Transient convective burning of a periodic fuel-droplet array

The transient convective burning of fuel-droplets interacting within 3-D infinite periodic arrays in a hot gas stream is numerically studied for the first time, with considerations of droplet regression, deceleration due to the drag of the droplets, internal liquid motion, variable properties, non-uniform liquid temperature, surface tension, and n-octane one-step oxidation kinetics. Depending upon the initial conditions and other constraints, a flame is established early as either a wake flame or an envelope flame. An initial envelope flame remains an envelope flame, and an initial wake flame has a tendency to develop from a wake flame to an envelope flame. The flame shows no strong tendency to modify significantly the standoff distance during the lifetime of the droplet. For an initial wake flame, the moment of wake-to-envelope transition is advanced as the initial droplet spacing (intermediate) is decreased, but tends to be postponed as the initial droplet spacing is further reduced. The burning rate at smaller initial droplet spacing or smaller initial Reynolds number might be greater for some period during the lifetime because of an earlier wake-to-envelope transition which elevates the average surface temperature. Lower ambient temperature yields a later wake-to-envelope transition time and smaller mass burning rate. At the lower ambient pressure with the same initial relative stream velocity, the average surface temperature is reduced, the wake-to-envelope transition is later, and the mass burning rate is smaller. Validation of our analysis is made by comparing with the results of an isolated droplet Wu and Sirignano [11].     

Identification of droplet burning modes in lean, partially prevaporized swirl-stabilized spray flames

Experimental and numerical investigations of single droplet burning modes in a lean, partially prevaporized swirl-stabilized spray flame are reported. In the experiment single droplet flames have been visualized by CH-PLIF and simultaneous recording of the Mie signal. Two single droplet burning modes were identified: the envelope flame is a spherical diffusion flame burning at near-stoichiometric conditions. The wake flame is a potentially lean, partially premixed flame located downstream of the droplet. The droplet burning mode is of practical relevance, since it has significant impact on NO formation due to incomplete prevaporization.The droplet burning mode is determined by the ratio of chemical and convective time scales. The convective time scale is related to the droplet slip velocity. The impact of turbulent gas phase velocity fluctuations on droplet mechanics and droplet burning is discussed, based on a previous numerical investigation. In the present study the droplet slip velocity was measured with the 3D Phase Doppler (3D-PD) technique. For the measured slip velocities and ambient conditions in the hot gas region of the spray flame, simulations of single droplet burning were performed utilizing detailed models for chemical reaction, diffusive transport and vaporization. An agreement between the droplet burning modes predicted by the simulation and the droplet burning modes observed in the experiments was found.     

Transient convective burning of interactive fuel droplets in single-layer arrays

The transient convective burning of n-octane droplets interacting within single-layer arrays in a hot gas flow perpendicular to the layer is studied numerically, with considerations of droplet surface regression, deceleration due to the drag of the droplets, internal liquid motion, variable properties, non-uniform liquid temperature and surface tension. Infinite periodic arrays, semi-infinite periodic arrays with one row of droplets (linear array) or two rows of droplets, and finite arrays with nine droplets with centers in a plane are investigated. All arrays are aligned orthogonal to the free stream direction. This paper compares the behavior of semi-infinite periodic arrays and finite arrays with the behavior of previously studied infinite periodic arrays. Furthermore, it identifies the critical values of the initial Damköhler number for bifurcations in flame behavior at various initial droplet spacing for all these arrays. The initial flame shape is either an envelope flame or a wake flame as determined by the initial Damköhler number, the array configuration and the initial droplet spacing. The critical initial Damköhler number separating initial wake flames from initial envelope flames decreases with increasing interaction amongst droplets at intermediate droplet spacing (when the number of rows in the array increases or the initial droplet spacing decreases for a specific number of rows in the array). In the transient process, an initial wake flame has a tendency to develop from a wake flame to an envelope flame, with the moment of wake-to-envelope transition advanced for the increasing interaction amongst droplets at intermediate droplet spacing. For the array with nine droplets with centers in a plane, the droplets at different types of positions have different critical initial Damköhler number and different wake-to-envelope transition time for initial wake flame.     

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.     

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.     

