Hot Metal Particles Research Articles

Interdependence of metal vapor plume and melt pool dynamics during laser beam welding under vacuum

Deep penetration laser beam welding involves the emission of hot metal vapor and particles from the keyhole, which interact with the laser beam by means of scattering, absorption, and phase front deformation. The combination of scattering and absorption leads to an extinction of the laser beam, while the phase front deformation adversely affects the beam quality. The capillary formation depends on the local distribution of the absorbed irradiance of the laser beam, which in turn is related to the material process emissions. Additionally, the process atmosphere, in particular the oxygen content, and the working pressure influence the capillary fluctuations and cause variations in welding penetration. This study introduces a measurement setup using simultaneous high-speed observation of the melt pool and the keyhole while measuring the interaction mechanisms between the laser beam and the metal vapor plume during laser beam welding under vacuum of stainless steel. The obtained measurement provides a quantification of the various interaction mechanisms between the laser beam, vapor plume, keyhole, and interdepended melt pool dynamics. These quantitative high-frequency measurements at various working pressures and laser parameters provide useful insights that are crucial for understanding the formation of weld defects, resulting in process monitoring and validation of process-oriented welding simulations.

Spotting ignition of plastic foam by a fast-moving hot metal particle

Spotting ignition involves dynamic interaction between fuel bed and hot particles, but the scientific understanding of the ignition by a fast-moving hot particle is still limited. Herein, a hot steel particle with various horizontal velocities, temperatures, and sizes is shot to ignite vertically oriented low-density expandable polystyrene foam. A high-speed particle can directly get embedded into the foam to achieve flash-point, fire-point, or no ignition, while a low-speed particle bounces away from the foam without ignition. Results show that for a particle of 1150 °C, its minimum velocity for embedding is 12.00 m/s. Such a critical velocity for hot-particle embedded or ignition slightly decreases as particle temperature increases. Minimum ignition temperature of these high-speed particles is 200 °C higher than that of near-static or with a low free-fall velocity, due to the shorter residence time and insufficient to produce a flammable mixture. Moreover, when the particle is neither too slow to bounce away nor too fast to get embedded, it will be partially embedded on the sample surface to burnout the fuel, posing the biggest fire hazard. It deepens our knowledge of the complex interaction between hot moving particles and insulation foam to reduce spotting fire risk for building façade.

Experimental Study on the Ignition Behavior of Wildland Fuels Ignited by Heated Stainless-Steel Particles

ABSTRACT Hot metal particles from activities like machining, grinding, and welding are significant ignition sources for wildland fuels. Thus, this study examines the effects of stainless-steel particle geometries and wildland fuel beds on wildfire ignition behavior. To this purpose, experiments have been conducted to investigate the impact of particle type, composition, diameter, number, and wildland fuel bed species on ignition behavior. Three common California wildland grass fuel species were studied: Bromus (Bromus diandrus), Tall Wheatgrass (Thinopyrum ponticum), and Avena Wildoats (Avena fatua). Various stainless-steel grades were tested as ignition particles. The findings from this study support prior research and indicate that increasing the quantity and diameter of ignition particles reduces ignition temperature and affects the type of ignition (flaming, smoldering, or none). Moreover, results from this study showed that decreasing the carbon content in stainless steel may lower the temperature required to reach wildland fuel ignition. In addition, wildland fuel species influenced ignition type and temperature. For instance, Bromus fuels had the lowest flaming ignition temperature at 320°C whereas Tall Wheatgrass and Avena ignited at 400°C and 380°C, respectively. Flaming ignition was primarily observed in Bromus fuels, while Avena and Tall Wheatgrass primarily supported smoldering ignition. This work asserts that species-specific fuel morphology, chemical, and thermal properties affect the flammability of grassy fuels when these are ignited by metal particles and observes that in addition to particle geometry, the chemical properties of ignition particles, such as carbon content, may also lay a key role in shapping ignition behavior.

