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Theoretical study of non-adiabatic counter-flow diffusion flames propagating through a volatile biomass fuel taking into account drying and vaporization processes
Abstract
Due to important advantageous of non-premixed flames such as controllability and safety, a proper investigation can be highly beneficial for application of these flames in medical and power generation industries. The current paper attempts to provide a promising analytical model for non-adiabatic counter-flow diffusion flames propagating through volatile biomass particles using an asymptotic method. In order to offer a reliable model for analysis of the flames, a multi-zone flame structure including preheat, drying, vaporization, reaction and oxi-dizer zones, is considered. In this work, lycopodium particles and air are taken as biofuel and oxidizer, respectively. For following the influences of effective dimensionless numbers, such as fuel and oxidizer Lewis numbers on the flame structure, dimensionalized and non-dimensionalized forms of mass and energy conservation equations are derived for each zone. In order to observe the heat loss effects, a linear term is added to the energy conservation equation. The conservation equations are solved by Mathematica and Matlab software applying accurate boundary and jump conditions. Finally, variations of flame temperature, flame front position, gaseous fuel and oxidizer mass fractions with fuel and oxidizer Lewis numbers, mass particle concentration, particle size, equivalence ratio and heat loss effect are elaborately elucidated.
... Recently, Bidabadi et al. [18] have initialized the derivation of a relatively simple mathematical model to simulate non-adiabatic counter-flow diffusion flames for non-premixed combustion of a volatile biomass fuel, considering drying and vaporization processes. This numerical analysis way was adopted in this paper and extended to the cases of multiple biomass particles. ...
... This numerical analysis way was adopted in this paper and extended to the cases of multiple biomass particles. One improvement to the mathematical model proposed by Bidabadi et al. [18] is that all heat losses, including both convective and radiative losses, were involved in our study. In this work, a counterflow non-premixed flame fueled by moisty porous biomass particles was numerically modeled using our improved analytical model. ...
... x-direction velocity for the fuel and oxidizer are the same [17]; density, specific thermal capacity, and other transport coefficients are constant [23]; drying and evaporating processes occur in limited surface fronts [18,20]; surface reactions on the fuel particles prior to evaporating zone are neglected; ambient temperature is 300K; lycopodium particles through the solving field are equally distributed; size and shape for all particles are the same over the system; the value of Zeldovich number is very high. Therefore, thickness of the flame zone is very small and an asymptotic method can be used. ...
In this study, non-premixed multi-zone combustion of porous lycopodium particles in a counter-flow configuration at steady state was modeled using a derived analytical model. To model the porosity, a reasonable series form is considered. In the derived analytical model, both convective and radiative heat losses were included. The overall combustion process was mathematically divided into pre-heating, drying, vaporization, flame, and post-flame zones and lycopodium particles and oxidizer are injected from opposite sides locating infinity away the stagnation plane. The particles are firstly dried and then vaporized in a limitedly finite region to produce a gaseous fuel of special chemical composition. Relevant conservation equations were formulated in both dimensional and dimensionless forms. Applying suitable jump and boundary conditions, the corresponding continuity and energy equations were solved using Mathematica and Matlab software. The derived analytical model was first validated through comparison against literature data and a satisfactory agreement was achieved. Then temperature distribution and mass fractions of fuel and oxidizer along the x direction were presented to describe the combustion characteristics. Finally, a series of parametric studies were conducted to study the effects of key operating conditions.
... Contrary to the match condition, due to instantaneous phenomena, the temperature and mass fraction gradients are not continuous at the interfaces between the zones. To obtain the jump conditions, convective terms are eliminated from equations (9) and (10) and then by integrating them over the domain of examined zone, the conditions are obtained [36,37]. These conditions are written as follows: ...
... In this paper, an analytical investigation of lean premixed magnesium-air combustion is conducted to obtain the flame propagation rate and temperature. In this section, based on the obtained mathematical relations in previous section, equations (19)- (26) and (32)- (37), the effects of involved parameters on combustion characteristics are scrutinized. For this purpose, the thermophysical properties of magnesium and oxidizing medium, air, are given in Table 2. ...
Due to shortage and environmental pollution of fossil fuels and environment-dependent production of renewable energies, the metal fuels are promising candidates for alternative energy sources. In this study, the micron-sized magnesium particles are considered as energy carrier and the flame propagation of lean premixed magnesium-air combustion is investigated under zero gravity condition. To analyze dust cloud combustion, an asymptotic model of flame structure is proposed. According to the combustion behavior of single particle, the flame structure consists of four different zones including preheat, liquid magnesium, vaporized magnesium and post-flame zones in which the melting, vaporization and flame occur instantaneously. Afterwards, the non-dimensional forms of governing equations including mass, energy and gaseous fuel mass fraction conservations and the appropriate boundary conditions are derived and analytically solved. Subsequently, as the important achievements of the present study, the explicit formulas are obtained for flame velocity, location and temperature. Eventually, the effects of involved parameters on combustion characteristics are examined. The results indicate that as the particle diameter enhances from 15 to 60 µm, the flame front moves to a place 2.5 times farther away. The flame temperature increases linearly with concentration, while it decreases with the inverse of square of the diameter.
... Lewis number, one of the effective combustion parameters, represents the ratio of thermal diffusivity to mass diffusivity as presented below [38] : ...
... In this study, Arrhenius one-step reaction is considered [38] : ...
Due to coal's vital role as a well-known fuel in electricity production worldwide, promising prediction of the behavior of this fuel in different combustible systems can be an effective prerequisite for augmentation of efficiency of coal-fuelled power plants. In this research, planar non-premixed combustion of moisty coal particles in counter-flow design is mathematically elucidated using an asymptotic concept. To develop a reliable model, preheat, drying, pyrolysis and oxidation processes are included. In the pyrolysis process, effects of produced char and gas are examined. In the flame region, homogeneous and heterogeneous reactions are considered. Mass and energy conservation equations are mathematically characterized to yield the temperature and mass fraction profiles of the fuel particles and oxidizer taking into account the porosity of the coal particles. A system of coupled governing equations are solved using Matlab and Mathematica software. Eventually, effects of crucial parameters including flame front temperature and position, strain rate, fuel and oxidizer mass fractions with Lewis number, and initial temperature are evaluated. Maximum temperature of the counter-flow non-premixed flame fed with coal particles with diameter of 20 µm is found to be 3097 K for fuel Lewis number of 0.3 for unity values of porosity factor and oxidizer Lewis number.
... The properties of Lf and BSA nanoparticles, and 2L-AN@RB4 were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), and dynamic light scattering (DLS). To investigate the behaviors of adsorbent in the expanded bed column, the effects of contact time (4 hours), kinetic adsorption (Pseudo-second order equation), pH (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12), initial concentration (0.5-4 mg/mL adsorbent), and adsorption isotherms were assessed. ...
