Average Propagation Velocity Research Articles

Calculation of Wave Velocities in Segmented Pipelines with Flexible Joints

Engineering methods for finding the average (averaged) velocity of propagation of longitudinal waves in pipelines with flexible joints are presented. By accurate analysis of the problem of oscillations of a one dimensional periodically inhomogeneous structure it is shown that the results of engineering approaches for rod velocity are the first or long-wave asymptotic approximation which valid when the period of external influence (the length of the seismic wave) significantly exceeds the size of the periodicity cell of the pipeline (the length of the pipe with a joint). Thus, it is established that when this condition is met, the problem of pipeline dynamics with joints is reduced to a much simpler problem of vibrations of a homogeneous pipeline, the velocity of wave propagation in which is equal to the found average value. Numerical examples are given that demonstrate a significant (sometimes by an order of magnitude) decreasing of the rod velocity in the presence of flexible joints.

Flame propagation mechanism of nano-scale PMMA dust explosion

Experiments of nano-scale polymethyl methacrylate dust explosions were conducted to reveal the flame propagation characteristics and evolution mechanism. The results showed that the flame preheat zone front was approximately smooth. The average pulsating flame propagation velocity and maximum temperature both increased with an increase in dust concentration. The thermal heat transfer rate and pyrolysis rate dominated the propagating flame. A transition from pyrolysis rate control to combustion reaction rate control occurred as the dust concentration increased during the combustion process. Speculating from the physical model of nano dust flame propagation including a post-flame zone, a luminous spot-flame reaction zone, a combustible premixed-gas reaction zone, a preheat zone, an unburnt particle stagnation zone, and an unburnt zone away from the flame front, the propagating flame was macroscopically analogous to the kinetic combustion of premixed gas. In addition, it was microscopically coupled with the localised diffusion combustion of luminous spot flames.

In Situ Observation and Phase-Field Modeling of Peritectic Solidification of Low-Carbon Steel

In the present study, both in situ experiment and multiphase field modeling are adopted to investigate the peritectic solidification of a low-carbon steel. The results show that the peritectic reaction occurs at the temperature of 3.4 K lower than the equilibrium peritectic temperature, the γ-austenite first nucleates at the δ/L boundary, and then rapidly propagates along the δ/L boundary by the advance of the L/γ/δ triple point till the δ-ferrite is encircled by the γ austenite. The whole peritectic reaction process is very fast, and the measured and predicted average propagation velocity of L/γ/δ triple point along the δ/L boundary are, respectively, 1.36 and 1.09 mm/s. This small difference between the measurement and prediction means that the developed multiphase field model is capable of predicting the peritectic reaction and peritectic transformation during the peritectic solidification process of Fe-C alloy, and the peritectic reaction can be regarded as a solute diffusion-controlled process. With the increase of cooling rate and undercooling, the advancing velocities of L/γ/δ triple point, L/γ interface and γ/δ interface increase, and thus the γ-austenite between the liquid phase and δ phase becomes longer and thicker for the same elapsed time.

Flame propagation and pressure characteristics of polymethyl methacrylate dust explosions in a horizontal pipe

To reveal the characteristics of flame propagation and pressure in organic dust explosions during pneumatic transportation, 30 μm polymethyl methacrylate (PMMA) dust explosions with different airflow velocities were experimentally conducted in a horizontal pipe. The results showed that dust flame transited from the luminous flame with a continuous structure to discrete flames as flame propagating. After the appearance of the discrete flames, flame propagation velocity increased sharply owing to the positive feedback coupling between combustion and expansion. With the increase of dust concentration, the average flame propagation velocity, maximum flame temperature, temperature rising rate, maximum pressure, and maximum rate of pressure rise increased and then decreased. These explosion characteristics (except for the maximum flame temperature) increased significantly with the increase of airflow velocity. When airflow velocity increased from 6.39 m/s to 13.3 m/s, the peaks of average flame propagation velocity increased from 23.9 m/s to 38.2 m/s; the peaks of maximum pressure and maximum rate of pressure rise were increased by 1.9 and 6.1 times respectively. In addition, it was found that the optimum dust concentration decreased as airflow velocity increased. Furthermore, the relationship among flame propagation, temperature, and pressure was discussed in detail.

