Dynamic behavior of particles across flame propagation through micro-iron dust cloud with thermal radiation effect

In this investigation, a theoretical study is performed to analyze the dynamic behavior of particles across flame propagation through a two-phase mixture consisting of micro-iron particles and air. In the first step for calculation of the particle velocity profile, the Lagrangian approach of particle motion is employed, and then thermophoretic, gravitational and buoyancy forces are taken into consideration. In order to simulate the temperature profile for the thermophoretic force, it is assumed that the flame structure consists of three zones: preheat, reaction, and post flame (burned). It should be noted that the radiative heat-transfer equation is employed to describe the thermal radiation exchanged between the burned zone and the preheat zone. In the resumption, a control volume above the leading edge of the combustion zone is considered and the change in the particle number density in this volume is obtained via the balance of particle mass fluxes passing through it. The results show that the induced thermal radiation plays a significant role in increasing the mixture temperature all over the preheat zone, and that the particle velocity profile and the concentration distribution of particles as a function of distance from the leading edge of the combustion zone also have considerable consistency with published experimental data.     

Approximate flame temperature distribution during combustion of oil and a coal-oil mixture

The flame temperature field has been analytically determined and theoretically calculated for the cases where the air supply is uniform along the flame and supplied completely at the flame beginning. The two ways of determining the flame heating ability have been analysed, first formulated as a fraction of the local supply of the chemical energy of fuel, and second as a fraction of the local physical enthalpy of the flame substance passing through the flame cross section. The present analysis confirms practical observations that the more uniform the supply of combustion air, the more equalized is the temperature in the flame. Present calculation procedures can be applied to design or control the flame temperature field by using the proper distribution of combustion air delivered along the flame.     

Distribution of iron dust particles across the unburned zone during flame propagation in a vertical duct

The homotopy analysis method is employed in this study to obtain the concentration and velocity profiles of particles during flame propagation over a two-phase medium consisting of iron dust microparticles and air. The particle concentration is demonstrated to increase when the leading edge of the combustion zone is approached. This increase in the particle concentration above the combustion zone has a remarkable effect on the lower flammability limit of the combustible particle cloud. The results obtained with the use of this approach are in good agreement with the published experimental data.     

Experimental and mathematical modeling of three-dimensional natural convection in an enclosure

Three-dimensional natural convection patterns in an enclosure under conditions simulating flow in glass-making furnaces have been calculated using an approximate method in which solutions for the temperature and velocity fields in orthogonal two-dimensional planes are superposed. The computational method was tested by comparison with measurements of the temperatures and velocities in an enclosure heated and cooled from above and with a controlled heat loss from the sides. Good agreement was found between measurements and computations under conditions of interest in glass-making furnaces, wherein the motion in one of the two orthogonal planes is dominant.     

Relationship between the dust flame propagation velocity and the combustion mode of fuel particles

A possibility of determining the regime of combustion of individual fuel particles on the basis of the dependence of the flame velocity on the fuel and oxidizer concentrations is considered by an example of a dust flame of microsized metal particles with diameters d ₁₀ < 15 μm and particle concentrations from ≈10¹⁰ to 10¹¹ m^?3 in oxygen-containing media at atmospheric pressure. The combustion mode (kinetic or diffusion) is responsible for the qualitative difference in the character of the normal velocity of the flame as a function of the basic parameters of the gas suspension. The analysis of such experimental dependences for fuel-rich mixtures shows that combustion of zirconium particles (d ₁₀ = 4 μm) in a laminar dust flame is controlled by oxidizer diffusion toward the particle surface, whereas combustion of iron particles of a similar size is controlled by kinetics of heterogeneous reactions. For aluminum particles with d ₁₀ = 5–15 μm, there are no clearly expressed features of either kinetic or diffusion mode of combustion. To obtain more information about the processes responsible for combustion of fine aluminum particles, the flame velocity is studied as a function of the particle size and initial temperature of the gas suspension. It is demonstrated that aluminum particles under the experimental conditions considered in this study burn in the transitional mode.     