In this study, adsorption of lactoferrin (Lf) and bovine serum albumin (BSA) nanoparticles on pellicular two-layer agarose-nickel immobilized by reactive blue 4 dye-ligand ([email protected]) was investigated. [email protected] was prepared using the three-phase emulsion method. The dynamic light scattering was first employed to quantify size distribution of Lf and BSA nanoparticles. Then, scanning electron microscopy (SEM) and atomic force microscope (AFM), respectively, were used to characterize structures of the two types of nanoparticles and adsorbent beads. SEM and AFM images demonstrate that shapes of the nanoparticles are globular and relatively uniform, where no adhesions were observed. Finally, adsorption behaviors of both Lf and BSA nanoparticles on [email protected] in affinity chromatography were investigated, with focus on the adsorption kinetics. Influences of contact time, pH, and initial concentration were analyzed to investigate the adsorbent behaviors in the expanded bed column. The influence of contact time on adsorption of Lf versus BSA nanoparticles indicates that 4 h is enough to adsorb protein models equal to 45%. Results indicate that Lf nanoparticles have a higher rate of around 83% than that of BSA nanoparticles. The increases in pH have negative effect on adsorption while opposite trend was found for initial concentration. Adsorption isotherm results reveal that the Langmuir and Freundlich isotherm models fit both adsorption kinetics of Lf and BSA very well.
... In the view of the background associated with suitability of dual-fuel combustion systems, implied from the literature review, this works presents an analytical study on a dual-fuel counter-flow premixed combustion system fueled with lycopodium/air and propane/air, which are simultaneously injected into the system. Analytical modeling has been comprehensively proven to be effective to investigate singlefuel combustion for many types of particles, such as coal [15], and biomass [19,20]. Compared with experimental approaches, analytical modeling has the advantages of clarification of underlined mechanisms and reduction of development cycle. ...
In this paper, an analytical study was conducted on two possible cases of dual-fuel combustion systems operating in premixed mode in counter-flow configuration fueled simultaneously with lycopodium particles/air and propane/air. Combustion behaviors of two distinct cases were discussed whose difference is whether there is an overlap between the reaction zone formed by combustion of propane and that formed by combustion of lycopodium particles. Non-dimensional forms of mass and energy conservation equations were formulated and analytically solved using Matlab and Mathematica. The derived analytical solution was first validated against experimental data and good agreement was achieved. Then, the profiles of temperature and species distributions for the two cases were discussed to evaluate the effects of overlap between reaction zones. It was found that in both cases, dual fuel mode can be considered as an effective technique for improving the combustion characteristics of lycopodium particles. However, the flame temperature in case 1 is higher than that in case 2. Eventually, influences of fuel and oxidizer Lewis numbers and particle size on the flame temperature and position were para-metrically analyzed. Furthermore, flame temperature and position inversely varied with Lewis number of reactants and particle size.
... Recently, mathematical modelling based upon computational fluid dynamics (CFD) models that have been validated with data from laboratory-scale setups has proved to be promising and they have become a powerful tool used to address real-world explosion scenarios pertinent to the mining industry (Arntzen 1998;Bidabadi et al. 2018;Hjertager 1993;Makarov et al. 2009;Mehrabian et al. 2012;Park and Lee 2009;Venetsanos et al. 2008). Moreover, complex scenarios of fire and explosion for different scales, fuels, geometries and configurations can be simulated and studied with ease. ...
Deflagration of fire and explosion caused by ignition of a methane-air mixture is a major safety concern in industrial settings, such as the mining and petrochemical industries. The aim of this study is to investigate numerically the effect of tube size on the flame and pressure wave propagations for methane-air deflagration in a tube closed at one end. A partially premixed combustion model that avoids the need to specify the flame speed is deployed based upon the Flamelet Generated Manifold (FGM) model. Good agreement is achieved between the predicted results and the benchmark experimental data collected using a large-scale detonation tube (L/D = 66). Subsequently, the explosion behaviour immediately after the ignition of methane-air mixtures and the propagation characteristics of the flame front and pressure wave through the tube are examined, covering a broad range of L/D, that is from 26 to 526, in which the diameter is changed and the length is kept fixed. The results show that the pressure wave propagates significantly faster in narrower tubes and hence decouples from the flame front shortly after ignition, which in turn results in a low overpressure at the flame front. Moreover, abrupt changes in gas properties are observed in narrow tubes with L/D ≥ 132. The peak overpressure increases as the tube diameter increases; however, the local maximum pressure decreases substantially in large tubes when approaching the tube vent but remains almost constant throughout in the narrow tubes. Similarly, the flame propagates faster in narrower tubes. A correlation that estimates the distance the flame propagates in the exponential acceleration stage is proposed as a function of tube size and time. Deviations of less than 7% are obtained when comparing the predicted results using the correlation against the experimental data. The results provide local information that aids the theoretical interpretation of experimental observations and the understanding of the fuel combustion and explosion phenomena in different sized tubes.
"Background and Aim: One of the most effective ways to improve efficiency in organizations is human resources management. Job Anchors and Organizational Commitment are constructs having important implications for human resources management practices, especially in efficiency. The purpose of this perusal was to study a structural equation model (SEM) of the impact of job anchors and organizational commitment on efficiency in a military hospital. Methods: The statistical population of this study was the personnel of a military hospital, which gathered accomplishing simple random sampling method. As a result, the research used two hundred and thirty questionnaires so to analyse, accomplishing as survey was developmental-applied, that was, requirement data with questionnaires. To measure efficiency, organizational commitment and job anchors variables respectively the Persian version of Hersey & Goldsmith, Meyer & Allen, and Schein questionnaires standard were used. In order to test the research conceptual model, structural equations modelling based on partial least squares method was used, and the model was analysed using AMOS statistical software. Results: The results of data analysis demonstrated that hidden variables such as organizational commitment, job anchors, and efficiency had acceptable convergent validity, and the fitness of the conceptual model was confirmed. Based on the research model analysis, organizational commitment did not have impact on efficiency, and job anchors had a positive and significant impact on efficiency. Eventually, the modified model showed that organizational commitment variable moderates the connection between job anchors and efficiency. Conclusion: In light of the findings of this research, managers of military organizations should develop programs to create and improve organizational commitment in personnel. Furthermore, they should pay attention to employees' characteristics and the proportion of their occupation with job anchors in cases of changing and promoting employees. Implementing aforementioned items will have a positive impact on improving the efficiency of military organizations. "
Jet fires generally leave patterns on the surface of the plate, which can be helpful for fire investigators to find the source and cause of fire incidents. However, the interaction between jet fires and wood plates still lacks research. This paper investigates the flame extension and pattern of jet fire impinging wood plates under strong plume conditions. The flame extension length beneath the plate is intensely studied. Moreover, an attempt is made to relate the fire patterns left by the jet fire impinging on the wood plate with the fire characteristic parameters. The results indicate that the flame extension length under the plates is strongly related to the visible free height, nozzle-plate height, and gas fuel velocity. The radius of the fire pattern is highly correlated with the flame extension length, and a new relationship equation between the radius of the fire pattern and the flame extension length is proposed. The area of the fire pattern is related to three characteristic parameters, the fuel flow rate, nozzle diameter, and nozzle-plate height, and the prediction equation of the fire pattern area is given to collapse all the experimental data well to provide theoretical support for fire investigation work.
The study of the pine sawdust drying kinetics was carried out using thermal analysis. The evaporation rate of free and bound moisture was determined using thermogravimetric data. Differential scanning calorimetry allows to evaluate the effect of evaporation on the deviation of the sample temperature from the heating gas temperature. In experiments, this temperature deviation increases with the heating rate and can be several tens of degrees. Using the developed theoretical approach, the values of the energy of moisture sorption were obtained from the data on the rate of evaporation at different temperatures under conditions of diffusional evaporation (the period of decreasing drying rate). This approach operates with the thermodynamic parameters of interfacial interaction, which affect the change in the saturated pressure of water vapor above the biomass surface. The results show that with a decrease in the moisture content of the pine wood, the sorption energy of moisture increases rather sharply, and at a moisture content of less than 5%, this dependence is close to exponential.
