A spherical cell model for multi‐component droplet combustion in a dilute spray

A model for sphericosymmetric thin‐flame combustion of a multi‐component fuel droplet in a dilute spray has been developed using a unit cell approach. The gas‐phase transport has been modelled as convective–diffusive while the liquid‐phase processes as transient–diffusive. Convective heat and mass transfer condition has been used at the cell surface. The results indicate that evaporation and combustion characteristics of the droplet are strongly affected by the variation of both ambient conditions and convective transfer coefficients. Using the model, the effects of droplet spacing in spray, ambient oxidizer concentration, ambient temperature and pressure have been considered. Droplet life increases with decrease in droplet spacing, ambient temperature and ambient oxidizer concentration. However, droplet life has a weak dependence on ambient pressure. Copyright © 2001 John Wiley & Sons, Ltd.     

A parametric study of burning of multicomponent droplets in a dilute spray

The model for sphericosymmetric thin‐flame combustion of a multicomponent fuel droplet in a dilute spray using a unit cell approach, developed in the companion paper, has been used for studying the interaction effect between droplets. The effects of droplet spacing, ambient oxidizer concentration, ambient temperature and pressure have been considered. Droplet life increases with decrease in droplet spacing, ambient temperature and ambient oxidizer concentration. However, droplet life has a weak dependence on ambient pressure. Copyright © 2001 John Wiley & Sons, Ltd.     

Flame propagation through an air-fuel spray mixture with transient droplet vaporization

Flame initiation and propagation through an air/fuel vapor/fuel drop system is numerically modeled in a cylindrical one-dimensional closed combustor. An unsteady formulation of the flow problem eliminates the cold-boundary difficulty and gas-phase ignition problem. A velocity lag between the gas and the liquid phase is allowed and unsteady heat transfer to the droplets is taken into account. The surface temperature of the droplet is evaluated by using an unsteady spherically symmetric formulation of the droplet heat conduction problem with no internal motion and with a time-varying heat flux specified at the surface as a boundary condition. Results have been obtained for two commercially important fuels, namely, n-octane and n-decane. The activation energy and the preexponential factor in the Arrhenius-type expression for chemical rate, along with initial temperature, initial droplet size, stoichiometric ratio, and diffusivity are parametrically varied and flame speed and flame temperatures are observed. Flame speed is seen to increase with increasing preexponential factor, decreasing activation energy, increasing ambient temperature, decreasing initial droplet radii, and increasing diffusivities. It is also observed that unlike premixed combustion, heterogeneous combustion gives rise to local variations of equivalence in the axial direction. This phenomenon could give rise to a secondary diffusion flame in the wake of a propagating flame and produce local variations in the flame temperature.     

Combustion of liquid fuel inside inert porous media: an analytical approach

This paper presents a numerical analysis of combustion of liquid fuel droplets suspended in air inside an inert porous media. A one-dimensional heat transfer model has been developed assuming complete vaporization of oil droplets prior to their entry into the flame. The effects of absorption coefficient, emissivity of medium, flame position on radiative energy output efficiency and optimum oil droplet size at the entry, defined as the maximum size for complete vaporization before entering the combustion zone, have been presented. The inert porous medium with low absorption coefficient will produce high downstream radiative output with large oil droplet sizes.     

Droplet combustion in presence of airstream oscillation: Mechanisms of enhancement and hysteresis of burning rate in microgravity at elevated pressure

The enhancement and hysteresis behavior of the burning rate of single droplet combustion in the presence of airstream oscillation observed in previously performed microgravity experiments at elevated pressure up to 1.0 MPa were numerically investigated. Excellent agreement with the experimental results was obtained and the mechanisms of these phenomena were examined based on precise numerical data on instantaneous droplet diameter variations corresponding to the unsteady airstream velocity, flow fields around the droplet, and flame movement during combustion. Results show that, depending on the oscillation Reynolds number, which is a function of pressure, flow amplitude and droplet diameter, there are three mechanisms involved in the enhancement of burning rate. In the cases of low oscillation Reynolds numbers, a diffusion-time-delay has a significant effect on the flame front movement and thus, on heat from the flame to the droplet. In the cases of high oscillation Reynolds numbers, a vortex generated outside the droplet flame promotes the motion of the flame, especially in the wake region, and thus enhancing the droplet burning rate. In addition to these two mechanisms, the forced convection during the acceleration period of the flow oscillation causes overshooting of the droplet burning rate due to instantaneous imbalances of the airflow momentum with the Stefan flow. These three mechanisms explain the predominant role of the highest velocity of the oscillatory airstream in determination of the mean burning rate constant and droplet lifetime. Results also show that the hysteresis behavior of the burning rate is a consequence of the different responses of the flame to the deceleration period compared with the responses to the acceleration period under the existence of those three mechanisms.     

