Von Neumann Spike Research Articles

Experimental and Numerical Study of Pulse Detonation Engine with Ethylene as a Fuel

This paper presents an insight on the study of transient, compressible, intermittent pulsed detonation engine with one-step overall reaction model to reduce the computational complexity in detonation simulations. Investigations are done on flow field conditions developing inside the tube with the usage of irreversible one-step chemical reactions for detonations. In the present simulations one and two-dimensional axisymmetric tubes are considered for the investigation. Studies are also performed with different grid sizes which influence the prediction of Von-Neumann spike, CJ Pressure and detonation velocity. Numerical studies are conducted using 66mm diameter tube and compared with the literature values. The simulation result from the single-step reaction model agrees well with the previous published literature of multi-step reaction models. The present studies shows that one-step overall reaction model is sufficient to predict the flow properties with reasonable accuracy. Experiments are conducted in a 25mm dia. tube with Ethylene/Air mixtures. The reactants are premixed and injected into the detonation tube using solenoid valves. The equivalence ratio of the fuel/oxidizer mixture and the length of the detonation tube are varied to understand their effects on detonation parameters. Finally the results from the present numerical study are compared and validated using NASA CEA and experiments.

Effect of charge diameter on detonation parameters in the mixture of gelled nitromethane/microballoons

In this work, the detonation wave structure and detonation parameters of mixtures of gelled nitromethane (NM) with hollow glass microballoons (GMBs) are investigated with a laser velocimetry technique. It is shown that the addition from 0.5 to 13 wt. % GMB with the mean diameter of 70 μm does not qualitatively change the reaction zone structure of gelled NM, and it corresponds to the Zeldovich–von Neumann–Döring theory, predicting the formation of the von Neumann spike. The detonation parameters of the explosive mixture monotonically decrease—in particular, the ideal detonation velocity drops from 6.3 km/s for gelled NM to 3.7 km/s for its mixture with 13 wt. % GMB. The addition of GMB leads to a significant reduction in the critical detonation diameter, which decreases from 16.5 mm in gelled NM to 3.5 mm in the mixture with 8 wt. % GMB. The influence of polymethylmethacrylate concentration in the mixture of gelled NM with GMB on the value of critical diameter of the mixture is observed.

Thermochemical release mechanisms and circumferential initiation affecting detonation in an aluminized explosive

ABSTRACT A modified thermochemical energy term and γ-law extension are incorporated into the JWL equation of state for predicting PBXN-111 detonation behavior under self- and externally supported conditions. The modifications resolve previous speculations regarding the causes affecting non-scalability of the reaction zone and detonation velocity with respect to explosive diameter, and a problem encountered to quantify increased detonation power gain by circumferential initiation. The modifications accurately consider “static effects” of size and the additional hydrodynamic compression resulting from a consecutive 2-step shock jump during convergent front collapse, leading to an elevated von Neumann spike and Chapman-Jouguet state between which physical and chemical transformations take place affecting shock regeneration and detonation power release. Necessary modifications to the CYLEX test are recommended to ensure that wall expansions are measured under steady-state conditions.

Chemical reaction zone measurements in pressed trinitrotoluene (TNT) and comparison with triaminotrinitrobenzene (TATB)

Photonic Doppler velocimetry and digital high-speed shadowgraphy have been used to characterize the chemical reaction zone parameters of pressed trinitrotoluene (TNT) samples with an initial density of 1.568 g cm−3. Comparison of the nanosecond time-resolved particle velocity histories of the free surfaces of detonating charges in air and light vacuum and of the interfaces between TNT detonation products and lithium fluoride or polymethyl methacrylate windows allow one to bracket the von Neumann spike pressure between 24.8 and 28.8 GPa. Our velocity waveforms confirm the two-step reaction pathway already observed in TNT, triaminotrinitrobenzene (TATB), and nitromethane, with a first fast energy release over 80 ns followed by a slower release over 250 additional ns. We consider the end of the first release zone as the locus of the Chapman–Jouguet (CJ) state, and the CJ pressure thus lies between 17.6 and 17.7 GPa. The energy release is not completed in this Jouguet plane but only after about ≈280 ns when the carbon cluster formation process ends. This corresponds to both the end of the slow release part of the interface velocity profiles and to the moment at which the free surface velocity profiles reach their maximum. Our shadowgraphy images confirm that carbon formation occurs very rapidly after the detonation breakout, in good agreement with previous time-resolved small-angle x-ray scattering measurements. The comparison with similar results previously obtained on TATB allows one to further highlight their similarities, which mainly result from their excess carbon production at late times.

