Burning Rate Of Particles Research Articles

µAl-based reactive materials with improved energy efficiency by using the fluorine-containing oxidizer perfluoropolyether as an interfacial layer

To enhance the reactivity of micro-sized aluminum (µAl) particles in the energetic materials, the fluorine-containing oxidizer perfluoropolyether (PFPE) was chosen as an interfacial layer based on the molecular dynamics simulation results to construct µAl-based reactive materials. It was determined that PFPE was uniformly distributed on the surface of the µAl particles by various characterization techniques, including scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). This characteristic microstructure could improve the interfacial contacts between fuels and oxidizers. Besides, the thermal analysis and combustion performance evaluation experiment indicated that fluorine-containing oxidizer PFPE participated in the oxidation and combustion of µAl particles. The burning rates of PFPE-functionalized µAl particles were 1.5 to 3.0 times higher than that of µAl particles. Experimental results revealed that the environmental resistance and the overall reactivity of µAl-based reactive materials was improved owing to the characteristic structure and synergistic effects of the components, respectively. This work provides a deeper understanding towards oxidation process and combustion properties.

Condense-luminescence and global characterization of metal particle suspension combustion

Thermal processing of aluminum (Al) particles such as annealing followed by rapid quenching had been previously shown to affect single metal particle burning rates. This study extends single particle combustion to a global material-based energy exchange model. Experiments were designed to investigate the global energy exchange resulting from Al powder suspensions processed to induce different (fast and slow) burning regimes. Thermally processed and untreated Al particles were reacted as suspended powder in a closed bomb calorimeter. The calorimeter monitored the transient temperature changes resulting from energy release upon powder combustion. The product residue was analyzed for species concentration using X-ray diffraction. Results link the phase fractions of the aluminum oxide combustion products with global radiant fluxes in the calorimeter system. Metastable alumina associated with nano-oxide formation is in substantially higher concentration for thermally processed powder reactions and also produces greater energy transfer rates. The increased energy transfer rates correspond to higher radiant energy emission which may result from condensation energy associated with nano-oxide particle formation. This study qualifies condense-luminescence as a means for increasing the energy release rates of aluminum particles. By strategically altering metal fuels to control formation of nano-oxide particles upon combustion, appreciable increases in the radiant energy flux can transform energy release rates.

A coupled finite difference and material point method for simulation of gas–solid flow of thermobaric explosions

A coupled finite difference and material point method (CFDMP) was presented to simulate the dynamic of the gas–solid flow of thermobaric explosions. The coupled method takes advantages of both the two methods where the finite difference method captures the wave structures of gas phase and the material point method tracks the movement of solid phase. Particle collision is easily realized in MPM framework. By using a uniform grid system, the grid nodes have both the two phases’ information and their interactions are directly calculated. The simulation of thermobaric explosions in different containers is carried out and the results validate the accuracy of CFDMP program. The particle collision reduce the compression degree of the solid phase in the initial expansion stage but slightly influence the pressure history at the gauge. The process of particle movement in the container is demonstrate and the wall-reflected wave enhances the burning rate of particles.

