Fuel Droplets Research Articles

Numerical Simulation of Combustion for Freely Falling Gelled Fuel Droplets under Normal Gravity Conditions

Understanding the evaporation and combustion mechanisms of single droplets of gel propellant is the first stage to predict the burning characteristics in the combustion chamber. This paper, taking into account convection heat for freely falling gelled fuel droplets under normal gravity conditions, as well unsteady mass diffusion and thermal diffusion inside droplet, a theoretical model was developed to understand mass and heat transport mechanisms, and bubble growth within the gel droplet during processes of droplet combustion. The results show that at the first stage, shrinkage of the radius obeys the d2-law; steep temperature gradient and fuel mass concentration gradient appear within droplet, especially region near droplet surface. At the second stage, liquid fuel near the gellant layer within droplet starts to boiling, gellant layer formation resist the vaporizing fuel gas flow to extent; the vapor region appears between gellant layer and vaporizing surface within the droplet, and the droplet expands, swells, the layer thickness decreases until it ruptures.

Experimental Study on the Combustion and Microexplosion of Freely Falling Gelled Unsymmetrical Dimethylhydrazine (UDMH) Fuel Droplets

The increasing demand for high energy density fuels and the concern for their safety have propelled research in the field of gelled propellants, where understanding the combustion of single gelled fuel droplets is the first stage to predict the spray combustion characteristics. The experiments utilized single-isolated freely falling gelled unsymmetrical dimethylhydrazine (UDMH) droplets instead of the conventional suspended droplet approach, in order to eliminate the perturbation associated with the suspension mechanism. Morphological transformations of the gelled droplet involved in the combustion processes were monitored by employing a high-speed digital camera, while the effects of ambient pressure and oxygen concentration on burning rate constants were also studied. The experimental results show that four main phenomena (droplet deformation, bubble formation and growth, vapor jetting and luminous jetting flame with “horn” shape) and three distinct phases were identified in the droplet combustion process; the high yield stress and polymer chain structure of polymer gellant are responsible for the appearance of bubbles with almost the same order of magnitude as the droplets. Increasing the ambient pressure can increase the burning rate, postpone the appearance of microexplosions, and weaken microexplosion intensity; while increasing the ambient oxygen concentration can promote the appearance of microexplosions, strengthen microexplosion intensity and increase the burning rate.

Thermodynamic Analysis of Evaporation of Levitated Binary and Ternary Liquid Fuel Droplets under Normal Gravity

The present study presents a thermodynamic model for predicting the vaporization characteristics of binary and ternary hydrocarbon fuel droplets under atmospheric pressure and normal gravity conditions. The model employs activity coefficients based on UNIFAC group contribution method and evaluates the vapor-liquid equilibrium of binary and ternary droplets. The gas-phase properties have been evaluated as a function of temperature and mixture molecular weight. The model has been validated against the experimental data available in literature. The validated model is used to predict the vaporization characteristics of binary and ternary fuel droplets at atmospheric pressure under normal gravity. Results show multiple slopes in the droplet surface regression indicating preferential vaporization of fuel components based on their boiling point and volatility. The average evaporation rate is dictated by the ambient temperature and also by composition of the mixture.

A Simple Model of Off-Stoichiometric Premixed Fuel Rich Spray Flames with Droplet Evaporation and Pyrolysis

Fuel droplets that manage to traverse the main reaction zone of a one-dimensional off-stoichiometeric fuel-rich laminar spray flame enter a hot, oxygen-free environment under which conditions their pyrolysis occurs. Under such circumstances the droplets release pyrolysis products which may diffuse back towards the flame front thereby effectively supplying a second (pool of) fuel different to the (first) fuel that was released due to pre-flame droplet evaporation. A new simple mathematical model of the propagation of a spray flame that includes this phenomenon is presented in this work for the first time and a formula is given for the burning velocity. It is shown that under certain operating conditions flame velocity enhancement may occur as a result of the additional fuel supply. Although the effect is limited to a few percent it is pointed out that it may be significant in cases of borderline flame stability and/or ignition/extinction.

Physical Continuous Model of Heating, Evaporation and Ignition of Polydisperse Fuel Droplets

A new modeling of heating and evaporation of fuel droplets and ignition of a fuel vapour/ air mixture in continuousform is suggested. The size distribution of fuel droplets is assumed to be continuous and found from thesolution of the kinetic equation for the probability density function (PDF). The semi-transparency of droplets, thedifference between the gas temperature and the external temperature are take into account. The model represent indimensionless from, and the dynamics of the system is present in term of the dynamics of a multi-scale, singularlyperturbed system (SPS)

Comparative Analysis of Gas Phase Models for Fuel Droplet Evaporation in Forced Convection

In this study, mathematical model of fuel droplet evaporation is developed. Basic relationship of gas phase model is proposed by summarizing a large number of droplet evaporation gas phase models. Predictions of gas phase model are compared in forced convection. The results show that: Predictions are strongly dependent on the choice of gas phase models. Predictions of Ranz model and Haywood model are accurate with the experimental results. Evaporation time which is predicted by the gas phase models considering Stefan flow is longer than those without considering Stefan flow. For different gas phase models, droplet evaporation time is directly proportional to environmental pressure and inversely proportional to ambient temperature and Reynolds number.