Preliminary experimental study on the mechanisms of explosion-electric coupling

Electric energy is one of the most widely used forms of energy. It is valuable to couple electric energy with an explosion to boost detonation performance. The phenomenon of electric energy enhancing an explosion is referred to herein as explosion-electric coupling (EEC) by the author. An experimental device was designed to explore this phenomenon, and the device was equipped with measuring devices to collect data related to the electric and detonation characteristics of explosives. Trinitrotoluene and aluminized explosives were selected as test explosives and pressed into flat and cylindrical charges. Thus, different kinds of explosives with different shapes were able to be studied. The change of the current-voltage curve of the EEC was analyzed, and the process of the EEC was divided into three stages: explosion initiation, detonation growth, and steady-state detonation propagation. The results showed that the enhanced pressure of the von Neumann spike varied with the explosive type, the same as for the increase of the detonation velocity. However, the increased pressure of the Chapman-Jouguet point mattered for the shape of the explosive charges. According to the features of the electrical characteristics and the increases of the detonation properties, a hypothesis was proposed to explain how the electric energy affected the explosion. The external electric field and the large pulse current were two forms of electric energy that influenced the explosion, and these forms of electric energy had different effects on different detonation properties.

URGENT COMMUNICATION The Deceleration Curve of the von Neumann Spike Is the New Feature of Detonating Explosives

A new characteristic of detonating explosives is proposed—the deceleration curve of the von Neumann spike. An example of plotting such a curve on the basis of experimental data obtained at the All-Russian Scientific Research Institute of Technical Physics for detonating plastic-bobded TATB is given.

Methane–oxygen detonation characteristics at elevated pre-detonation pressures

The detonation characteristics of methane–oxygen mixtures at pre-detonation pressures of 101–1,013 kPa were investigated in a detonation tube. Both pure methane–oxygen mixtures and mixtures with argon dilution were explored. Measurements made include cell sizes via soot foil, wave speed via high speed ion probes / pressure transducers, and temperature / H2O molar concentration profiles via 100 kHz absorption spectroscopy. Measured cell widths agreed with predicted cell widths based on a ZND length correlation. In addition, the power law fit of cell width with pre-detonation pressure agreed with previous data at less than 101 kPa. Measured detonation wave speeds agreed within 3% of Chapmen-Jouguet for all cases. H2O molar density and temperature were successfully captured up to 507 kPa. However, above 507 kPa pre-detonation pressure, low signal to noise ratio and poor spectral fits at the extreme conditions of the von Neumann spike resulted in unacceptable uncertainty. These results provide a unique dataset to validate kinetics models and high-fidelity computation fluid dynamics codes for methane-oxygen detonations at elevated pre-detonation pressures relevant to rotating detonation rocket engines.

Structure of Detonation Waves in Mixtures of Tetranitromethane with Nitrobenzene and Methanol

Experimental studies of the structure of detonation waves in mixtures of tetranitromethane with methanol and nitrobenzene have been performed. Particle velocity profiles at the arrival of detonation waves at the interface with a water window were measured by a laser interferometer. It has been shown that the flow pattern in the reaction zone changes sharply at a concentration of diluents in the vicinity of stoichiometry, resulting in a decrease in the amplitude of the von Neumann spike up to its complete disappearance. The detonation waves are stable to the formation of a cellular structure of the front over almost the entire concentration range, except in the range near the limiting values. At the same time, the amplitude values of particle velocity show poor reproducibility from experiment to experiment performed under the same conditions. The obtained experimental dependences of detonation velocity on the concentrations of methanol and nitrobenzene are in good agreement with thermodynamic calculations.