ГОРІННЯ ПАЛИВА НА ОСНОВІ АЛКАНІВ ПІД ДІЄЮ ЕЛЕКТРИЧНОГО ПОЛЯ

The relevance of research on the combustion processes of alkane-based fuels is due to the search for new, efficient, environmentally friendly fuels for the energy and aerospace industries. Paraffin (a mixture of alkanes) is a low-melting material that can be used as an environmentally friendly and high-energy fuel. An important task is to intensify the processes of fuel combustion in the reaction volume. One of the factors of intensification is the electric field. The purpose of this work is to study the laws of the influence of the electric field on the processes of high-temperature heat and mass transfer, melting and burning drops of liquid fuel based on alkanes.The paper developed a methodology for studying the kinetics of melting and burning of liquid fuel droplets using digital imaging of the object of study and computer processing of the images obtained.Studies have been conducted on the effect of a uniform electric field on the combustion of alkane particles in the air. The stages of heat and mass transfer in the combustion process are determined. It is shown that the melting of a particle is the main component of the induction period — the time during which the particle ignites. It was revealed that the melting rate significantly decreases if the electric field strength exceeds a certain critical value. It is established that the effect of the electric field on melting is more pronounced at high gas temperatures. It was found that when an octadecane particle melts in an electric field, the unmelted residual solid phase rotates in the melt. It is proved that this is a special case of the Quincke effect.The kinetics of combustion of octadecane drops of various initial diameters was studied. It has been proven that a linear law of variation of the square of the diameter of a droplet with time (Sreznevsky’s law) is fulfilled in a certain area of burning. The influence of the electric field on the burning rate of particles of different initial diameters was studied. It has been established that under the action of an electric field an increase in the rate of burning of alkane droplets occurs. In particular, for octadecane drops, the burning rate in a constant electric field increases by 10 ÷ 23% depending on the initial diameter of the drop. The burning rate constants of octadecane are found.The influence of the electric field on the geometric dimensions of the flame is studied. It is established that under the influence of the field the flame becomes asymmetrical and shifts towards the negatively charged plate of the capacitor. This reduces the height of the flame and increases its width. These phenomena are associated with the action of "ion wind" in the flame and the transformation of the energy of the electric field into heat. As a result of the action of these physical mechanisms, the combustion front approaches the surface of the drop, increasing its rate of evaporation.

Numerical and experimental studies on nozzle two-phase flow characteristics of nanometer-scale iron powder metal fuel motor

Metal iron powder is a promising new type of energy source that is of enormous practical and research interest for future automotive power systems. To better optimize engine design, this study was devoted to the characteristic investigation of a two-phase flow. Experimental studies involving nanometer iron powder particle combustion and engine thrust measurement were conducted to confirm the results obtained from numerical calculations that were performed using a fourth-order Runge–Kutta–Gill method. Governing equations for nozzle two-phase flow were established to perform a theoretical study to analyze the combustion properties of iron oxide particles and flow in the nozzle. The results indicate that variations in the size and coagulation content of particles play a significant role in the loss of two-phase flow. Significant emphasis was placed on the effect of particle size (0.4–1.0 μm) and condensate content (10–40%) of ultrafine particles on the specific impulse. To further validate the theoretical results, the burning rates of particles of three different sizes were experimentally measured. In addition, the motor thrust and the specific impulse with the particle size of 50 nm were tested through combustion experiment, and the results show excellent agreement.

Combustion of Mg and composite Mg·S powders in different oxidizers

Micron-sized, spherical magnesium powders were ignited by a CO2 laser beam and by injecting them in the products of air–C2H2 and air–H2 flames. The same experiments were performed with composite Mg·S powders prepared by mechanical milling magnesium and elemental sulfur powders. The non-spherical Mg powder used to prepare composites was also explored in selected combustion tests. Flow conditions were varied in experiments performed in air with all materials. The combustion products were collected for particles burning in air; the products were studied using electron microscopy. Optical emission produced by burning particles was recorded using filtered photomultipliers. The emission pulses were processed to recover the particle burn times and their temperatures. Fine Mg particles burn in air very rapidly, with the burn times under 1 ms for particles finer than ca. 10 µm. The apparent trend describing burn time as a function of the particle size for such particles is t∼d0.5. The particles burn without generating a detectable standoff flame zone or producing smoke; combustion products are particles of MgO with dimensions comparable to those of the starting Mg powder particles. Both the particles burn times and their measured flame temperatures decrease slightly when particles are carried by faster air flows. The present experimental results is interpreted qualitatively assuming that the reaction occurs at or very near the boiling Mg surface and its rate is affected by both surface kinetics and the inward diffusion of oxygen. It is further proposed that the fine, solid MgO particles form either directly on surface of Mg droplet or in its immediate vicinity. Deposition of MgO crystals on liquid Mg causes little change in the particle burn rate. Combustion of Mg in air–C2H2 and air–H2 flames occurs much slower than in air. Combustion of composite Mg·S particles follows a two-step process. In the first step, sulfur is evaporated. When the particles are heated by a CO2 laser beam, rapid evaporation of sulfur leads to a sudden change in the particle velocity. Once sulfur is removed, the particles burn similarly to the pure Mg.

