Average Propagation Velocity Research Articles

Crack propagation velocity and fracture toughness of hydroxyl-terminated polybutadiene propellants with consideration of a thermo-viscoelastic constitutive model: Experimental and numerical study

Solid rocket motors are subject to numerous temperature loads during their service life, so it is essential to investigate the sensitivity of propellant mechanical properties to temperature, particularly fracture properties. This paper presents fracture characteristics of hydroxyl-terminated polybutadiene (HTPB) propellant with mechanical-thermal coupled responses. This work investigated the fracture morphology of the HTPB propellant at different temperatures (233.15 K–333.15 K) using a self-developed thermostatic in-situ video imaging. The crack propagation velocity and J-integral were measured using single-edge notched tension (SENT) specimens. In the numerical study, a thermo-viscoelastic constitutive model combined with 1/r singular elements was used to perform calculations on J-integral. The thermo-viscoelastic constitutive model combined with a cohesive zone model was used to simulate crack propagation; meanwhile, load–displacement curves and crack propagation velocity were obtained. All simulation results were analyzed in comparison to the experimental results. The results revealed that temperature could significantly affect the fracture properties of the HTPB propellant. The fracture morphology of the HTPB propellant changed significantly at different temperatures. As the temperature decreases, the strength of the material gradually increases, with a consequent increase in the critical J-integral and a decrease in the average crack propagation velocity. The time ratio governed the regularity of average instantaneous crack propagation velocity with temperature. Furthermore, three simulation methods for calculating the J-integral were analyzed and discussed.

Ionization Waves, Propagating in Opposite Directions, as in Red Sprites

The paper is devoted to the study of a pulsed streamer discharge, similar to that in the Earth’s upper atmosphere. An experimental setup providing the formation of two ionization waves propagating in opposite directions from a region filled with a plasma formed by the capacitive discharge in low-pressure atmospheric air was created. In a physical experiment, the process of propagation of red ionization waves (streamers) was simulated. It was established that the average propagation velocities of their fronts correspond to those of red sprites. This was shown as a result of spectral studies that at air pressures of 0.4-3 Torr, the radiation color radiation observed visually and captured on an integral photograph from the region of passage of ionization waves is determined by the spectral transitions of the first positive system (FPS) of nitrogen molecules, similar to what occurs for red sprites. In this case, the spectral energy density of radiation in the most intense band of the second positive system (SPS) of the nitrogen molecule with a wavelength of 337.13 nm is an order of magnitude or higher than that in the most intense band of the FPS with the wavelength of 775.32 nm. Using the emission spectra and methods of optical emission spectroscopy (OES), the main parameters of the discharge plasma are estimated. Thus, the created setup makes it possible to simulate the process of the formation of red sprites propagating in opposite directions under laboratory conditions.

Observation of Rayleigh-Lamb waves generated by the 2022 Hunga-Tonga volcanic eruption with the POLA detectors at Ny-Ålesund

The eruption of the Hunga-Tonga volcano in the South Pacific Ocean on January 15, 2022, at about 4:15 UTC, generated a violent explosion, which created atmospheric pressure disturbances in the form of Rayleigh-Lamb waves detected all over the globe. Here we discuss the observation of the Hunga-Tonga shock-wave performed at the Ny-Ålesund Research Station on the Spitsbergen island, by the detectors of the PolarquEEEst experiment and their ancillary sensors. Online pressure data as well as the results of dedicated offline analysis are presented and discussed in details. Results include wave arrival times, wave amplitude measurements and wave velocity calculation. We observed five passages of the shock wave with a significance larger than 3 sigma and an amplitude up to 1 hPa. The average propagation velocity resulted to be (308 ± 0.6) m/s. Possible effects of the atmospheric pressure variation associated with the shock-wave multiple passages on the cosmic-ray rate at ground level are also investigated. We did not find any significant evidence of this effect.