Flame propagation characteristics and explosion behaviors of aluminum dust explosions in a horizontal pipeline

The effects of airflow velocities, particle sizes, and dust concentrations on the flame propagation characteristics and explosion behaviors of aluminum dust explosions in a horizontal pipeline were experimentally investigated. The results showed that the distributions of the airflow velocity and turbulence integral length both exhibited convex parabolic distributions. After ignition, the flame propagation velocities of 5μm and 30μm aluminum dust explosions both experienced acceleration processes. The positive feedback coupling among the combustion, expansion, and turbulence governed the acceleration mechanism. The flame propagation characteristics and explosion behaviors were significantly affected by the airflow velocity. As the airflow velocity increased from 6.4m/s to 13.3m/s, the maximum average flame propagation velocity increased from 60m/s to 140m/s. Moreover, the maximum explosion pressure increased from 36kPa to 83kPa, and the maximum explosion impulse increased from 78Pa.s to 576Pa.s.

Peridynamics simulation of crack propagation of ring-shaped specimen like rock under dynamic loading

The split Hopkinson pressure bar (SHPB) system with a spindle-shaped striker is widely used to test the characteristics of rock-like materials subjected to dynamic loading, and the crack propagation of ring-shaped specimens under dynamic loading is a research focus in rock mechanics. In this paper, a model of the SHPB system, including the striker, incident bar, specimen, and transmitted bar, is established based on peridynamics theory and is validated by comparing two kinds of stress waves obtained by a peridynamics simulation and experiment. Based on these results, the crack propagation of a ring-shaped specimen with an aperture is simulated, and the influences of the apertures of the specimen on the failure patterns are discussed. The results show that there are four cracks distributed on the specimen with an aperture of 24 mm: crack-1 first initiates at the end of the inner diameter near the incident bar, crack-2 then occurs at the end of the inner diameter near the transmitted bar, and then crack-3, also called a secondary crack, initiates on the upper and lower outer boundaries of the specimen; these secondary cracks are perpendicular to the other cracks. The sequence of starting velocities and average propagation velocities of cracks from high to low is crack-3, crack-2, and crack-1, and the failure of the ring-shaped specimen occurs due to tension. Controlled by the structure effect of the ring-shaped specimen, the loading rate and the peak stress decrease gradually, and the failure patterns of the specimens transform from splatted by one initial crack to broken by two cracks that are perpendicular to each other and then to asymmetrical failure with an increase in the apertures of the specimen. The comparison results of peak stresses and failure patterns between the peridynamics simulation and the experimental results verify the accuracy of peridynamics theory.

Realization of methane-air continuous rotating detonation wave

Methane-air Continuous Rotating Detonation (CRD) has been firstly achieved in this paper in the hollow chamber with a Laval nozzle, and the diameter of the chamber is just 100 mm. The contraction ratio of the Laval nozzle is a key factor for the CRD realization. CRD can only be obtained when the contraction ratio is no less than 4, but its ER operating range decreases in the increase of contraction ratio from 4 to 10. For all the success cases, the average propagation frequency and velocity are in the ranges of 5.32–5.65 kHz and 1670.48–1774.10 m/s, respectively, and the velocity deficits are less than 10%. Based on the high-speed photography images, the approach of chemiluminescence intensity integral is proposed in this paper, and the propagation characteristics of the flame are analyzed quantitatively. The propagation velocities of the flame and shock wave are agreed well with each other, indicating that the typical feature of detonation wave, i.e., the coupling of the flame and the shock wave, is verified quantitatively.

Catastrophic damage in hollow core optical fibers under high power laser radiation.

The propagation of an optical discharge (OD) along hollow-core optical fibers (HCFs) is investigated experimentally. Silica-based revolver-type HCFs filled with atmospheric air were used as test samples. We observed that the average propagation velocity of an OD along the HCF (VAV) depends on the properties of the medium around the silica structure of the fiber. It is shown that the value of VAV changes by approximately a factor of three, depending on whether the optical discharge is moving along a polymer coated or uncoated fiber. The value of VAV practically does not change when the polymer is replaced by an immersion liquid (such as glycerol) or liquid gallium. By analyzing the destruction region's patterns that appear in the fiber cladding after an OD propagation, we propose the physical picture of the phenomenon.