On evaluation of high burnout performance in a rocket-engine-based incinerator for solid-particles burning

To dispose of solid biomass particles such as wood, municipal solid wastes (MSW), and animal carcass, burning of solid particles in the chamber of a rocket-engine-based incinerator (RBI) was suggested for the purposes of both high-performance burnout and mobility. For high burning performance, the chamber of an RBI has the shape of a rocket combustor and in-chamber swirl flow is formed by peripheral injectors. But, its high burnout performance was verified by only numerical analyses without experimental evidence. In this study, both burning tests and simulations are conducted for several operating conditions with a target performance, which is higher than that of conventional incinerators by a factor of 10. For the tests, bituminous coal is employed as a simulant fuel for solid particles from wastes. It is supplied into the chamber with air as in previous numerical simulations. The calculated burning rate of coal particles is validated by a measured one, which confirms the feasibility of higher burning rate with an RBI by one order of magnitude than with conventional incinerators or boilers. Burnout ratio, a quantitative parameter for burnout performance, is evaluated numerically and experimentally for a comparative study. For experimental evaluation of burning performance, burnout ratio based on CO2 mole fraction is suggested and verified. Gaseous methane is supplied into the chamber to facilitate ignition of solid particles and its co-burning with coal affects burnout ratio significantly, which is increased by around 15% when methane of 20% is fed in mole fraction.

Comments on: “Combustion of nano-sized aluminum particles in steam: Numerical modeling”, by V.B. Storozhev and A.N. Yermakov

This comment points out the large discrepancy between the predictions of the recently published model by Storozhev and Yermakov (Storozhev and Yermakov, Combust. Flame 162 (2015) 4129–4137) and the large body of accumulated literature on aluminum combustion. Their model predicts burning times between 40-63 ms for a cloud of nanometric aluminum particles (20 or 50 nm) burning in steam at initial temperatures around 2300 K. These predictions are roughly two orders of magnitude larger than the measured combustion times of micron and submicron aluminum particles from various experiments, which are on the order or hundreds of microseconds to milliseconds. Experimentally measured flame speeds of aluminum flames in the products of hydrocarbon flames, and the burn rates of metal-water propellants, also point to much faster burning rates of aluminum particles in steam than predicted by this model. As such, the predictions of the model by Storozhev and Yermakov cannot be reconciled with the available experimental evidence.

고체입자의 높은 연소율을 갖기 위해 고안된 로켓 엔진 기반 소각로의 냉각 해석

고체입자의 높은 연소율을 갖기 위해 고안된 로켓 엔진 기반 소각로의 냉각 해석

A two-dimensional analytical model of laminar flame in lycopodium dust particles

A two-dimensional analytical model is presented to determine the flame speed and temperature distribution of micro-sized lycopodium dust particles. This model is based on the assumptions that the particle burning rate in the flame front is controlled by the process of oxygen diffusion and the flame structure consists of preheat, reaction and post flame zones. In the first step, the energy conservation equations for fuel-lean condition are expressed in twodimensions, and then these differential equations are solved using the required boundary condition and matching the temperature and heat flux at the interfacial boundaries. Consequently, the obtained flame temperature and flame speed distributions in terms of different particle diameters and equivalence ratio for lean mixture are compared with the corresponding experimental data for lycopodium dust particles. Consequently, it is shown that this two-dimensional model demonstrates better agreement with the experimental results compared to the previous models.