An experimental study on the dynamic frosting characteristics on the edge zone of a horizontal copper plate under forced convection

To retard the negative effects of frosting, it is necessary to better understand the frosting mechanism on cold surfaces. In this study, a systematical study on the dynamic frosting characteristics on the edge zone of a horizontal copper plate under forced convection was conducted. The results showed that the air velocity effect and edge effect on droplet condensation, frozen and frost layer growth characteristics were significant. The duration of droplet condensation stage decreased from 365 to 113 s with a decrease of 69.0% when air velocity increased from 0.5 to 2.5 m/s. Besides, the average freezing wave propagation velocity for Region I (an area where a row of water droplets closest to plate edge) was significantly larger than that for Region II (the remained area). As a result, the duration of droplet frozen stage for Region I was much shorter than that for Region II. Accordingly, frost layer amount for Region I was notably larger than that for Region II. Besides, the frost surface roughness for Region I increased as air velocity increased at the early frosting stage, but decreased as air velocity increased at the later frosting stage. The general turning point for frost surface roughness at 1.5 m/s surpassed the value at 0.5 m/s was around 21.4 × 10−6 m at 390 s, and that between 2.5 m/s and 1.5 m/s was around 30.3 × 10−6 m at 450 s. This study can help to establish a better relationship among different frosting stages, and better understand the plate edge effect on frosting.

Preparation and characterization of flower-like Mg-Al hydrotalcite powder for suppressing methane explosion

A novel flower-like Mg-Al hydrotalcite powder was successfully prepared using coprecipitation method and then characterized for suppressing methane explosion. The influences of Mg-Al hydrotalcite on the explosion pressure and flame propagation characteristics of methane were studied using a 5 L pipeline explosion experimental setup, and its explosion suppression performance was compared with that of magnesium hydroxide and aluminum hydroxide. The results show that the novel Mg-Al hydrotalcite powder prepared in this study has an excellent methane explosion suppression performance. The maximum explosion pressure of methane after adding 0.2 g/L Mg-Al hydrotalcite powder was decreased by 77.72%, and the average flame propagation velocity of methane explosion was decreased by 66.34%. With an increase of the Mg-Al hydrotalcite powder concentration, the main methane flame structure became progressively discrete. Combined with the physical and chemical properties of Mg-Al hydrotalcite powder, its explosion suppression mechanism on 9.5% methane was discussed.

Passive Tomographic Study on Velocity Changes in Underground Mines Using Tabular Mesh Grids

Abstract Double difference tomographic inversion on measurements of travelling time and location are performed to analyze the velocity structure within rock mass in underground mining. Residuals of each iteration are estimated to evaluate the conversion of computation. Average wave propagation velocities in tabular areas are assessed to compare the velocity change affected by mainshocks. It is summarized that velocity increases before mainshocks and then reduces with temporal evolution after them. Possible explanations include static stress buildup that enhances the wave propagation before mainshocks and static stress reduction that weakens the waveform propagation. Additionally, wave propagation is attenuated by the dynamic-shaking induced fractures and ruptures within rock masses. Velocity change is shown to be of importance in assessing the stress redistribution and stability of rock masses. It is found that P-wave velocity increased by 1% ~5% before the occurrence of mainshocks. After the mainshocks, the velocity turned into decreasing and eventually dropped to a level that was even lower than the average level before the mainshocks. It can be inferred that stress increased and formed a stress concentration region before the mainshocks. The occurrence of mainshocks caused damage and stress relaxation in the rock mass, leading to a significant velocity decrease.

Flame Propagation Characteristics of Hybrid Explosion of Ethylene and Polyethylene Mixture under Pressure Accumulation

In order to study the flame propagation characteristics of a ethylene/polyethylene hybrid explosion under pressure accumulation, a visual pressure-bearing gas/power hybrid-explosion experimental platform was built. The flame propagation characteristics of polyethylene and ethylene/polyethylene hybrid explosions in the closed vessel were analyzed. The results show that the flame brightness, flame front continuity and average flame propagation velocity of polyethylene dust explosion in the closed vessel increased first and then decreased when the polyethylene dust concentration increased. The curve of the flame propagation velocity with time had obvious pulsation characteristics. Adding 1% and 3% ethylene to different concentrations of polyethylene dust significantly improved its explosion flame brightness, flame front continuity and average flame propagation velocity. Moreover, it also improved the fluctuation amplitude of the explosion flame propagation velocity with time curve. The explosion flame of the polyethylene dust and ethylene/polyethylene hybrid mixture included four zones during the propagation process, which were denoted as the unburned zone, preheated zone, premixed flame zone and dust flame zone. The addition of ethylene to polyethylene dust can significantly increase its thickness of premixed flame zone and preheated zone, and the thickness increased when the ethylene concentration increased.