519 不均一な燃料液滴群の蒸発および燃焼挙動(GS-5・6 燃焼)

519 不均一な燃料液滴群の蒸発および燃焼挙動(GS-5・6 燃焼)

Effect of asymmetric radiant heating on monodisperse acetone/ethanol and acetone/2-propanol bi-component droplets

Fuel droplet vaporization is an integral element ofmany combustion systems and extensive research has been carried out in the field to explore and model the droplet heating and vaporization process. Many numerical and most experimental droplet studies have considered isolated droplets or a stream of droplets exposed to uniform and symmetric vaporization conditions. However, few numerical and even fewer experimental studies have addressed the influence of radiant heating on droplet vaporization. In particular, there are likely to exist combustion environments where radiant heating is both non-negligible as well as asymmetric. Asymmetric radiant heating of droplets may exist in counter-flow, liquid-fuelled diffusion flames, conventional spray flames, and in liquid-fuelled microcombustors. Ammigan et al. (in press) provides a review of droplet vaporization literature with a focus on radiant heating. Recent works by the authors have considered single component (acetone) (Ammigan and Clack, 2009) and bi-component (acetone and hydrocarbons octane and hexane) (Ammigan et al., in press) droplets vaporizing under asymmetric thermal radiation. PLIF images consistently revealed that acetone vapor mole fraction distributions on the irradiated side were punctuated by local concentration maxima, whose extent varied with droplet composition, diameter, and incident radiant flux. Such behavior is the result of localized radiant heating of the droplet and the influence of droplet thermophysical properties and the phase equilibria of the droplet

A Numerical Study of the Effect of Injection Velocity on Fuel Droplets Sizing in a Three-Dimensional Side-Dump Combustor

The effects of injection velocity on propulsive droplets sizing and efficient mass fraction in a three dimensional side-dump combustor with dual opposite curved side-inlet duct are numerically investigated in the present paper. The mass of fuel vapor inside the flammability limit is named efficient mass fraction. The air flow comes from side-inlet ducts into the cylindrical combustor and four nozzles which are located in the top of the cylinder have the duties of fuel injection. The injection velocity is varied as 20, 40, 60 and 80 [m/s] respectively to examine its effects on propulsive droplets sizing and efficient mass fraction which provides worthwhile information for the combustor design work. As well, by increasing entrance air flow velocity from 35 to 100 and 280[m/s] correspondingly, these computations are repeated. To fulfill the calculations a modified version of KIVA-3V code which is a transient, three-dimensional, multiphase, multicomponent code for the analysis of chemically reacting flows with sprays, is used.

Effect of Electrode Distance on Combustion Behavior of Fuel Droplet in Vertical DC Electric Field

Flames are affected by external electric fields because they contain ions and electrons. The movement of ions and electrons affects the external electric field owing to their charge. In this context, this paper discusses the change in the electric field on the basis of experimental results obtained at different electrode distances. Employing one-dimensional (1D) steady-state analysis, we assume that if the electric field is changed spatially, the effect of the electric field on combustion behavior is aligned with V2/L3, where V is the applied voltage between the electrodes and L is the distance between the electrodes. Because a flame deforms to a cathode owing to electric body force in an electric field, the change in the flame shape of burning ethanol droplets observed in a vertical DC electric field and the electric current during combustion are measured as flame characteristics. The results reveal that applied electric voltage exists where the flame becomes vertically symmetrical to balance the buoyancy due to the electric body force. The relationship between m and n of Vm/Ln for flame symmetry is around n/m = 1.5. On the basis of these results, all experimental results for different electrode distances are rearranged with eV2/L3, which is a representative electric body force, and it is proved that the use of parameter eV2/L is effective. These results indicate that the change in the electric field due to the existence of a flame should be considered when examining the effect of an external electric field on combustion behavior.

G060063 赤外吸収法を用いた燃料液滴近傍の燃料蒸気濃度計測([G06006]熱工学部門一般セッション(6)二燃焼の分光計測および輻射)

G060063 赤外吸収法を用いた燃料液滴近傍の燃料蒸気濃度計測([G06006]熱工学部門一般セッション(6)二燃焼の分光計測および輻射)

Modelling biodiesel droplet evaporation using continuous thermodynamics

A model for the evaporation of droplets of biodiesel fuels based on continuous mixture theory is presented. The fuel is represented by three chemical groups – two fatty acid methyl ester (FAME) fractions and one monoglyceride fraction – and the parameters for each distribution function are derived by conducting a numerical simulation of a distillation test. Properties correlations are presented for saturated and unsaturated FAME’s and for monoglycerides, and it is demonstrated that the evaporation behaviour of saturated and unsaturated FAME’s is essentially identical. The model is tested against experiments on single suspended droplets and found to agree with the measurements to within 3% in d2.