Combustion of magnesium powders in products of an air/acetylene flame

This paper is aimed at characterizing combustion dynamics of fine magnesium powders. Two spherical, micron-sized Mg powders with different particle size distributions were introduced into an air–acetylene flame. Particles were observed to burn in laminar flames as well as in flames with turbulence induced by an auxiliary swirling air flow. Particle emission was detected above the flame emission background and emission pulses for individual particles were recorded using an array of three filtered photomultiplier tubes. Particle size distributions were correlated with their emission times, interpreted as burn times, for different turbulence levels. Ratios of the recorded filtered emission signals were used to obtain temperatures of the burning particles. Partially burned particles were collected and examined. Particle burn times were approximately proportional to d0.8, where d is the particle diameter. The particle spherical shapes were not preserved. For the coarser powder, halos of condensed ultrafine MgO crystals were observed around quenched particles. Presence of slightly greater amounts of MgO on the surface of as received particles of the finer powder resulted in their longer burn times and elevated temperatures. These observations are interpreted assuming that the initial small MgO particles adhered to the metal surface result in the formation of solid MgO islands and inclusions on surface of the burning Mg droplets. Such islands block evaporation of magnesium and thus reduce the burn rates. In addition, they serve as condensation centers for the combustion products and grow rapidly during combustion. As a result, a relatively small number of the initial fine MgO particles can cause substantial disruption in the burning particle shape, surface morphology, and burn rate. Larger MgO inclusions can be heated above the Mg boiling point resulting in an increased measured temperature.

Char burning kinetics from imaging pyrometry: Particle shape effects

Biomass combustion becomes increasingly important in nowadays energy production. One of the major differences between biomass and coal is the particle shape. Therefore the effect of particle shape variations was calculated to determine the effect of spheroidal or cylindrical particle shapes when determining char burning kinetics of pulverized fuel particles from pyrometric measurements. A model frequently used for char kinetics determination was adapted to spheroidal and cylindrical particles under typical combustion conditions. The impact of the particle shape on the energy balance, which is needed for the determination of kinetics parameters, was calculated for typical particle sizes and length to diameter ratios. As the calculations show, the influence of particle shape effects is not negligible for the determination of biomass char burning parameters by pyrometric measurements. The application of the adapted model on measured data shows, that particle shape effects can be resolved by pyrometric techniques. The results presented indicate that a sound analysis of the particle burning rate should include a three-dimensional particle shape measurement.

Comparative study of four alternative models for CO oxidation around a burning coal char particle

The steady state combustion of a quiescent char particle is investigated by means of a detailed model which accounts for heterogeneous oxidation and gasification, as well as homogeneous reactions (GRI-Mech 3.0) in the particle’s boundary layer. First, the way and extent in which the mass and energy transfer are altered due to the oxidation of CO are examined by comparison with the predictions of a single-film model in the case of anthracite particles within 60–1000μm. Then, four alternative descriptions of the gas phase (single-film, double-film, global kinetics and detailed kinetics) are evaluated for two coals of very different reactivity towards O2 and CO2, at low and high O2 concentration and in the same broad range of sizes. The overall influence of the CO conversion modeling on the particle burning rate and temperature (i.e. whether the reduction in O2 surface concentration or the heat and CO2 provided by the flame dominate over each other) is found to depend on the conditions considered. The single-film approach reasonably fits the predictions of the most complete model in all cases (and especially in the pulverized-coal size range), whereas the double-film hypothesis and the global kinetics generally overestimate the effects of the flame on the consumption rates and the particle temperatures.

On burnout performance of a rocket-engine-based incinerator devised for high burning rate of solid particles

A rocket-engine-based incinerator is proposed as a compact device of a solid-particle incinerator for high-performance burnout of wastes with solid phase. Burnout performance is investigated numerically in the chamber devised for this purpose. The present chamber employs design concepts of swirl flow and nozzle shape applied to a furnace of power-plants and a rocket combustor, respectively. As the first step, non-reactive flow field is analyzed in the incinerator with primary injectors and secondary injectors through which coal fuel and air are supplied. Deflection angle of a primary injector, incline angle of a secondary injector, gap between two-type injectors are selected as design parameters. Swirl number is adopted for evaluation of the degree of swirl flow and it is estimated over wide ranges of three parameters. Swirl number varies non-monotonically depending on the combination of the parameters. Burnout ratio increases with swirl number. High swirl number is attained over broad ranges of deflection and incline angles. From the numerical results, the design points with high burning performance can be found by maximizing swirl number. The proposed chamber has been verified to attain higher burnout performance than a conventional incinerator by a factor of 10.