Influence of front time on positive switching impulse discharge characteristics of UHVDC tower gaps

The front time of switching impulses has a significant influence on long air gap discharge performance, but most testing experiments have been conducted using standard impulses. Here, we carried out experiments investigating the discharge characteristics of a ±800 kV transmission line-tower gap (5–8 m) under positive switching impulses with different front times and obtained important physical parameters, such as the time and voltage of the leader inception, the leader propagation velocity, and the 50% breakdown voltage (U50). The results show that (1) the critical wave front time increases with the air gap distance; (2) for the tested gaps, the time and voltage of the leader inception show minimum dispersion when the front time is 250 μs; (3) an approximately U-shaped curve occurs between the front time and the average velocity of the continuous leader propagation, and the velocity is the slowest with a front time of 500 μs. The results illustrated that choosing standard switching impulses (250 μs/2500 μs) to conduct a discharge test in a ±800 kV transmission line-tower gap is both suitable and effective; however, when the operation voltage further increases, the front time should be increased appropriately when conducting switching impulse tests.

Energy Partitioning During Subcritical Mode I Crack Propagation Through a Heterogeneous Interface

AbstractThe energy budget during the propagation of tensile fractures typically includes two major energy dissipation modes: the fracture energy to create a new interface and the radiated energy. Since seismic hazard is related to the amount of seismic energy released, it is important to evaluate precisely the energy budget during fracture propagation and see in particular if it is constant or not. However, two aspects are typically limiting a precise estimate of the energy budget: first, the measurement of the nonseismic energy release and second, the rock heterogeneity‐like asperities or barriers. We conducted here laboratory experiments, using an analog material (Polymethyl methacrylate (PMMA)), of a stable mode I interfacial crack propagation close to brittle‐creep transition through a heterogeneous interface for different macroscopic rupture velocities to evaluate carefully their energy budget. Both acoustic and optical advances of the crack front were measured simultaneously, providing precise estimates of both types of dissipated energy. We computed the radiation efficiency ηR (ratio of the radiated energy to the available energy for driving the fracture) and observed a nonlinear increase of ηR with the average fracture propagation velocity v over 2 orders of magnitude independently of the initial quenched disorder. The experimental observations are supported by a model based on the fluctuations of the local rupture velocity induced by the crack front pinning on local asperities which leads to ηR ∝ v0.55. We discuss implications for slow shear rupture modes, seismicity rate evolution, and induced seismicity.

Traveling-wave fault location algorithms in hybrid multi-terminal networks with a tree-like structure

Traveling-wave methods of the fault location prove their practical efficiency for the power transmission lines (TL) with an arbitrary configuration of any voltage class. This paper formulates algorithms to automate the process of the fault location in the TLs with a tree-like structure. The error of the simplified traveling-wave fault location (TWFL) algorithm based on the average propagation velocity of the transient signals (TS) is analyzed. A tabular algorithm for TWFL, which considers different propagation velocities of the TS in different segments of a hybrid network is proposed. An adaptive TWFL algorithm that considers the registration of the TSs with known places of their occurrence is proposed, to reduce the impact of the inaccuracy of the initial different network segments’ lengths and TS’s propagation velocity determination.

Suppression of nano-polymethyl methacrylate dust explosions by ABC powder

The suppression effects of ABC powder on 100 nm polymethyl methacrylate (PMMA) dust explosions with different mass densities were experimentally studied. Results showed that the suppressed flames were dim and the flame propagation was slowed down with the addition of ABC powder. When the proportion of suppressant increased, the average propagation velocity was decreased. After adding 30% ABC powder to 650 g/m3 PMMA dust cloud, the maximum suppression ratio for velocity was 93%. After adding 10% ABC powder, the temperatures of PMMA dust cloud with the mass densities of 250 g/m3, 450 g/m3 and 650 g/m3 were decreased to 19.8%, 27.4% and 33.1%, respectively. Meanwhile, the critical suppression proportion of PMMA dust explosion with three mass densities were 25%, 35% and 35%. Thermal analysis results showed that the addition of suppressant could increase the thermal stability of PMMA. The suppression mechanisms were comprised of physical suppression and chemical suppression which reflected on the consumption of H and OH radicals. The chemical kinetic models indicated that PO2 and HOPO played a catalytic role in the combination of H and OH.