Experimental investigation and mathematical modelling of wood combustion in a moving grate boiler

The use of biomass to generate energy offers significant environmental advantages for the reduction in emissions of greenhouse gases. The main objective of this study was to investigate the performance of a small scale biomass heating plant: i.e. combustion characteristics and emissions. An extensive series of experimental tests was carried out at a small scale residential biomass heating plant i.e. wood chip fired boiler. The concentrations of CO, NO_x, particulate matter in the flue gas were measured. In addition, mathematical modelling work using FLIC and FLUENT codes was carried out in order to simulate the overall performance of the wood fired heating system. Results showed that pollutant emissions from the boiler were within the relative emission limits. Mass concentration of CO emission was 550-1600 mg/m³ (10% O₂). NO_x concentration in the flue gas from the wood chips combustion varied slightly between 28 and 60 ppmv. Mass concentration of PM₁₀ in the flue gas was 205 mg/m³ (10% O₂) The modelling results showed that most of the fuel was burnt inside the furnace and little CO was released from the system due to the high flue gas temperature in the furnace. The injection of the secondary air provided adequate mixing and favourable combustion conditions in the over-bed chamber in the wood chips fired boiler. This study has shown that the use of wood heating system result in much lower CO₂ emissions than from a fossil fuel e.g. coal fired heating system.     

The effects of coal dust concentrations and particle sizes on the minimum auto‐ignition temperature of a coal dust cloud

Flash fires and explosions in areas containing an enriched combustible dust atmosphere are a major safety concern in industrial processing. An experimental study was conducted to analyse the effects of atmospheric coal dust particle sizes and concentrations on the minimum auto‐ignition temperature (MAIT) of a dust cloud. Two different coal samples from Australian coal mines were used. The coal dust particles were prepared and sized in 3 ranges, of below 74 μm, 74 to 125 μm and 125 to 212 μm, by using a series of sieves and a sieve shaker. A humidifier was used to increase the moisture content of the particles to the required level. All the experiments were conducted in accordance with the ASTM E1491‐06 method in a calibrated Goldbert‐Greenwald furnace. The results from this study indicate that coal dust properties, such as the chemical nature (H/C), concentration, particle size (D₅₀), and moisture content, impact on the MAIT. For coal dust concentrations less than 1000 g.m^?3, the MAIT decreases with increasing coal dust concentrations. On the other hand, for low concentrations of 100 to 15 g.m^?3, the MAIT becomes more reliable for particle size D₅₀ rather than for volatile matters.     

On the problem of laminar flame propagation in a gas with an inert dust

This paper presents a mathematical model and the results of calculation of the velocity of propagation of the flame front in a fuel gas with suspended inert particles taking into account the thermal expansion of the gas and the dynamic relaxation of particles. Dependences of the steady-state flame velocity on the particle size and mass concentration are obtained.     

Effect of convective motion on the flame structure in combustion waves propagating in heterogeneous systems under natural and artificial gravity conditions

Convective motion in combustion waves arises from natural or artificial gravity (centrifugal effect) for a wide range of heterogeneous systems, such as ammonium perchlorate containing solid additives, metal and nonmetal powders with a gas reagent, thermite mixtures, and hybrid layered structures. The paper summarizes the results obtained by the author and coworkers in studies of the effect of convective motion in combustion of heterogeneous systems for the period from the early 1970s to the present. It is shown that convective processes in combustion waves of heterogeneous systems may determine the structure and propagation mechanism of the combustion waves and the concomitant heat and mass transfer. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 4, pp. 86–92, July–August, 2009.     

Experimental investigation and modeling of the tension behavior of polypropylene at different temperature

The mechanical behavior of polypropylene polymer was investigated under the effect of various temperatures. Mechanical properties of polymer were carried out through uniaxial tensile tests for low and high temperatures respectively. The results showed that both yield stress and the elastic modulus of the material decrease with the increase of temperature. Similarly, the post-yielding behavior of the material can be clearly observed at low temperature, and this behavior gradually disappears as the temperature increases. A phenomenological constitutive model is proposed in order to understand the mechanical behavior of polymer by combining the hyperbolic and multi-linear relationships. It is based on the experiments, and the proposed constitutive model is successfully validated by the excellent agreement between model prediction and experimental results.     

Simultaneous measurements of two-dimensional temperature and particle concentration distribution from the image of the pulverized-coal flame

This paper presented a model for simultaneously measuring the two-dimensional temperature and particle concentration distribution from the images of the flame. In order to determine the relationship between a point in the three-dimensional space and its image in the camera, the optical image-formation process was analyzed. The inverse problem of the radiation transfer in the participating medium was studied. The mathematics method to simultaneously solve the temperature and the particle concentration was discussed. To validate the model presented in this paper, a test furnace with the fuels mixed by pulverized-coal and oil was set up. The temperature and particle concentration of a cross section were measured under different coal feed rates. The comparison between the measured temperature by the pyrometer and the calculated temperature according to the flame image proved that the two-dimensional distribution of temperature can be obtained accurately. The particle concentration distribution was reasonable under different cases.