Ignition of pine needle fuel bed by the coupled effects of a hot metal particle and thermal radiation

Spot fire and flame radiation are two important causes of Wildland-Urban interface fire. Previous studies have mainly focused on one single cause. However, the ignition of fuels by firebrands (or hot particles) coupled with flame radiation is unknown due to the lack of experimental and theoretical study. In this paper, the ignition behavior of pine needle fuel bed under the coupled action of a hot particle and thermal radiation was studied for the first time, and the critical ignition conditions were analyzed. The experimental results show that the ignition risk of a hot particle coupled with thermal radiation is much higher than that of a hot particle or thermal radiation alone. Under the coupled condition, the ignition probability increases with the increase of particle temperature and radiation heat flux, and the critical radiation heat flux required for ignition decreases with the increase of particle diameter and temperature. The ignition delay time decreases as the radiation heat flux increases for different particle diameters and temperatures. The theoretical analysis revealed that there is a good linear relationship between the critical radiation heat flux required for ignition and the parameters of the hot particle d(Tp-Tsm). This study may help provide the first step to understand the ignition mechanism of fuels by the coupled effect of firebrands and flame radiation.

Abrasion protection of silicate enamel coating of engineering network nodes in field conditions

The paper proposes methods for protecting the internal silicate-enamel coating of pipes in field conditions. It is shown that the proposed methods are highly efficient for manual mechanical cutting, chamfering, cutting edges with abrasive tools with vulcanite cutting discs for welding, in manufacturing and assembling the engineering systems, transportation of aggressive products, and fire fighting systems.It is shown that the most appropriate method for preservation of the internal silicate-enamel coating is its preliminary annular incision and sizing of the cut place with a thin cloth (calico) impregnated with quick-drying glue, and clay coating protection from hot metal particles. This method is recommended to be used when cutting pipes, fittings, segments, processing their chamfers and cutting edges, in manufacturing of network nodes (fire-fighting systems) in field conditions.The proposed methods will significantly increase the operating life and reliability of engineering networks with silicate-enamel coating for the transportation of aggressive products and, as a result, technical safety of trouble-free facilities.

Ignition of dispersed coal in local conductive heating conditions

Present work studies the ignition of a brown coal by a hot metal particle. The experiments establish the limits of the gas-phase ignition and ignition delay times in conditions whee the metal particle situated on the surface of dispersed coal layer. The coal particle size was different in different series of experiments, and it was ranged from 0.1 to 1 mm; the shapes of the metal particles were sphere, disk, and cube; their initial temperature varied between 1000 and 1400 K. Three modes of gas-phase ignition of coal were established with the ignition zone of volatiles located in the vicinity of the hot particle. The practical application of obtained results is the development of fire prevention guidelines for tightening fire safety management at productions dealing with coal mining, transportation, storage, processing, and combustion.

Spot Fire Ignition of Natural Fuels by Hot Aluminum Particles

The ignition of combustible material by contact with hot metal particles is an important pathway by which wildland and urban spot fires are started. This work examines how fuel characteristics such as density, morphology and chemical composition effect the ability of the fuel to be ignited by a hot metal particle. Fuels were prepared out of three materials: alpha-cellulose, a barley/wheat/oat grass blend, and pine needles. Each material was prepared as a powder and as larger, long pieces: strips of cellulose paper, loose grass, and pine needles. These fuels are representative of thermal insulation (cellulose strips), dry grasses (grass blend), forest litter (pine needles) and duff (powders). Aluminum particles ranging from 2 mm to 8 mm in diameter heated to temperatures between 575°C and 1100°C were dropped onto these fuels. The particle temperature required for ignition becomes higher as the particle size decreased. The results show that the required temperatures for ignition of powders were lower, with this trend particularly pronounced for the alpha-cellulose fuels. The biomass fuels required higher temperature particles to ignite, indicating that the presence of other ligno-cellulosic materials make ignition more difficult.

