Semi-analytical modeling of non-premixed counterflow combustion of metal dust

In this study, a semi-analytical model is developed for non-premixed combustion of metal dusts in counterflow configuration. Combustion domain is divided into three separate zones, each of which possesses corresponding mass and energy conservation equations as well as boundary and jump conditions. Metal dust, assumed to be aluminum, undergoes an Arrhenius-type reaction with oxidizer, when it is heated enough to reach the ignition temperature. Dimensionless forms of conservation equations are derived and utilized to elucidate the combustion characteristics. The effects of oxidizer Lewis number and fuel mass concentration on the flame position and temperature are discussed thoroughly. In addition, temperature distribution of the whole domain is calculated by numerically solving the system of partial differential equations. In order to track particles through combustion domain, Lagrangian equations of motion are solved either mathematically or numerically, considering thermophoretic, weight, buoyancy and drag forces. The effects of thermophoretic force on the particle path are investigated, and the deviation of particle from carrier neutral gas direction is obtained. The results showed a great agreement with the data reported in the literature highlighting the fact that the presented model is an efficient one to accurately model the non-premixed counterflow combustion of metal dust.

     

The effect of inclination angle on fire behaviors of stay cable in an intercepted double-layer cable model

Understanding the fire characteristics of stayed-cable bridge, such as fire growth, during accidental fire is essential to develop prevention strategies for potential damage. This study focuses on the effects of high cable fire that may result from a short circuit or a lightning strike since high flammability heat release characteristics of HDPE sheath causes burning of nearby cables. Fire propagation behaviors on one single cable and between two adjacent cables under working conditions with different inclination angles were obtained. The temperature distribution, drop ignition behaviors and flame spread rate were analyzed. The results show that flame propagation characteristics of stay cables seriously changed as the angle of inclination increases, which explain the fracture sequence of cables to some extent for the Red Stone Bridge fire event. The particle size of molten substance formed by combustion of the upper-layer cables increases, and the ignition position for the under-layer cable gradually moves down as the increase of inclination angle. Moreover, increased inclination angle also resulted in increase in flame height and molten drops flow rate and reduce in the duration of the prosperity stage for cable fire. Although HDPE sheath is a protective device, it does prevent the combustion for inner strands from becoming intense, but once ignited, would become the main fire load, promoting fire development.

     

The influence of gravity levels on soot formation for the combustion of ethylene-air mixture

The reduced mechanism coupled with 2D flame code using CHEMKIN II to investigate the effect of gravity on flame structure and soot formation in diffusion flames. The results show that the gravity has a rather significant effect on flame structure and soot formation. The visible flame height and peak soot volume fraction in general increases with the gravity from 1g decreased to 0g. The peak flame temperature decreases with decreasing gravity level. Comparing the calculated results from 1g to 0g, the flame shape becomes wider, the high temperature zone becomes shorter, the mixture velocity has a sharp decrease, the soot volume fraction has a sharp increase and CO and unprovided species distribution becomes wider along radial direction. At normal and half gravity, the flame is buoyancy controlled and the axial velocity is largely independent of the coflow air velocity. At microgravity (0g), the flame is momentum controlled.     

Experimental and numerical simulation of effects of CO2/N2 concentration and initial temperature on combustion characteristics of biomass syngas

Biomass syngas is a form of renewable energy with very broad application prospects, and it has different combustion characteristics according to the fuel composition and processing technology of biomass syngas. The influence of combustion composition, diluent and temperature variation on combustion characteristics were studied in this paper. The FFCM-1 mechanism was used to investigate the combustion characteristics of CO/CH₄/H₂ under varied diluents CO₂/N₂ and temperature by using spherical expansion flame method and ANSYS CHEMKIN-PRO. The experimental laminar burning velocity was compared with the simulation results of FFCM-1 mechanism. The results reveal that the experimental data are in good agreement with the simulation results, which are somewhat different under the condition of rich fuel. The laminar burning velocity decreases significantly with the increase of diluent CO₂/N₂, with the effect of diluent CO₂ being more significant. The laminar burning velocity increase dramatically with the increase of initial temperature, and the adiabatic flame temperature also decreases with the increase of diluent. The reduction caused by diluent CO₂ is much larger than that caused by diluent N₂. The change of initial temperature also affects the adiabatic flame temperature, but the range of variation is not as pronounced as that of diluent. Not only was the interaction between the combustion characteristics of CO/CH₄/H₂ under different diluents and temperature changes explored in this paper, but the influence mechanism was also revealed in depth.     

