Inertial effects in nonstationary models of flame front evolution

The objective of the present study is to formulate flame front evolution models capturing the effects of flame extinction, ignition, and oscillations in addition to the conventional regime of flame propagation. The basic equations constituting the one-dimensional thermodiffusion model of combustion are reduced to a system of two ordinary differential equations for the flame front coordinate and flame temperature. These equations admit solutions that describe, for example, ignition, extinction, and nonlinear oscillations of the flame, which are observed during premixed gas combustion in microchannels with an elevated wall temperature or during gasless combustion of condensed systems. Similarity of the basic thermodiffusion models assuming an in-finitely small thickness of the chemical reaction zone allows application of a universal method to derive reduced equations in physically different systems. It is demonstrated that modeling of flame oscillations requires at least considering effects associated with flame acceleration (flame front “inertia”) and the rate of flame temperature variation. The accuracy of the proposed model with inertial effects is checked by results of direct numerical simulations of the original equations.     

Effect of fuel properties on biomass combustion: Part I. Experiments—fuel type,equivalence ratio and particle size

Moving bed combustion is commonly used for energy conversion of biomass. Conditions on the moving bed can be conveniently represented by a time dependent fixed bed. The present work experimentally investigates the combustion of four biomass materials having different fuel properties in a fixed bed under fuel-rich conditions. Temperature, gas composition and mass loss curves identified two distinct periods as the combustion progresses in the bed: the ignition propagation and char oxidation. The effects of bulk density, particle size and air flow rate on the combustion characteristics during the two periods are interpreted by using the ignition front speed, burning rate, percentage of mass loss, equivalence ratio and temperature gradient. Different channelling of air was observed for small miscanthus pellets and large wood particles due to the fast propagation of the ignition front around a channel. The elemental ash composition was also analysed, which explained the sintered agglomerates of miscanthus ashes in terms of alkali index.     

Numerical study on the realization of compression ignition in a type of porous medium engine fueled with Isooctane

The working process of a porous medium (PM) engine, characterized as periodic contact type and fueled with liquid fuel as Isooctane, is simulated by using an improved version of KIVA-3V. A modified volume-averaged method is proposed for describing the interaction between fuel droplets and the solid phase of the PM. The improved version of KIVA-3V was validated by simulating the experiment of Zhdanok for the superadiabatic combustion of CH₄-air mixtures under filtration in a packed bed. Good agreement between experimental data and computational results for the speed of combustion wave is achieved. The influences of initial PM temperature, PM structure and valve opening timing on the realization of compression ignition in the PM engine are also verified. Initial PM temperature is the crucial factor in guaranteeing the realization of the compression ignition of the PM engine. Considering influential factors, such as the properties of the PM, the compression ratio, the equivalence ratio, and the heat transfer between gas and solid phase of the PM should obtain optimized initial PM temperature. The variation in PM structure affects the convective heat transfer between the gas and solid phase and the dispersion effect of the PM. Compression ignition all can be realized in PM engines with four kinds of PM. Compression ignition is achieved at the considered four valve opening timings. Value opening timing has influence on the average temperature of the PM engine and the working of the PM engine does not allow earlier or later valve opening timing.     

Unsteady gasless combustion of planar symmetrical samples

Influence of ignition conditions on the modes of unstable combustion of planar symmetrical green samples was numerically explored in terms of 2D theoretical model. Within the range of instability, ignition conditions were found to define the number of self-propagating hot spots and their trajectories. In case of uniform distribution of ignition temperature over the inner boundary of annular sample, the combustion front propagates in the form of expanding ring.     

Ignition and unstable regimes of gasless combustion of a disk-shaped sample

The effect of ignition conditions and the parameters of the melting of the inert component on unstable combustion modes is studied numerically using a model of solid-flame combustion of a disk. It is shown that the shape of the heated region initiating combustion of the disk determines the number and trajectories of hotspots of the self-propagating combustion zone. The effect of the phase transition on the combustion characteristics is the more pronounced the closer the phase transition temperature to the combustion temperature. In this case, the combustion front takes the shape of a ring.     