Detonation properties of the mixtures of tetranitromethane/methanol and tetranitromethane/nitrobenzene

In this work, the reaction zone structure, stability of detonation waves and limits of detonation propagation in the mixtures of tetranitromethane/methanol and tetranitromethane/nitrobenzene depending on the diluent concentration, were studied by a VISAR laser interferometer with a nanosecond time resolution. In the experiments, a sharp change in the reaction zone at the concentration of diluents in the vicinity of stoichiometry is observed – the amplitude of von Neumann spike decreases significantly. This is due to an increase in the initial reaction rate, which leads to the decomposition of HE directly in the shock wave front. It was found that the addition of diluent resulted in an increase in the detonation velocity of the tetranitromethane/diluent mixture in the vicinity of the stoichiometric concentration (about 25% of methanol or nitrobenzene). In almost the entire concentration range, the detonation waves are stable with respect to the formation of the cellular structure of the front. At the same time, in the range of diluent concentration of 15-35%, a poor reproducibility of the amplitude values of the particle velocity from shot to shot is observed. This is probably due to the longitudinal instability of the detonation waves in the mixture.

Effects of Turbulence and Temperature Fluctuations on Knock Development in an Ethanol/Air Mixture

The effects of turbulence on knock development and intensity for a thermally inhomogeneous stoichiometric ethanol/air mixture at a representative end-gas autoignition condition in internal combustion engines are investigated using direct numerical simulations with a skeletal reaction mechanism. Two- and three-dimensional simulations are performed by varying the most energetic length scale of temperature, $$l_T$$ , and its relative ratio with the most energetic length scale of turbulence, $$l_T/l_e$$ , together with two different levels of the turbulent velocity fluctuation, $$u'$$ . It is found that $$l_T$$ / $$l_e$$ and the ratio of ignition delay time to eddy-turnover time, $$\tau _{ig}/\tau _t$$ , are the key parameters that control the detonation development. An increase in either $$l_T$$ or $$l_e$$ enhances the detonation propensity by allowing a longer run-up distance for the detonation development. The characteristic length scale of the temperature field, $$l_T$$ , is significantly modified by high turbulence intensity achieved by a large $$l_e$$ and $$u'$$ . The intense turbulence mixing effectively distributes the initial temperature field to broader scales to support the developing detonation waves, thereby increasing the likelihood of the detonation formation. On the contrary, high turbulence intensity with a short mixing time scale, achieved by a small $$l_e$$ and a large $$u'$$ , reduces the super-knock intensity attributed to the finer broken-up structures of detonation waves. Either $$\tau _{ig}/\tau _t$$ less than unity or $$l_e = l_T$$ even with a large $$u'$$ is found to have no significant effect on super-knock mitigation. Finally, high turbulent intensity may induce high-pressure spikes comparable to the von Neumann spike. Increased temperature and pressure by combustion heating, noticeably after the peak of heat release rate, significantly enhance the collision and interaction of multiple emerging autoignition fronts near the ending combustion process, resulting in localized high-pressure spikes.

Shock Initiation Microscopy with High Time and Space Resolution

AbstractWe describe studies of shock initiation and shock‐to‐detonation transitions in energetic materials using a tabletop shock compression microscope with nanosecond time resolution and micrometer spatial resolution. Planar input shocks with durations of 4–20 ns are produced using 0–4.5 km/s laser‐launched flyer plates. Emphasis is on measurements of temperature, velocities, pressure, and microstructure using photon Doppler velocimetry (PDV), optical pyrometry and high‐speed videography. Techniques are discussed for fabricating disposable shock target arrays of tiny plastic‐bonded explosives (PBX), liquid and powder explosives, and single‐crystal explosives for high‐throughput studies. Optical temperature measurements of shocked triaminotrinitrobenzene (TATB) are discussed. Since TATB is yellow, we developed methods to correct for the blue absorption to obtain more accurate temperatures. Hot spots in shocked polymer‐encased HMX (octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine) crystals are observed in real‐time, showing a hot spot produced in a collapsing void that ignites a deflagration. Despite the small dimensions of our explosive charges (typically 1 mm diameter and 250 μm length), we produced reproducible detonation states in solid and liquid explosives using short‐duration shocks near the von Neumann spike (VNS) pressure. We show the VNS pressure is associated with a transition to high‐efficiency gas production from the explosives. In studies of NM, prior to detonation, we see reaction originating at hot spots which coalesce to form a superdetonation.

