On the behaviour of organic gel multi-size spray diffusion flames

The recently reported, experimentally observed, unusual behaviour of organic gellant-based fuel droplets which, under appropriate ambient thermal conditions, evaporate and burn in an oscillatory fashion is incorporated in a phenomenological manner in a model of a two-dimensional arbitrary multi-size spray diffusion flame. Non-unity Lewis numbers are permitted for the fuel vapour and oxidant. A combined analytical/numerical solution of the governing equations is presented and used to investigate how a spray's initial polydispersity and the frequency of oscillatory evaporation influence the combustion field. It is demonstrated that the initial droplet size distribution and the frequency of evaporation of the burning gel droplets can have an acute impact both on the homogeneous diffusion flame shape, height and width and on the thermal field downstream of the flame front. Hot spots of individual (or clusters of) burning droplets can be created and under certain operating conditions can lead to hotter temperatures than experienced in the main homogeneous flame. The intensity of these hotspots, their number and location are sensitive to spray related parameters. In realistic combustion chambers there is a danger inherent in the existence of hotspots in undesirable regions as they can damage the structural integrity. Other computed results demonstrate that, in relation to the spray diffusion flames obtained using an equivalent purely liquid fuel spray, the use of a gel fuel spray can lead, under certain operating conditions, to a reduction in flame height and temperature. The latter effect is critical when considering flame extinction.     

Laminar flame propagation on a horizontal fuel surface: Verification of classical Emmons solution

This work analyses the classical Emmons (1956) solution of flat plate laminar flame combustion on a film of liquid fuel. A two-dimensional (2D) numerical model developed for this purpose has been benchmarked with experimental results available in the literature for methanol. In the parametric study, numerical predictions have been compared with Emmons classical solution. The study shows that the Emmons solution is valid in a range of Reynolds numbers where flame anchors near the leading edge of the methanol pool and the combustion zone is confined around the hydrodynamic and thermal boundary layers. However, in cases of low free stream velocities the combustion zone is beyond the boundary layer zone and the Emmons solution deviates. In cases of very high free stream velocities, the flame moves away from the leading edge and anchors at a location downstream. The Emmons solution is not applicable in this case as well. For the fuel considered in this study (methanol), accounting for thermal radiation, employing an optically thin radiation model, allows better agreement between experimental and numerical temperature profiles but does not affect the mass burning rates.     

Modeling of Generation and Growth of Non-Spherical Nanoparticles in a Co-Flow Flame

Generation and growth of polydisperse non-spherical silica nanoparticles in an oxy-hydrogen co-flow diffusion flame have been simulated for the first time. A complete set of Navier–Stokes equations describing multi-component chemically-reacting fluid flows was first solved considering the detailed H₂/O₂ chemistry and oxidation/hydrolysis reactions of SiCl₄. A recently developed aggregate sectional model (Jeong & Choi, 2001) was employed to solve the dynamics of particles undergoing generation, convection, diffusion, coagulation and coalescence in a spatially two-dimensional flame system. Non-uniform spatial distributions of flame temperatures and non-spherical particle sizes were successfully simulated. Comparison on flame temperature and particle size between the numerical simulation and the experimental data has also been done. Performance of a simple monodisperse model was also studied by comparing with the detailed sectional model.     

Numerical study of methanol flames in laminar forced convective environment using short chemical kinetics mechanism

Due to recent interest in methanol economy, it is seen that a numerical study of combustion of methanol in a comprehensive manner is necessary. Motivated from this interest and based on the studies from literature, a numerical study on prediction of structures of non-premixed methanol-air flames in laminar forced convective environment is reported. Two-dimensional, planar and axisymmetric, computational domains are considered. Corresponding governing equations for conservation of mass, momentum, species and energy have been solved using Ansys FLUENT. The numerical model incorporates multi-component diffusion, variable thermal and physical properties, a short chemical kinetics mechanism with 18 species and 38 elementary reactions, and a non-luminous thermal radiation model. Homogeneous flames in opposed flow and heterogeneous flames in cross-flow and co-flow configurations are studied. For heterogeneous flames, interface conditions at the liquid methanol surface are defined systematically using a user-defined function. Numerical results are validated against the experimental results available in literature. Results in terms of mass burning rates, flow, species and temperature fields have been presented to describe the flame characteristics.     

