Front Propagation Research Articles

Dynamic characteristics of density-graded cellular materials for impact mitigation

Due to their exceptional impact resistance capabilities, density-graded cellular materials have immense potential in applications where crashworthiness requirements are of prime importance. Under impact loading, the deformation in these materials is characterized by compaction front propagation. Previous studies have utilized numerical techniques to solve the equations governing compaction shock propagation for cellular materials with density gradation. In this study, analytical solutions are formulated using compaction front position as the independent variable. The expressions for the velocity of the impinging rigid mass, energy absorption, incident stress, and transmitted stress are determined. The analytical solutions are shown to be in excellent agreement with the cell-based finite element solutions. The effect of density gradient on energy absorption and stresses is studied. The impact resistance factor is employed to assess different density-graded cellular materials for their effectiveness in impact mitigation. This study demonstrates that density-graded cellular materials can offer superior impact protection to objects at both the incident and transmitted ends. Lower density toward the incident end enhances impact resistance to objects located there, and likewise, lower density at the transmitted end offers more impact resistance to objects at that end.

Self-catalysed frontal polymerisation enables fast and low-energy processing of fibre reinforced polymer composites

Frontal polymerisation has the potential to bring unprecedented reductions in energy demand and process time to produce fibre reinforced polymer composites. Production of epoxy-based fibre reinforced polymer parts with high fibre volume content, commonly encountered in industry, is however hindered by the heat sink created by the fibres and the mould, overcoming the heat output of the chemical reaction, thus preventing front propagation. We propose a novel self-catalysed frontal polymerisation manufacturing method based on the integration of thin resin channels in thermal contact with the composite stack as a strategy for low-energy production of high fibre volume fraction polymer composites without the need for a continuous energy input. Frontal polymerisation inside the resin channel proceeds faster and preheats the fabric stack, thus catalysing the process. Parts with up to 60% fibre content are successfully produced independently of the sample thickness. Fillers added within the resin channels provide means to tailor the frontal polymerisation process kinetics. The parts have a significantly higher glass transition temperature than those produced in a conventional oven, and comparable mechanical properties while energy consumption is reduced by over 99.5%.

Thermal and compositional driven convection in thin reaction fronts.

Chemical reaction fronts separate regions of reacted and unreacted substances as they propagate in liquids. These fronts may induce density gradients due to different chemical compositions and temperatures across the front. In this paper, we investigate buoyancy-induced convection driven by both types of gradients. We consider a thin front approximation where the normal front velocity depends only on the front curvature. This model applies for small curvature fronts independent of the specific type of chemical reaction. For density changes due only to heat variations near the front, we find that convection can take place for either upward or downward propagating fronts if density gradients are above a threshold. Convection can set in even if the fluid with lower density is above the higher density fluid. Our model consists of Navier-Stokes equationscoupled to the front propagation equation. We carry out a linear stability analysis to determine the parameters for the onset of convection. We study the nonlinear front propagation for liquids confined in narrow two-dimensional domains. Convection leads to fronts of steady shape, propagating with constant velocities.

Impact of air entrapment on capillary absorption in porous building materials

When the water content of a porous material is high, air entrapped in the pore space is expected to affect water transfer through the pores. To understand the effects of air entrapment on water transfer in porous building materials in the high-water-saturation region, we examined the water transfer characteristics corresponding to significantly small air entrapment effects. First, we conducted two sets of water uptake experiments. In the first experiment, using three building materials, the time evolution of the amount of water absorption was measured at a low air pressure near vacuum (several kPa). In the second experiment, the water content profile during water uptake was measured using the gamma-ray attenuation method. The experimental results showed that low air pressure accelerated the water uptake by the brick and aerated concrete specimens, whereas water uptake by the calcium silicate board specimens was not significantly affected. These differences among materials were analyzed from a pore structure viewpoint. Moreover, gamma-ray attenuation measurements confirmed that the obtained water content profile was qualitatively similar at atmospheric and low air pressures, although the low air pressure increased both the water content of the material after capillary absorption and the wetting front propagation rate. Finally, simultaneous water and air transfer calculations based on the air and liquid water balance in a material reproduced the measured water absorption rates well, confirming that air entrapment and pressure development in the pores can significantly reduce the rate of water uptake and water content after capillary absorption. The calculation results also indicated that the air pressure in a material did not significantly increase at early water uptake stages where local water content was not high, which supported the general assumption that treating the liquid-water transfer in porous building materials as a one-component flow is valid in most cases.