低温燃料化学特性对湍流预混火焰传播的影响

大分子碳氢燃料的低温化学反应及两阶段点火特性会显著影响火焰的分区及燃烧情况。本文采用数值模拟的方法探究了正庚烷/空气预混混合气在RATS燃具上的湍流火焰传播,与试验结果具有一致性。模拟使用的是44种物质,112步的正庚烷简化动力学机理。使用Open FOAM的reacting Foam求解器建立了简化模拟流道及出口的三维模型,模拟了在大气环境下,初始反应温度450–700 K、入口速度6 m·s~(-1)与10 m·s~(-1)、焰前流动滞留时间100 ms及60 ms、当量比φ=0.6的正庚烷/空气混合气湍流火焰燃烧情况。结果发现,标准化湍流燃烧速度与混合气初始温度以及流动滞留时间有关。在低温点火阶段,正庚烷氧化程度受到初始温度与速度的影响,燃料分解并在预热区中产生大量中间物质如CH_2O,继而会影响湍流火焰燃烧速度。随着初始反应温度的升高,湍流燃烧火焰逐渐由化学反应冻结区过渡到低温点火区;温度超过一定数值后,燃料不再发生低温反应,此时燃烧位于高温点火区域。     

Features of the propagation of laminar spherical flames initiated by a spark discharge in mixtures of methane,pentane, and hydrogen with air at atmospheric pressure

Using high-speed digital color cinematography, we studied the propagation of a laminar spherical flame in stoichiometric mixtures of hydrogen, methane, and pentane with air in the presence of additives at atmospheric pressure in constant-volume reactors, and derived quantitative data on the time of formation of a stable flame front. Cellular flames caused by gas-dynamic instability attributable to convective flows arising during the afterburning of gas were observed in hydrocarbon-air stoichiometric mixtures diluted with inert additives. It was found that the effect of additives of carbon dioxide and argon (>10%) and minor additives of CCl₄ on the combustion of hydrocarbons, and of propylene on the combustion of hydrogen-rich mixtures, lead to periods of delay in the development of a laminar spherical flame; in addition, additives of propylene promote the combustion of hydrogen poor mixtures.     

Molecular dynamics study of the response of nanostructured Al/Ni clad particles system under thermal loading

Molecular dynamics simulations are used to study the exothermic alloying reactions by imposing a thermal loading on a local area of nanostructured Al/Ni clad particles. The combustion parameters, such as particles size, density, and ignition temperature, are characterized. Reducing the size of Al/Ni clad particles makes the propagation velocity of reaction front increase but lowers both the adiabatic combustion temperature and pressure of the system. However, increasing either mass density or ignition temperature makes the propagation velocity of reaction front increase and raises the adiabatic temperature and pressure as well. We estimate the propagation velocity of the chemical reaction front to range from 35.70 to 44.06 m/s.     