Dynamic Propagation and Electro-Mechanical Characteristics of New Microcracks in Notched Coal Samples Studied by the Three-Point Bending Test System and AFM

This work extends research on the mechanism of electromagnetic radiation (EMR) induced by coal or rock fractures to the category of microscopic dynamic experimental research. A custom-made three-point bending test system and atomic force microscope (AFM) were integrated to obtain the microdynamic loading test system. The notched coal samples were prepared specially. The dynamic propagation of new microcracks in coal samples was measured, and the propagation velocity was calculated. The morphology and electro-mechanical characteristics of new microcracks were tested. More importantly, the causes of the changes in the electro-mechanical characteristics before and after fracture were analyzed, and the effects of these changes on the EMR were discussed. The results showed that the average propagation velocities during the same time interval are 9.5 μm/s, 12.1 μm/s, and 16.2 μm/s. The elastic modulus of the material at the microcrack edge is generally smaller than that of the material in other locations, while the adhesion and deformation are larger. Moreover, the closer the material is to the microcrack, the higher its surface potential. The electrons generated at the microcrack edge and emitted into the atmosphere, which made the greater potentials of the microcrack edge. Many electrons with different velocities and directions migrate in similar parallel-plate capacitors, which are formed by the relative microscale surface of the coal microcrack tip and have different field strengths, resulting in EMR with complex frequencies and different intensities. This study provides a micro-dynamic experimental basis for research on the electromagnetic radiation mechanism.

Influence of impact loading orientations on the mechanical behaviour of rocks around a tunnel

The surrounding rock of tunnels may suffer from dynamic disturbances caused by impact loading or blasting at different locations nearby, leading to the instability of fractured rock masses. To investigate the mechanical behaviour of the surrounding rock subjected to various impact loading directions, an improved drop weight system is introduced for experimental study. To better understand the mechanisms in fractured rock mass, the extended peridynamic (XPD) method is adopted for auxiliary analysis. Both the numerical and experimental results consistently show that the initiation time, initiation toughness and fracture toughness (KI and KII) of precracks are notably affected by the loading directions. As the crack propagates towards the tunnel, the crack growth speed gradually decreases and is inversely proportional to the fracture toughness. The average errors between the test results and numerical results for crack fracture time, average propagation toughness and crack propagation velocity are 4.85%, 3.76% (KI), 4.08% (KII) and 6.79%, respectively, demonstrating the effectiveness of the XPD model in experimental prediction. Moreover, by using the XPD model, the distribution of deformation is analysed and the failure trend of tunnels on the shadow side can be identified. The precracks exhibit a tensile failure mode shortly after initiation, but then change to a mixed tensile–shear failure mode because of the influence of different loading directions. When the cracks break through the upper side of the tunnel, the crack initiation points of the shadow side are notably different for different loading directions, mainly occurring at the corner (E2), roof (A), spandrel (C2) and tunnel arc-side 45° (B2), providing a valuable reference for tunnel engineering.

Experimental study and mechanism analysis on the suppression of flour explosion by NaCl and NaHCO3

ABSTRACT The suppression effect of adding different proportions of NaCl and NaHCO3 powder on flour explosion was studied by using Hartmann experimental device and 20 L spherical explosive device. The results showed that with the increase of NaCl and NaHCO3 concentrations, the flame speed and propagation distance decreased gradually. The maximum explosion pressure Pmax and the maximum pressure rise rate (dP/dt)max decreased significantly. When 80% NaCl and NaHCO3 were added, the average flame propagation velocity reduced to 2.73 m/s and 0.56 m/s, respectively, and the maximum explosion pressure less than 0.15MPa, the flour explosion could be inhibited by NaCl and NaHCO3. NaHCO3 was more effective than NaCl in inhibiting flame propagation and explosion overpressure. Finally, combined with the pyrolysis characteristics of NaCl and NaHCO3, the mechanism of inhibitors inhibiting the explosion of flour was further studied. The endothermic decomposition of NaHCO3 produced inert gas, diluted oxygen concentration, captured active free radicals, and terminated the combustion chain reaction. NaCl inhibits the explosion of flour by absorbing heat and coating it.