In this study, a mathematical modeling using asymptotic solution method was performed to a multi-region premixed combustion of moist moso bamboo particles under adiabatic condition. The analytical model assumes and divides the modeling system into multi-regions as preheating, drying, pyrolysis, and homogeneous and heterogeneous reactions. The formulated mass and energy conservation equations were written for each region and solved analytically using specified jump and boundary conditions. The experimental validation using temperatures of homogeneous flame and heterogeneous reaction fronts confirmed that the prediction accuracy is promising. Then, combustion characteristics such as distributions of temperature and species mass fractions were clarified. Influences of particle diameter, gaseous fuel Lewis number, and equivalence ratio were finally explored on crucial quantities such as homogeneous flame temperature and heterogeneous reaction temperatures, burning velocity, and pyrolysis front location. The results showed that increasing bamboo particle diameter leads to lower burning velocity, lower flame temperatures, and prolonged reaction fronts. Fuel Lewis number showed similar trends for burning velocity and flame and reaction temperatures as those of particle diameter, while opposite conclusions were found for reaction front locations. The impacts of equivalence ratio are opposite for burning velocity, flame temperature, and reaction front locations as those of particle diameter.
In this study, a comprehensive analysis is conducted to evaluate the effects of heat recirculation and particle porosity on combustion characteristics of multizone counterflow nonpremixed flames fed with porous biofuel particles. For this purpose, the structure of flame contains preheat, postvaporization, and oxidizer zones. Additionally, Lycopodium is considered as the volatile biofuel, especially due to its appreciable flammability and dispensability. Dimensionalized and nondimensionalized forms of mass and energy conservation equations are scrutinized in each zone. To explore of the thermal recirculation effect, a specific term is included in the energy conservation equation. The variation of several parameters, including flame temperature, particle radius, mass fraction of the gaseous fuel and oxidizer, mass particle content, equivalence ratio, and particle porosity, is studied in this work considering and ignoring the thermal recirculation impact. As a result, increasing heat recirculation coefficient from k = 0 to 1 will rise the flame temperature and shift the flame position to the left side (fuel nozzle). Furthermore, consideration of the thermal heat recirculation will improve the gaseous fuel production in the preheat and postvaporization zones. Additionally, an increase in mass concentration and reduction of particles radius and porosity would lead to a rise in the flame temperature.
Highlights
• Nonpremixed combustion of dust particles in a counterflow arrangement is studied.
• Particles porosity and heat recirculation modeling are investigated.
• Effects of fuel Lewis number and particle porosity on flame temperature are studied.
• Effects of equivalence ratio and particle radius on flame temperature are studied.
Owing to their safety, stability and controllability, diffusion flames have found extensive applications in medicine and power generation. Regarding the significance of recirculation impact on micro-combustors, an efficient method should be developed for better analysis of the micro-combustors performance. In this paper, an asymptotic method is developed to model diffusion flames propagation through a biofuel in counterflow configuration with the consideration of heat recirculation effect. The flame structure includes pre-heat, post-vaporization and oxidizer zones. Micron-sized lycopodium particles and air can be regarded as biofuel and oxidizer, respectively. Mass and energy conservation equations are investigated in each zone. For evaluation of the thermal recirculation impact, a specific term is included in the energy conservation equation. Furthermore, the effects of changes in the flame temperature, mass fraction of the gaseous fuel and oxidizer (relative to fuel and oxidizer Lewis numbers), mass particle content, particle radius and equivalence ratio were examined considering and ignoring the thermal recirculation effect. The results indicate that increase of heat recirculation coefficient will rise the flame temperature and shift the flame position to the fuel nozzle side. Also, consideration of thermal heat recirculation will enhance the gaseous fuel production in the pre-heat and post-vaporization zones.
Identifying the thermal-diffusive instability boundaries of flames is of great significance in the field of combustion. In this paper, a detailed mathematical analysis is performed to detect the threshold of thermal-diffusive instability in planar counter-flow non-premixed flames using an asymptotic concept. Uniformly-scattered micron-sized lycopodium particles and air are applied as organic fuel and oxidizer, respectively. In order to suggest a basic combustion structure, preheat, vaporization, flame and oxidizer zones are considered. In the first phase of this investigation, time-dependent forms of mass and energy conservation equations are derived considering appropriate boundary and jump conditions. In each of the considered zones, governing equations are solved by Matlab and Mathematica software using perturbation method. To predict the onset of instability, critical values of frequency of the wrinkled flame front, Zeldovich and Lewis numbers are calculated. Eventually, the effects of wave number on the critical Zeldovich and Lewis number are explained. For validation purposes, results of this analysis obtained for critical flow strain rate (corresponding to extinction of the counter-flow non-premixed flame) are compared to those reported in prior studies. Based on the comparisons, proper compatibility is observed between the mathematical results of current study and the data reported in the literature.
Controllability, safety and stability are benefits of diffusion flames which make them highly advantageous to be used in medical and power generation industries. Radiation heat transfer is a dominant phenomenon in flame propagation through organic particle clouds. In this research, a promising thermal radiative model is presented for non- premixed counterflow combustion (diffusion flame) propagating through volatile organic particles using an asymptotic method. In order to propose a credible model to analyze the flame, a multi-zone flame structure comprised of pre-heat, post-vaporization and post-flame zones, is considered. In addition, vaporization and reaction zones are supposed to have thin asymptotic fronts. Lycopodium particles and air are taken as biofuel and oxidizer, respectively. To follow the influences of dimensionless numbers, such as fuel and oxidizer Lewis numbers on the flame structure, dimensionless and non- dimensionless equations of mass and energy conservation are extracted for each zone. So as to observe the thermal radiation effects, a special term is added to the energy conservation equation. The governing equations are solved applying elaborate boundary and jump conditions. Eventually, variations of flame temperature, gaseous fuel and oxidizer mass fractions with respect to fuel and oxidizer Lewis numbers, mass particle concentration, particle radius and equivalence ratio are investigated considering thermal radiation and compared to those in which thermal radiation heat transfer is not considered. As a result, by applying radiation term in the left side of the flame zone, temperature increases and flame position moves toward the fuel nozzle. Moreover, considering thermal radiation increases the amount of produced gaseous fuel in pre-heat and post-vaporization zones.
The main aim of this research is focused on determining the velocity and particle density profiles across
the flame propagation of micro-lycopodium dust particles. In this model, it is tried to incorporate the
forces acting on the particle such as thermophoretic, gravitational and buoyancy in the Lagrangian equation of motion. For this purpose, it is considered that the flame structure has four zones (i.e., preheat,
vaporization, reaction and post flame zones) and the temperature profile, as the unknown parameter in
the thermophoretic force, is extracted from this model. Consequently, employing the Lagrangian equation
with the known elements results in the velocity distribution versus the forefront of the combustion region.
Satisfactory agreement is achieved between the present model and previously published experiments. It is
concluded that the maximum particle concentration and velocity are gained on the flame front with the
gradual decrease in the distance away from this location.