Aluminum particle combustion in turbulent flames

Predictive mechanisms for particle ignition and combustion rates are required in order to develop optimized propellant and energetic formulations using micron-sized metal powders, such as aluminum. Most current descriptions are based on laboratory experiments performed in stationary or laminar combustion configurations. However, turbulent environments exist in most applications and validity of the present descriptions for such environments has not been established. This experimental study is aimed to measure burn times for aluminum particles burning in environments with different levels of turbulence. A laminar air-acetylene flame is produced, and auxiliary tangential jets of air with adjustable flow rates are used to achieve different controlled levels of turbulence. Fine spherical aluminum powder is injected in the flame axially using a flow of nitrogen. The streaks of burning particles are photographed using a camera placed behind a mechanical chopper interrupting the photo-exposure with a pre-set frequency. The obtained dashed streaks are used to measure the particle burn times for different flow conditions. The particle burn times are correlated with the particle size distribution to obtain the burn time as a function of the particle size. The results are processed to obtain a correction for the Al particle burn rate as a function of the turbulence intensity, I. The measured burn times are longer than predicted for the micron-sized Al particles using a correlation based on survey of earlier experiments, mostly with coarser Al powders. Increased turbulence intensity results in substantial reduction of the particle burn time. Present data suggest that the burn rate for particle combustion in a laminar environment should be multiplied by 1+18.2I, to estimate the acceleration of aluminum combustion in turbulent environments.

Numerical study on operating characteristics of a magnesium-based fuel ramjet

A two-dimensional axis symmetrical model for multi-phase turbulent combustion simulation has been developed in a magnesium-based fuel ramjet, the stochastic model is applied to track the motion and transportation of discrete phase in the whole flow field in Lagrangian frame. Firstly, the flow field parameters distribution including temperature, velocity, Mg particle burning rate, main components are obtained to describe the basic operating characteristics, and the thermoacoustic oscillation during the combustion process is predicted due to the observed vortex. Then combined with the developed thermodynamic calculation program, the influencing rules of water/fuel ratio and magnesium-based fuel composition ratio on ramjet operating characteristics are analyzed. The results obtained show that the distribution of two water/fuel ratios exerts a direct impact on the temperature fluctuations in combustor, further influences the combustion stability, as for theoretical specific impulse, a maximal value is observed along with the total water/fuel ratio, while the monotonous trends for thermal efficiency and propulsive efficiency are achieved respectively. Moreover, the magnesium-based fuel composition ratio is a dominant factor to influence the operating characteristics, and it is proved that the high-proportion Mg in fuel is beneficial to specific impulse augment through validation of both simulation results and thermodynamic calculation results.

Modes of particle combustion in iron dust flames

The so-called argon/helium test is proposed to identify the combustion mode of particles in iron dust flames. Iron powders of different particle sizes varying from 3 to 34 μm were dispersed in simulated air compositions where nitrogen was replaced by argon and helium. Due to the independence of the particle burning rate on the oxygen diffusivity in the kinetic mode, the ratio between the flame speeds in helium and argon mixtures is expected to be smaller if the particle burning rate is controlled by reaction kinetics rather than oxygen diffusion. Experiments were performed in a reduced-gravity environment on a parabolic flight aircraft to prevent particle settling and buoyancy-driven disruption of the flame. Uniform suspensions of the iron powders were produced inside glass tubes and a flame was initiated at the open end of the tube. Quenching plate assemblies of various channel widths were installed inside the tube and pass or quench events were used to measure the quenching distance. Flame propagation was recorded by a high-speed digital camera and spectral measurements were used to determine the temperature of the condensed emitters in the flame. The measured flame speeds and quenching distances were in good agreement with previously developed one-dimensional, dust flame model where the particles are assumed to burn in a diffusive mode and heat losses are described on a volumetric basis. However, a significant drop of the ratio of flame speeds in helium and argon mixtures was observed for finer 3 μm particles and was attributed to a transition from the combustion controlled by diffusion for larger particles to kinetically controlled burning of micron-size particles. In helium mixtures, the lower flame temperatures measured in suspensions of fine particles in comparison to larger particles reinforces this assumption.