A numerical study of quasi-steady burning characteristics of a condensed fuel: effect of angular orientation of fuel surface

A numerical study of laminar diffusion flames established over a condensed fuel surface, inclined at several angular orientations in the range of –90°?θ?+90° with respect to the vertical axis, under atmospheric pressure and normal gravity environment, is presented. Methanol is employed as the fuel. A numerical model, which solves transient gas-phase, two-dimensional governing conservation equations, with a single-step global reaction for methanol–air oxidation and an optically thin radiation sub-model, has been employed in the present investigation. Numerical results have been validated against the experimental data from the present study. Thereafter, the model is used to investigate the influence of angular orientation of fuel surface on its quasi-steady burning characteristics. Results in terms of fuel mass burning rate, flame stand-off distances, temperature field, velocity profiles and oxygen contours have been presented and discussed in detail. It is observed that orientation angles in the range of –45°?θ? –30° (fuel surface facing upwards), yield the maximum mass burning rates. The flame anchoring location near the leading edge of the fuel surface, normal gradient of fuel vapor mass fraction at the surface and oxygen contours have been used to explore this unique behavior. Based on the numerical results, a theoretical correlation to predict the mass burning rate as a function of fuel surface orientation is also proposed. Furthermore, a discussion on the differences in the structure of laminar diffusion flame established over fuel surface as a function of its angular orientation is included.     

Propagation and extinction of an unsteady spherical spray flame front

Experimental evidence seems to indicate that the life of a laminar spherical flame front propagating through a fresh mixture of air and liquid fuel droplets can be roughly split into three stages: (1) ignition, (2) radial propagation with a smooth flame front and (3) propagation with flame front cellularization and/or pulsation. In this work, the second stage is analysed using the slowly varying flame approach, for a fuel rich flame. The droplets are presumed to vaporize in a sharp front ahead of the reaction front. Evolution equations for the flame and evaporation fronts are derived. For the former the combined effect of heat loss due to droplet vaporization and radiation plays a dominant explicit role. In addition, the structure of the evaporation front is deduced using asymptotics based on a large parameter associated with spray vaporization. Numerical calculations based on the analysis point to the way in which the spray modifies conditions for flame front extinction. Within the framework of the present simplified model the main relevant parameters turn out to be the initial liquid fuel load in the fresh mixture and/or the latent heat of vaporization of the fuel.     

A spray flamelet/progress variable approach combined with a transported joint PDF model for turbulent spray flames

A spray flamelet/progress variable approach is developed for use in spray combustion with partly pre-vaporised liquid fuel, where a laminar spray flamelet library accounts for evaporation within the laminar flame structures. For this purpose, the standard spray flamelet formulation for pure evaporating liquid fuel and oxidiser is extended by a chemical reaction progress variable in both the turbulent spray flame model and the laminar spray flame structures, in order to account for the effect of pre-vaporised liquid fuel for instance through use of a pilot flame. This new approach is combined with a transported joint probability density function (PDF) method for the simulation of a turbulent piloted ethanol/air spray flame, and the extension requires the formulation of a joint three-variate PDF depending on the gas phase mixture fraction, the chemical reaction progress variable, and gas enthalpy. The molecular mixing is modelled with the extended interaction-by-exchange-with-the-mean (IEM) model, where source terms account for spray evaporation and heat exchange due to evaporation as well as the chemical reaction rate for the chemical reaction progress variable. This is the first formulation using a spray flamelet model considering both evaporation and partly pre-vaporised liquid fuel within the laminar spray flamelets. Results with this new formulation show good agreement with the experimental data provided by A.R. Masri, Sydney, Australia. The analysis of the Lagrangian statistics of the gas temperature and the OH mass fraction indicates that partially premixed combustion prevails near the nozzle exit of the spray, whereas further downstream, the non-premixed flame is promoted towards the inner rich-side of the spray jet since the pilot flame heats up the premixed inner spray zone. In summary, the simulation with the new formulation considering the reaction progress variable shows good performance, greatly improving the standard formulation, and it provides new insight into the local structure of this complex spray flame.     

Diffusion-controlled combustion of gases in the absence of forced convection

A mathematical model for the laminar diffusion combustion of gases in the absence of forced convection is developed. This combustion mode is realized near an orifice in the partition that separates the fuel and oxidizer. The stationary solution, size and shape of the flame, temperature distribution, and profiles of the concentrations of fuel, oxidizer, and combustion products are determined. It is shown that the limiting diameter of the diffusion flame is inversely proportional to the burning rate of an equivalent premixed mixture of the same fuel and oxidizer, whereas the flame length is proportional to the diameter of the orifice. The unsteady (quasi-stationary) solution to this problem for the case where the gas is confined in a finite-volume vessel is obtained. The time it takes the flame to approach the partition of the vessel with fuel and enter inside is determined. Experiments on studying the diffusion combustion of natural gas in the absence of forced convection near orifices and a slit in the partition separating the gaseous fuel and oxidizer in a finite-volume vessel are performed. The time of combustion is obtained. Depending on the orifice diameter and slit width, three modes of diffusion combustion were identified: combustion above the partition ending in flame extinction, combustion with a breakthrough, and combustion inside the vessel (submerged flame).     