Detonation properties of the mixture of tetranitromethane/nitrobenzene

The experimental investigation of the reaction zone structure and stability of detonation waves was performed for the mixture of tetranitromethane/nitrobenzene (TNM/NB) by a VISAR laser interferometer. At the near stoichiometric concentration of NB, it is observed that the amplitude of von Neumann spike decreases significantly, whereas the detonation parameters increase. The instability of detonation waves in the TNM/NB mixture is shown, both with respect to one-dimensional longitudinal perturbations and to the curvature of the front, which results in the formation of a cellular structure. The dependence of detonation velocity of TNM/NB on the diluent concentration and limits of detonation propagation were found.

Measuring the detonation reaction zone structure ofJOB-9003 explosive using PDV

In order to obtain the chemical reaction zone of HMX based JOB-9003 explosive, experimental measurements on the detonation wave profile of solid explosives using photon Doppler velocimetry (PDV) have been performed. Planar detonations were produced by impacting the explosive with sapphire flyer launched from a powder gun. Particle velocity wave profiles were measured at the explosive/window interface. LiF windows with very thin vapor deposited aluminum mirrors were used in the experiments. The time resolution of PDV is about 1 ns, and the velocity uncertainty is less than 2%. The measurements show distinct end to the reaction zone indicating a CJ point in JOB-9003. The results show that the reaction time of JOB-9003 is (11±2) ns, and the corresponding reaction length is (0.075±0.014) mm. The CJ pressure is (35.6±0.9) GPa, and the pressure at Von-Neumann spike is (47.9±1.2) GPa.

Measuring detonation wave profiles in plastic-bonded explosives using PDV

To obtain reaction zone parameters for several high explosives, experimental measurements of the detonation wave profiles in HMX-, RDX- and PETN-based explosives were performed using photon Doppler velocimetry (PDV). Planar detonations were produced by impacting the explosive with a sapphire flyer in a gas gun. Particle velocity wave profiles were measured at the explosive/window interface. LiF windows with very thin vapor-deposited aluminum mirrors were used for experiments. All measurements show a distinct end to the reaction zone, indicating a Chapman-Jouguet (CJ) point. For HMX-based explosives, the presented measurements show that the fast reaction time is approximately 10±2 ns, whereas for RDX- and PETN-based explosives, the values are 14±3 ns and 7±2 ns, respectively. The reaction times or reaction zone lengths obtained in the present study are smaller than previously reported data but much closer to the estimated values in theory. Additionally, the velocity at the Von Neumann (VN) spike was analyzed using the “beat cycles” method, and the pressure at the VN spike was obtained.

Detonation on a tabletop: Nitromethane with high time and space resolution

Nitromethane (NM), a flammable liquid, has been a model system for the shock-to-detonation transition in homogeneous condensed-phase explosives for over 50 years, but we do not understand the fast processes at the molecular scale in the detonation front at the molecular scale. That is largely because prior studies triggered detonations in bomb-sized charges with input shock durations and times-to detonation that were typically microseconds, which made it impossible to observe the faster processes in real time. We studied NM shocked with 4 ns duration input pulses using a tabletop apparatus with laser-launched flyer plates and arrays of tiny disposable optical cuvettes, where the pressure and temperature were probed in real time (1 ns) with photon Doppler velocimetry, optical pyrometry, and high-speed video. Using a 4 ns shock with an input pressure close to the von Neumann spike pressure of 19 GPa, we achieved the minimum time-to-detonation, about 12 ns, where the time-to-detonation is controlled by fundamental molecular processes. We demonstrated the reproducibility of our detonations and showed that they had the same properties as in bomb-sized charges: our detonation velocity, von Neumann spike and Chapman-Jouguet pressures, temperatures, and reaction zone lengths were the same as in bomb-sized charges. Being able to trigger realistic reproducible detonations from a short pulse makes it possible to investigate molecular and fluid dynamics in the detonation by measuring transient responses in real time. We found that it took 6 ns for the temperature to reach 3430 K. The high pressure was observed at about 8 ns, when there was a volume explosion to nearly twice the von Neumann spike pressure before settling down to a steady detonation.

Non-classical detonation regimes of liquid high explosives

This paper presents experimental data of studies of the liquid explosives characteristics, in the reaction zone of which the distribution of parameters does not correspond to the classical detonation theory. An increase of the pressure and particle velocity behind a shock jump in liquid explosives and the possibility of the existence of a steady-state detonation wave without a von Neumann spike are interpreted within the framework of models that take into account the possibility of chemical reactions directly in the shock wave front.