Effect of Pristine Palygorskite Powders on Explosion Characteristics of Methane-Air Premixed Gas

In this study, pristine palygorskite powders were used as the inhibition materials to suppress the explosion of methane-air premixed gas for the first time. The composition, porosity and pyrolysis characteristics of the powders were tested by X-ray diffraction (XRD), energy dispersive spectrometry (EDS), N2 adsorption-desorption and Thermogravimetry-differential scanning calorimetry (TG-DSC) techniques. The effects of pristine palygorskite powders concentration on the explosion pressure and the average velocity of flame propagation of the 9.5% methane-air premixed gas were tested by a 20 L spherical explosion system and a 5 L pipeline explosion system. The results indicated the pristine palygorskite powders possess a considerable suppression property on methane explosion. When the mass concentration of pristine palygorskite powders was 0.20 g·L−1, the max-pressure of methane explosion was decreased by 23.9%. The methane explosion flame propagation velocity was inhibited obviously. Owing to the excellent inhibitory performance and the advantage of low-cost and environmental harmlessness, pristine palygorskite powders are potential new materials for the application on gas explosion suppression.

An ordinary state-based peridynamic model for the fracture of zigzag graphene sheets.

This study develops an ordinary state-based peridynamic coarse-graining (OSPD-CG) model for the investigation of fracture in single-layer graphene sheets (SLGS), in which the peridynamic (PD) parameters are derived through combining the PD model and molecular dynamics (MD) simulations from the fully atomistic system via energy conservation. The fracture failure of pre-cracked SLGS under uniaxial tension is studied using the proposed PD model. And the PD simulation results agree well with those from MD simulations, including the stress-strain relations, the crack propagation patterns and the average crack propagation velocities. The interaction effect between cracks located at the centre and the edge on the crack propagation of the pre-cracked SLGS is discussed in detail. This work shows that the proposed PD model is much more efficient than the MD simulations and, thus, indicates that the PD-based method is applicable to study larger nanoscale systems.

Hydrogen cloud explosion evaluation under inert gas atmosphere

This paper is aimed at evaluating the hydrogen cloud explosion under inert gas atmosphere experimentally and numerically. The results demonstrate that only under Ar, N2 and CO2 atmosphere, the lean and stoichiometric hydrogen flame tends to be unstable. The Markstein length of Ar, N2 and CO2 is less than zero and the decreasing order is Ar, N2 and CO2. Except for CO2, the average flame propagation velocity, maximum explosion pressure, maximum pressure rise rate, positive pressure impulse and explosion pressure attenuation decrease monotonously in the order of He, Ar and N2. For various equivalence ratios and inert gases, the explosion pressure releases into the far field at the sound speed and attenuates with distance. Due to the fact that the explosion pressure is closely related to laminar burning velocity, the explosion pressure suppression by inert gas is revealed by analyzing the factors affecting laminar burning velocity. As equivalence ratio increases, the adiabatic flame temperature plays a more important role in affecting laminar burning velocity than thermal diffusivity. For different inert gases, the effect of thermal diffusivity on laminar burning velocity is obviously greater than adiabatic flame temperature. Besides, the third-body effect of various inert gases increases gradually until Φ = 1.4. and then decreases with increasing equivalence ratio. For a given equivalence ratio, the increasing order of third-body effect is He, Ar and CO2.

Propagation of Cellular Modes of Combustion of Porous Media under Nonadiabatic Conditions

The formation and propagation of cellular combustion of porous media under near-critical combustion existence conditions were investigated. The sizes and structure of cells in which the condensed component reacted with the gas depended nonlinearly on the heat loss to the environment. With increasing heat loss, the oscillation frequency of the cellular front, its average propagation velocity, and the intensity of the exothermic transformation of the condensed phase increased, and the cell sizes decreased. Cellular combustion was extinguished only when the transformation of the condensed component within a cell was completed.

Ethylene/polyethylene hybrid explosions: Part 1. Effects of ethylene concentrations on flame propagations

To reveal the explosion regimes in ethylene/polyethylene dust hybrid explosions, flame propagations in ethylene/polyethylene hybrid explosions with different ethylene concentrations were experimentally studied. The results showed that minimum explosible concentration of polyethylene particles significantly decreased with the increase of ethylene concentration due to the synergistic effect of ethylene and polyethylene particles. The MEC was reduced from 200 g/m3 to 42 g/m3 with the addition of 2.3% ethylene gas. Flame propagation velocities, maximum flame temperatures, and temperature rising rates obviously increased as ethylene concentration increased. The maximum average flame propagation velocities of 0.5%, 1.2%, and 2.3% mixtures were 6.22 m/s, 8.31 m/s, and 12.91 m/s respectively. With the increase of ethylene concentration, flame propagation mechanism was transited from the devolatilization-controlled regime to the kinetics-controlled regime. Hybrid explosions of ethylene/polyethylene dust mixtures can be identified into four different regimes, which are no-explosion, synergistic explosion, gas-driven explosion, and dust-driven explosion. In the synergistic explosion region, maximum flame temperatures were obviously sensitive to the ethylene concentration and dust concentration. Hybrid explosion regime was transited from the dust-driven explosion to the gas-driven explosion with the increase of ethylene concentration (below LFL).