Droplet combustion in the presence of acoustic excitation

This experimental study focused on droplet combustion characteristics for various liquid fuels during exposure to external acoustical perturbations generated within an acoustic waveguide. The alternative liquid fuels include alcohols, aviation fuel (JP-8), and liquid synthetic fuel derived via the Fischer–Tropsch process. The study examined combustion during excitation conditions in which the droplet was situated in the vicinity of a pressure node (PN). In response to such acoustic excitation, the flame surrounding the droplet was observed to be deflected, on average, with an orientation depending on the droplet’s relative position with respect to the PN. Flame orientation was always found to be consistent with the sign of a theoretical bulk acoustic acceleration, analogous to a gravitational acceleration, acting on the burning system. Yet experimentally measured acoustic accelerations based on mean flame deflection differed quantitatively from that predicted by the theory. Phase-locked OH^∗ chemiluminescence imaging revealed temporal oscillations in flame standoff distance from the droplet as well as chemiluminescent intensity; these oscillations were especially pronounced when the droplets were situated close to the PN. Simultaneous imaging and pressure measurements enabled quantification of combustion-acoustic coupling via the Rayleigh index, and hence a more detailed understanding of dynamical phenomena associated with acoustically coupled condensed phase combustion processes.     

Modeling of quasi-steady sodium droplet combustion in convective environment

In the event of accidental leakage of sodium from the systems of a Liquid Metal Fast Breeder Reactor (LMFBR), a spray of liquid sodium droplets may be formed which will burn by reacting with the surrounding atmospheric oxygen. In order to understand the burning characteristics of the complete spray, combustion of an individual sodium droplet forms the basis and this has been investigated in the present study. A comprehensive numerical model has been developed to analyze the isolated sodium droplet combustion in a mixed convective environment. The governing equations for mass, momentum, species and energy conservation have been solved in axisymmetric cylindrical coordinates using the Finite Volume Method (FVM). Finite rate kinetic mechanisms have been incorporated to simulate droplet burning, using available kinetics data for basic sodium oxidation reactions. Salient features of the numerical model include a global single-step reaction for sodium oxidation in air and the incorporation of property variations with temperature and concentration. An equilibrium mixture of sodium peroxide, sodium monoxide and sodium vapor is considered as the final reaction product, taking into account the effects of dissociation reactions. The numerical model has been validated with experimental results available in literature. Results for the fuel mass burning rates and flame shapes are presented for different sizes of droplets burning under different free-stream conditions. The model predicts the occurrence of envelope flame around a sodium droplet even at fairly high free-stream velocities.     

On the burning and microexplosion of collision-generated two-component droplets: miscible fuels

The combustion and microexplosion of freely falling two-component droplets, generated either independently or through the collision or merging of two droplets of the two fuels have been studied experimentally. The results show that the non-disruptive combustion characteristics, including the ignition delay, the flame shrinkage and the burning rates are largely similar for droplets generated with these two different modes, hence indicating the efficiency of mixing through the internal motion produced when droplets coalesce. Microexplosion induced by internal superheating and hence nucleation, however, was only observed for the collision-generated droplets, and is believed to be initiated by the air bubbles entrained during a collision. The potential importance of bubble entrainment during droplet and spray generation on spray atomization and burnout is emphasized.     

Numerical study for the combustion characteristics of Orimulsion fuel in a small-scale combustor

A numerical and experimental study has been made in order to figure out the combustion characteristics of Orimulsion fuel for the possible substitution of traditional fuels in existing boilers. To this end, a comprehensive computer model has been developed using Patankar’s SIMPLE algorithm for the complex process associated with the turbulent reaction of this fuel. The program includes several phenomenological models such as behavior of fuel droplet, multi-phase reaction and formation of pollutant species such as NO and SO₂ in radiation-participating, turbulent flows.The calculation results of species and temperature profiles have been successfully compared with the experimental data measured in a small-scale combustor. Comparing the combustion characteristics of Orimulsion with that of heavy fuel oil, one of the noticeable features of Orimulsion combustion is relatively low flame temperature together with the delayed appearance of the peak flame. This is considered to be attributed to the effect of moisture content and the increased flow rate of Orimulsion fuel on the basis of an equivalent calorific value. Further this feature of Orimulsion flame observed in calculation and experiment are consistent with other experimental findings reported in open literature.It is found that the flame characteristics of Orimulsion fuel can be changed to some extent by the variation of operation conditions such as the injection velocity of Orimulsion fuel and the swirl strength of air stream. Further, other parametric study has been made in terms of the type of atomizing fluid, equivalence ratio, radiation model and grid size. In general the results are physically acceptable and consistent. Therefore the computer program developed is considered as a viable tool for the advanced design and determination of optimal operating condition for the large scale of Orimulsion combustor.     