Numerical Simulation of Polyacrylamide Hydrogel Prepared via Thermally Initiated Frontal Polymerization.

Traditional polymer curing techniques present challenges such as a slow processing speed, high energy consumption, and considerable initial investment. Frontal polymerization (FP), a novel approach, transforms monomers into fully cured polymers through a self-sustaining exothermic reaction, which enhances speed, efficiency, and safety. This study focuses on acrylamide hydrogels, synthesized via FP, which hold significant potential for biomedical applications and 3D printing. Heat conduction is critical in FP, particularly due to its influence on the temperature distribution and reaction rate mechanisms, which affect the final properties of polymers. Therefore, a comprehensive analysis of heat conduction and chemical reactions during FP is presented through the establishment of mathematical models and numerical methods. Existing research on FP hydrogel synthesis primarily explores chemical modifications, with limited studies on numerical modeling. By utilizing Differential Scanning Calorimetry (DSC) data on the curing kinetics of polymerizable deep eutectic solvents (DES), this paper employs Malek's model selection method to establish an autocatalytic reaction model for FP synthesis. In addition, the finite element method is used to solve the reaction-diffusion model, examining the temperature evolution and curing degree during synthesis. The results affirm the nth-order autocatalytic model's accuracy in studying acrylamide monomer curing kinetics. Additionally, factors such as trigger temperature and solution initial temperature were found to influence the FP reaction's frontal propagation speed. The model's predictions on acrylamide hydrogel synthesis align with experimental data, filling the gap in numerical modeling for hydrogel FP synthesis and offering insights for future research on numerical models and temperature control in the FP synthesis of high-performance hydrogels.

Numerical investigation of the stress regime effect on injection-induced fault reactivation and associated seismicity

The occurrence of many seismic events in energy production projects is caused by fluid injection-induced reactivation of critically stressed faults. The distribution of local stress regime has a significant influence on natural earthquakes, while its role in controlling injection-induced fault slip behavior and seismicity magnitude remains poorly understood. In this paper, we present a three-dimensional hydro-mechanical coupled fault reactivation study using unified pipe-interface element method (UP-IEM). The accuracy and reliability of the numerical method are evaluated and confirmed by numerical verification and actual observation data in Pohang EGS project, South Korea. The relationship between fault aseismic slip, seismic slip and seismicity magnitude is analyzed. Results show that both aseismic and seismic slip have different spatiotemporal distribution with the variation of stress regime. The seismic slip zone will exceed the pressurized area with the increase of initial slip tendency under the strike-slip-faulting stress regime (σH>σv>σh). Compared with fluid pressure, the seismic slip is mainly determined by a sudden increase of the shear stress at the aseismic slip zone boundary. In addition, the location of the microseismic events moves towards the fluid pressure propagation front. The larger slip area and seismicity magnitude are easier to be induced under strike-slip-faulting stress regime than thrust-faulting stress regime (σH>σh>σv). The tensile failure and higher permeability of the fault induced at injection phase for normal-faulting stress regime (σv>σH>σh) leads to a greater seismogenic hazard at the shut-in stage.

Poroacoustic front propagation under the linearized Eringen–Cattaneo–Christov–Straughan model

Abstract Employing the Laplace transform and its properties, we investigate the evolution of the coupled thermal and fluid-acoustic waveforms that arise in a signaling problem under the Eringen–Cattaneo–Christov–Straughan model of poroacoustic phenomena. Assuming a Heaviside temperature input, we determine the effects of what we term the “Eringen coefficient” by examining the propagation and evolution of the resulting shock and/or acceleration waveforms. Special/limiting cases are also discussed and, in the final section of this Note, possible follow-on studies are suggested.