The combustion of droplets of liquid fuels and biomass particles

Studies have been made of the combustion of droplets of liquid hydrocarbons, including kerosene and Diesel fuels, biofuels such as FAME and the alcohols, especially ethanol and n-butanol, and of pulverised solid biomass materials such as pine wood and Miscanthus which burn in an analogous fashion. Information is given on the burning rates of both the liquids and the solids and data given on soot formation yields for the different fuels. The mechanism of soot formation is discussed in relation to (1) volatile liquid fuels such as n-heptane, alcohols and aviation fuels, (2) liquid fuels having higher aromatic levels such as Diesel fuels, and (3) biomass particle combustion.

An aerosol rapid compression machine for studying energetic-nanoparticle-enhanced combustion of liquid fuels

The use of energetic nanoparticles offers a promising means of adjusting the reactivity of liquid fuels for enhanced combustion stability in next generation propulsion systems. This work outlines the development of a novel aerosol rapid compression machine (RCM) for studying the impact of energetic nanoparticles on reducing the ignition delay of liquid fuels, and a proof-of-concept demonstration is presented using ethanol and JP-8. Fuel droplets are generated using an ultrasonic nozzle. The seeding of 50 nm aluminum nanoparticles in the liquid fuel is achieved by using a combination of chemical surfactants in addition to mixing in an ultrasonic bath. The autoignition delay is measured for neat and nanoparticle-enhanced mixtures at compressed conditions of 772–830 K and 12–28 bar in the RCM. The results show that significant changes in the ignition delay can be observed using a low concentration (2%-weight) of energetic nanoparticles. For ethanol and JP-8, ignition delays were reduced by 32% and 50%, respectively. Measurements to verify the uniformity of aerosol dispersion in the RCM, the reproducibility of the RCM data, and a method for approximating compressed temperature are also presented.

Evaporation of Hydrocarbon Fuel Droplet under Elevated Temperatures

Evaporation of Hydrocarbon Fuel Droplet under Elevated Temperatures

Theoretical Model of the Transient Combustion of Organic-Gellant-Based Gel Fuel Droplets

Experimental evidence of the combustion process of an all-organic gel fuel droplet indicates that at a certain time after ignition, evaporation of the liquid fuel results in the formation of an elastic layer of high-viscosity gellant around the droplet, which prevents further vaporization. As a result, constantly expanding vapor bubbles are produced within the droplet. Eventually, the layer ruptures and jets of fuel vapor are released. A theoretical, time-dependent model of organic-gellant-based gel droplet combustion has been developed and numerically solved. The results indicate that the evaporation rate of the liquid fuel from the droplet surface depends on droplet size and strongly affects the thickness of the gellant layer. The tensile stress, applied to the gellant layer during the formation of the bubbles, reaches high levels in short periods of time and causes the droplet to rupture when it exceeds the layer material rupture stress. The stage during which the gellant layer is formed is almost three orders of magnitude longer than the stage of bubble formation and layer rupture.

G0700-5-1 FAME燃料単一油滴の燃焼およびスート生成特性(エンジンシステム部門一般講演(5))

G0700-5-1 FAME燃料単一油滴の燃焼およびスート生成特性(エンジンシステム部門一般講演(5))

Effects of Carbon Dioxide on Unsteady Combustion of Isolated Fuel Droplet in Microgravity

Effects of Carbon Dioxide on Unsteady Combustion of Isolated Fuel Droplet in Microgravity

Visualization of Internal Behavior in Emulsified Fuel Droplets and the Effect of Surfactant on Secondary Atomization

現在，窒素酸化物やすすといった環境汚染物質の同時排出削減法として，エマルジョン燃料の研究が行われている．加熱過程におけるエマルジョン燃料液滴では，蒸気吹き出し（puffing）およびミクロ爆発といった2次微粒化現象が発生することが知られているが，2次微粒化挙動の詳細が解明されているとは言い難い．本研究では，界面活性剤のHydrophile-Lipophile Balance（HLB）値がエマルジョン燃料の乳化特性および2次微粒化特性に及ぼす影響を検討した．加熱過程における分散滴の挙動を観察するために，水相のみに着色したエマルジョン燃料を用いて観察を行う内部挙動の可視化方法を提案した．この可視化方法を用いて，着色した水およびHLB値の異なる界面活性剤を用いて調製したエマルジョン燃料液滴を熱電対に懸垂し，集光加熱を行い，2次微粒化挙動を高速度カメラにより観察した．その結果，Oil-in-Water（O/W）型エマルジョン燃料においては，水中油滴が凝集し，O/W型からWater-in-Oil（W/O）型への転相が生じた後に，分散した油中水滴が再凝集し，ミクロ爆発が発生することが示された．W/O型エマルジョン燃料においては，HLB値の増加に伴い，ミクロ爆発発生確率が減少するが，O/W型エマルジョン燃料においては，HLB値にかかわらず，ミクロ爆発発生確率に大きな差異はないことが示唆された．

1210 エマルション燃料液滴列の燃焼挙動に及ぼす含水率の影響(GS-6 噴霧・液滴)

1210 エマルション燃料液滴列の燃焼挙動に及ぼす含水率の影響(GS-6 噴霧・液滴)