Experimental and numerical study of coal dust ignition by a hot particle

This work studies the ignition of a layer of brown coal dust by hot metal particles experimentally and numerically. The experiments establish the limits of the flaming ignition of gas and ignition delay times when the parameters of solid fuel and metal particles varied in a wide range. The particle size of coal ranged from 0.1 to 1 mm; the shapes of the metal particles were sphere, disk, and cube; their initial temperature varied between 1000 and 1400 K. A mathematical model was developed for describing the processes involved in heat and mass transfer as well as chemical reactions around the local heat source. The results of the numerical simulation are in good agreement with the experimental data: boundaries of the flaming ignition of coal; coal ignition delay times; three modes of flaming ignition of coal with the ignition zone of volatiles located in the vicinity of the hot particle. The mathematical model is good at predicting the ignition conditions during interaction between a hot metal particle and a layer of coal dust. The model can also be used for developing the promising technology of steam boiler start-up by highly reactive coal instead of flammable liquid combustion. Another application is the development of fire prevention guidelines for tightening fire safety management at productions deal with coal mining, transportation, storage, processing, and combustion. Finally, the paper includes the analysis of limitations for the practical use of the predictive mathematical model in thermal power engineering and fire safety management.

Wildland fire spot ignition by sparks and firebrands

Wildland and Wildland Urban Interface (WUI) fires are an important problem in many areas of the world and may have major consequences in terms of safety, air quality, and damage to buildings, infrastructure, and the ecosystem. It is expected that with climate changes the wildland fire and WUI fire problem will only intensify. The spot fire ignition of a wildland fire by hot (solid, molten or burning) metal fragments/sparks and firebrands (flaming or glowing embers) is an important fire ignition pathway by which wildfires, WUI fires, and fires in industrial settings are started and may propagate. There are numerous cases reported of wildfires started by hot metal particles from clashing power-lines, or generated by machines, grinding and welding. Once the wildfire or structural fire has been ignited and grows, it can spread rapidly through ember spotting, where pieces of burning material (e.g. branches, bark, building materials, etc.) are lofted by the plume of the fire and then transported forward by the wind landing where they can start spot fires downwind. The spot fire problem can be separated in several individual processes: the generation of the particles (metal or firebrand) and their thermochemical state; their flight by plume lofting and wind drag and the particle thermo-chemical change during the flight; the onset of ignition (smoldering or flaming) of the fuel after the particle lands on the fuel; and finally, the sustained ignition and burning of the combustible material. Here an attempt has been made to summarize the state of the art of the wildfire spotting problem by describing the distinct individual processes involved in the problem and by discussing their know-how status. Emphasis is given to those areas that the author is more familiar with, due to his work on the subject. By characterizing these distinct individual processes, it is possible to attain the required information to develop predictive, physics-base wildfire spotting models. Such spotting models, together with topographical maps and wind models, could be added to existing flame spread models to improve the predictive capabilities of landscape-scale wildland fire spread models. These enhanced wildland fire spread models would provide land managers and government agencies with better tools to prescribe preventive measures and fuels treatments before a fire, and allocate suppression resources and issue evacuation orders during a fire.

Interaction between flaming and smouldering in hot-particle ignition of forest fuels and effects of moisture and wind

Ignition of natural fuels by hot metal particles from powerlines, welding and mechanical processes may initiate wildfires. In this work, a hot steel spherical particle (6–14 mm and 600–1100°C) was dropped onto pine needles with a fuel moisture content (FMC) of 6–32% and wind speed of 0–4 m s–1. Several ignition phenomena including direct flaming, smouldering and smouldering-to-flaming transition were observed. The critical particle temperature for sustained ignition was found to decrease with the particle size (d) and increase with FMC as (°C), and the maximum heating efficiency of particle was found to be . As the particle size increases, the influence of FMC becomes weaker. The flaming ignition delay times for both direct flaming and smouldering-to-flaming transition were measured, and decreased with particle temperature and wind speed, but increased with FMC. The proposed heat-transfer analysis explains the ignition limit and delay time, and suggests that the hot particle acts as both heating and pilot sources like a small flame for direct flaming ignition, but only acts as a heating source for smouldering. This study deepens the fundamental understanding of hot-particle ignition, and may help provide a first step to understanding the mechanism behind firebrand ignition.

