon chemical times, is not. By accounting for the chemistry, the perturbation problem determines the delay until reignition of the detonation occurs. This problem is almost identical to the problem of initiation in the region between a shock and contact surface, which is created by the collision of two shock waves. The main difference between that problem and the current analysis is that the downstream boundary condition now consists of radiating acoustics into hot burned products, at the location of the surface discontinuity. When the Newtonian limit is applied, that is, for a ratio of the specific heats approaching unity, the hot spot at which reignition occurs approaches the location of the contact surface. The time and length to reignition are then found to vary exponentially with the activation energy of the mixture. However, the Newtonian limit is not a very realistic model, because it makes the interval between a Mach number of 1/√γ and 1 disappear: in this range of Mach numbers, adding energy to a steady flow lowers the temperature, hence the reaction rate.

Experimental investigation of detonation of nitric acid solutions

Detonation of mixtures of concentrated (94–100%) nitric acid with nitromethane, diethylene glycol dinitrate, nitroglycol, trinitrotoluene, dinitrotoluene, acetic anhydride, and dichlorethane was studied experimentally. The detonation failure diameter was measured in glass tubes. For mixtures of nitric acid with nitromethane, dinitrotoluene, and trinitrotoluene, its minimum values are smaller than 1 mm and correspond to zero oxygen balance of the mixtures (A=0). Nitroglycol (A=0) and its mixtures containing less than 20% nitrogen acid have the same detonation diameter (2 mm), which is larger than that for a mixture of nitric acid with diethylene glycol dinitrate (1 mm forA=0). The minimum values of the detonation diameter for mixtures of nitric acid with acetic anhydride and dichlorethane (2 and 3 mm) are shifted towardA<0. Comparison of the detonation diameter with calculated values of the heat of explosion and analysis of the experimental results within the framework of Dremin’s theory of the detonation failure diameter show that nitric acid enhances the reactivity of nitrocompounds in a shock wave more considerably than the reactivity of nitroesters.

Decoupling and recoupling of detonation waves associated with sudden expansion

Detonation propagation behavior associated with sudden expansions has been investigated both experimentally and numerically. Different mechanisms, from sustained propagation to detonation failure and reinitiation including shock and flame front decoupling and recoupling have been observed with the schlieren technique. The shock-induced flame propagation has been modeled with two-step chemistry and structured two-dimensional CFD on arbitrary geometries. The results of the numerical simulations show good correspondence to the variety of phenomena observed in experiments. Thus the numerical simulation can be used to study detonation propagation in complex geometries. It provides a tool for the design of safety devices and aids experimental investigations.

Sulfuric Acid Influence on the Nitrocompounds Detonation Reactions

The detonation failure diameter d f and detonation velocity D of mixtures of nitromethane, trinitrotoluene, dinitrotoluene, and trinitrobenzene with sulfuric acid and oleum have been measured in the wide range of concentrations. It was shown that the detonation ability of the nitrocompounds depends significantly on the sulfuric acid content. The minimum value of d f for the mixture TNT/oleum is about 2 mm, i.e., 30 times less, than that for pure melted TNT, and practically equal to d f of nitroglycerine. In some cases, the temperature dependencies of failure diameter have been determined. Dremin theory of detonation failure diameter was used to treat quantitatively the results of experiments with the NM and TNT solutions in the frameworks of the Arrhenius chemical kinetics.

Effect of diethylenetriamine sensitization on detonation of nitromethane in porous media

The effect of varying the explosive sensitivity for a heterogeneous explosive formed of a packed bed of glass beads impregnated with liquid nitromethane (NM) sensitized with diethylenetriamine (DETA) is investigated. The changes in the dependence of critical charge diameter on bead size is investigated at concentrations of 5%, 10%, and 15% DETA over a range of bead diameters from 66 μm to 2.4 mm. Additional measurements are also performed on the critical diameter of the homogeneous liquid explosive itself. Three regions of behavior are identified for the bead-liquid explosive mixture. For bead sizes above approximately 1.5 mm, the critical diameter increases drastically as the amount of DETA is reduced from 15% to 10% and 5%. For bead sizes below approximately 1 mm, the critical diameter is nearly unchanged at concentrations of 10% and 15%. Between 1 and 1.5 mm, a region where the critical diameter increases sharply is found and detonation failure was observed in charge diameters of up to 60.5 mm. This transition region between the two others is found to be wider at lower DETA concentrations. This supports the previous proposition of two distinct mechanisms of detonation propagation which depend on the bead size: propagation through the pores of the media and propagation through the medium material itself.