Dynamic laser ignition characteristics of solid fuel and oxygen for hybrid rocket system

The transient laser ignition process in a slab burner that is similar to hybrid rocket motors is investigated in this study. The coupling characteristics between solid fuel properties, pyrolysis rate, and laser power were analysed. The results show that the laser ignition process can be divided into four stages: preheating, pyrolysis, ignition, and combustion, with the preheating and pyrolysis stages primarily controlled by the solid heating rate and the mixing characteristics in the fuel-rich combustion zone. Meanwhile, the ignition and combustion stages were characterized by ignition kernel growth and flame front propagation. The ignition delay time and the establishment of steady-state combustion were significantly affected by the ignition energy and fuel properties, i.e., The ignition delay time increases exponentially with decreasing laser energy. An increase in oxidizer flow flux reduces the demand for mixing during ignition, but the corresponding increase in combustion pressure poses challenges to the ignition process. A two-dimensional transient numerical model was established based on optical-thermal coupling theory to further understand the dynamic pyrolysis process of solid fuel particles. The model was validated through fire experiments under different laser ignition energy conditions. The results indicate that the deviation of the maximum penetration depth and ignition delay time between the experiment and the simulation is within 7 %.

Synthesis and characterization of foams generated via gel state frontal polymerization

AbstractIn conventional foams, anisotropy within the microcellular structure can only be achieved through confinement in one or more directions during the foaming process. Herein, we present a novel, versatile, and energy efficient method to create unique high‐performance foams where the anisotropy of the microcellular and the microcellular structure itself is not dictated by external confinement during the foaming process. To generate such foams, an initial crosslinked gel is first created where the crosslink density of the gel can be tuned by UV intensity and cure time. Next, the gel can be foamed while simultaneously creating a second network via radically induced cationic frontal polymerization (RICFP). In this case, the anisotropy in the foam is dictated by propagation front during the RICFP and the microcellular morphology and cell surface area are dictated by the crosslink density of the gel. These two unique qualities lead to rigid foams that can be generated from a gel without the need for confinement where the microcellular size and shape can be tuned by the crosslink density of the gel precursor. A one‐pot liquid system composed of miscible epoxies, acrylates, and chemical blowing agents (CBAs), allows for control over the foam physical, mechanical, and thermal properties with glass transition temperatures ranging from 74.7 to 125.8°C. Additionally, by patterning the crosslink density of the initial gel through controlled exposure of UV, complex microcellular structures can be formed that are not possible in conventional foaming processes. This positions gel frontal polymerization and foaming as an advanced technique to produce high performing anisotropic foams with a wide array of tunable physical, mechanical, and thermal properties.

Crack tunnelling analysis of P91 steel under creep-fatigue loading with various force ratios and hold times at 600 ºC

ABSTRACT The advent of non-conventional and renewable energy sources places a tremendous strain on generic sources of power generation, which have been historically conceived and designed for constant-load criteria, as they are compelled to conform to a ‘fluctuating’ mode of operation so as to comply with modern-day energy requirements. Power plants undergo a starting schedule, followed by sustained operation at intermediate loads when the energy requirement is mediocre. This renders the evaluation of material behaviour under creep-fatigue conditions, a critical component of failure prevention and prediction. ASTM has come up with a standard E-2760 for creep-fatigue crack growth (CFCG) testing, wherein Compact tension, C(T) specimens, are subjected to trapezoidal loading waveforms. In this article, the crack tunnelling phenomenon has been objectively evaluated along with exploring the propagation of crack growth fronts and the crack tip opening angles (CTOA).

A phenomenological design method of the parallel packed bed reactors for chemical looping combustion of gas fuels

The parallel packed bed reactor (PPBR) is a promising route for industrial application of chemical looping combustion. This paper introduces a phenomenological approach for the rational design of PPBR, serving as a manner to efficiently obtain comprehensive design results. Based on the macroscopic propagation of heat and reaction fronts, it elucidates the dynamic heat/mass transfer within the bed. Subsequently, it is applied to design a 1.5 MWth reactor. The risk of carbon deposition in the bed is firstly investigated from a thermodynamic perspective. Then, the study reveals that the height-to-diameter ratio of the reactor must meet certain constraints to ensure stable and cost-effective operation, and these constraints are extended to the scale-up process. Sensitivity analysis is utilized to investigate the effect of key parameters on the design schemes and performance. Effects of particle size on intra-particle gas diffusion is analyzed. Finally, detailed design schemes and performance of the reactor are determined.

Laser tracking system for real-time monitoring the combustion of energetic nanomaterials

This paper discusses a technique for monitoring the combustion of high-energy materials in real time using a laser monitor modified to track the combustion front at the relatively long distance between the object of study and optical system. Options for moving the platform with the sample at a fixed beam of a laser monitor and a system with a motorized rotary mirror are considered. The system provides moving the observation area over the surface of the sample at an adjustable speed up to 22 cm/s. The advantages of the proposed technique are the possibility of high-speed control the combustion front propagation through the sample, full suppression of background lighting and the distance increasing when monitoring high-temperature objects. The paper presents the results of imaging the combustion in air of nanothermite compositions in a real time with movable field of view. The video images allow observing the changes in morphology of the nanopowder surface during combustion. In contrast to previously used laser monitors, the system allows studying the uniformity and linearity of combustion front propagation including the local burning rate at different moments of time.