Air-breathing rotating detonation fueled by liquid kerosene in cavity-based annular combustor

This study experimentally demonstrates the practicality of air-breathing/ramjet rotating detonation engines fueled by liquid kerosene and air. Experiments are performed, via a direct-connect scheme, in an annular combustor with a specific air-heater for generating supersonic air with a total temperature of 860 K, which approximates the Mach-4 flight condition at an altitude of 20 km. To address the difficulty in stabilizing the liquid-kerosene-air fueled rotating detonations, an optimized cavity that mimics the functionality of a scramjet is adopted in the rotating detonation combustor. The results indicate that, while maintaining the mass flow rate of the supersonic air at approximately 1000 g/s, the kerosene-air detonations can be successfully obtained in a self-sustaining manner with the equivalence ratio located within a wide range of 0.77–1.47. To the best of the authors' knowledge, these are the first experimentally obtained liquid-kerosene-air rotating detonations operating in the air-breathing mode. The average propagation velocities of the rotating detonation waves are experimentally measured to be approximately 60% of the corresponding theoretical Chapman–Jouguet velocity. Such significant velocity deficits, typically reported for liquid-fueled detonations, are indicative of considerable non-ideal effects experienced by the rotating detonation. Additionally, both the maximum average propagation velocity and largest statistically averaged pressure peak are obtained when the equivalence ratio approaches 1.10. Moreover, owing to the high-speed air inflow and the characteristic distance required for the evaporation and mixing of liquid kerosene, the detonation wave is located significantly downstream of the cavity, rather than typically being attached directly or close to the combustor inlet. The findings of this study provide insights into rotating detonations fueled by liquid kerosene, as well as demonstrate the high potential of their application to air-breathing propulsion.

Influence of Temperature on Lightning Performance of Mineral Oil

This article deals with the influence of temperature on the lightning impulse breakdown voltage (LI BDV) of mineral oil in a non-uniform electric field. The experiments were performed in point–plane electrode arrangements under a standard lightning impulse voltage of negative polarity. The LI BDV for the temperatures of 20, 40, 60, 80, 100 and 110 °C was measured using a step method with registration of the light signals emitted during pre-breakdown and breakdown phases. The average propagation velocity was also determined. The results confirm that the mineral oil tested under LI voltage behaves similarly to AC voltage-based stress. The LI BDV increases with temperature up to 100 °C and then tends to decrease. The results demonstrated that viscosity dependent processes are responsible for increasing the LI BDV with temperature up to the 100 °C; and liquid vapor formation processes are responsible for decreasing the LI BDV at temperatures equal to and higher than 100 °C.

The Normal Velocity of the Population Front in the “Predator–Prey” Model

The propagation of one and two-dimensional waves of populations are numerically investigated in the framework of the “predator-prey” model with the Arditi - Ginzburg trophic function. The propagation of prey and predator population waves and the propagation of co-existing populations’ waves are considered. The simulations demonstrate that even in the case of an unstable quasi-equilibrium state of the system, which is established behind the front of a traveling wave, the propagation velocity of the joint population wave is a well-defined function. The calculated average propagation velocity of a cellular non-stationary wave front is determined uniquely for a given set of problem parameters. The estimations of the wave propagation velocity are obtained for both the case of a plane and cellular wave fronts of populations. The structure and velocity of outward propagating circular cellular wave are investigated to clarify the local curvature and scaling effects on the wave dynamics.

Explosion characteristics of aluminum-based activated fuels containing fluorine

Measuring the dust explosion characteristics of aluminum-based activated fuels was a prerequisite for developing effective prevention and control measures. In this paper, ignition sensitivity, flame propagation behaviors and explosion severity of aluminum/polytetrafluoroethylene (Al/PTFE) compositions including 2 PT (2.80 wt.% F), 4 PT (7.18 wt.% F) and 8 PT (11.90 wt.% F) were studied. When the content of F increased from 2.80 wt.% to 11.90 wt.%, the minimum explosive concentration MEC decreased from 380 g/m3 to 140 g/m3, due to the dual effects of increased internal active aluminum and enhanced reactivity. The average flame propagation velocities increased as the percentage of F increased. The maximum explosion pressure Pm of 500 g/m3 aluminum-based activated fuels increased from 247 kPa to 299 kPa. Scanning electron microscopy demonstrated that with the increase of PTFE content, the reaction was more complete. On this basis, the explosion mechanism of aluminum-based activated fuels was revealed.