Development and application of the drop number size moment modelling to spray combustion simulations

This work presents the development and implementation of spray combustion modelling based on the spray size distribution moments. In this spray model, the droplet size distribution of spray is characterised by the first four moments related to number, radius, surface-area and volume of droplets, respectively. The governing equations for gas phase and liquid phase employed are solved by the finite volume method based on an Eulerian framework. These constructed equations and source terms are derived based on the moment-average quantities which are the key concept for this work. The sub-model employed for ignition and combustion is the coupling reaction rate between Arrhenius model and Eddy Break-Up model (EBU) via a reaction progress variable. The results obtained from simulation are compared with the experimental and simulation data in the literature in order to assess the accuracy of present model. Comparing with the experimental results, present approach is capable to provide a qualitatively reasonable prediction for auto-ignition. In addition, the flame area developed during the combustion progress corresponds with the experimental data.     

Phenomenology of electrostatically charged droplet combustion in normal gravity

Experimental findings are provided on the effect of electrostatically charging a fuel on single-burning droplet combustion in normal gravity. It was established that significant modification of the flame morphology and the droplet burning time could be achieved, solely by the droplet charge, without the application of external electric fields. Negative charging of the droplets of mixtures of isooctane with either ethanol or a commercially available anti-static additive generated intense motion of the flame and abbreviated the droplet burning time by as much as 40% for certain blend compositions. Positive charging of the droplets generated almost spherical flames, because electrostatic attraction toward the droplets countered the effect of buoyancy. By comparing combustion of droplets of the same conductivity but different compositions, coupling of electrostatics with combustion chemistry was established.     

Combustion of suspended fine solid fuel in air inside inert porous medium: A heat transfer analysis

Present work is a numerical analysis of combustion of submicron carbon particles inside an inert porous medium where the particles in form of suspension in air enter the porous medium. A one-dimensional heat transfer model has been developed using the two-flux gray radiation approximation for radiative heat flux equations. The effects of absorption coefficient, emissivity of medium, flame position and reaction enthalpy flux on radiative energy output efficiency have been presented. It is revealed that in porous medium the combustion of suspended carbon particles is similar to premixed single phase gaseous fuel combustion except the former has shorter preheating temperature zone length. Use of porous ceramic having high porosity and made of Al₂O₃ or ZrO₂ with stabilized flame position operated nearer to downstream end will ensure radiative output maximum and minimum at downstream and upstream end, respectively.     

Interactive combustion of two-dimensionally arranged quasi-droplet clusters under microgravity

To investigate the mutual interactions between droplets in the spray combustion, combustion of 2-dimensionally arranged quasi-droplet clusters is studied under microgravity. Quasi-droplet samples, which are solid in room temperature and change into liquid just after the ignition, consist of alcohol (propanol, butanol, pentanol, or hexanol) and polyethylene glycol with a volumetric ratio of 2:1. Seven samples sustained by glass rods form a 2-dimensional quasi-droplet cluster. Electrically heated nichrome wires ignite all samples in the cluster simultaneously. Single envelope flames that surround the clusters appeared. The results show that the sample spacing has a strong effect on the shape and movement of the flame. Sample clusters with large sample spacings come to the external group combustion through the scavenging combustion mode, whereas the small spacing clusters start directly with the external group combustion. At large sample spacings, the distance from the edge of the sample cluster to the flame (flame distance) increases to a maximum value and then decreases with time. The period of flame growth is prolonged with decreasing sample spacing and finally, at a small enough sample spacing, the flame distance keeps increasing until the flame disappears. This flame movement is attributed to the fuel vapor accumulation effect, which becomes more dominant with decreasing sample spacing. The burning lifetime decreases monotonically and approaches the value of the single flame with increasing sample spacing. The flame distance decreases monotonically and approaches the single flame radius with increasing sample spacing also. These results render important confirmations of the external group combustion phenomena and prove the importance of the two kinds of unsteadiness, that is, the scavenging combustion with large droplet interval and the fuel vapor accumulation effect with small droplet interval, in group combustion.     

Microgravity experiments on flame spread along fuel-droplet arrays at high temperatures

Microgravity experiments on droplet-array combustion were conducted under high-ambient-temperature conditions. n-Decane droplet arrays suspended on SiC fibers were inserted into a high-temperature combustion chamber and were ignited at one end to initiate the flame spread in high-temperature air. Flame-spread modes, burning behavior after the flame spread, and flame-spread rate were examined at different ambient temperatures. Experimental results showed that the appearance of flame-spread modes and the flame-spread rate were affected by the ambient temperature. The flame-spread rate increased with the ambient temperature. These facts are discussed based on the temperature effects on the droplet heating and the development of a flammable-mixture layer around the next droplet. A simple model was introduced to analyze these effects. The effects of the ambient temperature on the appearance of group combustion of the array after the flame spread and the scale effect in the flame spread are also discussed.     

