Instantaneous Propagation Research Articles

Flame propagation and spectrum characteristics of CH4-air gas mixtures in a vertical pressure relief pipeline

The explosion characteristics, flame propagation mechanism, and spectrum characteristics of five free radicals (C2·, CHO·, CH·, O·, and OH·), formaldehyde (CH2O), and one reagent (O2) under various concentrations (w) of methane were investigated using a vertical rectangular pressure relief pipeline. The experimental results indicated that the instantaneous flame propagation speed (v), maximum flame temperature (Tex), maximum explosion pressure, and relative spectrum intensity (I) first increased and subsequently decreased with methane concentration. The maximum instantaneous flame propagation speed (vmax), maximum flame temperature (Tmax), maximum explosion pressure (Pmax), and maximum relative spectrum intensity (Imax) were achieved, and the time taken to form a tulip-shaped flame was the shortest at w = 10 vol%. The experiment also revealed that Tex and w had a polynomial relationship. In the explosion process, positive correlations were noted among the T, P, and I. Furthermore, the spectrum signal characteristic times of the intermediate species was considerably influenced by v. When the methane concentration was 9.5 vol%, the I of the seven intermediate generation rates exhibited two distinct trends. This finding contributes to the analysis and validation of the microscopic mechanism underlying gas explosion flame propagation. Further, a reference for selecting new gas explosion suppressants based upon the attenuation level of free radical spectrum intensity.

Efficient Instantaneous Channel Propagation Modeling for Aeronautical Communications Systems With Compressed Sensing

Establishing an instantaneous channel propagation model in aircraft engineering simulators is important for aircraft systems development. This article investigates an instantaneous channel propagation modeling scheme with compressed sensing (CS) recovery and electromagnetic (EM) interpolation for aeronautical communications systems to reduce the cost of flight experiments. Specifically, the system architecture introduces the proposed modeling process of the instantaneous aeronautical channel. The mathematical models are analyzed to show the sparsity of the aeronautical channel with a limited scattering environment. The channel modeling algorithm is detailed by explaining the CS algorithm and the EM interpolation scheme. The experimental results show the effectiveness of the proposed method. Most recovery relative errors between the recovering channel model and the real sampled channel responses are lower than 0.1, i.e., −10 dB. More than 75% of the sampling points can be reduced by the proposed method compared with the real flight experiments. Moreover, the proposed single-input-single-output (SISO) approach can also be applied in the potential aeronautical communications systems with multiple-input-multiple-output (MIMO) transmission.

Relativistic dynamics of the motion of heat

The motion of heat can be described by a thermal energy–momentum tensor. A general description of heat conduction in the geometrical language is developed based on a two-phase continuum model within the framework of relativistic continuum dynamics, and the momentum balance equation of heat, or the general heat conduction equation (GHCE), is derived by the balance equation of the thermal energy–momentum tensor. We demonstrate that the low-speed limit of the GHCE coincides the former version of GHCE of the thermomass theory that is based on the energy–mass equivalence and Newtonian dynamics. Numerical solutions of the GHCE are also provided to demonstrate that the GHCE not only overcomes the paradox of instantaneous heat propagation of the Fourier’s law, but also resolves the defect of the CV model that temperatures may drop below absolute zero under some conditions. The present work formulates a general description of the thermal transport processes, and can provide a deeper insight into the heat transfer discipline from the relativistic point of view.

A Simple Proof of Locality in Quantum Mechanics

While quantum mechanics allows spooky action at a distance at the level of the wave-function, it also respects locality since there is no instantaneous propagation of real physical effects. We show that this feature can be proved in the standard interpretation of quantum mechanics by a simple general result involving commuting Hermitian operators corresponding to distant (causally disconnected) observables. This is reminiscent of satisfying the locality condition in relativistic quantum field theories.

Analysis of coupled-waves structure and propagation characteristics in hydrogen-assisted kerosene-air two-phase rotating detonation wave

In order to investigate the structure and propagation characteristics of hydrogen-assisted kerosene (liquid phase)-air rotating detonation, a modified solver based on OpenFOAMⓇ was proposed and utilized to simulate detonation in the mixture of hydrogen, liquid kerosene and air. The obtained results show that evaporation waves exist in the discrete liquid mist detonation, and the coupled-waves structure formed by the evaporation wave (EW) and the incident shock wave (ISW) propagates upstream. By analyzing the overall and instantaneous propagation characteristics, it is found that the propagation speed of the detonation wave increases with the increase of the equivalence ratio at the fuel rich state. Increasing the droplet size will cause the evaporation wave to be further downstream relative to the ISW, resulting in lower evaporating pressure. The effect of EW on ISW is accomplished by the blast wave traveling upstream which is generated by droplet reaction, while EW is influenced by the ISW by modulating the droplet enthalpy and the pressure attenuation following ISW. Part of the droplet reaction energy is held in the gap between the EW and ISW, consequently the gap acts as an energy storage piston, regulating the ISW and EW propagation speeds. The combustion of droplets with smaller sizes imposes more positive feedback on their evaporation, and increasing the total temperature of the premixed gas promotes the interaction between the EW and the ISW, resulting in a more steady propagating overall structure.