Особенности распространения стоячих электромагнитных и электронных волн в металлическом проводнике с электрическим переменным током проводимости

The paper demonstrates the results of approximate calculations on the establishment of basic features of the propagation of standing transversal electromagnetic waves (EMWs) and standing longitudinal de Broglie electronic waves in a homogeneous not massive non-magnetic metallic conductor of finite dimensions (the radius r0 and the length l0 >>r0) with the alternating axial-flow current of conductivity of i0(t) of different peak-temporal parameters. The correlation for the rated estimation of the average velocity of propagation of the standing transversal EMWs and standing longitudinal de Broglie electronic waves in a metal (alloy) of the indicated conductor is presented. It is shown that quantized standing transversal EMWs arising in a metallic conductor of finite dimensions substantially differ from ordinary transversal EMWs, propagated in the conducting environments of unlimited dimensions. An important feature of the standing transversal EMWs in the examined conductor is the fact that their tension of an axial-flow electric-field advances by a phase their tension of an azimuthal magnetic-field on the corner of π/2. It was established that in the standing transversal EMWs of the used conductor the energy of their electric field only passes into the energy of their magnetic field and vice versa. Therefore the standing transversal EMWs do not transfer the flows of the electromagnetic energy on the surface of the studied conductor.

Spreading of disturbances in realistic models of transmission grids in dependence on topology, inertia and heterogeneity

The energy transition towards more renewable energy resources (RER) profoundly affects the frequency dynamics and stability of electrical power networks. Here, we investigate systematically the effect of reduced grid inertia, due to an increase in the magnitude of RER, its heterogeneous distribution and the grid topology on the propagation of disturbances in realistic power grid models. These studies are conducted with the DigSILENT PowerFactory software. By changing the power generation at one central bus in each grid at a specific time, we record the resulting frequency transients at all buses. Plotting the time of arrival (ToA) of the disturbance at each bus versus the distance from the disturbance, we analyse its propagation throughout the grid. While the ToAs are found to be distributed, we confirm a tendency that the ToA increases with geodesic distance linearly. Thereby, we can measure an average velocity of propagation by fitting the data with a ballistic equation. This velocity is found to decay with increasing inertia. Characterising each grid by its meshedness coefficient, we find that the distribution of the ToAs depends in more meshed grids less strongly on the grid inertia. In order to take into account the inhomogeneous distribution of inertia, we introduce an effective distance r_{mathrm{eff}}, which is weighted with a factor which strongly depends on local inertia. We find that this effective distance is more strongly correlated with the ToAs, for all grids. This is confirmed quantitatively by obtaining a larger Pearson correlation coefficient between ToA and r_{mathrm{eff}} than with r. Remarkably, a ballistic equation for the ToA with a velocity, as derived from the swing equation, provides a strict lower bound for all effective distances r_{mathrm{eff}} in all power grids. thereby yielding a reliable estimate for the smallest time a disturbance needs to propagate that distance as function of system parameters, in particular inertia. We thereby conclude that in the analysis of contingencies of power grids it may be advisable that system designers and operators use the effective distance r_{mathrm{eff}}, taking into account inhomogeneous distribution of inertia as introduced in Eq. (12), to locate a disturbance. Moreover, our results provide evidence for the importance of the network topology as quantified by the meshedness coefficient beta.