The diffusive-thermal (D-T) instability of opposed nonpremixed tubular flames near extinction is investigated using two-dimensional (2-D) direct numerical simulations together with the linear stability analysis. Two different initial conditions (IC), i.e. the perturbed IC and the C-shaped IC are adopted to elucidate the effects of small and large amplitude disturbances on the formation of flame cells, similar to conditions found in linear stability analysis and experiments, respectively. The characteristics of the D-T instability of tubular flames are identified by a critical Damkohler number, Da
Dust flames are associated with two-phase combustion phenomena where flame characteristics depend on interactions between solid and gas phases. Since organic dust particles can be effectively utilized in energy production systems, investigation of this phenomenon is essential. In this study, an analytical model is presented to simulate the combustion process of moist organic dust. The flame structure is divided into three zones: preheat zone, reaction zone, and postflame zone. To determine the effects of moisture content and volatile evaporation, the preheat zone is also divided into four subzones: first heating subzone and drying subzone, second heating subzone, and volatile evaporation subzone. The results obtained from the presented model are in reasonable agreement with experimental data for lycopodium particles. An increase in moisture content causes a reduction in burning velocity owing to moisture evaporation resistance. Consequently, the effects of some important parameters, like volatilization temperature, volatilization Damköhler number and drying Damköhler number are investigated. In special cases, like high moisture content, low volatilization temperature, and high drying resistance, the second heating subzone is omitted.
Organic dust flames deal with a field of science in which many complicated phenomena like pyrolysis or devolatization of solid particles and combustion of volatile and char particles take place. One-dimensional flame propagation in the cloud of fuel mixture has been analyzed in which flame structure is divided into three zones: preheat zone, reaction zone and post flamezone.It is assumed that particles pyrolyze first to yield a fuel mixture consisting of gaseous and charry fuel. In this research, the effect of char content on pyrolysis process has taken into account and a novel non-linear burning velocity correlation is obtained.Our results are in a reasonable agreement with experimental data.
The structure and extinction characteristics of counterflow diffusion flames with flame radiation and nonunity Lewis numbers of the fuel and oxidant are examined using multiscale asymptotic theory, and a model expressed in terms of the jump relations and reactant leakages with the proper consideration of the excess enthalpy overlooked in previous analyses is developed. The existence of the dual extinction limits in the presence of radiative heat loss, namely the kinetic limit at small Damkoehler number (high stretch rate) and the radiative limit at large Damkoehler number (low stretch rate), are identified. It is found that the former is minimally affected by radiative loss, while a substantial amount of heat loss is associated with the radiative limit. Reactant leakage, however, is the root cause for both limits. The influence of radiative loss on the extinction Damkoehler numbers is found to be through its effects on the flame temperature, the excess enthalpy, and the reduced extinction Damkoehler number. At both extinction limits, the contribution from the flame temperature is always important and dominant. The contributions from the other two, however, could be important in some special cases. At small Le{sub F}, the contribution from the reduced extinction Damkoehler number is large and even dominant under small radiative loss. The contribution from the excess enthalpy is important for small Le{sub O} and it may be comparable to the contribution from the flame temperature when radiative loss is small. Thus, overlooking the excess enthalpy in previous analyses may have resulted in rather large error in the predicted extinction Damkoehler numbers, especially the kinetic one. (author)
We investigated computationally and experimentally the structure of steady axisymmetric, laminar methane/enriched-air diffusion flames. Experimentally, we imaged simultaneously single-photon OH LIF and two-photon CO LIF, which also yielded the forward reaction rate (RR) of the reaction CO + OH → CO2+ H. In addition, particle image velocimetry (PIV) was used to measure the velocity in the proximity of the fuel and oxidizer nozzles, providing detailed boundary conditions for the simulations. Computationally, we solved implicitly the steady state equations in a modified vorticity–velocity formulation on a non-staggered, non-uniform grid. We compared the results along the axis of symmetry from the two-dimensional simulations with those from the one-dimensional model, and showed consistency between them. The comparison between the experimental and computational data yielded excellent agreement for all measured quantities. The field of these two-dimensional flames can be roughly partitioned into two regions: the region between the two reactant nozzles, in which viscous and diffusive effects are confined to the mixing layer and to the nozzle walls, where separation occurs; and a radial development region, which is initially confined by recirculation zones near the burner flanges. Buoyancy is virtually irrelevant in the first region at all but the smallest, and practically irrelevant, strain rates. Buoyancy, on the other hand, does play a role in the growth of the recirculation zones, and in determining the flame location in the outermost region.
High temperature behavior of spray-dried NiO-based oxygen carrier samples for Chemical Looping Combustion (CLC) under oxidizing and reducing conditions was studied using a ceramic fluidized bed furnace. Differently from the common CLC process, high enough temperatures were obtained by burning air and methane mixture directly in the bed of carriers, changing the stoichiometric air to fuel ratio in the range of 0.70 to 1.30 and using external electric heating.None of the five particle samples tested showed any sign of defluidization in oxidizing atmosphere at any of the temperatures tested, i.e. up to 1175–1185 ∘C. However, under reducing conditions or in the oxidizing-reducing sequences, shortly after changing the atmosphere from oxidative to reducing, some of the spray-dried samples agglomerated causing defluidization. To study the structure of the carrier particles and to clarify the agglomeration phenomena, SEM and EDX analysis of the initial and treated samples was carried out. It was shown that agglomerates are formed through small bridges between the particles which consist of pure Ni phase. Three samples which were produced using MgO as an additive were tested. For these no defluidization was observed at any of the temperatures tested, i.e. up to 1175–1185 ∘C, neither in oxidizing, nor in reducing atmosphere. So, addition of proper components to the carrier particles can significantly improve reliability of the CLC process at higher temperatures.
The suppression of low strain rate non-premixed flames was investigated experimentally in a counterflow configuration for laminar flames with minimal conductive heat losses. This was accomplished by varying the velocity ratio of fuel to oxidizer to adjust the flame position such that conductive losses to the burner were reduced and was confirmed by temperature measurements using thermocouples near the reactant ducts. Thin filament pyrometry was used to measure the flame temperature field for a curved diluted methane-air flame near extinction at a global strain rate of 20 s−1. The maximum flame temperature did not change as a function of position along the curved flame surface, suggesting that the local agent concentration required for suppression will not differ significantly along the flame sheet. The concentration of N2, CO2, and CF3Br added to the fuel and the oxidizer streams required to obtain extinction was measured as a function of the global strain rate. In agreement with previous measurements performed under microgravity conditions, limiting non-premixed flame extinction behavior in which the agent concentration obtained a value that insures suppression for all global strain rates was observed. A series of extinction measurements varying the air:fuel velocity ratio showed that the critical N2 concentration was invariant with this ratio, unless conductive losses were present. In terms of fire safety, the measurements demonstrate the existence of a fundamental limit for suppressant requirements in normal gravity flames, analogous to agent flammability limits in premixed flames. The critical agent volume fraction in the methane fuel stream assuring suppression for all global strain rates was measured to be 0.841 ± 0.01 for N2, 0.773 ± 0.009 for CO2, and 0.437 ± 0.005 for CF3Br. The critical agent volume fraction in the oxidizer stream assuring suppression for all global strain rates was measured as 0.299 ± 0.004 for N2, 0.187 ± 0.002 for CO2, and 0.043 ± 0.001 for CF3Br.