Numerical analysis on evolution process of multiple rotating detonation waves with ethylene–oxygen–nitrogen mixture

Two-dimensional numerical simulations of a rotating detonation combustor (RDC) fueled by ethylene–oxygen–nitrogen were performed to investigate the evolution process of multiple rotating detonation waves (RDWs), that is, initial chaotic stage, self-adjustment stage, and the final stable stage. The number of stable RDWs corresponding to the initial conditions was in good agreement with that predicted by an empirical model proposed by Daniau et al. from Lavrenty’ev Institute of Hydrodynamics (LIH). From the simulation results, the spontaneous formation of a reverse RDW and the transition of trailing waves to detonation waves contribute to the emergence of the chaotic stage. The self-adjustment stage comprises three distinctive parts, which are the self-adjustment of the propagation direction, the number, and the propagation characteristics of RDWs. Furthermore, the double-wave collision was found to play a significant role in modulating the propagation direction as well as the number of RDWs. The same self-adjusting trend of different instantaneous propagation characteristics (e.g., peak pressure, propagation velocity, and wave front height) of the RDWs was observed, suggesting the self-similarity property of the adjusting mechanism demonstrated in the evolution of rotating detonation dynamics.

Spatio-temporal modelling of the first Chikungunya epidemic in an intra-urban setting: The role of socioeconomic status, environment and temperature.

Three key elements are the drivers of Aedes-borne disease: mosquito infestation, virus circulating, and susceptible human population. However, information on these aspects is not easily available in low- and middle-income countries. We analysed data on factors that influence one or more of those elements to study the first chikungunya epidemic in Rio de Janeiro city in 2016. Using spatio-temporal models, under the Bayesian framework, we estimated the association of those factors with chikungunya reported cases by neighbourhood and week. To estimate the minimum temperature effect in a non-linear fashion, we used a transfer function considering an instantaneous effect and propagation of a proportion of such effect to future times. The sociodevelopment index and the proportion of green areas (areas with agriculture, swamps and shoals, tree and shrub cover, and woody-grass cover) were included in the model with time-varying coefficients, allowing us to explore how their associations with the number of cases change throughout the epidemic. There were 13627 chikungunya cases in the study period. The sociodevelopment index presented the strongest association, inversely related to the risk of cases. Such association was more pronounced in the first weeks, indicating that socioeconomically vulnerable neighbourhoods were affected first and hardest by the epidemic. The proportion of green areas effect was null for most weeks. The temperature was directly associated with the risk of chikungunya for most neighbourhoods, with different decaying patterns. The temperature effect persisted longer where the epidemic was concentrated. In such locations, interventions should be designed to be continuous and to work in the long term. We observed that the role of the covariates changes over time. Therefore, time-varying coefficients should be widely incorporated when modelling Aedes-borne diseases. Our model contributed to the understanding of the spatio-temporal dynamics of an urban Aedes-borne disease introduction in a tropical metropolitan city.

Combustion characteristics of zirconium particles coated with ferrite nanoparticles

To improve the properties of Zr powder for specific applications in aerospace and military fields, Zr powders were coated with ferrite nanoparticles, namely FeOOH, Fe2O3, and Fe3O4. An instantaneous flame propagation system and scanning electron microscopy, X-ray diffraction, thermogravimetry, and differential scanning calorimetry techniques was used to characterized the flame propagation characteristics, micromorphology, phase composition, crystal structure, thermal stability, and reactivity of the three types of nano ferrite-coated Zr particles. The results showed that, the flame propagation velocity and maximal temperature exhibited the following order: Fe2O3-coated > FeOOH-coated > Fe3O4-coated Zr dust cloud. With an increase in the proportion of coating materials, the combustion characteristics all decreased. The contrastive analysis of surface, ingredient and heating process revealed that, during combustion, in addition to inducing oxidation–reduction, inner Zr led to a replacement reaction with the outside ferrite coating layer, and then, Fe generated was oxidized at a high temperature.