Combustion behavior of a falling sodium droplet: Burning rate‐constant and drag coefficient

The combustion behavior of a single sodium droplet has been studied experimentally, by use of a falling droplet. It was found that D²‐law can hold for the sodium droplet combustion after the ignition, which can be observed to occur through an increase in the droplet temperature under a condition without a gaseous flame, suggesting that a surface reaction plays an important role in the ignition of sodium. It was also found that the burning rate‐constant without forced convection has nearly the same value as those for conventional hydrocarbon droplets, although it is considered that the sodium combustion proceeds in an oxidizer‐rich environment even in the air. This can be judged by comparing a temporal variation of the flame/droplet diameter ratio for the sodium droplet with that for the hydrocarbon droplet. A micro‐explosion of the burning droplet is also observed when oxygen concentration in the ambience exceeds 0.33 in mass fraction. As for the falling velocity and/or distance of the burning droplet, it turned out that the use of the drag coefficient for solid sphere under isothermal condition is inappropriate in obtaining accurate values. It was also found in another experiment that when Re > 500, the drag coefficient of the falling droplet undergoing combustion is as high as 2 depending on combustion situation and/or droplet temperature, while that of the solid sphere under an isothermal condition is 0.44. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(7): 481–495, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20084     

Combustion and microexplosion of freely falling multicomponent droplets

The combustion characteristics of isolated, low-Reynolds number, multicomponent droplets freely falling in a hot, oxidizing gas flow are studied. Experimental results on two-component fuels substantiate a three-staged combustion behavior, with diffusion being the dominant liquid-phase transport mechanism: that is, there exists an initial period during which the volatile component in the surface layer is preferentially gasified while the droplet temperature is relatively cold; a short, transition, period during which the droplet temperature increases rapidly, the burning rate is extremely low, the flame size shrinks, and the flame temperature diminishes; and a final, quasi-steady period during which the droplet interior concentration distribution remains constant, the surface region is more concentrated with the less volatile component, and the droplet temperature is relatively high. Experimental results on microexplosion show that its occurrence depends sensitively on the mixture concentration as well as the stability of the droplet generation mode, and that frequently nucleation appears to initiate in the vicinity of the droplet center.     

A study of thin‐flame quasisteady sphericosymmetric combustion of multicomponent fuel droplets. Part I: modelling for droplet surface regression and non‐unity gas‐phase Lewis number

A model for sphericosymmetric thin‐flame combustion of multicomponent fuel droplets has been developed in the first part of this two‐part work. The model incorporates effects of droplet surface regression and gas‐phase Lewis number. It is observed that both these effects affect the results substantially. The study also reveals the transient nature of the combustion process. Copyright © 1999 John Wiley and Sons, Ltd.     

微重力燃烧及固体表面火蔓延的研究

本文对微重力条件下单滴燃烧和固体可燃物表面火焰传播过程的研究作了简要评述。微重力条件下单滴燃烧的研究对于认识燃烧过程的物理机理具有重要价值。对于微重力条件下固体可燃物表面的火焰传播过程,指出其主要物理机制是气化表面的“表面燃料喷射”效应,文中还给出了作者对火蔓延过程进行的三维非稳态数值模拟得到的部分计算结果。     

Flame structure in fuel rich mono-dispersed sprays

The flame structure and the mechanism of the flame propagation in fuel sprays is studied through a one-dimensional, laminar, monodispersed spray of n-octane fuel. A hybrid Eulerian-Lagrangian method is used in this analyses. The results show that for sprays with small initial droplet radii (e.g., γ_κ = 29.6 μm), the flame structure resembles that of a premixed gaseous combustion. However, as the droplet size is increased, the flame becomes more heterogeneous. When droplet interaction is added to the model the heterogeneity of the flame is enhanced and large fluctuations both in the flame temperature and fuel vapor concentrations are observed.     

Numerical Simulation of Microgravity Flame Spread Over Solid Combustibles

Ｎｕｍｅｒｉｃａｌ　Ｓｉｍｕｌａｔｉｏｎ　ｏｆ　Ｍｉｃｒｏｇｒａｖｉｔｙ　Ｆｌａｍｅ　Ｓｐｒｅａｄ　Ｏｖｅｒ　Ｓｏｌｉｄ　ＣｏｍｂｕｓｔｉｂｌｅｓＮｕｍｅｒｉｃａｌＳｉｍｕｌａｔｉｏｎｏｆＭｉｃｒｏｇｒａｖｉｔｙＦｌａｍｅＳｐｒｅａｄＯｖｅｒＳｏｌｉ...