Modes of particle combustion in iron dust flames

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

Investigation of droplet combustion in strained counterflow diffusion flames using planar laser-induced fluorescence

Experimental investigation of an isolated droplet burning in a convective flow is reported. Acetone droplets were injected in a steady laminar diffusion counterflow flame operating with methane. Planar laser-induced fluorescence measurements applied to OH radical and acetone was used to measure the spatial distribution of fuel vapour and the structure of the flame front around the droplet. High-magnification optics was used in order to image flow areas with a ratio of 1:1.2. The different combustion regimes of an isolated droplet could be observed from the configuration of the envelope flame to that of the boundary-layer flame, and occurrence of these regimes was found to depend on the droplet Reynolds number. Experimental results were compared with 1D numerical simulations using detailed chemistry for the configuration of the envelope flame. Good agreement was obtained for the radial profile of both OH radical and fuel vapour. Influence of droplet dynamics on the counterflow flame front was also investigated. Results show that the flame front could be strongly distorted by the droplet crossing. In particular, droplets with high velocity led to local extinction of the flame front whereas droplets with low velocity could ignite within the flame front and burn on the oxidiser side. PACS 33.50.-j; 42.62.-b; 47.55.D-; 47.70.Pq; 47.80.Jk     

A semi-analytical,multizone model of droplet combustion with varying properties

A multizone droplet burn model is developed to account for changes in the thermal and transport properties as a function of droplet radius. The formulation is semi-analytical – allowing for accurate and computationally efficient estimates of flame structure and burn rates. Zonal thermal and transport properties are computed using the Cantera software, pre-tabulated for rapid evaluation during run-time. Model predictions are compared to experimental measurements of burning n-heptane, ethanol and methanol droplets. An adaptive zone refinement algorithm is developed that minimizes the number of zones required to provide accurate estimates of burn time without excess zones. A sensitivity study of burn rate and flame stand-off with far-field oxygen concentration is conducted with comparisons to experimental data. Overall agreement to data is encouraging with errors typically less than 20% for predictions of burn rates, stand-off ratio and flame temperature for the fuels considered.     

High-speed visualization of flame propagation in explosions

Flow visualization data is presented to describe the structure of flames propagating in methane-air explosions in semi-confined enclosures. The role of turbulence is well established as a mechanism for increasing burning velocity by fragmenting the flame front and increasing the surface area of flames propagating in explosions. This area increase enhances the burning rate and increases the resultant explosion overpressure. In real situations, such as those found in complex process plant areas offshore, the acceleration of a flame front results from a complex interaction between the moving flame front and the local blockage caused by presence of equipment. It is clear that any localised increase in flame burn rate and overpressure would have important implications for any adjacent plant and equipment and may lead to an escalation process internal to the overall event. To obtain the information required to quantify the role of obstacles, it is necessary to apply a range of sophisticated laser-based, optical diagnostic techniques. This paper describes the application of high-speed, laser-sheet flow visualization and digital imaging to record the temporal development of the flame structure in explosions. Data is presented to describe the interaction of the propagating flame with a range of obstacles for both homogeneous and stratified mixtures. The presented image sequences show the importance of turbulent flow structures in the wake of obstacles for controlling the mixing of a stratified concentration field and the subsequent flame propagation through the wake. The data quantifies the flame speed, shape and area for a range of obstacle shapes.     

Electrostatic charging of a conducting sphere using another conducting sphere at constant voltage

This study uses results of classical electrostatics to predict the resulting voltage of a conducting sphere that is charged by a second sphere maintained at a constant voltage. The voltage of the charged sphere is calculated as a function of the charging voltage and the size ratio of the two spheres. Theoretical predictions are verified experimentally using conducting spheres of various sizes and a Faraday ice pail apparatus. The results may seem somewhat counterintuitive. For example, a charging sphere held at 100 V will charge another sphere of equal size only to 69.3 V by contact.     

Force-coupling method for particulate two-phase flow: Stokes flow

In this paper we describe a force-coupling method for particle dynamics in fluid flows. The general principles of the model are described and it is tested on three different Stokes flow problems; a single isolated sphere, a pair of otherwise isolated spheres, and a single sphere in a channel. For sphere to sphere or sphere to wall distances larger than 1/4 of the sphere radius the force-coupling results compared well with analytical and accurate numerical values. For smaller distances the results agree qualitatively, but lubrication effects are not included and this leads to a quantitative discrepancy.     