Effect of CO 2 gasification reaction on oxy-combustion of pulverized coal char

For oxy-combustion with flue gas recirculation, as is commonly employed, it is recognized that elevated CO 2 levels affect radiant transport, the heat capacity of the gas, and other gas transport properties. A topic of widespread speculation has concerned the effect of the CO 2 gasification reaction with coal char on the char burning rate. To give clarity to the likely impact of this reaction on the oxy-fuel combustion of pulverized coal char, the Surface Kinetics in Porous Particles (SKIPPY) code was employed for a range of potential CO 2 reaction rates for a high-volatile bituminous coal char particle (130 μm diameter) reacting in several O 2 concentration environments. The effects of boundary layer chemistry are also examined in this analysis. Under oxygen-enriched conditions, boundary layer reactions (converting CO to CO 2, with concomitant heat release) are shown to increase the char particle temperature and burning rate, while decreasing the O 2 concentration at the particle surface. The CO 2 gasification reaction acts to reduce the char particle temperature (because of the reaction endothermicity) and thereby reduces the rate of char oxidation. Interestingly, the presence of the CO 2 gasification reaction increases the char conversion rate for combustion at low O 2 concentrations, but decreases char conversion for combustion at high O 2 concentrations. These calculations give new insight into the complexity of the effects from the CO 2 gasification reaction and should help improve the understanding of experimentally measured oxy-fuel char combustion and burnout trends in the literature.

Numerical investigation of the oxy-fuel combustion in large scale boilers adopting the ECO-Scrub technology

The present paper presents a three-dimensional numerical investigation of a pulverized-fuel, tangentially-fired utility boiler located at Florina/Greece under air, partial and full oxy-fuel conditions. Heat and mass transfer and major species concentration, such as CO 2, CO and O 2 are calculated; whilst the results for the reference air case scenario studied are in good agreement with the corresponding operational data measured in the plant, both for combustion calculations and NO x emissions. Results for the partial and full oxy-fuel operation scenarios are in line with similar experimental and numerical investigations found in the recent literature. This numerical investigation of oxy-fuel conditions scenarios prior to their implementation under real scale conditions demonstrates the utmost of its importance, since significant results regarding the operation of a boiler in terms of lignite particle trajectories and burning rates are attained. Furthermore, NO x calculations have been performed for all the examined case studies.

An analytical study of radiation effects on the premixed laminar flames of aluminium dust clouds

A new analytical model of a quasi one-dimensional non-adiabatic dust flame is developed with the assumption that the particle burning rate in the flame front is controlled by the process of oxygen diffusion. In this model, the flame propagation mechanism is considered to be radiation, conduction, and convection. Algebraic equations defining the laminar flame speed were obtained in two limiting cases: lean and rich mixtures. The flame structure is assumed to consist of a preheat zone, a reaction zone, and a postflame zone for lean mixtures and a preheat zone and a reaction zone for rich mixtures. Under the lean mixture approximation, values of the flame speed, lean limit, and flame temperature were calculated by adding the radiation term; flame temperature in the preheat zone increased, while it decreased in the postflame zone. This phenomenon may be attributed to the radiative heat transfer from the postflame zone to the preheat zone. Also, when the radiation term was considered, the flame speed increased but the lean limit decreased. In addition, radiation in the rich mixture resulted in the increase of the flame speed and the gas phase temperature in the preheat zone, whereas in the flame zone, the gas phase temperature decreased. Calculated values of the flame speed and flame temperature are in a good agreement with the experimental data in the literature.