Due to the finite stock of fossil fuels and its negative impact on the environment, many countries across the world are now leaning toward renewable sources energies like solar energy, wind energy, biofuel, hydropower, geothermal and ocean energy to ensure energy for the countries development security. Biodiesel is one kind of biofuel that is renewable, biodegradable and has similar properties of fossil diesel fuel. The aim of this paper is to provide the substantial information on biodiesel to the researchers, engineers and policy makers. To achieve the goal, this paper summarizes the information on biofuel development, feedstocks around the world, oil extraction technic, biodiesel production processes. Furthermore, this paper will also discuss the advantages of biodiesel compared to fossil fuel. Finally, the combustion behavior of biodiesel in an internal combustion engine is discussed and it will help the researchers and policy maker and manufacturer. To determine the future and goal of automotive technology the study found that, feedstock selection for biodiesel production is very important as it associates 75% production cost. Moreover, the test of fuel properties is very important before using in the engine which depends on the type of feedstocks, origin country, and production process. Most of the researchers reported that the use of biodiesel in diesel engine reduces engine power slightly but reduces the harmful emission significantly. Finally, the study concludes that biodiesel has the potential to be used as a diesel fuel substitute in diesel engines to solve the energy and environment crisis.
In the present article an analytical approach is employed to study the counterflow non-premixed dust flame structure in air. Lyco-podium is assumed as the organic fuel in in this paper. First, it is presumed that the fuel particles vaporize in a thin zone to form a gaseous fuel to react with the oxidizer. The reaction rate is presumed to be of the Arrhenius type in first order respect to oxidizer and fuel. Mass conservation equations of dust particles, oxidizer, gaseous fuel and the energy conservation equation for non-unity Lewis numbers of oxidizer and fuel are presented as the governing equations. Boundary conditions are applied for each zone for the purpose of solving the governing equations analytically. The flame temperature in terms of oxidizer and fuel’s Lewis numbers is calculated. Furthermore, the variation of flame position based on the fuel and oxidizer’s Lewis numbers is evaluated .Also, mass fraction and temperature profiles of oxidizer and fuel are presented. In addition, the variation of the ratio of critical extinction values of strain rates as a function of non-unity Lewis numbers of oxidizer and fuel to the unity Lewis numbers of them is investigated.
In this investigation, the structure of one-dimensional flame propagation in uniform cloud of volatile organic particles has been analyzed in which the structure of flame is divided into three zones. The first zone is preheat zone which is divided into three subzones itself. In first subzone (heating), particle cloud is heated until the moist particles reach to vaporization temperature (water vapor). In the next subzone (drying), particle moisture comes out, and in the final subzone the pyrolysis phenomenon happens. The second zone is reaction zone, and the last zone is post-flame zone. In this research, an analytical method is used in order to solve the governing equations of particle cloud combustion in aforementioned zones. The overall investigation of this study leads to a non-linear burning velocity correlation. Consequently, the results show that decrease in particle moisture content or increase in equivalence ratio or Lewis number causes to increase in moisture evaporation and devolatization rates, and consequently both flame temperature and burning velocity increase.
In this article, we calculate effect of various effective dimensionless numbers and moisture content on initiation of instability in combustion of moisty organic dust. To have reliable model, effect of thermal radiation is taken into account. One dimensional flame structure is divided into three zones: preheat zone, reaction zone and post-flame zone. To investigate pulsating characteristics of flame, governing equations are rewritten in dimensionless space-time coordinates. By solving these newly achieved governing equations and combining them, which is completely discussed in body of article, a new expression is obtained. By solving this equation, it is possible to predict initiation of instability in organic dust flame. According to the obtained results by increasing Lewis number, threshold of instability happens sooner. On the other hand, pulsating is postponed by increasing Damköhler number, pyrolysis temperature or moisture content. Also, by considering thermal radiation effect, burning velocity predicted by our model is closer to experimental results.
This paper reports a numerical investigation of syngas flame structure and NO reaction pathways over a wide range of operating conditions (H2/CO ratio between 0.4 and 2.4, scalar dissipation rate from equilibrium to extinction and ambient pressure from 1 to 10 atm) in mixture fraction space. An analysis of optimal operation conditions for syngas combustion in regard to NO index emissions is also provided.
Flame structure is characterized by solving flamelet equations with the consideration of radiation. The chemical reaction mechanism adopted is GRI-Mech 3.0.
The computational predictions showed that flame temperature exhibits a peak at an intermediate scalar dissipation rate for a given value of H2/CO ratio. From hydrogen-lean syngas to hydrogen-rich syngas fuels, maximum flame temperature increases for scalar dissipation rate values lower than the intermediate value whereas decreases at higher values. Zeldovich route is found to be the main NO formation route and its contribution to the NO production continually increases with the increase of hydrogen content and pressure. Hydrogen-rich syngas flames produce more NO at lower scalar dissipation rates while NO levels increase towards hydrogen-lean syngas flames at higher scalar dissipation rates.
This study analyzes a premixed dust–air flame, under conditions where a homogeneous gas-phase reaction front can exist. Discussion on four possible flame types is provided. A solution is obtained for the burning velocity of a flammable dust–air flame in both fuel and oxygen limiting cases. A sensitivity analysis is used to analyze the features controlling the dust burning. It is shown that vaporization is significant for fuel limiting conditions; however, does not play a major role in oxygen limiting cases. The calculated burning velocity shows good agreement with available experimental data for coal–dust–air flames.
In this study large eddy simulation (LES) technique has been used to predict the fuel variability effects and flame dynamics of four hydrogen-enriched turbulent nonpremixed flames. The LES governing equations are solved on a structured non-uniform Cartesian grid with the finite volume method, where the Smagorinsky eddy viscosity model with the localised dynamic procedure is used to model the subgrid scale turbulence. The conserved scalar mixture fraction based thermo-chemical variables are described using the steady laminar flamelet model. The Favre filtered scalars are obtained from the presumed beta probability density function approach. Results are discussed for the instantaneous flame structure, time-averaged flame temperature and combustion product mass fractions. In the LES results, significant differences in flame temperature and species mass fractions have been observed, depending on the amount of H2, N2and CO in the fuel mixture. Detailed comparison of LES results with experimental measurements showed that the predicted mean temperature and mass fraction of species agree well with the experimental data. The high diffusivity and reactivity of H2 largely affect the flame temperature and formation of combustion products in syngas flames. The study demonstrates that LES together with the laminar flamelet model is capable of predicting the fuel variability effects and flame dynamics of turbulent nonpremixed hydrogen-enriched combustion including syngas flames.
This paper presents a new analytical model for predicting non-adiabatic dust flame with the assumptions that the flame structure is composed of a preheat zone, a reaction zone and a post flame. It is presumed that part of fuel particles vaporize to yield a gaseous fuel, and therefore the vaporization term is added into the preheat zone. Then, the radiation and heat loss effects are applied in the governing equations for lean mixtures. It is declared that the flame temperature decreases at the post flame zone while the radiation and heat loss effects are considered. Consequently, it is observed that the calculated values of flame temperature are in the good agreement with the experimental data in the literature.
We examine the structure and oscillatory instability of low Peclet number, non-premixed edge-flames in a fixed rectangular channel, closed at one end, with constant side-wall mass injection, one surface supplying fuel, the other oxidizer. Both reactant components have unit Lewis numbers. This study is motivated by issues regarding the nature of combustion that may occur in a propellant crack formed at the interface between the fuel-binder and oxidizer in a heterogeneous propellant. Flux conditions are imposed on the fuel and oxidizer at the injection walls, while the temperature on the boundary walls is held constant. For situations in which steady burning occurs, the flame has two components: an edge that faces towards the closed end of the channel and a trailing one-dimensional diffusion flame. A large Damköhler number study of the trailing, planar, strained diffusion flame structure is conducted and a new solvability condition uncovered in this limit, whereby the flame may not exist if the supply mixture strength is sufficiently far from stoichiometric. Numerical calculations also reveal that axial oscillations of the edge-flame in the channel may occur, but for a finite range of mixture strengths sufficiently far from stoichiometric values. The importance of mixture strength, heat losses to the relevant injection surface and the channel end-wall, and the role of the injection surface reactant flux conditions in inducing the oscillations are emphasized. Finally we explore the effect on the combustion structure of varying Peclet number and of different injection velocity magnitudes on the side-wall surfaces. This article was chosen from Selected Proceedings of the Third International Symposium on Turbulence and Shear Flow Phenomena (Sendai, Japan, 24 27 June 2003) ed N Kasagi et al.