Aggregation of particles settling in shear-thinning fluids

We have experimentally studied the coaxial settling of three identical non-Brownian spheres in a shear-thinning fluid at small Reynolds numbers. While settling, the particles create corridors of reduced viscosity in their wake and, if they are initially close enough to one another, they can form stable clusters. By analogy with previous results obtained on two-particle interaction in the first part of this work, we show that the particle velocities can be satisfactorily described using a first-order expression and assuming that the reduced viscosity remains constant. We report systematic experiments performed at different initial separation distances between particles and the use of our simple model allows the prediction of the settling behaviour and in particular the conditions for clusters formation. We thus show that particle aggregation can occur even for large initial distances between particles and within times that are small compared to the time scales in Newtonian fluids.     

Using level sets for creating virtual random packs of non-spherical convex shapes

Random packs of spheres have been used to model heterogeneous and porous material morphologies during simulations of physical processes such as burning of coal char, convective burning in porous explosives, and regression of solid rocket propellant. Sphere packs have also been used to predict thermo-mechanical properties, permeability, packing density, and dissolution characteristics of various materials. In this work, we have extended the Lubachevsky–Stillinger (LS) sphere packing algorithm to create polydisperse packs of non-spherical shapes for modeling heterogeneity in complex energetic materials such as HMX and pressed gun propellants. In the method, we represent the various particle shapes using level sets. The LS framework requires estimates of inter-particle collision times, and we predict these times by numerically solving a minimization problem. We have obtained results for dense random packs of various convex shapes such as cylinders, spherocylinders, and polyhedra, and we show results with these various particles packed together in a single pack to high packing fraction.     

约束条件下胶球间排空力的研究

用MonteCarlo方法对处于两平行硬板约束下三个浓度的大小胶球系统进行了模拟,通过对大胶球表面小胶球密度的统计,由密度积分公式获得了大胶球所受的排空力.研究结果显示,因为平行硬板的存在或当改变两平行硬板的距离时,同浓度下,排空力在硬板距离小的时候最明显;三个浓度中,浓度高的,排空力受硬板距离影响最大;有硬板约束比无该约束的时候,排空力效果更显著.     

Interactions of flame balls

Flame ball interactions are numerically investigated in a reaction–diffusion system characterized by single-step Arrhenius kinetics and radiative heat losses. It is found that the interactions of two neighbouring flame balls are characterized by two distinct regimes – a repulsion regime and an attraction regime, depending upon the separation distance. The two regimes join at a critical separation distance, which corresponds to an unstable equilibrium state. For supercritical separation distances, the two flame balls repel and drift apart from each other; whereas for sub-critical separation distances, they move towards each other and eventually merge into a single stationary flame ball. In this connection, flame ball interactions are found to exhibit a qualitatively reverse character in comparison with the well-known van der Waals curve which characterizes intermolecular forces.     

Autoignition and early flame behavior of a spherical cluster of 49 monodispersed droplets

Autoignition and early flame behavior of a spherical cluster of 49 monodispersed droplets in a high-temperature air were examined in microgravity. The monodispersed suspended-droplet cluster (MSDC) model with which both droplet spacing and initial droplet diameter were well-controlled was developed, and the solidified-fuel fiber-suspension technique was utilized for making the MSDC model. The tested 3D MSDC models had the HCP (hexagonal closest packing) structure. Individual flames, which enveloped each droplet, or group flame, which enveloped the whole droplet cluster, were formed immediately after ignition. The flame changed from the group flame to a cluster of the individual flames either with increasing the droplet spacing or decreasing the initial droplet diameter. Each of the individual flames merged into the group flame with the lapse of time. Burning sphere diameter decreased at the beginning, and then increased. The transition from the individual flames to the group flame occurred around the time period at which the burning sphere diameter reached its minimum. The time period at which the burning sphere diameter reached its maximum was delayed and the expansion rate of the burning sphere was enhanced with decreasing the droplet spacing or with increasing the initial droplet diameter.     

Aggregation of particles settling in shear-thinning fluids

We have experimentally studied the coaxial settling of three identical non-Brownian spheres in a shear-thinning fluid at small Reynolds numbers. While settling, the particles create corridors of reduced viscosity in their wake and, if they are initially close enough to one another, they can form stable clusters. By analogy with previous results obtained on two-particle interaction in the first part of this work, we show that the particle velocities can be satisfactorily described using a first-order expression and assuming that the reduced viscosity remains constant. We report systematic experiments performed at different initial separation distances between particles and the use of our simple model allows the prediction of the settling behaviour and in particular the conditions for clusters formation. We thus show that particle aggregation can occur even for large initial distances between particles and within times that are small compared to the time scales in Newtonian fluids. Received 10 July 2002 RID="a" ID="a"e-mail: talini@fast.u-psud.fr     

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.     

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.