A novel method for the measurement of the von Neumann spike in detonating high explosives

We present detonation wave profiles measured in T2 (97 wt. % TATB) and TX1 (52 wt. % TATB and 45 wt. % HMX) high explosives. The experiments consisted in initiating a detonation wave in a 15 mm diameter cylinder of explosive using an explosive wire detonator and an explosive booster. Free surface velocity wave profiles were measured at the explosive/air interface using a Photon Doppler Velocimetry system. We demonstrate that a comparison of these free surface wave profiles with those measured at explosive/window interfaces in similar conditions allows to bracket the von Neumann spike in a narrow range. For T2, our measurements show that the spike pressure lies between 35.9 and 40.1 GPa, whereas for TX1, it lies between 42.3 and 47.0 GPa. The numerical simulations performed in support to these measurements show that they can be used to calibrate reactive burn models and also to check the accuracy of the detonation products equation of state at low pressure.

Nonideal detonation regimes in low density explosives

Measurements using Velocity Interferometer System for Any Reflector (VISAR) were performed for three high explosives at densities slightly above the natural loose-packed densities. The velocity histories at the explosive/window interface demonstrate that the grain size of the explosives plays an important role. Fine-grained materials produced rather smooth records with reduced von Neumann spike amplitudes. For commercial coarse-grained specimens, the chemical spike (if detectable) was more pronounced. This difference can be explained as a manifestation of partial burn up. In fine-grained explosives, which are more sensitive, the reaction can proceed partly within the compression front, which leads to a lower initial shock amplitude. The reaction zone was shorter in fine-grained materials because of higher density of hot spots. The noise level was generally higher for the coarse-grained explosives, which is a natural stochastic effect of the highly non-uniform flow of the heterogeneous medium. These results correlate with our previous data of electrical conductivity diagnostics. Instead of the classical Zel'dovich–von Neumann–Döring profiles, violent oscillations around the Chapman–Jouguet level were observed in about half of the shots using coarse-grained materials. We suggest that these unusual records may point to a different detonation wave propagation mechanism.

Numerical Study of Pulse Detonation Engine with One-step Overall Reaction Model

<p>This paper presents an insight for the study of transient, compressible, intermittent pulsed detonation engine with one-step overall reaction model to reduce the computational complexity in detonation simulations. Investigations are done on flow field conditions developing inside the tube with the usage of irreversible one-step chemical reactions for detonations. In the present simulations 1-D and 2-D axisymmetric tubes are considered for the investigation. The flow conditions inside the detonation tube are estimated as a function of time and distance. Studies are also performed with different grid sizes which influence the prediction of Von-Neumann spike, CJ Pressure and detonation velocity. The simulation result from the single-cycle reaction model agrees well with the previous published literature of multi-step reaction models. The present studies shows that one-step overall reaction model is sufficient to predict the flow properties with reasonable accuracy. Finally, the results from the present study were compared and validated using NASA CEA.</p><p><strong>Defence Science Journal, Vol. 65, No. 4, July 2015, pp. 265-271, DOI: http://dx.doi.org/10.14429/dsj.65.8730</strong></p>

Feasibility analysis of SPH method in the simulation of condensed explosives detonation with ignition and growth model

The smoothed particle hydrodynamics (SPH) method has the advantage in modeling detonation process. But in most previous applications of SPH method in the simulation of explosives, the detonation process of explosives is usually regarded as steady-state detonation in which the reaction rate of explosive is ignored and propagation of detonation wave is at the Chapman–Jouguet (C–J) state. The simulation of steady-state detonation process cannot predict the von Neumann spike state of explosives which is another important feature in studying condensed explosives. The simulation of condensed explosives response under impact load also cannot base on the steady-state detonation assumption. In this paper, the SPH method is applied to simulate the detonation process of condensed explosives with reaction rate. The ignition and growth model is incorporated into the SPH method to represent the reaction model of condensed explosives. One-dimension detonation process of condensed explosives PBX-9404 and TATB are taken as numerical examples to analyze the feasibility of SPH method in the simulation of detonation with ignition and growth model. Numerical results show that the SPH method combined with ignition and growth model can give good prediction for the von Neumann spike state of detonation in condensed explosives. Propagation and interaction of cylindrically divergent detonation wave in PBX-9404 have also been successfully simulated by SPH method combined with ignition and growth model.