Heat and Mass Transfer Investigation of Hydrocarbon Droplet Evaporation under Rotatory Movement

The steady-state boundary layer equations of a rotatory movement around hydrocarbon droplets saturated of pure fuel are numerically solved. The transfer equations are schemed by using an implicit finite difference method. The system of algebraic equations is solved by using the Thomas algorithm. The model is compared to the Kreith one and a good agreement is observed between both models. The dimensionless physical parameters of the evaporation phenomena of droplet in rotation are calculated and presented under the effect of the convection.     

Coupling of Evaporation and Thermocapillary Convection in a Liquid Layer with Mass and Heat Exchanging Interface

We propose and analyse a new model of thermocapillary convection with evaporation in a cavity subjected to horizontal temperature gradient, rather than the previously studied model without evaporation. The pure liquid layer with a top free surface in contact with its own vapour is considered in microgravity condition. The computing programme developed for simulating this model integrates the two-dimensional, time-dependent Navier-Stokes equations and energy equation by a second-order accurate projection method. We focus on the coupling of evaporation and thermocapillary convection by investigating the influence of evaporation Biot number and Marangoni number on the interracial mass and heat transfer. Three different regimes of the coupling mechanisms are found and explained from our numerical results.     

Influence of pressure on near nozzle flow field and soot formation in laminar co-flow diffusion flames

Soot formation from combustion devices, which tend to operate at high pressure, is a health and environmental concern, thus investigating the effect of pressure on soot formation is important. While most fundamental studies have utilised the co-flow laminar diffusion flame configuration to study the effect of pressure on soot, there is a lack of investigations into the effect of pressure on the flow field of diffusion flames and the resultant influence on soot formation. A recent work has displayed that recirculation zones can form along the centreline of atmospheric pressure diffusion flames. This present work seeks to investigate whether these zones can form due to higher pressure as well, which has never been explored experimentally or numerically. The CoFlame code, which models co-flow laminar, sooting, diffusion flames, is validated for the prediction of recirculation zones using experimental flow field data for a set of atmospheric pressure flames. The code is subsequently utilised to model ethane-air diffusion flames from 2 to 33 atm. Above 10 atm, recirculation zones are predicted to form. The reason for the formation of the zones is determined to be due to increasing shear between the air and fuel steams, with the air stream having higher velocities in the vicinity of the fuel tube tip than the fuel stream. This increase in shear is shown to be the cause of the recirculation zones formed in previously investigated atmospheric flames as well. Finally, the recirculation zone is determined as a probable cause of the experimentally observed formation of a large mass of soot covering the entire fuel tube exit for an ethane diffusion flame at 36.5 atm. Previously, no adequate explanation for the formation of the large mass of soot existed.     

Simulation of the ignition of a liquid fuel with a hot particle

A two-dimensional gas-phase model of ignition of a flammable liquid by a single particle heated to a high temperature with consideration given to heat conduction, evaporation, diffusion, and convection of fuel vapor in an oxidizer medium was developed. Numerical simulations made it possible to determine the dependences of the ignition delay time for the liquid on the size and initial temperature of the particle. The minimum size and initial temperature of the particle at which ignition still occurs were estimated.     

Tabulated chemistry approaches for laminar flames: Evaluation of flame-prolongation of ILDM and flamelet methods

The present study considers the performance of tabulation methods for numerical simulation of complex chemical kinetics in laminar combusting flows and compares their predictions to results obtained by direct calculation. Two tabulation methods are considered: the Flame Prolongation of Intrinsic low-dimensional manifold (FPI) method and Steady Laminar Flamelet Model (SLFM). The FPI method is of current interest as it is a potentially unifying approach capable of dealing with both premixed and non-premixed flames for gaseous fuels. SLFM tabulation methods are popular for non-premixed flames and form a good basis for comparing the performance of the FPI approach. The performance of each method is also evaluated by comparing the results to the direct simulation of the laminar flames using two chemical kinetic schemes: simplified chemistry involving five species and one reaction and detailed chemistry involving 53 species and 325 reaction steps. As part of the evaluation process, the computational cost of each method is also assessed. The laminar flames considered in this study include: freely propagating laminar premixed flames, a two-dimensional axisymmetric methane–air opposed-jet diffusion flame, and a two-dimensional axisymmetric methane–air co-flow diffusion flame. Both tabulation methods are implemented in a parallel adaptive mesh refinement (AMR) framework for solving the complete set of governing partial differential equations. These equations are solved using a fully-coupled finite-volume formulation on body-fitted multi-block quadrilateral mesh. Significant improvements in terms of reduced computational requirements, as measured by both storage and processing time, are demonstrated for the tabulated methods.     