The asymptotic structure of counterflow and stagnant diffusion flames are analyzed in the limit for large values of the overall, nondimensional activation energy, Ta, characterizing the rate of the reaction and results are given for small values of the stoichiometric fuel to oxygen mass ratio. The chemical reaction between the fuel and the oxidizer is represented by a one-step, irreversible process. A new approach is developed to characterize the influence of the Lewis number of the fuel, LF and the Lewis number of the oxidizer, L0, on the outer and inner structure of near equilibrium diffusion flames. Explicit algebraic formulas to predict the critical conditions of flame extinction are also given.For counterflow diffusion flames at fixed values of L0, the flame moves significantly toward the oxidizer stream and the heat losses toward the oxidizer region of the flame increase significantly with decreasing values of LF. The value of the maximum flame temperature is relatively insensitive to the variations in LF although the value of the rate of strain at extinction, A, increases significantly with decreasing values of LF and increasing values of Ta. At fixed values of LF and decreasing values of L0, the flame moves slightly toward the fuel stream; the heat losses toward the fuel stream increase slightly and there is a moderate increase in the value of the maximum flame temperature. The value of A increases with decreasing values of L0 for large values of Ta and is relatively insensitive to variations in L0 for moderate values of Ta.The inner and outer structure for stagnant diffusion flames where convection is absent are qualitatively similar to those for counterflow diffusion flames. However, the value of the maximum flame temperature increases significantly with decreasing values of L0 and fixed values of LF.The results developed here are used to obtain overall chemical kinetic rate parameters characterizing the gas phase oxidation of methane using previously measured values of the critical conditions of flame extinction.
Results of a theoretical experimental study of the structure of a methane-air counterflow diffusion flame are reported. Concentration profiles of the stable species were measured using gas sampling techniques with quartz microprobes. The samples were analyzed with a gas chromatograph. Temperature profiles were measured using coated thermocouples. Numerical calculations including C2 chemistry were performed with an adaptive nonlinear boundary value solver at conditions identical to those used in the experiment. The results are compared using both the physical coordinate and the mixture fraction as the independent variable. Excellent agreement is obtained for concentration profiles of CH4, O2, N2, CO2, H2O, H2, CO, C2H2, C2H4, AND C2H6, for the peak value of the temperature and for flame standoff distances.
This paper explores some fundamental issues involved in flame-acoustic interaction in the context of non-premixed flames. The combustion model considered is a two-dimensional co-flowing non- premixed flame in a uniform flow field, as in the Burke-Schumann geometry. Both finite-rate and infinite-rate chemistry effects are examined. First, the velocity-coupled response of the flame to an externally imposed velocity fluctuation is studied at various frequencies of interest. The Damkohler number plays an important role in determining the amplitude and phase of the heat release fluctuations with respect to the velocity fluctuations. Second, the combustion model is coupled with the duct acoustics. The one-dimensional acoustic field is simulated in the time domain using the Galerkin method, taking the fluctuating heat release from the combustion zone as a compact acoustic source. The combustion oscillations are shown to cause exchange of acoustic energy between the different natural modes of the duct over several cycles of the acoustic oscillations.
We present an asymptotic study of the effect of volumetric heat loss on the propagation of triple flames in a counterflow configuration at constant density. Analytical results for the speed, the local burning rate, the shape and the extent of the flame front are derived in the asymptotic limits of weak strain rates and large activation energies and for Lewis numbers that are near unity. The results account for the combined effects of strain, heat loss, composition gradients and non-unit Lewis numbers and provide Markstein-type relationships between the local burning speed (or local flame temperature) and the local flame stretch and can be useful for future investigations in deriving such relationships in non-homogeneous non-adiabatic mixtures under more general flow conditions. The analytical predictions are complemented by and compared with numerical predictions focusing on the low strain regime and allowing for non-unit Lewis numbers. The numerical findings are found to be in good qualitative agreement with the asymptotics, both in predicting extinction (e.g. as the burning leading- front of a triple flame becomes vanishingly small) and in the dependence of the propagation speed on heat loss, strain and the Lewis numbers. Quantitative discrepancies are discussed and are found to be mainly attributable to the infinite activation energy assumption used in the asymptotics.
This article presents the structure of laminar, one-dimensional, and steady-state flame propagation in uniformly premixed wood particles. In order to predict the effect of radiation and particle size on the pyrolysis of biomass particles, the flame structure is divided into three regions: a preheat vaporization zone where the rate of the gas-phase chemical reaction is small; a narrow reaction zone composed of three zones (gas, tar, and char combustion) where convection and the rate of vaporization of the fuel particles are small; and finally a convection zone where diffusive terms in the conservation equation are small. In this model, it is assumed that fuel particles vaporize first to yield a gaseous fuel of known chemical structure. The analysis is performed in the asymptotic limit, where the value of the characteristic Zeldovich number is large and the equivalence ratio is larger than unity (i.e. ϕu≥1). The overall investigation of this study leads to a novel non-linear burning velocity correlation. Consequently, the impacts of radiation and particle size as determining factors on the combustion properties of biomass particles are declared in this research.
An experimental investigation of turbulent counterflow nonpremixed flames has been undertaken in order to clarify the interaction between the properties of the nonpremixed flames and the characteristics of the turbulent counterflow field. In particular, to distinguish between the effects of turbulence caused by the air and fuel streams, the turbulent characteristics of each flow in an opposed jet flow were controlled individually. From the visualization by laser tomographic technique, it was found that the width of the diffusion region along the centerline regarded as a macroscopic parameter of the local structure of nonpremixed flames was not changed by the flow turbulence, and was determined by the mean flow condition characterized by the bulk velocity gradient, while whole diffusion regions spatially showed the typical wrinkled motion within the turbulent counterflowing stream. On the other hand, the mixture fraction fluctuations which were estimated by measurements of the behavior of the flame and the diffusion region, depended mainly on turbulence and were not affected by the bulk velocity gradient. The mean scalar dissipation rate χturb due to the turbulence, estimated by combining the turbulent strain rate of the air side stream and the rms of mixture fraction fluctuation, increased with an increase in the turbulent strain rate of the air side stream, that is, with a decrease in the turbulent Damköhler number, Da. However, it is known that in a counterflow field the strain caused by the mean flow is also effective for properties such as the transport phenomena. Then, the total scalar dissipation rate χtotal, which is derived from the turbulence and the mean flow velocity gradient, was suggested as the characteristic quantity of nonpremixed flames formed in counterflow geometry. The total scalar dissipation rate of flames at extinction showed almost constant value regardless of the initial turbulent conditions. The present results agree with the laminar flamelet concept.