Smoldering spot ignition of natural fuels by a hot metal particle

The spotting ignition of combustible material by hot metal particles is an important pathway by which wildland and urban spot fires are started. Upon impact with a fuel, such as dry grass, duff, or saw dust, these particles can initiate spot fires by direct flaming or smoldering can transition to flaming. In spite of the relative frequency that fires are initiated by hot metal particles, there is little work published that addresses the ignition capabilities of hot metal particles landing on natural fuels, especially regarding smoldering ignition. This work is an experimental and analytical study of how the flaming and smoldering propensities of powdered natural fuel beds in contact with hot metal particles are affected by differences in the particle characteristics, particularly the effect of particle melting, which adds energy to the particle. In the experiments, stainless steel and aluminum particles ranging in size from 1.6 to 8 mm in diameter are heated to various temperatures between 500 and 1100°C and dropped onto a fuel bed composed of a powder grass blend. It is observed that the ignition boundary both for flaming and smoldering follows a hyperbolic relationship between particle size and temperature, with smaller particles requiring higher temperatures to ignite the fuel. For both metal particles smoldering ignition occurs at significantly lower temperatures than flaming ignition. A simplified numerical model is developed to help understand smoldering ignition by a metal particle and to examine how the melting influences the ignition process. Good qualitative agreement is obtained between the model predictions and the experiments suggesting that the model provides a first step toward the theoretical modeling of this complex problem.

Temperature measurements in metalized propellant combustion using hybrid fs/ps coherent anti-Stokes Raman scattering.

We apply ultrafast pure-rotational coherent anti-Stokes Raman scattering (CARS) for temperature and relative oxygen concentration measurements in the plume emanating from a burning, aluminized ammonium-perchlorate propellant strand. Combustion of these metal-based propellants is a particularly hostile environment for laser-based diagnostics, with intense background luminosity and scattering from hot metal particles as large as several hundred micrometers in diameter. CARS spectra that were previously obtained using nanosecond pulsed lasers in an aluminum-particle-seeded flame are examined and are determined to be severely impacted by nonresonant background, presumably as a result of the plasma formed by particulate-enhanced laser-induced breakdown. Introduction of femtosecond/picosecond (fs/ps) laser pulses improves CARS detection by providing time-gated elimination of strong nonresonant background interference. Single-laser-shot fs/ps CARS spectra were acquired from the burning propellant plume, with picosecond probe-pulse delays of 0 and 16 ps from the femtosecond pump and Stokes pulses. At zero delay, nonresonant background overwhelms the Raman-resonant spectroscopic features. Time-delayed probing results in the acquisition of background-free spectra that were successfully fit for temperature and relative oxygen content. Temperature probability densities and temperature/oxygen correlations were constructed from ensembles of several thousand single-laser-shot measurements with the CARS measurement volume positioned within 3 mm or less of the burning propellant surface. The results show that ultrafast CARS is a potentially enabling technology for probing harsh, particle-laden flame environments.

Processes of conversion of a hot metal particle into aerogel through clusters

Processes are considered for conversion into a fractal structure of a hot metal micron-size particle that is located in a buffer gas or a gas flow and is heated by an external electric or electromagnetic source or by a plasma. The parameter of this heating is the particle temperature, which is the same in the entire particle volume because of its small size and high conductivity. Three processes determine the particle heat balance: particle radiation, evaporation of metal atoms from the particle surface, and heat transport to the surrounding gas due to its thermal conductivity. The particle heat balance is analyzed based on these processes, which are analogous to those for bulk metals with the small particle size, and its high temperature taken into account. Outside the particle, where the gas temperature is lower than on its surface, the formed metal vapor in a buffer gas flow is converted into clusters. Clusters grow as a result of coagulation until they become liquid, and then clusters form fractal aggregates if they are removed form the gas flow. Subsequently, associations of fractal aggregates join into a fractal structure. The rate of this process increases in medium electric fields, and the formed fractal structure has features of aerogels and fractal fibers. As a result of a chain of the above processes, a porous metal film may be manufactured for use as a filter or catalyst for gas flows.

Ignition of a composite propellant by a hot particle under conditions of a nonideal thermal contact

A mathematical modeling of the solid-phase ignition of a condensed material by a single diskshaped hot metal particle under conditions of a nonideal thermal contact between the particle and the propellant due to the natural surface roughness of the latter is performed. The ranges of values of the initial temperature of the local energy source and of the parameter characterizing the propellant surface roughness at which radiative heat transfer across the gas gap between the propellant and particle significantly (up to 25%) influences the ignition delay time, the main integrated characteristic of the process, are determined from numerical simulations.