Topological transitions of Jet A-1 lean azimuthal flames (LEAF)

Prior studies about liquid fuel combustion in a vitiated air environment have shown increased combustion efficiency with reduced NOx, CO, and soot emissions. The concept of lean azimuthal flame (LEAF), which can be associated to the latter combustion mode, is based on opposed injections of air and liquid fuel sprays in an axisymmetric chamber with a central outlet, which can result in a highly turbulent toroidal reaction zone. The mixture of fresh air and hot combustion products of each spray provides a vitiated cross-flow configuration to the next spray distributed along the chamber circumference, leading to ignition and sequential combustion of the sprays by the others. The present paper deals with a LEAF combustor with air-assisted spray atomization, which has not been investigated so far. The combustor is fueled with Jet A-1 and operated from 15 to 25 kW with variations in the atomization-air to liquid mass flow ratio (ALR). This study focuses on the flame topology transitions as a function of atomizer ALR. Experimental results based on flame chemiluminescence and OH planar laser-induced fluorescence show two flame topologies: tubular and LEAF topology for ratio of 2 and 4, respectively (denoted ALR2 and ALR4). The spray Mie scattering indicates a significant presence of unburnt droplets for ALR2, whereas quick evaporation is observed for ALR4 cases. In this paper, we propose and validate a basic model based on the spray droplet size distribution, and the evaporation and convection timescales, which are the prominent factors governing the flame topology. Indeed, for ALR2, the evaporation timescale is longer than the convective timescale, which causes incomplete spray evaporation and insufficient vitiated environment, leading to a tubular flame topology and preventing a LEAF to develop. In contrast, for ALR4, the spray evaporation timescales are smaller than the convective timescales, which aids the LEAF topology.     

湿空气扩散燃烧火焰结构特性研究

利用二维粒子成像速度仪(PIV)对钝体燃烧器中的甲烷／湿空气扩散燃烧的速度场进行测量,考察其火焰的结构特性及其内部流动状况。通过对湿空气燃烧流场与普通燃烧流场的对比分析表明,湿空气燃烧情况下,两种燃烧状态的火焰(回流燃烧火焰和中心射流主导火焰)相互转换的燃空速度比(γ)值要比普通燃烧的小;湿空气燃烧使得喷嘴后的同流空气的速度降低,空气的回流作用减弱,燃料更容易冲出回流区,火焰的稳定性能变差。     

The burning rate’s effect on the flame length of weak fire whirls

This paper discusses why the visibly-determined flame length of a weak fire whirl increases as compared with the corresponding pool fire without spin. Here, a fire whirl is called weak when the pure aerodynamic effect of flow circulation has a negligible influence on the flame length. Split cylinders were used to apply a flow circulation to a 3-cm-diameter methane burner flame and a 3-cm-diameter ethanol pool fire. After applying the flow circulation, the flame length of the ethanol pool fire increased about three times, while little change was observed in the flame length of the methane burner flame. The difference is explained by the fact that the burning rate of the methane burner flame was fixed constant, whereas that of the ethanol pool fire increased due to the increased heat input to the fuel surface caused by a change in flame shape pushed toward the fuel surface. The experimental observations thus demonstrate that the burning-rate effect can significantly increase the flame length even under a weak circulation condition. Computational fluid dynamics (CFD) simulations were conducted to understand the detailed flow structure of a fire whirl. An analytical model was then developed based on the experimental observations and CFD calculations; the predicted relationship between the flame height and the burning rate agreed with experimental data.     

Effects of pressure on flame propagation in a premixture containing fine fuel droplets