Time-resolved imaging of CaF2 poly-crystal response following 355 nm nanosecond laser irradiation

The evolution of 355 nm nanosecond laser-induced damage to C a F 2 poly-crystals is investigated by using the time-resolved pump-probe shadowgraph technique. The damage morphologies of the front surface, rear surface, and interior of the C a F 2 crystal are imaged by optical microscopy. When the rear surface is focused by one laser pulse throughout the front surface, three shock waves (SWs) and one SW are observed in the air beside the front and rear surfaces, respectively. When the laser energies are 40 mJ and 60 mJ, at delay time of 1000 ns the radii of SW fronts beside the front surface are 2569.8 µm and 2831.7 µm, while those beside the rear surface are 1012.9 µm and 1078.1 µm, respectively. The filamentary channels inside the C a F 2 crystal are established before the end of a laser pulse at energies of 25, 40, or 60 mJ. The average propagation velocities of SWs along the filamentary channel are approximately 8.2 µm/ns. The maximum diameters of channels can reach approximately 53 µm and 128 µm for 25 mJ energy and 40 mJ energy, respectively. The experimental results help to explore the mechanism of laser-induced C a F 2 poly-crystal damage in nanosecond regime.

Explosion propagation in a dust removal pipeline under dust collector explosion

The shockwave and high-temperature flame of dust explosion in the dust collector are often transmitted through the dust removal pipeline, and then cause secondary harm. In this study, the dust explosion propagation in a typical dry dust collector connected with dust removal pipelines was studied by CFD. The impacts of static activation pressure of the pressure relief panels on the explosion propagation were also addressed. The dust explosion propagation in the dust removal pipeline showed multiple accelerated combustion processes and severe pressure shocks. The explosion pressure and temperature first increase and then decrease along with the pipeline. Under a higher static activation pressure, the maximum explosion pressure along with the pipeline increases by 144%, where the average velocity of the dust propagation, dust consumption, pressure shock wave, and flame propagation increase by 82.2%, 32.1%, 25.3%, and 70.1%, respectively. When the static activation pressure increases from 0.01 MPa by more than 25%, the changes of the dust explosion pressure and flame propagation in the pipeline are observed limited. The explosion-proof valve should be set between 5 m and 10 m in the dust removal pipeline. And the lower the static activation pressure, the closer the explosion-proof valve. This research provides a scientific basis for the effective control of dust explosion disasters in dust removal systems.

Experimental verification of cylindrical air-breathing continuous rotating detonation engine fueled by non-premixed ethylene

An especial air-breathing continuous rotating detonation engine without a centerbody is proposed to reduce energy loss and simplify the structure, which is analogous to a typical scramjet combustor with the cavity. Non-premixed ethylene/air rotating detonation experiments are performed in a direct-connect experimental facility with an air inflow of 860 K total temperature (Ma 2.02). Two combustion modes (continuous rotating detonation and deflagration mode) are discovered with varying equivalent ratio (ER). A proper accumulation layer of the combustible mixture in the cavity re-circulation region attaching the outer wall is critical for rotating detonation realization. The average propagation velocity of the detonation wave is 1297.1 m/s (78.5% C-J velocity) at ER = 0.64 with the combustor pressure of 230 kPa. Its motion of the reaction zone is quantitatively verified to be rotating through the integral of chemiluminescence intensity. Besides, it is confirmed by the optical observation that deflagration could also self-sustain in the short combustor with the axial-moving flame. The combustor pressure of deflagration mode can reach 220 kPa with ER = 0.56. Though the combustor pressure of detonation is slightly higher than that of deflagration, the impact of combustor high-pressure on the inflow of detonation mode is much stronger through the induced oblique shock wave.

Spall-Velocity Reduction in Double-Pulse Impact on Tin Microdroplets

We explore the deformation of tin microdroplets of various diameters induced by two consecutive laser pulses having pulse durations of 0.4 ns. Impact of laser pulses with this duration mainly leads to shock-wave-induced cavitation and spallation. The main result obtained in this work is the observation of a strong reduction of the spall velocity that depends on the time delay between the two pulses. This reduction reveals a complex interplay between plasma recoil pressure and shock-wave-driven deformation, and enables an estimation of the moment of spall formation and the average shock-wave propagation velocity. We find that the shock wave traverses the droplet with an average velocity ranging from 1.2 to 1.6 times the speed of sound. We study the effects of the energy of the second pulse on the deformation and qualitatively discuss the formation of microjets. Crucially, we demonstrate the ability to manipulate the microdroplet expansion and spallation with double-pulse sequences, thereby increasing the portfolio of obtainable target shapes for droplet-based extreme ultraviolet light sources.