Splitting flames in a narrow channel with a temperature gradient in the walls

A possibility of simultaneous formation of two chemical reaction fronts during nonstationary combustion of a gas in a microchannel with a temperature gradient in the walls is demonstrated. Combustion in a straight tube and in a gap between two disks with radial fuel injection is considered. In both cases, the characteristic transverse size of the channel is smaller than the critical diameter determined for the ambient temperature, and gas combustion occurs in the region where the wall temperature is higher than the ambient temperature. A numerical study of flame repetitive extinction/ignition (FREI) demonstrated a possibility of simultaneous formation of two chemical reaction fronts in the hot region of the channel. One front corresponds to conventional flame propagating upstream from the hot to the cold part of the channel, and the other front moves in the downstream direction and decays as the fuel burns out. Based on this study, a new mechanism of ignition and incomplete combustion of the combustible mixture in microsystems is proposed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 2, pp. 12–19, March–April, 2008.     

IGNITION DELAY OF DROPLET CLOUDS: RESULTS FROM GROUP COMBUSTION THEORY

The ignition and evaporation of spherical cloud of droplets in a hot quiescent atmosphere is examined numerically using transient group combustion analysis. Ignition delay times are calculated as a function of cloud radius, ambient temperature, drop size and droplet number density. The ignition temperature for a cloud of drops was found to be less than that obtained from a single drop. The results indicated an interaction between chemical and physical effects resulting in the possibility of an optimal interdrop spacing for ignition of a fuel with a high boiling point. The model results indicate that for interdrop spacing to radius ratio of less than 5, the ignition and evaporation of a cloud of drops is confined to a thin layer at the surface of the cloud. For drops spaced farther apart thermal penetration from the hot ambient is possible resulting in vaporization within the cloud.     

Numerical modeling of straw combustion in a fixed bed

Straw is being used as main renewable energy source in grate boilers in Denmark. For optimizing operating conditions and design parameters, a one-dimensional unsteady heterogeneous mathematical model has been developed and experiments have been carried out for straw combustion in a fixed bed. The straw combustion processes include moisture evaporation, straw pyrolysis, gas combustion, and char combustion. The model provides detailed information of the structure of the ignition flame front. Simulated gas species concentrations at the bed surface, ignition flame front rate, and bed temperature are in good agreement with measurements at different operating conditions such as primary air-flow rate, pre-heating of the primary air, oxygen concentration, moisture content in straw, and bulk density of the straw in the fixed bed. A parametric study indicates that the effective heat conductivity, straw packing condition, and heat capacity of the straw have considerable effects on the model predictions of straw combustion in the fixed bed.     

Smoldering combustion of rice husk dusts on a hot surface

An accumulated combustible dust layer on some hot process equipment, such as grinders, dryers, hot bearings, etc., can be ignited and lead to fires if the hot surface temperature is sufficiently high. Experimental tests are used to determine the minimum hot surface temperature for dust ignition, the ignition temperature of dust itself, and the ignition times in this study. Egyptian rice husk dust is sieved into different sizes (particle diameters) to be used in this investigation. The effects of the dust particle size and the sample size (depth of the dust layer) on ignition parameters are tested. The boundary between the ignition and non-ignition conditions is investigated precisely through a large number of tests. The results show that the minimum hot plate temperature for ignition of dusts decreases as the dust layer depth increases.     

Numerical study of coal particle cluster combustion under quiescent conditions

Behavior of ignition and combustion of coal particle cluster under a quiescent condition was numerically simulated by solving balance equations of mass and enthalpy with combustion kinetic models of volatiles and char. Two-flame structure, one flame penetrating into the cluster and the other moving out of the cluster, was predicted during the combustion of coal particle cluster. Effects of radiative heat transfer, group number, ambient temperature, coal particle size, and oxygen concentration on ignition and combustion of coal particle clusters were also analyzed. Simulations indicated that the gas volume fraction of coal particle cluster increases with time after devolatilization. Gas velocity passing through the cluster surface varied significantly at volatile liberation. The ignition time delay was reduced with the increase of ambient temperature. The cluster devolatilization rate and char burning rate increased while the ignition time delay decreased with the increase of ambient oxygen concentration.     