Combustion experiments on fuel droplet–vapor–air mixtures have been performed with a rapid expansion apparatus which generates monodispersed droplet clouds with narrow diameter distribution using the condensation method. The effects of fine fuel droplets on flame propagation were investigated for ethanol droplet–vapor–air mixtures at various pressures from 0.2 to 1.0 MPa. A stagnant fuel droplet–vapor–air mixture, generated in a rapid expansion chamber, was ignited at the center of the chamber using an ignition wire. Spherical flame propagation under constant-pressure conditions was observed with a high-speed video camera and flame speed was measured. Total equivalence ratio, and the ratio of liquid fuel mass to total fuel mass, was varied from 0.6 to 1.4 and from zero to 56%, respectively. The mean droplet diameter of fuel droplet–vapor–air mixtures was set at 8.5 and 11 μm. It was found that the flame speed of droplet–vapor–air mixtures less than 0.9 in the total equivalence ratio exceeds that of premixed gases of the same total equivalence ratio at all pressures. The flame speed of fuel droplet–vapor–air mixtures decreases as the pressure increases in all total equivalence ratios. At large ratios of liquid fuel mass to total fuel mass, the normalized flame speed (the flame speed of droplet–vapor–air mixtures divided by the flame speed of the premixed gas with the same total equivalence ratio), increases with the increase in pressure for fuel-lean mixtures, and it decreases for fuel-rich mixtures. The outcome is reversed at small ratios of liquid fuel mass to total fuel mass; the normalized flame speed decreases with the increase in pressure for fuel-lean mixtures, and increases for fuel-rich mixtures. The results suggest that the increase in pressure promotes droplet evaporation in the preheat zone.     

Investigation of interaction between methanol fed tandem porous spheres burning in a mixed convective environment

Flame interaction during the burning of two porous spheres in tandem arrangement fed with methanol and subjected to a mixed convective environment, has been studied experimentally and numerically. Porous sphere technique is employed for experimentally simulating the burning characteristics of methanol transpired spheres of different sizes, separated by fixed distances. The mass burning rates from both the spheres and visible flame stand-off distances from the sphere surfaces have been measured in the experiments. In the numerical simulations, transient, axisymmetric, mass, momentum, species and energy conservation equations are solved using a finite volume technique based on non-orthogonal semi-collocated grids. Features of the numerical model include finite rate chemistry and temperature and mixture composition dependent thermo-physical properties. Burning of tandem porous spheres in an air stream flowing vertically upwards, at atmospheric pressure has been simulated for different sphere sizes, separation distances and free stream velocities. The numerical predictions have been compared with experimental results. Results reveal that when two spheres burn one over the other, the transition from envelope to wake flame is delayed when compared with that of an isolated sphere. For two spheres of same diameter burning one over the other, depending on the separation distance, flame blows-off after the occurrence of transition from envelope to wake flame in the bottom sphere. For the case of larger sphere at the top, either the flame stabilises in the recirculation zone formed in between the spheres or the flame from the smaller sphere lifts off and stabilises near the front portion of the larger sphere, depending on the separation distance. At higher separation distances, around four times the diameter of the sphere, both the spheres burn independently. The burning rate undergoes complex variations with air stream velocity depending on the sphere sizes and separation distances.     

3D problem of heat and mass transfer at the ignition of a combustible liquid by a heated metal particle

A nonlinear nonstationary 3D problem of heat and mass transfer at gas phase ignition of a combustible liquid spread on the surface of a solid body by a metal particle heated to a high temperature is solved. This is done within the framework of a model taking into account the heat conduction and evaporation of the liquid, the diffusion and convection of the combustible vapors in the oxidizer medium, the crystallization of the ignition source, the kinetics of the processes of evaporation and ignition of liquids, the dependence of the thermophysical characteristics of the interacting substances on the temperature, and the moisture content of the oxidizer—air. The dependences of the ignition delay time of the liquid on the temperature and sizes of the heating source are established. Limiting values of the temperature and particle sizes at which the ignition conditions take place are determined. The influence of the air humidity on the inertia of the process being investigated is analyzed. A comparison of numerical values of typical parameters of the process under investigation for 2D and 3D models is performed.     

Investigation of the transition from lightly sooting towards heavily sooting co-flow ethylene diffusion flames

Laminar, sooting, ethylene-fuelled, co-flow diffusion flames at atmospheric pressure have been studied experimentally and theoretically as a function of fuel dilution by inert nitrogen. The flames have been investigated experimentally using a combination of laser diagnostics and thermocouple-gas sampling probe measurements. Numerical simulations have been based on a fully coupled solution of the flow conservation equations, gas-phase species conservation equations with complex chemistry and the dynamical equations for soot spheroid growth. Predicted flame heights, temperatures and the important soot growth species, acetylene, are in good agreement with experiment. Benzene simulations are less satisfactory and are significantly under-predicted at low dilution levels of ethylene. As ethylene dilution is decreased and soot levels increase, the experimental maximum in soot moves from the flame centreline toward the wings of the flame. Simulations of the soot field show similar trends with decreasing dilution of the fuel and predicted peak soot levels are in reasonable agreement with the data. Computations are also presented for modifications to the model that include: (i) use of a more comprehensive chemical kinetics model; (ii) a revised inception model; (iii) a maximum size limit to the primary particle size; and (iv) estimates of radiative optical thickness corrections to computed flame temperatures.