Flammability and Thermal Oxidative Degradation Kinetics of Magnesium Hydroxide and Expandable Graphite Flame Retarded Polypropylene Composites

The flammability and the thermal oxidative degradation kinetics of expandable graphite (EG) with magnesium hydroxide (MH) in flame‐retardant polypropylene (PP) composites were studied by limiting oxygen index (LOI), UL‐94 test, and thermogravimetric analysis (TGA). The results show that EG is a good synergist for improving the flame retardancy of PP/MH composite and the effect is enhanced with decreasing EG particle size. The Kissinger method and Flynn‐Wall‐Ozawa method were used to determine the apparent activation energy (E) for degradation of PP and flame retarded PP composites. The data obtained from the TGA curve indicate that EG markedly increases the thermal degradation temperature of PP/MH composites and improves the thermal stability of the composites. The kinetic results show that the values of E for degradation of flame retarded PP composites is much higher than that of neat PP, especially PP/MH composites with suitable amount of EG, which indicates that the flame retardants used in this work have a great effect on the mechanisms of pyrolysis and combustion of PP.     

预混天然气在多孔介质燃烧器中的燃烧与传热

在一台小型渐变型多孔介质燃烧器上进行了预混天然气燃烧与传热试验研究,探讨了天然气速度和多孔介质厚度对多孔介质燃烧室的温度分布、排烟温度和流动阻力的影响。结果表明,天然气在渐变型多孔介质燃烧器中燃烧稳定,燃烧室与水冷夹套间的换热受天然气速度和多孔介质厚度影响,换热效果比空管中燃烧明显增强,同时预混天然气通过多孔介质的进出口压差随着天然气速度和多孔介质厚度的增加而增加。     

Accelerated diffusion of chain carriers and kinetic features of heterogeneous processes in gas-phase chain reactions

In gas-phase combustion processes, the regeneration of free atoms and radicals in chain propagation reactions enhances the diffusion flux of these species from the flame zone. In flame propagation in tubular reactors and in filtration combustion, this effect facilitates the access of chain carriers to the surface even at atmospheric pressure, increases the role of heterogeneous reactions (primarily chain termination), and enhances heat removal due to heterogeneous recombination.     

Characteristics of steady burning over inclined polymethyl methacrylate surface in different pressure environments

This paper aims to examine the characteristics of steady burning of inclined polymethyl methacrylate (PMMA) slabs under different ambient pressures. A sequence of experiments concerning steady burning intensity and combustion efficiency of the sample were conducted under both normal pressure (Hefei: altitude 30 m, 100 kPa) and reduced pressure (Lhasa: altitude 3650 m, 64 kPa). An in situ calorimeter based on oxygen consumption method was employed to measure the heat release rate of materials. A semi-theoretical model based on convective heat feedback from the flame was proposed and fitted well with laminar-dominated flame over the inclined PMMA surface. The critical maximum side length of sample for laminar-dominated flame in Hefei is lower than Lhasa. The trend of combustion efficiency with increasing inclination angle shows a great difference under different pressure environments.

     

Concentration Limits of Flame Propagation in H<Subscript>2</Subscript>—C<Subscript>3</Subscript>H<Subscript>8</Subscript>—Air Mixtures at Elevated Pressures

Upper concentration limits of flame propagation in H₂—C₃H₈—air mixtures at elevated initial pressures are determined. It is revealed that the flame propagation area widens substantially with the initial pressure rise. It is found that the presence of hydrogen promotes combustion of rich propane—air mixtures at concentrations of propane that exceed its upper concentration limit. It is concluded that research results can be explained from consideration of features of chain branching reactions, which are responsible for hydrogen and hydrocarbons combustion in air.     

Effects of gas composition and flame sheathing on the spatial velocity profiles of laminar analytical acetylene flames

A recently developed technique has been employed to spatially map the rise velocity pronies (horizontal and vertical) of commonly used laminar analytical flames and to determine the influence on the profiles of fuel-to-oxidant ratio and the presence of a flame sheath. The rise velocities for fuel-rich, lean, and stoichiometric flames were found to differ substantially, the entire profile being greatest for the fuel-rich condition and lowest for the fuel-lean flame. In addition, the change in rise velocity with the addition of a solid (quartz tube) or gas (N₂) sheath was studied. The flowing sheath, at several different flow rates, affected each rise velocity profile principally by altering atmospheric entrainment and thereby changing secondary combustion in the flame. In contrast, a solid quartz tube used as a sheath produces an additional increase in the entire velocity profile of each flame, since it constrains gas expansion to the direction of flame propagation. The degree to which the velocity of each flame is affected by either a flowing N₂ and quartz sheath is strongly influenced by fuel-to-oxidant ratio.     