Dynamic modes of infiltration-mediated combustion in a tubular reactor

In this work, we analyzed some regularities of frontal steady-state filtration combustion in moving porous media in tubular reactors. We analyzed the profiles temperature, concentration, pressure, rate of heat release, and velocity of combustion front propagation. Analysis was carried out in terms of 1D model from the moment of reaction ignition until onset of steady-state frontal regime. Special emphasis was made on the processes of front stabilization, non-uniqueness of steady-state regimes, and stable periodical regimes. Analyzed is the stability on combustion waves arising in the reactor. The results can be expected to extend the applicability range for the theory of filtration combustion.      

An analysis of the ignition of premixed gases by a hot spherical surface with catalytic reforming reaction☆

A study of the unsteadiness problem of the ignition of static premixed gases that contain CH₄ and steam by a catalytic hot sphere and a non-catalytic hot sphere were conducted, and a comparison between calculated and experimental results was done in the paper. The catalytic reforming reaction of CH₄ with steam on the surface of the sphere produced a small amount of H₂, CO and CO₂, at the same time there occur oxidizing reactions of CH₄, H₂ and CO in the space. Both experimental and calculated results show that a small quantity of H₂ produced by catalytic reforming reaction can greatly reduce the ignition temperature. In traditional catalytic combustion precious metals is applied to catalyse oxidizing reaction between oxygen and fuel to reduce ignition temperature. In this paper, a study on a ‘indirect’ catalytic combustion is conducted. The cheap catalytic material of Ni with rare earth is used and reforming reaction between steam and fuel is catalyzed, so hydrogen is generated on the surface of hot sphere and utilized to improve combustion. Calculation indicates that the high reactivity and high diffusivity of H₂ remarkably affect ignition.     

Ignition kernel development studies relevant to lean-burn natural-gas engines

In lean-burn premixed natural-gas engines, ignition and combustion can be accelerated by burning a small fraction of the mixture in a pre-chamber. High pressure generated in the pre-chamber results in the discharge of burned products into the main chamber. This creates multiple ignition kernels along the surface of the resulting jet. In this work, lean-burn characteristics of methane under the high pressure and high temperature conditions of a hot-jet ignited combustion chamber are investigated numerically by initializing a kernel of specified composition, temperature and size in a lean premixed gas, and following the development of the flame. In the case of hot-jet ignition the kernel temperature is limited by the temperature of the hot products. The influence of variations in ignition energy, affected by both temperature and size, and equivalence ratio, on the flame development is studied in an initially quiescent gas. It is shown that as long as the available ignition energy is greater than a minimum, the duration in which a steady flame speed is achieved is a strong function of kernel temperature; it is not a function of kernel size.     

Aerosol and Carbon Monoxide Emissions from Low-Temperature Combustion in a Sawdust Packed-Bed Stove

Low-temperature combustion in biomass-burning stoves used for cooking results in poor thermal efficiency and high emissions. A sawdust packed-bed stove has been shown to give more stable combustion at higher temperatures than woodstoves. The study examines pollutant emissions from this stove and their dependence on stove dimensions, specifically the vertical port radius and the stove-pot spacing. Emission rates of particulate matter (PM)—along with size resolution—and of carbon monoxide (CO) were measured during steady-state combustion. The stove power increased with increased spacing and vertical port radius. However, the air-flow rate, combustion temperature, and air-fuel ratio showed complex variations with stove dimensions from the described coupling among the pyrolysis, combustion, induced air flow, and mixing. Emission rates of PM (0.21–0.36 gh^?1 and CO (3–8 gh^?1 and were a factor of ten lower than those previously measured from woodstoves. Emission rates of CO decreased, while PM increased, with increasing combustion temperature. Aerosol size distributions were unimodal with mass median aerodynamic diameters (MMAD) of 0.24–0.40 𝛍 a factor of two smaller than from woodstoves. Cool combustion at 534–625°C gave lower PM emission rates but particles of larger MMAD, while hot combustion at 625–741°C gave higher PM emission rates with smaller particle MMAD. The OC/EC ratio obtained for cool combustion was higher (1.20) than that for hot combustion (0.96). Greater elemental carbon formation was seen at the higher temperatures. PM and CO emission rates followed opposite trends with combustion temperature and stove configuration, resulting in no single configuration at which both CO and PM emissions were minimized. However, its superior thermal efficiency and significantly lower emissions than wood stoves should motivate further study of this device to optimize thermal and emissions performance.     