Quench dynamics in MgB2 Rutherford cables

The generation and propagation of quench induced by a local heat disturbance or by overcurrents in MgB2 Rutherford cables have been studied experimentally. The analysed cable is composed of 12 strands of monocore MgB2/Nb/Cu10Ni wire and has a transposition length of about 27 mm. Measurements of intra- and inter-strand voltages have been performed to analyse the superconducting-to-normal transition behaviour of these cables during quench. In case of external hot-spots, two different time-dynamic regimes have been observed, a slow stage for the formation of the minimum propagation zone (MPZ), and a fast dynamics once the quench is triggered and propagates to the rest of the cable. Significant local variations of the quench propagation velocity across the strands around the MPZ have been observed, but with average quench propagation velocities closely correlated with the predictions given by one-dimensional-geometry models. For quench induced by overcurrents (i.e. with applied currents higher than the critical current) the nucleation of many normal zones distributed within the cable, which overlap during quench propagation, gives a distinctive and faster quench dynamics.

Combustion behaviors and temperature characteristics in pulverized biomass dust explosions

Abstract Flame propagation behaviors and temperature characteristics of four types of biomass with two different particle size distributions were studied experimentally. Results show that the flame front of a 50–70 μm biomass is nearly spherical and smooth, the flame zone is characterized by yellow or dark red spotted flames, and luminous flames are present behind it. The flame morphology of 100–200 μm biomass dust is irregular and discrete. The average flame propagation velocity and the amplitude of the velocity fluctuation are functions of the mass density of the biomass particles and depend on the particle size distributions. The flame-speed oscillation of biomass particles is caused by the velocity slip between the volatile gases and particles. Flame temperatures of 50–70 μm and 100–200 μm biomass dust reach the maximum value at 1000 g/m3, and show a slight dependence on the particle size distribution. An analysis of the Knudsen number indicates that the combustion characteristics of biomass particles with particle size distributions within the range studied are characterized by a continuum regime. It is indicated that 100–200 μm poplar sawdust will be the “best” option as a biomass replacement feedstock for coal powered plants.

Flame propagation behaviors and temperature characteristics in polyethylene dust explosions

To reveal the flame propagation mechanism in polyethylene (PE) dust explosions, the flame propagation behaviors and temperature characteristics of polyethylene dust clouds were experimentally studied in an open duct. Flame propagations in polyethylene dust clouds with different concentrations and particle size distributions were recorded using high-speed photography. The flame temperatures were measured using two fine thermocouples comprising Pt–Pt/Rh13% wires of diameter 25 μm. Because of severe agglomeration, the minimum explosible concentration of polyethylene particles with diameter < 75 μm was higher than that of the particles with diameter 75–100 μm. It was observed that the flame front gradually became continuous as the particle size increased with the same polyethylene dust concentration of 300 g/m3. With higher polyethylene dust concentration, the flame front of the polyethylene particles of size <75 μm could quickly become discrete, and the floating flame appeared early, while the flame front of the 100–212 μm polyethylene particles transformed from a continuous flame into several independent diffusion flames. It was demonstrated that the flame propagation velocities were not constant, and that they fluctuated owing to the turbulence and expansion of the combustion products. The average flame propagation velocity increased initially, and then decreased with increasing dust concentration. The peak velocity of the polyethylene particles of size <75 μm was 4.79 m/s at a mass density of 300 g/m3, while the peak velocity of the 75–100 μm particles was 4.55 m/s at a mass density of 400 g/m3, and that of the 100–212 μm particles was 2.67 m/s at a mass density of 500 g/m3. In addition, it was found that the maximum flame temperatures of the different polyethylene particles were approximately the same; the temperatures were 1585.4 °C, 1511.8 °C, and 1508.4 °C for the polyethylene particles of sizes of 100–200 μm, <75 μm, and 75–100 μm, respectively. This phenomenon may be caused by the combustion behavior of the molten polyethylene particles. When the dust concentration is increased within the optimum concentration, flame temperature increased with the increase in flame propagation velocity. The flame temperature of the polyethylene particles of size <75 μm decreased as the flame propagation velocity decreased, while the flame temperature of the 75–100 μm polyethylene particles did not change significantly.