The effect of the temperature difference between the gas and the particles on propagation of premixed flames in a combustible mixture containing volatile fuel particles uniformly distributed in an oxidizing gas mixture is analyzed in this paper. It is presumed that the fuel particles vaporize first to yield a gaseous fuel, which is oxidized in the gas phase. The analysis is performed in the asymptotic limit, where the value of the characteristic Zel’dovich number is large, which implies that the reaction term in the preheating zone is negligible. Required relations between the gas and the particles are derived from equations for premixed flames of organic dust. Subsequently, the governing equations are solved by an analytical method. Finally, the variation of the dimensionless temperatures of the gas and the particles, the mass fraction of the particles, the equivalence ratio ϕ
g
as a function of ϕ
u
, the flame temperature, and the burning velocities of the gas and the particles are obtained. The analysis shows that the calculated value of ϕ
g
is smaller than unity for certain cases, even though ϕ
u
⩾1.
The structure of premixed flames propagating in combustible systems, containing uniformly distributed volatile fuel particles, in an oxidizing gas mixture, is analyzed. It is presumed that the fuel particles vaporize first to yield a gaseous fuel of known chemical structure, which is subsequently oxidized in the gas phase. The analysis is performed in the asymptotic limit, where the value of the characteristic Zeldovich number, based on the gas-phase oxidation of the gaseous fuel is large, and for values of φu ≥ 1.0, where φu is the equivalence ratio based on the fuel available in the fuel particles. The structure of the flame is presumed to consist of a preheat vaporization zone where the rate of the gas-phase chemical reaction is small, a reaction zone where convection and the rate of vaporization of the fuel particles are small and a convection zone where diffusive terms in the conservation equations are small. For given values φu the analysis yields results for the burning velocity and φg, where φg is the effective equivalence ratio in the reaction zone. The analysis shows that even though φu ≥ 1.0, for certain cases the calculated value of φg is less than unity. This prediction is in agreement with experimental observations.
The laminar flamelet concept covers a regime in turbulent combustion where chemistry (as compared to transport processes) is fast such that it occurs in asymptotically thin layers—called flamelets—embedded within the turbulent flow field. This situation occurs in most practical combustion systems including reciprocating engines and gas turbine combustors. The inner structure of the flamelets is one-dimensional and time dependent. This is shown by an asymptotic expansion for the Damköhler number of the rate determining reaction which is assumed to be large. Other non-dimensional chemical parameters such as the nondimensional activation energy or Zeldovich number may also be large and may be related to the Damköhler number by a distinguished asymptoiic limit. Examples of the flamelet structure are presented using onestep model kinetics or a reduced four-step quasi-global mechanism for methane flames.For non-premixed combustion a formal coordinate transformation using the mixture fraction Z as independent variable leads to a universal description. The instantaneous scalar dissipation rate χ of the conserved scalar Z is identified to represent the diffusion time scale that is compared with the chemical time scale in the definition of the Damköhler number. Flame stretch increases the scalar dissipation rate in a turbulent flow field. If it exceeds a critical value χq the diffusion flamelet will extinguish. Considering the probability density distribution of χ, it is shown how local extinction reduces the number of burnable flamelets and thereby the mean reaction rate. Furthermore, local extinction events may interrupt the connection to burnable flamelets which are not yet reached by an ignition source and will therefore not be ignited. This phenomenon, described by percolation theory, is used to derive criteria for the stability of lifted flames. It is shown how values of ∋q obtained from laminar experiments scale with turbulent residence times to describe lift-off of turbulent jet diffusion flames. For non-premixed combustion it is concluded that the outer mixing field—by imposing the scalar dissipation rate—dominates the flamelet behaviour because the flamelet is attached to the surface of stoichiometric mixture. The flamelet response may be two-fold: burning or non-burning quasi-stationary states. This is the reason why classical turbulence models readily can be used in the flamelet regime of non-premixed combustion. The extent to which burnable yet non-burning flamelets and unsteady transition events contribute to the overall statistics in turbulent non-premixed flames needs still to be explored further.For premixed combustion the interaction between flamelets and the outer flow is much stronger because the flame front can propagate normal to itself. The chemical time scale and the thermal diffusivity determine the flame thickness and the flame velocity. The flamelet concept is valid if the flame thickness is smaller than the smallest length scale in the turbulent flow, the Kolmogorov scale. Also, if the turbulence intensity v′ is larger than the laminar flame velocity, there is a local interaction between the flame front and the turbulent flow which corrugates the front. A new length scale LG=vF3/∈, the Gibson scale, is introduced which describes the smaller size of the burnt gas pockets of the front. Here vF is the laminar flame velocity and ∈ the dissipation of turbulent kinetic energy in the oncoming flow. Eddies smaller than LG cannot corrugate the flame front due to their smaller circumferential velocity while larger eddies up to the macro length scale will only convect the front within the flow field.Flame stretch effects are the most efficient at the smallest scale LG. If stretch combined with differential diffusion of temperature and the deficient reactant, represented by a Lewis number different from unity, is imposed on the flamelet, its inner structure will respond leading to a change in flame velocity and in some cases to extinction. Transient effects of this response are much more important than for diffusion flamelets. A new mechanism of premixed flamelet extinction, based on the diffusion of radicals out of the reaction zone, is described by Rogg. Recent progress in the Bray-Moss-Libby formulation and the pdf-transport equation approach by Pope are presented. Finally, different approaches to predict the turbulent flame velocity including an argument based on the fractal dimension of the flame front are discussed.
In this research, a mathematical model is performed to analyze the structure of flame propagation through a two-phase mixture consisting of organic fuel particles and air. In contrast to previous analytical studies, thermal radiation effect is taken into consideration, which has not been attempted before. In order to simulate of the dust combustion phenomenon, it is assumed that the flame structure consists of four zones: preheat, vaporization, reaction and post flame (burned). Furthermore, radiative heat transfer equation is employed to describe the thermal radiation exchanged between these zones. The obtained results show that the induced thermal radiation from flame interface into the preheat and vaporization zones plays a significant role in the improvement of vaporization process and burning velocity of organic dust mixture, compared with the case in which the thermal radiation factor is neglected. According to present results, flame structure variables such as the burning velocity, mixture temperature, mass fraction of volatile fuel particles and gaseous fuel mass fraction strongly depend on radiative heat transfer. These predictions have reasonable agreement with published experimental data.
The structure of flame propagating through lycopodium dust clouds has been investigated experimentally. Upward propagating laminar flames in a vertical duct of 1800 mm height and 150×150 mm square cross-section are observed, and the leading flame front is also visualized using by a high-speed video camera. Although the dust concentration decreases slightly along the height of duct, the leading flame edge propagates upwards at a constant velocity. The maximum upward propagating velocity is 0.50 m/s at a dust concentration of 170 g/m3. Behind the upward propagating flame, some downward propagating flames are also observed. Despite the employment of nearly equal sized particles and its good dispersability and flowability, the reaction zone in lycopodium particles cloud shows the double flame structure in which isolated individual burning particles (0.5–1.0 mm in diameter) and the ball-shaped flames (2–4 mm in diameter; the combustion time of 4–6 ms) surrounding several particles are included. The ball-shaped flame appears as a faint flame in which several luminous spots are distributed, and then it turns into a luminous flame before disappearance. In order to distinguish these ball-shaped flames from others with some exceptions for merged flames, they are defined as independent flames in this study. The flame thickness in a lycopodium dust flame is observed to be 20 mm, about several orders of magnitude higher than that of a premixed gaseous flame. From the microscopic visualization, it was found that the flame front propagating through lycopodium particles is discontinuous and not smooth.