油页岩及其半焦混烧特性的热重试验研究和动力学分析

利用热重分析仪对桦甸油页岩燃烧时的双峰现象以及与其500℃半焦按不同比例混烧时单样相互影响程度、颗粒特性、着火温度和燃烧特性指数进行了考察;利用DAEM模型解析了混烧动力学参数。结果表明：油页岩燃烧DTG曲线在500℃附近的双峰活性参数值相差不大,分别为0.79和0.73,反映油页岩燃烧DTG曲线双峰归因于脂类有机质与干酪根中芳香烃的燃烧;混样中各单样在不同温度段相互影响程度不同,表观影响程度系数f的值均小于1。在燃烧过程中颗粒的缩核形状和表面分形维数并不是固定不变的,在低温段接近长圆柱体,在高温段接近平板,当浓度级次β=0.6时,低温段分形维数接近3,在高温段接近1;着火温度随半焦掺混量的增加呈线性递增;混样燃烧特性指数介于单烧之间并且存在掺混效果明显区;试样表观活化能在燃烧前期变化比较缓慢并呈下降趋势,大约在60～90 kJ·mol^-1之间,在后期剧烈增加,当转化率在0.60～0.75之间时表观活化能从80 kJ·mol^-1剧增到200 kJ·mol^-1。     

Counter‐intuitive temperature excursions during regeneration of a diesel particulate filter

A major technological challenge in the regeneration of diesel particulate filters (DPFs) is that sometimes local high temperature excursions melt the cordierite ceramic filter. The cause of this melting is still an open question as the highest temperature attained under stationary (constant feed) combustion of the accumulated particulate matter is too low to cause this melting (melting temperature ～1250°C). We recently conjectured that the high temperature excursions are a counterintuitive response to a rapid deceleration, which decreases the exhaust gas temperature and flow rate and increases the oxygen concentration. Infrared measurements of the spatiotemporal temperature during soot combustion on a single‐layer DPF showed that a simultaneous step change of the feed temperature, flow rate, and oxygen concentration can lead to a transient temperature that exceeds the highest attained for stationary operation under either the initial or the final operation conditions. The experiments revealed that the magnitude of the temperature rise depends in a complex way on several factors, such as the direction of movement of the propagating temperature front. The amplitude of the temperature rise is a monotonic decreasing function of the distance that the temperature front moved before the step change. The rapid response to the feed oxygen concentration increases initially the moving front temperature. The slow response of the ceramic DPF to a decrease in the feed temperature may eventually decrease the moving front temperature and even lead to premature extinction and partial regeneration. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Formation and reduction of nitric oxide in fixed-bed combustion of straw