Formation, propagation and conversion of transport barriers triggered by dynamical critical gradient in tokamak plasmas

In this work, we propose a reduced model with a dynamical critical gradient to study the formation, propagation, and conversion of the transport barriers. In contrast to the commonly adopted static critical gradient, an evolving critical gradient self-consistently softens the profile stiffness, so as to facilitate the generation of transport barriers. This is especially crucial to the internal transport barrier (ITB) formation. Numerically, we show that the inhomogeneity of turbulent and neoclassical diffusivities can induce the global wave front propagation of the transport barrier. When the heating power ramps quickly, the ITB propagates unidirectionally to the edge region and converts into an edge transport barrier. For slow power ramping, the propagating ITB will bifurcate into bidirectional wavefronts and finally convert into a steady double transport barrier state. Our model uncovers the vital role of a dynamical ‘profile-stiffness’ in depicting the global dynamics of the transport barrier.

Thenovel numerical solutions for conformable fractional Kuramoto-Sivashinsky equations by using Cq-HATM and CHPETM

The present study used the novel conformable fractional methods to examine the nonlinear Kuramoto-Sivashinsky equations of the conformable fractional derivative. The Kuramoto-Sivashinsky equation is a mathematical model employed to elucidate a diverse range of chemical and physical phenomena, encompassing chemical reaction-diffusion processes, plasma instabilities, viscous flow issues, flame front propagation, and magnetized plasmas. The present inquiry provides an examination of convergence and error for the future scheme. The conformable q-homotopy analysis transform method (Cq-HATM) provides h-curves that illustrate the convergence range of the series solution achieved. In order to validate the effectiveness and suitability of the Cq-HATM, we examine separate instances. In this study, we introduce an application that demonstrates the potential benefits and effectiveness of the proposed approach. Furthermore, an error analysis is performed in order to verify the accuracy of the scheme. Numerical simulations are conducted to validate the precision of the forthcoming approach. The findings obtained from the numerical and graphical analyses are presented in this study. The method provided in this study showcases a notable degree of computational precision and ease in examining and resolving intricate phenomena related to conformable fractional nonlinear partial differential equations within the domains of science and technology.

Analysis of the behavior of TCAD and SPICE diode models in ESD protection circuits

To protect the interfaces of high-speed ICs from ESD, it is necessary to develop and optimize specialized protection circuit designs to minimize parasitic capacitance. The classic widely used designs based on ggMOS and SCR have too large parasitic capacitance, which degrades the parameters of the interface pins of the ICs. It is necessary to develop and optimize other ESD protection designs. One of the possible promising options is optimized diode protection. In this paper, an analysis of the effect of the design of a diode protection on its electrical characteristics is carried out. An analysis of the possibility of modeling protection using SPICE models was also performed and an assessment of the need to use TCAD simulation was carried out. Based on a comparison of the results of SPICE and TCAD simulations, it is shown that widely used SPICE diode models not optimal for designing and optimizing diode ESD protection. For this task, it is necessary to use TCAD simulation, taking into account the self-heating of the structure and the dynamics of the propagation of the thermal front. The results of this work are used for the development of ESD protection for interfaces of high–speed microcontrollers, DACs/ADCs, and transceivers.

Plasma evolution in laser powder bed fusion using a double pulse format: Time-resolved measurements and physics-based modeling