LES investigation of cycle-to-cycle variation in a SI optical access engine using TFM-AMR combustion model

Multi-cycle large-eddy simulations (LES) are performed to investigate combustion cycle-to-cycle variability (CCV) in a gasoline spark ignited optical access engine operating under homogeneous stoichiometric conditions. Combustion is addressed with the Thickened Flame Model (TFM) and finite rate chemistry is accounted for through a reduced oxidation reaction mechanism. In view of the fact that computational costs of LES engine simulations are still very high today, this work investigates the use of adaptive mesh refinement (AMR) in the flame zone in conjunction with the artificial flame thickening applied by the TFM model. The paper discusses how the resulting coupled TFM-AMR combustion model allows good resolution of the flame, maintaining accuracy at acceptable costs. First, the details of the coupled model are presented and the effects of the parameters are explored, highlighting their impact on the combustion prediction. Then, computational fluid dynamics (CFD) simulation results are validated against experimental data collected in a low-speed low-load engine point, by comparing 20 LES cycles and 100 measured cycles, for mass fraction burned, combustion phasing, flame images and CCV indices. Lastly, a detailed investigation on the fastest and slowest numerical cycles is presented, analyzing instantaneous flame structures, ignition behaviors, propagation speeds, and probability density function (PDF) of the instantaneous velocity fluctuation around the spark region. The results show that combustion variability is highly correlated to the resolved velocity field and the resolved turbulence intensity, which is found to be the main cause of CCV and affects the early flame kernel growth. This work is an early attempt to use TFM-AMR combustion model for LES simulations of internal combustion engines.

The Reattachment Process of a Lifted Jet Diffusion Flame by Repetitive DC Pulse Discharges

On research of plasma assisted combustion, effects of electric and plasma discharges in DC, AC and pulse forms on reattachment of a lifted flame have attracted extensive attention. However, the detailed plasma assisted reattachment process and mechanism and roles of induced corona discharge and corona-induced ozone on the reattachment process are still unclear and undocumented. The forced reattachment process of a lifted diffusion jet flame by repetitive DC electric pulse discharges was experimentally investigated in this study using high-speed flame imaging, conditioned particle image velocimetry (PIV), and planar ozone concentration imaging. The forced reattachment process can be divided into three stages in sequence: ionic wind prior to corona initiation, corona initiation, and corona enhancement propagation. The conditioned PIV results showed that the instantaneous flame base propagation velocity is sufficiently enhanced beyond the laminar burning velocity for high pulse-repetition-frequency (PRF) cases at the instant of pulse discharge (on pulse) due to the enhanced oxidation of the corona induced ozone. By observing the dynamic flame-base behavior and evolution characteristics of the short-lived corona induced ozone for various PRFs, the novel forced reattachment process and mechanism of a lifted jet flame induced by repetitive DC electric pulse discharges is proposed.

Modelling quench of a 50 kA REBCO conductor with soldered-twisted-stacked-tape-cable strands

A 50 kA REBCO conductor based on the Twisted-Stacked-Tape-cable concept is being developed at Swiss Plasma Center (SPC) for EU-DEMO central solenoid. The conductor contains 12 strands and each one is made of twisted REBCO tape stack soldered in round copper profile with rectangular slot. In this paper, first it is verified if the strands could be modelled as monolithic elements for quench simulation, which means instantaneous heat propagation within the domain and homogeneous current density, by studying the influence of the inter-tape and stack-profile resistance on their heat and current transfer. Then a local disturbance induced quench on the 50 kA REBCO conductor is studied in detail considering various inter-strand electrical resistance.

Combined Line Fault Location Method for MMC–HVDC Transmission Systems

A combined algorithm based on traveling wave (TW) theory and time-frequency characteristics is proposed to estimate the DC line fault location in modular multilevel converter-high voltage direct current (MMC-HVDC) systems. The relationship between instantaneous frequency and TW propagation velocity is analysed, and the local voltage signal is utilized to extract fault features. A TW distortion-based scheme is improved to obtain the approximate fault location and accurately detect TW arrival times. The arrival times of the initial TW from the fault point and opposite terminal are determined using intrinsic mode functions under variational mode decomposition. Given the attenuation property of the TW-based method, the Teager energy operator is used to overcome the weakness in fault feature extraction. The instantaneous frequency of the TW front is obtained via ensemble empirical mode decomposition and Hilbert-Huang transform. The fault location is predicted with the detected TW arrival times and corresponding TW propagation velocity. The effectiveness and superiority of this method are verified through simulations in PSCAD/EMTDC and computation in MATLAB.