正十二烷高温机理简化及验证

由于详细化学反应机理在模拟燃烧室燃烧时,计算量极大,很难被广泛运用。为了满足工程设计要求,采用替代燃料的简化机理进行计算不失为一种行之有效的方法。本文基于误差传播的直接关系图法和敏感性分析法对正十二烷180组分1962步高温机理(温度大于1100 K)进行简化,获得40组分234步化学反应机理。在温度为1100–1650 K,压力为0.1–4 MPa条件下,采用简化机理及详细机理对不同当量比、压力下着火延迟时间进行模拟,模拟结果与实验数据吻合得较好。通过对不同压力及温度下火焰传播速度进行模拟,验证了简化机理能够正确地反映正十二烷的燃烧特性。利用C_(12)H_(26)/OH/H_2O/CO_2等重要组分随时间变化的数据,验证了简化机理能够准确描述燃烧过程反应物消耗、基团变化、生成物产生的过程,并表明该机理具有较高的模拟精度。利用该简化机理对本生灯进行数值分析,结果表明该机理能够准确地反映火焰区温度和组分浓度的变化。紧凑的正十二烷高温简化机理不仅能够正确体现其物理化学特性,而且能够用于三维数值模拟,具有较高的工程运用价值和应用前景。     

Relative contributions from the kinetic and thermal characteristics of impurities and the inflammability of air-hydrogen mixtures

The effect of chemically active and inert impurities upon the concentration limits of flame propagation of air-hydrogen mixtures at atmospheric pressure is studied. The effect of impurities as a function of their thermal properties and the mechanism of the action on combustion is discussed. Combustion inhibition is considered a clear sign of the versatility of the one-stage reaction model used as the basis of thermal combustion theory.     

Thermal decomposition behaviors and dust explosion characteristics of nano-polystyrene

With the development of nano-powder technology, polymeric nano-materials are widely used in various industries, while not much research on their thermal decomposition and dust explosion characteristics has been conducted. The thermal behaviors and explosion characteristic parameters of the nano-polystyrene (nano-PS) with a typical particle size of 90 nm were studied by employing thermogravimetric analysis (TG), MIE-D 1.2 minimum ignition energy (MIE) test device, and 20-L spherical dust explosion test equipment. The results showed that the thermal decomposition of the nano-PS occurred in a two-step process which was different from the single process for conventional PS. Meanwhile, the reaction rate of the thermal decomposition for nano-PS increased with the heating rate. The TG and DTG curves shifted to the higher-temperature zone when the heating rate increased, and the initial temperature, final temperature, temperature at the maximum rate, and the maximum rate also increased. The sensitivity parameter of the minimum ignition energy of nano-PS varied as the dust concentration altered, and the most sensitive explosive concentration was about 200 g m⁻³. Also, nano-PS was proved to be quite sensitive to the electrostatic spark, as its calculated MIE value was as low as 11 mJ. For the severity parameters, the explosion pressure and its rising rate of nano-PS tended to increase at first and then decrease with the increase in dust concentrations. According to the risk classification standard, the explosion risk class of nano-PS was St2. The results were further extensively compared to other previous works. The results demonstrated both the higher explosion possibility and severity of nano-PS. This study could provide guidance for the safety management of nano-PS in its manufacture, storage, and handling process.