A novel laser powder bed fusion (LPBF) process utilizing a special double-pulse format was previously proposed. In this process, typically two different types of laser pulses are fired alternatively in time: the low-intensity “sintering laser pulses” intended to melt and coalesce particles, followed by the high-intensity “pressing laser pulse(s)” intended to induce plasma to generate high pressure onto the powder bed to suppress balling and enhance the densification of the sintered material. Laser-induced plasma plays a key role in the novel double-pulse LPBF process, but has not been studied sufficiently. Some critical questions remain to be better answered, such as: what is the minimum plasma-induced pressure needed to effectively suppress balling and how do the “sintering pulses” influence the plasma evolution? This paper reports a model-experiment integrative study of the plasma in DP-LMS, which is seldom reported in a paper in literature to the authors’ knowledge. The plasma evolution is observed in-situ with a high temporal resolution using an intensified CCD (ICCD) camera. The optical emission spectrum (OES) of the plasma is measured and the plasma temperature is deduced from the OES. A physics-based model for the plasma is developed by combining multiple modules. Under the investigated conditions, the model predictions show acceptable agreements with the measured plasma temperature and top front propagation. Utilizing the model, it has been found that under the conditions studied, the minimum plasma-induced pressure required to effectively suppress balling is approximately the pressure making the Weber number exceed ∼1 for the approximately analogized process of a droplet impacting a solid surface. The effect of the “sintering pulses” also influences the evolution of the plasma and obviously increase the plasma-induced pressure on the powder bed surface.

A Bayesian spatio-temporal level set dynamic model and application to fire front propagation

A Bayesian spatio-temporal level set dynamic model and application to fire front propagation

Polymer Patterning by Laser-Induced Multipoint Initiation of Frontal Polymerization.

Frontal polymerization (FP) is an approach for thermosetting plastics at a lower energy cost than an autoclave. The potential to generate simultaneous propagation of multiple polymerization fronts has been discussed as an exciting possibility. However, FP initiated at more than two points simultaneously has not been demonstrated. Multipoint initiation could enable both large-scale material fabrication and unique pattern generation. Here, the authors present laser-patterned photothermal heating as a method for simultaneous initiation of FP at multiple locations in a 2-D sample. Carbon black particles are mixed into liquid resin (dicyclopentadiene) to enhance absorption of light from a Ti:sapphire laser (800 nm) focused on a sample. The laser is time-shared by rapid steering among initiation points, generating polymerization using up to seven simultaneous points of initiation. This process results in the formation of both symmetric and asymmetric seam patterns resulting from the collision of fronts. The authors also present and validate a theoretical framework for predicting the seam patterns formed by front collisions. This framework allows the design of novel patterns via an inverse solution for determining the initiation points required to form a desired pattern. Future applications of this approach could enable rapid, energy-efficient manufacturing of novel composite-like patterned materials.

Controlling reaction transfer between Al/Ni reactive multilayer elements on substrates

Reactive multilayers produce exothermic reaction with definite velocity and maximum temperature after ignition, which are the fundamental properties of the reactive multilayer systems. The generated heat with certain velocity makes it widely used in joining, bonding in the packaging, thermal batteries and many more applications. In this work, a distinct approach for achieving a reaction transfer between the reactive multilayers and different materials is demonstrated which can affect the generated temperature and velocity from the self-propagating properties of the reaction. For these intensions, we fabricated the Al/Ni reactive elements with certain separations between elements which allow to observe the reaction front transfer and emitted temperature in the reaction chain. The created separation between reactive elements are periodical and ordered systems with different thermal conductive properties. The temperature and definite velocity were measured by time-resolved pyrometer and high-speed camera measurements. SEM analysis showed the characteristics of the reaction transfer between reactive multilayer elements. It is predicted that: (I) The reaction front stops at a space with critical length; (II) Reducing heat loss through the substrate supports reaction front propagation through spaces; (III) Thermal property design of the spaces between the reactive elements enables property modification of the self-propagating reaction.Graphical abstract

Imploding Detonation Waves in Self-gravitating Ideal Gas

In the present paper, the propagation of strong spherical imploding detonation waves in a reacting ideal gas under the effect of self-gravitation of the non-homogeneous medium has been investigated, by the help of Chester-Chisnell-Whitham (CCW) theory. It is considered that detonation wave is initially Chapman-Jouguet. Initial taking the density distribution law as power decreasing with distance, the analytical expressions for the detonation velocity just behind the front along with other flow variables are derived. Neglecting the effect of overtaking disturbances the variation of non-dimensional detonation velocity, the pressure and density with the propagation distance have been calculated numerically. The effect of change in density parameter at different Alfven Mach number on the convergence of detonation front have been discussed through graphs in details. Finally, it is found that density parameter and Alfven Mach number of gas have a significant role on propagation of strong spherical detonation front in reacting ideal gas with gravitation effect on all the post-flow variables. The software MATLAB have been used for computation of the problem. Keywords: Self-gravitating gas, Strong detonation waves, CCW theory.