Mechanisms of nitric oxide (NO) formation and reduction in fixed-bed combustion of straw have been modeled mathematically and verified experimentally. The model for the straw combustion and nitrogen chemistry consists of sub-models for evaporation, pyrolysis, tar and char combustion, nitrogen conversion, and energy and mass conservation. Twenty chemical reactions are included, of which 12 belong to the fuel nitrogen reaction network. Volatile nitrogen is assumed to be NO, NH₃, HCN and HNCO, and char nitrogen is converted to NO during char oxidation. The model predictions are in qualitative agreement with the measurements during the ignition phase, i.e. when the combustion front passes through the un-burnt fuel. The yield of NO can be reduced considerably by using a low primary air flow due to the longer gas residence in the fixed-bed, while the NO exhaust concentration is insensitive to the bed temperature. The NO exhaust concentration initially reaches a maximum and then decreases towards a stable value after the straw bed is ignited. Variations of NO, NH₃, HCN, and HNCO concentrations in the ignition flame front indicate that a large quantity of NO can be reduced in the thin flame front zone. The developed model is further validated by separate experiments in which NO or NH₃ was added at the middle through tubes or at the bottom of the bed with the primary air flow. Both the simulations and measurements showed that the variation of the NO exhaust concentration is small as compared with the injected NO or NH₃ concentration. According to the simulations and experiments, it is proposed that flue gas recirculation may be a very effective method of reducing NO emissions from flue gas in the fixed-bed combustion of straw. Calculations indicated that about 20% of the flue gas may be recirculated without significantly affecting the combustion behavior.     

Selection of a diesel fuel surrogate for the prediction of auto-ignition under HCCI engine conditions

Homogeneous charged compression ignition (HCCI) is a promising combustion concept able to provide very low NO_x and PM diesel engine emissions while keeping good fuel economy. Since HCCI combustion is a kinetically controlled process, the availability of a kinetic reaction mechanism to simulate the oxidation (low and high temperature regimes) of a diesel fuel is necessary for the optimisation, control and design of HCCI engines. Motivated by the lack of information regarding reliable diesel fuel ignition values under real HCCI diesel engine conditions, a diesel fuel surrogate has been proposed in this work by merging n-heptane and toluene kinetic mechanisms. The surrogate composition has been selected by comparing modelled ignition delay angles with experimental ones obtained from a single cylinder DI diesel engine tests. Modelled ignition angle results are in agreement with the experimental ones, both results following the same trends when changing the engine operating conditions (engine load and speed, start of injection and EGR rate). The effect of EGR, which is one of the most promising techniques to control HCCI combustion, depends on the engine load. High EGR rates decrease the n-heptane/toluene mixture reactivity when increasing the engine load but the opposite effect has been observed for lower EGR rates. A chemical kinetic analysis has shown that the influence of toluene on the ignition time is significant only at low initial temperature. More delayed combustion processes have been found when toluene is added, the dehydrogenation of toluene by OH (termination reaction) being the main kinetic path involved during toluene oxidation.     

Emissions from different alternative diesel fuels operating with single and split fuel injection

Few factors affect diesel combustion and emissions more significantly than the composition of the fuel and the fuel injection process. In this paper, both of these factors are considered by comparing conventional, synthetic and vegetable oil-derived diesel fuels and by comparing a single pulse injection and a split (pilot and main) injection process. This paper focuses on characterization of the combustion process and emissions produced by three substantially different diesel fuels: an ultra low sulfur diesel fuel (BP15), a pure soybean methyl ester (B100), and a synthetic, practically free of sulfur and aromatic compounds, Fischer-Tropsch fuel (FT) produced in a gas-to-liquid process. The study was carried out in a direct injection (DI) 2.5 L common-rail turbodiesel engine working at four engine operation modes, spanning conditions of most interest in the engine map. In all modes the engine was tested with single and split injection (pilot and main), with constant start of injection (SOI), and without exhaust gas recirculation (EGR). Using the results from thermodynamic analysis, this study confirms that the ignition character of the fuel affects the start of the combustion process, notably for the whole combustion process when the single injection is used, and during the combustion process after the pilot injection when the split injection is used. In general, the FT fuel can reduce both NOx and PM specific emissions in all modes under both single and split injection modes, bypassing the nitrogen oxides-particulate matter (NOx-PM) trade-off. Finally, this work confirms that biodiesel can reduce the particle concentration. However, in some cases an increase of PM mass emission has been observed and this increase of the PM mass emission is due to unburned or partially burned hydrocarbon (HC) emissions.