     

A simplified mathematical study of thermochemical preparation of particle oxide under counterflow configuration for use in biomedical applications

This study mathematically presents a counterflow non-premixed thermochemical technique for preparing a particle oxide used for cancer diagnosis and treatment. For this purpose, preheating, reaction, melting, and oxidation processes were simulated considering an asymptotic concept. Mass and energy conservation equations in dimensional and non-dimensional forms were solved using MATLAB^®. To preserve the continuity in the system and calculate the locations of melting and flame fronts, promising jump conditions were derived. In this research, variations in flame temperature, flame front location and mass fractions of the particle, particle oxide and oxidizer, with position, Lewis number and initial temperature of the particles were investigated. The simulation results were compared with those obtained from an earlier experimental study under the same conditions. Regarding the comparison, an appropriate compatibility was observed between the results. Based on the simulation results, flame temperature was found to be about 1310 K. Positions of flame and melting fronts were found to be ??1.8 mm and ??1.78 mm, respectively.

     

基于特征值分析的骨架机理获取方法

基于特征值分析法,建立了一套复杂化学反应动力学模型简化方法,并采用该方法对甲烷空气燃烧的详细化学反应动力学模型进行了简化.从GRI1.2得到了21个组分,83个反应的骨架机理.该机理与详细的GRI1.2机理和DRM19机理在不同化学计量比和不同压力下对比了点火延迟时间,结果表明简化机理能有效地再现详细反应动力学模型的反应机理,并具有更高的计算精度.从GRI3.0简化得到两种骨架机理分别为26个组分、120个反应和30个组分、140个反应.这两个机理都能很好地对火焰传播速度以及主要组分和NO浓度分布进行反应动力学模拟.     

Improvement of energy dissipative particle dynamics method to increase accuracy

In this article, dissipative particle dynamics with energy conservation eDPD is used for simulating hydrodynamic behavior and heat transfer of DPD particles in a two-dimensional channel with parallel planes. To this end, a Fortran programming code is developed and the results are presented as dimensionless velocity and temperature profiles on the cross section perpendicular to the flow direction inside the channel. For the indented geometry, thermal and dynamic boundary conditions have been considered. The dynamic boundary condition of solution domain in the flow’s direction is periodic, and for modeling the walls, freezing layers of DPD particles with Bounce-Back reflection has been used. For the thermal boundary condition, it is assumed that the wall temperature is constant and the temperature of each DPD particle in contact with the wall is the same as the wall temperature. In this article, for the first time, for modeling the walls four frozen layers with Bounce-Back reflection are used and the effect of particle exit on two and three-layers configurations is investigated. Furthermore, for the first time, modified velocity Verlet integration algorithm is improved by adding heat transfer equations. And considering λ?=?0.65 in the algorithm; the indented geometry is well simulated. In order to validate the results, first, the effect of regular and random initial distribution is compared. Furthermore, the results of wall alignment are compared with those obtained from CFD approach. In this paper, in addition to studying the effect of wall alignment and initial particle arrangement, the influence of the size of cells for averaging and the time steps in the output results are investigated.

     

Speed of flame propagation in CH<Subscript>3</Subscript>Cl + Cl<Subscript>2</Subscript> mixtures during combustion initiated by continuous UV radiation

Flame propagation in a closed tube over mixtures of chloromethane and chlorine of different compositions following ignition by continuous UV radiation is studied. It is found that the rate of combustion in all mixtures except limiting ones grows along with the propagation of the flame front up to its maximum values at nearly 1/3 the tube length and then slows. In limiting mixtures, the speed’s behavior is completely different. It is greatest near the source of UV light and gradually slows with distance from the source. The high speed in the initial section is due to the effect of UV light. The temperature of combustion is lowest in limiting mixtures, and the rate of chlorine molecule photodissociation at this temperature is comparable to and even faster than that of their thermal dissociation. The light in these mixtures thus contributes substantially to the initiation of the chemical reaction. It is concluded that when limiting mixtures are ignited by UV pulses, the speed of flame propagation falls markedly as it proceeds without the influence of radiation, and the character of changes in the speed’s behavior becomes identical to those for other mixtures.