Assessment of the Stability of Condensed Solid Substance Ignition by a Hot Particle

The paper presents numerical simulation of composite fuel ignition by disc-shaped hot metal particle. The study establishes the temperature and the source size limits, which are necessary and sufficient for condensed substance ignition. A diagram with coordinates a heat flow the amplitude - ignition delay time has been presented. The diagram shows the area of the sustainable initiate combustion of composite solid fuel at local heating by the source with limited energy capacity.

Spot Fire Ignition of Natural Fuel Beds by Hot Metal Particles, Embers, and Sparks

Wildland and wildland/urban interface fires are a serious problem in many areas of the world. It is expected that with global warming the wildfire and wildland/urban interface fire problem will only intensify. The ignition of natural combustible material by hot metal particles or embers is an important fire ignition pathway by which wildland and urban spot fires are started. There are numerous cases reported of wild fires started by hot metal particles from clashing power lines, or from sparks generated by machines or engines. Similarly there are many cases reported of industrial fires caused by grinding and welding sparks. Despite the importance of the subject, the topic remains relatively unstudied. The senior author of this article and his collaborators have been working for the past few years on this problem. In this article, we provide a comprehensive summary of that work to date. The work includes experimental and theoretical modeling of the ability of hot metal particles and embers to cause the ignition of cellulosic fuel beds. The metal particles studied are representative of clashing conductors (aluminum and copper) and those produced by machine friction and hot work such as welding (stainless steel and brass). In addition glowing and flaming wood embers are considered, as they represent an important source of fire spotting in wildfires. The overall results show a hyperbolic relationship between particle size and temperature, with the larger particles requiring lower temperature to ignite the fuel bed than the smaller particles. An important finding is that although particle energy is important in the capability of the particle to ignite the fuel, both energy and temperature are determining factors of the particle ignition capabilities. The thermal properties of the metal play a lesser role with the exception of the energy of melting if it occurs. It also appears that the controlling ignition mechanisms by large particles are different than those from the small particles. The former appear to be determined primarily by the particle surface temperature while the latter by the particle energy and surface temperature. Sparks are a specific type of particles with very small sizes and very high temperatures. Because of the small sizes, their energy is small and it is postulated that the sparks must accumulate for ignition of a fuel bed to occur. The results with embers indicate that the smoldering is the easier form of ignition, although flaming ignition can occur if the ember is flaming and the air velocities are moderate. To provide further information about the fire spot ignition process, both analytical and numerical modeling are used and compared with the experimental results. Although the models provide qualitative predictions further development is necessary to reach quantitative predictive capabilities.

Cellulose spot fire ignition by hot metal particles

The ignition of natural combustible material by hot metal particles is an important fire ignition pathway by which wildland and urban spot fires are started. There are numerous cases reported of wild fires started by clashing power-lines or from sparks generated by machines or engines. Similarly there are many cases reported of industrial fires caused by grinding and welding sparks. In this work, the effect of metal type on the ability of hot metal particles to cause flaming ignition of powdered cellulose fuel beds is studied experimentally. The materials studied are stainless steel, aluminum, brass and copper. These metals are representative of clashing conductors (aluminum and copper) and those involved in machine friction and hot work such as welding (stainless steel and brass). Cellulose powder is used as a surrogate for natural fuel beds. Particles of various sizes from 2 to 11mm in diameter are heated to various temperatures between 575 and 1100° C and dropped onto the fuel bed. The results show a hyperbolic relationship between particle size and temperature, with the larger particles requiring lower temperatures to ignite the cellulose than the smaller particles. For large particles of all the metals, the ignition boundary is not very sensitive to particle size. For small particles the ignition boundaries are similar for the different metals and sensitive to both energy and temperature. The thermal properties of the metal play a lesser role in determining ignition with exception of the energy release from melting when it occurs. It also appears that the controlling ignition mechanisms by large particles are different than those from the small particles. The former appears to be determined primarily by the particle surface temperature while the later by the particle energy and temperature.