Auto-ignitive deflagration speed of methane (CH4) blended dimethyl-ether (DME)/air mixtures at stratified conditions

Front propagation speeds from fully resolved unsteady one dimensional simulations with dimethyl-ether (DME)/methane (CH4)/air mixtures under engine relevant conditions are presented using complex kinetics and transport. Different time-scales of monochromatic inhomogeneities in DME concentration with varying DME/CH4 blending ratios are simulated to unravel the fundamental aspects of auto-ignition and flame propagation under the influence of reactivity stratification. To understand the influence of different stratification time-scales on the flame-ignition interaction, two sets of conditions are simulated such that low temperature chemistry is present in only one of them. For a given amplitude of stratification, it is found that the instantaneous propagation speed is significantly affected by the level of CH4 concentration in the binary fuel blend. Specifically, for cases with low temperature chemistry, at relatively smaller time-scales, the overall fluctuation in the instantaneous propagation speed is found to subside as the level of CH4 concentration in the mixture is increased. However, for both sets of conditions, at comparatively larger time-scales, a rapid change in the instantaneous propagation speed is observed with an increase in the level of CH4 concentration in the mixture. The intrinsic effects of stratification time-scales on the low temperature chemistry and the high temperature chemistry are further examined to assess the flame-ignition interaction. A displacement speed analysis is also carried out to elucidate the underlying combustion modes that are responsible for such a variation in flame response.

Large-eddy simulation study of combustion cyclic variation in a lean-burn spark ignition engine

Multi-cycle large-eddy simulation (LES) was performed to investigate combustion cyclic variability (CCV) in a single cylinder spark ignition engine with a homogeneous lean (λ = 1.25) isooctane-air mixture. The aim was to obtain physical insights into the early stage of combustion and its influence on CCV. Propagation of the flame was modeled by a transport equation for the filtered flame surface density within the LES framework. The ignition process was represented by the imposed stretch spark ignition model (ISSIM-LES). Ten consecutive cold flow LES cycles followed by two initialization cycles (12 cycles in total) were used to perform the reactive simulations concurrently. The simulation results were compared with experimental data. Although the number of computed cycles was fairly low, the LES was able to reproduce the cyclic variability observed in experiments both quantitatively and qualitatively. Firstly, validation of the simulation was done by comparing measured pressure traces. Secondly, correlations between the timing of the 10% fuel burnt mass fraction with early flame kernel growth and initial-to-turbulent transition period (in which there was an asymmetric flame kernel that persisted through the early development periods) were determined. Thirdly, calculated results of the flame propagation were analyzed at two cross-sections (in swirl and tumble planes) of the combustion chamber, which highlighted differences in instantaneous flame structures and propagation characteristics between the fastest and slowest cycles. Good overall agreement was obtained between the measurements and simulation data. The results revealed that the instantaneous velocity and fluctuation of flows around the spark vicinity affect growth of the early flame kernel and cause combustion cyclic variability.

Undetected non-conformities in material processing led to a service failure in a casing hanger during pre-fracture operation

The failure of a 5″ casing hanger at an oil well led to costly workover operations and lost production. The brittle failure was due to the instantaneous propagation of a circumferential fracture, initiated at a geometrical stress raiser. Combined operation stresses from internal pressure, casing weight and tightening torque were close to the yield strength of the material. If the material had sufficient ductility, plastic deformation would have led to stress relaxation and redistribution. But a SAE/AISI 4140 steel with a low forging ratio, plus a poor heat treatment and incomplete quenching, led to a banded microstructure with both low strength and toughness, and a consequent limited capacity for plastic deformation. Geometric discontinuities due to elongated inclusions and other microestructural defects justify crack propagation at the reported service loads. Quality assurance failed to prevent unacceptable pieces. Some recommendations are provided to reduce the likelihood of recurrence in new hangers: an appropriate microstructure must be assured, controlling the forging stages and heat treatment parameters. The representativeness of test coupons used for quality control must be ensured. Re-evaluating the geometry of the casing hanger and switching to a steel with higher hardenability would lead to more defect - tolerant components.

Thermal avalanche after non-simultaneous blow-up in heat equations coupled via nonlinear boundary

In this paper, we study a parabolic problem defined in the half line coupled via exponential-type boundary flux. Firstly, we prove the optimal classification of the two components of blow-up solutions when time reaches the blow-up time from left. Blow-up takes place only at the origin, and simultaneous blow-up rates are determined as well. Secondly, we study the weak extension after the blow-up time. Complete blow-up always occurs whether simultaneous blow-up arises or not. Moreover, an instantaneous propagation of the blow-up singularity to the whole spatial domain occurs at the blow-up time, which is the so-called thermal avalanche phenomenon. Finally, we use the evolution of the $k$-level set of solutions in the approximations to characterize the propagation of the singularity.