The purification of biomass-derived syngas via tar abatement by catalytic steam reforming has been investigated using benzene, toluene, naphthalene, anthracene and pyrene as surrogated molecules. The effects of temperature and steam-to-carbon ratio on conversion, and the tendency towards coke formation were explored for each model compound. Two commercial nickel-based catalysts, the UCI G90-C and the ICI 46-1, were evaluated. The five tar model compounds had very different reaction rates. Naphthalene was the most difficult compound to steam reform, with conversions from 0.008 gorg_conv/gcat min (790 °C) to 0.022 gorg_conv/gcat min (890 °C) at an S/C ratio of 4.2. The most reactive compound was benzene, with a conversion of 1.1 gorg_conv/gcat min at 780 °C and an S/C ratio of 4.3. The tendency towards coke formation grew as the molecular weight of the aromatic increased. The minimum S/C ratio for toluene was 2.5 at a catalyst temperature of 725 °C, and for pyrene at 790 °C ,it was 8.4. In general, catalyst temperatures and S/C ratios need to be higher than for naphtha in order to prevent the formation of coke on the catalyst.
Some results of measurements of laminar burning velocities and of maximum flame temperatures for combustible dust-air mixtures (starch dust-air mixtures, lycopodium-air mixtures and sulphur flour-air mixtures) are presented.Thin (25 and 50 μm) thermocouples have been used to measure maximum flame temperatures. The results are compared with those obtained with other devices such as resistors, pyrometers and are compared to the theoretical values. It appears that the observed discrepancies seem principally to come from the relatively poor efficiency of the burning processes inside the flame front than to heat losses by radiation as suggested before.Two methods for determining laminar burning velocities have been used: the classical ‘tube method’ and a ‘direct method’ based on the simultaneous determination of the flame speed and of the mixture velocity ahead of the flame front using a tomographic technique. Two different tube diameters are considered as well as additional results obtained with a small burner. The validity of these techniques is firstly assessed by comparing the results obtained with CH4-air mixtures and secondly by considering their relevancy for combustible dust-air mixtures (influence of the size of the apparatus). In particular, the influences of heat flame by radiation and of flame stretching are considered.
The structure of steady state diffusion flames is investigated by analyzing the mixing and chemical reaction of two opposed jets of fuel and oxidizer as a particular example. An Arrhenius one-step irreversible reaction has been considered in the realistic limit of large activation energies. The entire range of Damköhler numbers, or ratio of characteristic diffusion and chemical times, has been covered. When the resulting maximum temperature is plotted in terms of the Damköhler number (which is inversely proportional to the flow velocity) the characteristic S curve emerges from the analysis, with segments from the curve resulting from: 1.(a) A nearly frozen ignition regime where the temperature and concentrations deviations from its frozen flow values are small. The lower branch and bend of the S curve is covered by this regime.2.(b) A partial burning regime, where both reactants cross the reaction zone toward regions of frozen flow. This regime is unstable.3.(c) A premixed flame regime where only one of the reactants leaks through the reaction zone, which then separates a region of frozen flow from a region of near-equilibrium.4.(d) A near-equilibrium diffusion controlled regime, covering the upper branch of the S curve, with a thin reaction zone separating two regions of equilibrium flow.Analytical expressions are obtained, in particular, for the ignition and extinction conditions.
A great fraction of worldwide energy carriers and material products come from fossil fuel refinery. Because of the on-going price increase of fossil resources, their uncertain availability, and their environmental concerns, the feasibility of oil exploitation is predicted to decrease in the near future. Therefore, alternative solutions able to mitigate climate change and reduce the consumption of fossil fuels should be promoted. The replacement of oil with biomass as raw material for fuel and chemical production is an interesting option and is the driving force for the development of biorefinery complexes. In biorefinery, almost all the types of biomass feedstocks can be converted to different classes of biofuels and biochemicals through jointly applied conversion technologies. This paper provides a description of the emerging biorefinery concept, in comparison with the current oil refinery. The focus is on the state of the art in biofuel and biochemical production, as well as discussion of the most important biomass feedstocks, conversion technologies and final products. Through the integration of green chemistry into biorefineries, and the use of low environmental impact technologies, future sustainable production chains of biofuels and high value chemicals from biomass can be established. The aim of this bio-industry is to be competitive in the market and lead to the progressive replacement of oil refinery products.
The laminar flamelet concept views a turbulent diffusion flame as an ensemble of laminar diffusion flamelets. Work relevant to the flamelet concept is spread over various fields in the literature: laminar flame studies, asymptotic analysis, theory of turbulence and percolation theory. This review tries to gather and integrate this material in order to derive a self-consistent formulation. Under the assumption of equal diffusivities a coordinate-free formulation of the flamelet structure is given. This assumption is relaxed and flow dependent effects are considered. It is shown that the steady laminar counterflow diffusion flame exhibits a very similar scalar structure as unsteady distorted mixing layers in a turbulent flow field. Therefore the counterflow geometry is proposed to be the most representative steady flow field to study chemistry models and molecular transport effects in laminar flamelets. The conserved scalar model is interpreted as the most basic flamelet structure. Non-equilibrium calculations are reviewed.The coupling between non-equilibrium chemistry and turbulence is achieved by the statistical description of two parameters: the mixture fraction and the instantaneous scalar dissipation rate. The hypothesis of statistical independence of these two parameters is discussed. Calculation methods for the marginal distributions are reviewed. It is shown how local quenching of diffusion flamelets leads to a reduction of burnable flamelets. However, there are burnable flamelets in a turbulent flame which are not reached by an ignition source. This phenomenon is described by percolation theory. Complementary approaches related to local quenching effects and connectedness are combined to derive criteria for the stabilization of lifted flames and to blow out. Further applications of the flamelet concept are reviewed and work to be done is discussed.
The effects of radiation heat loss and variation in near-unity Lewis numbers on the structure and extinction of counterflow diffusion flame established near the stagnation plane of two opposed free streams of fuel and oxidizer are analyzed using the asymptotic method of large activation energy. Radiation heat loss from the reaction zone is accounted for using the optically thin assumption. The main concern of this study is the thermal effects of radiation heat loss and Lewis numbers on diffusion flame extinction, particularly at small stretch rates. The existence of two extinction limits, the radiation extinction limit at a small stretch rate and the conventional quenching limit at a large stretch rate, is theoretically reproduced. This simplified analysis is able to predict the existence of inflammable limits of counterflow diffusion flames in terms of the near-unity Lewis numbers and concentrations of the fuel and oxidizer streams. Crown copyright (C) 2000. Published by Elsevier Science Inc. The effects of radiation heat loss and variation in near-unity Lewis numbers on the structure and extinction of counterflow diffusion flame established near the stagnation plane of two opposed free streams of fuel and oxidizer are analyzed using the asymptotic method of large activation energy. Radiation heat loss from the reaction zone is accounted for using the optically thin assumption. The main concern of this study is the thermal effects of radiation heat loss and Lewis numbers on diffusion flame extinction, particularly at small stretch rates. The existence of two extinction limits, the radiation extinction limit at a small stretch rate and the conventional quenching limit at a large stretch rate, is theoretically reproduced. This simplified analysis is able to predict the existence of inflammable limits of counterflow diffusion flames in terms of the near-unity Lewis numbers and concentrations of the fuel and oxidizer streams. Yes
- M Bidabadi
M. Bidabadi et al.
Fuel Processing Technology 179 (2018) 184-196