Critical diameter for the transmission of a detonation wave into a porous medium

An experimental investigation has been undertaken to elucidate the existence of a critical diameter forthe transmission of gaseous detonation into a porous medium. A Chapman-Jouguet (CJ) detonation is first established in a tube and allowed to transmit through an orifice plate into a porous medium comprised of inert spheres of equal diameter. It is found that detonation can successfully transmit past the orifice for diameters much smaller than the normal critical diameter (dc) of the mixture. An immediate transition from detonation to quasi-detonation normally takes place upon wave entry in the porous medium. Failure of detonation is observed to take place downstream of the orifice in the near-limit regime and is followed by deflagration to detonation transition (DDT) within the porous medium. Wave velocities in the porous medium are found to be identical to the corresponding values measured for direct transmission (without an orifice). For subcritical conditions, there is complete quenching of combustion in the pores. The critical composition (lean and rich) for mixtures with high activation energy is found to be practically the same as the propagation limits in the porous medium without an orifice. This indicates that the phenomenon is governed by the smallest physical dimension of the pore size, and thus a local failure mechanism exists. In mixtures highly diluted with argon, i.e., (C2H2−O2)+75% Ar, which have a lower activation energy and for which the “dc=13λ” correlation (where λ is the cell size) is known to break down, the critical composition appears to depend on the orifice diameter. The orifice now introduces a larger controlling length scale at the limits compared to the pore size, indicating that a global failure mechanism may prevail for such mixtures. Present findings are consistent with a local and global failure mechanism for normal detonation failure recently proposed by Lee.

Detonation failure diameters and detonation velocities of nitric acid, acetic acid and water mixtures

Abstract Detonation failure experiments and detonation velocity measurements were carried out with homogeneous liquid compositions of nitric acid, acetic acid and water contained in steel tubes with different diameters. The criterion for failure or propagation of detonation was based upon the type of damage exhibited by the tubes after the experiments. Mixtures with the same critical diameter were determined by varying the composition in experiments performed in a set of tubes with the same diameter. These results were used to construct iso-critical diameter curves in the ternary diagram of the system of mixtures. The critical diameters were found to be strongly dependent on the mass fraction of water and the equivalence ratio. The detonation velocity measurements were performed on some oxygen - balanced mixtures contained in tubes with different diameters. The Detonation Velocities versus the Inverse of Tube Diameter curves were found to be linear, their slopes increasing with the mass fraction of water. The detonation velocities were found to be in the range 6000–6400 m/s.

Detonation Behavior of Emulsion Explosives

AbstractDetonation behavior of emulsion explosives sensitized by glass microballoons was studied. The detonation velocity and the failure diameter of emulsion explosives at various densities were measured in two charge configurations: cylindrical charges by ionization pins and conical charges by ionization pins and continuous resistance probes. The effect of microballoon size on detonation velocity and failure diameter was also determined.The behavior of emulsion explosives showed a marked difference from that of other high explosives; the detonation velocity reached its maximum far below the maximum density; the failure diameter increased with increasing density. The detonation velocity extrapolated to infinite diameter deviated increasingly from the predictions of the BKW code and of the Kamlet method as the initial density decreased. Diameter effect curves were downward concave and the diameter effect became more pronounced with increasing density. The temperature had a negligibie effect on the detonation velocity. At a given density, the detonation velocity increased linearly with decreasing microballoon size. The increase in detonation velocity was obscured at low densities by the increasing weight of glass added. I he failure diameter had a linear relationship with the spacing between microballoons.

Numerical simulations on failure processes of detonations propagating in a divergent nozzle.

The present paper studies processes of the failure of detonations propagating in a divergent nozzle by conducting numerical simulations. A finite-difference method, which combines the Random Choice Method with a chemical kinetics simulation code and a computational step for effects of nozzle cross section, is applied to simulate gasdynamic phenomena of one-dimensional detonations moving along a divergent nozzle in a hydrogen and oxygen diluted in argon mixture. Also, two-dimensional simulations are carried out and compared with the one-dimensional results. It is shown that the present one-dimensional calculations simulate averaged flow fields of the two-dimensional results over the cross section of the nozzle.

Steady detonation waves with losses

The Euler equations for a reacting polytropic gas with model losses, applied to unsupported steady detonation waves, lead to fast and slow detonation speeds for a given loss. The reaction rate is taken to have the Arrhenius form. The differential equations for the Mach number squared m and the reaction progress variable λ are integrated numerically using a fourth‐order Runge–Kutta–Fehlberg method. The calculations are presented as plots of trajectories in the λ‐log (m) plane. The main parameters that are varied are the activation temperature, the heat of reaction, the order of the reaction, and the polytropic exponent γ. The onset of detonation failure, and a continuum of solutions that appears in some parameter ranges, are explored in detail. The results are summarized in plots of the detonation Mach number squared m0 versus the logarithm of the loss parameter.

Dynamics of pressure changes accompanying the initiation of cast tg 50/50 by a diverging shock wave

The purpose of the work was to compare the dynamics of the development of initiation of TG 50/50 by diverging and plane detonation waves. The form of the wave accompanying the development of detonation was determined with a streak camera. The results show that the parameters of the flow behind the front of a hemispherical diverging detonation wave vary monotonically: the pressure decreases along the trajectory of a Langrangian particle and increases along the trajectory of the front. The zone directly adjacent to the front determines the development of the detonation. The generalized kinetic characteristic of heat liberation increases monotonically with the pressure in the range up to 20 GPa. The singularities in the dependences in the direction at an angle 45/sup 0/ to the x axis and the presence of a maximum on the curve of detonation failure are explained by the effect of hydrodynamic losses associated with the divergence of waves.