Effect of inhomogeneity on primordial black hole formation in the matter dominated era

We investigate the effect of inhomogeneity on primordial black hole formation in the matter dominated era. In the gravitational collapse of an inhomogeneous density distribution, a black hole forms if apparent horizon prevents information of the central region of the configuration from leaking. Since information cannot propagate faster than the speed of light, we identify the threshold of the black hole formation by considering the finite speed for propagation of information. We show that the production probability $\beta_{inhom}(\sigma)$ of primordial black holes, where $\sigma$ is density fluctuation at horizon entry, is significantly enhanced from that derived in previous work in which the speed of propagation was effectively regarded as infinite. For $\sigma \ll 1$, we obtain $\beta_{inhom}\simeq 3.70 \sigma^{3/2}$, which is larger by about an order of magnitude than the probability derived in earlier work by assuming instantaneous propagation of information.

Experimental investigation of reattachment behavior of turbulent lifted diffusion jet flames induced by repetitive DC electric pulse discharges with conditional PIV

ABSTRACTAmong the recently prevailing flame stabilization enhancement by electric field and plasma, repetitive DC electric pulse discharge is applied to a turbulent lifted diffusion jet flame to improve the reattachment in this study. The main objectove of this study is to characterize experimentally the stability and the reattachment behaviors of lifted flames under the electric pulse effect with a proposed electrode configuration. The effects of pulse repetition frequency (PRF) and voltage polarity on flame are investigated. The reattachment velocity increases by increasing PRF. The time history of flame base trace and absolute flame speed with positive-voltage-pulse are illustrated. The electric corona discharge is observed during upstream propagation process, and its role in flame acceleration is discussed. In addition, 2D-PIV measurement conditioned by high speed sequence images is conducted. Finally, the conditionally instantaneous flame propagation speed is enhanced and exceeds 3 times that of stoichiometric laminar flame speed, which leads to lifted flame reattachment.

Discrete‐Continuum Duality of Architected Materials: Failure, Flaws, and Fracture

Abstract3D nano‐ and micro‐architected materials are resilient under compression; their susceptibility to flaws and fracture remain unexplored. This work reports the fabrication and tensile‐to‐failure response of hollow alumina nanolattices arranged into 5 µm octet‐truss unit cells. Some specimens contained through‐thickness center notches oriented at different angles to the loading direction, with a length‐over‐sample‐width ratio of 0.45. In situ tensile experiments reveal that for all orientations, failure initiates at the notch root, followed by instantaneous crack propagation along lattice planes orthogonal to extension. A tensile strength of 27.4 ± 0.7 MPa is highest for unnotched samples and decreases as notch orientation varies from 0° to 90° to its minimum, 7.2 ± 0.4 MPa; their specific tensile strength is ≈4 × higher than that for all other low‐density materials. Finite element simulations reproduce observed strengths and failure mechanisms: initial cracks always initiate at the nodal junctions with highest stress concentrations by tearing of alumina walls at the nodes. Subsequent crack propagation shifts maximum stress concentration to the nodes along lattice plane orthogonal to the loading direction. A modified analytical fracture model based on the effective notch length predicts tensile strengths consistent with experiments. These findings imply that continuum fracture mechanics can predict failure in nano‐architected materials, which helps develop advanced materials through informed architectural design.

Unsteady deflagration speed of an auto-ignitive dimethyl-ether (DME)/air mixture at stratified conditions

The propagation speed of an auto-ignitive dimethyl-ether (DME)/air mixture at elevated pressures and subjected to monochromatic temperature oscillations is numerically evaluated in a one-dimensional statistically stationary configuration using fully resolved numerical simulations with reduced kinetics and transport. Two sets of conditions with temperatures within and slightly above the negative temperature coefficient (NTC) regime are simulated to investigate the fundamental aspects of auto-ignition and flame propagation along with the transition from auto-ignitive deflagration to spontaneous propagation regimes under thermal stratification. Contrary to the standard laminar flame speed, the steady propagation speed of an auto-ignitive front is observed to scale proportionally to its level of upstream reactivity. It is shown that this interdependence is primarily influenced by the characteristic residence time and the homogeneous auto-ignition delay. Furthermore, the unsteady reaction front in either of the two cases responds distinctly to the imposed stratification. Specifically, the results in both cases show that the dynamic flame response depends on the mean temperature at the flame base Tb and the time-scale of thermal stratification. It is also found that, based on Tb and the propensity of the mixture to two-stage chemistry, the instantaneous peak propagation speed and the overall time taken to achieve that speed differs considerably. A displacement speed analysis is carried out to elucidate the underlying combustion modes that are responsible for such a variation in flame response.