Flaming Ignition Behavior of Hot Steel and Aluminum Spheres Landing in Cellulose Fuel Beds

The ignition of combustible material by hot metal particles is an important fire ignition pathway that remains relatively unstudied. In this work, the flaming ignition behavior of powdered cellulose fuel beds by hot steel and aluminum spheres of various diameters and initial temperatures was studied. Understanding ignition in this scenario could offer insight into the mechanisms by which metal particles initiate wildland fires and fires in industrial settings. Earlier work on this topic has shown that ignition propensity has a relationship with the temperature and diameter of the sphere. However, little is known about the physical processes governing this relationship. This work provides further information regarding the conditions required for ignition, and useful observations for the development of a theoretical framework for predicting ignition propensity of combustible fuel beds. For the conditions tested, powdered cellulose ignition appears to exhibit limiting behavior in two regimes: for larger spheres, temperatures below 600 C did not ignite the cellulose and spheres with diameters below 2.38 mm for steel or 2.03 mm for aluminum and temperatures up to 1100 C did not ignite the cellulose either. We also observed that in the range of sphere diameters from 4-8 mm, aluminum spheres of a given diameter are more likely to cause ignition than their steel counterparts. This seems to be due to the fact that the aluminum spheres are molten at temperatures greater than 657.2 C; melting contributes to a spheres bulk energy through the latent heat of melting and allows for sphere deformation and splatter during impact. Furthermore, qualitative analysis of high speed schlieren videos shows differences in pyrolysis and ignition behavior and suggests that, different controlling processes may be at work for spheres of different sizes and for molten versus solid spheres.

Experimental research of heat transfer conditions influence on the distillate fuels ignition characteristics

The experimental studies results of diesel fuel ignition regularities by hot single metal particles and fuel droplets on a heated metal plate surface are presented. The ignition delay time dependences on the initial heat source temperature were determined. The results are a basis for the development of typical liquid fuels ignition models in terms of heat and mass transfer processes in a small neighborhood. Liquid fuels are widely used in road transport and under certain conditions (storage, transport, congestion) are sources of increased danger. They can cause a fire during thermal exposure of high intensity. Heat sources can be different, as well as the conditions of exposure (1-3). To date, mathematical models and solving methods for the ignition of solid (4, 5) and liquid (2, 3) fuels are worked out. Experimental conditions and the ignition characteristics of solid monolithic (9) and dispersed (7) fuels with a local heating source are established. Ignition regularities for films and drops of distillate fuels are studied much worse. The purpose of this work is an experimental study of diesel fuel ignition conditions and characteristics. Two of the most important options for practice are considered: a single fuel film heated up to high temperatures and particle droplets on the heated metal plate surface. Film fuel ignition process research was conducted on the plant (9), the concept of which is shown in Fig. 1, and drop method (1) (the experimental scheme is shown in Fig. 2). In all experiments a video registration of the investigated processes was conducted. The time moments of heat source (particle or plate) contact with a liquid (film or drop, respectively) and the ignition delay time were registered. As the heat sources the steel particles (diameter of dp = 6 · 10 −3 m and a height of hp = 3 · 10 −3 ÷7 · 10 −3 m) and the steel plate (size 5 · 10 −2 × 5 · 10 −2 m and a height hw = 5 · 10 −3 m) were used. Particles fall from a small height (0.01m) onto the fuel film surface. A possible change in its temperature is due to heat sink in the air. It is established that during the fall of the particles temperature is reduced by not more than 5K, which is about ±0.5% depending on the absolute value of the initial particle temperature. Experiments at constant heat source temperature T0 is repeated at least 7 times under identical conditions. Random error in ign determining is no more than ±10% in the experiments with the particle heat source and up to ±16% in the experiments with hot plate. The dependences of the diesel fuel ignition delay times from the initial heat source temperature obtained from the experiments results are shown in Fig. 3. Measurements have shown that the temperature of the massive plate during the experiment remained virtually unchanged (droplet sizes This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Analyzing the characteristic times of physical-chemical processes running at ignition of a liquid condensed substance under local heating

The ranges of times of heat and mass transfer processes, phase transitions, formation of a reactive vapor-gas mixture, and abruptly exponential acceleration of oxidation at ignition of a liquid condensed substance by a typical source with a limited heat content, that is, by a small hot metal particle, are found. Results of the performed numerical and experimental investigations are used to find the limiting values of the main parameters (temperature, sizes) of a local energy source, which are sufficient for ignition of a typical liquid fuel.