Burning Rate Constant Research Articles

A Burke–Schumann analysis of diffusion-flame structures supported by a burning droplet

A Burke–Schumann description of three different regimes of combustion of a fuel droplet in an oxidising atmosphere, namely the premixed-flame regime, the partial-burning regime and the diffusion-flame regime, is presented by treating the fuel and oxygen leakage fractions through the flame as known parameters. The analysis shows that the burning-rate constant, the flame-standoff ratio, and the flame temperature in these regimes can be obtained from the classical droplet-burning results by suitable definitions of an effective ambient oxygen mass fraction and an effective fuel concentration in the droplet interior. The results show that increasing oxygen leakage alone through the flame lowers both the droplet burning rate and the flame temperature, whereas leakage of fuel alone leaves the burning rate unaffected while reducing the flame temperature and moving the flame closer to the droplet surface. Solutions for the partial-burning regime are shown to exist only for a limited range of fuel and oxygen leakage fractions.

Nanoaluminum/Nitrocellulose microparticle additive for burn enhancement of liquid fuels

Addition of metal and metal oxide nanoparticles to hydrocarbon fuels has shown the ability to increase the volumetric energy density, decrease ignition delay, increase heat of combustion, and catalyze fuel decomposition in recent research. However, energetic metal nanoparticles are prone to aggregation, which occurs at an increased rate near the regressing surface of a burning liquid droplet where local concentrations increase and can form a transport-inhibiting shell, ultimately decreasing the droplet burning rate. Alternatively, gas ejections from the droplet can disrupt shell formation and transport nanoparticles from the droplet to the flame zone. This study quantifies up to a 12.1% decrease in the burning rate constant of Kerosene droplets when 6.1wt% nanoaluminum (nAl) particles are added (the maximum stable loading) with a hydrocarbon-based surfactant in a free-falling single droplet combustion experiment. Addition of nitrocellulose (NC) particles to the nanofuel diminishes or fully counteracts the burning rate decreases and provides a means of tuning the burning rate constant higher than that of pure Kerosene (maximum 13.8% increase over control with 2.3wt% nAl and 0.6wt% NC added). To reach stable nanofuels at higher particle loadings up to 15.0wt% solid additives, nAl and NC were electrosprayed into composite mesoparticles (MP) before suspending with surfactant in Kerosene. These MP-based nanofuels boast increased dispersibility and additive loadings and thus higher achievable burning rates (maximum 26.5% increase over control) than physically mixed analogs. A mechanism is proposed in which droplet disruptions influenced by NC addition include cyclical inflations, during which the liquid gasification rate increases, e.g. by expanding the outer surface area of the droplet.

Partial-Burning Regime for Quasi-Steady Droplet Combustion Supported by Cool Flames

A simplified model for droplet combustion in the partial-burning regime is applied to the cool-flame regime observed in droplet-burning experiments performed in the International Space Station with normal-alkanes fuels resulting in expressions for the quasi-steady droplet burning rate and for the flame standoff ratio. The simplified predictions are found to produce reasonable agreement with the experimentally measured values of burning-rate constants but not with their apparent dependencies on pressure or on the initial droplet diameter. Good agreement is found, however, with newly measured and numerically calculated flame standoff ratios in this droplet combustion supported by cool flames.

Research of High-Temperature Heat and Mass Transfer and Kinetics of Phase Transformations during combustion higher alkanes in the air

The paper presents a study of high-temperature heat and mass transfer and combustion kinetics of octadecane particle in room temperature air taking in consideration fuel heating, melting and evaporation. The consecutive stages of droplet combustion are described. Burning rate constants and flame heights are determined for droplets with different initial diameters

Combustion characteristics of alkane two-droplet array Part 2. Two droplet array of n-Docosane

Combustion of two n-Docosane droplets in air at room temperature is studied experimentally. The droplets are disposed vertically one above another and burning history of upper droplet is registered. A new accurate method for measuring an equivalent diameter of burning droplet is developed. As a result, burning rate constants of Docosane and Octadecane droplets are measured more accurately. The dependence of upper droplet burning rate on interdroplet spacing is analyzed.

Investigation of the Burning of Paraffin Droplets

The burning characteristics of pure n-alkanes (n-octadecane and n-docosane) are investigated. Combustion of single and double droplets is studied experimentally, the histories of burning droplets are obtained, and the burning rate constants are determined. Flame transfer is studied in a two- droplet system.

Droplet combustion studies of hydrocarbon-monopropellant blends

An experimental investigation was conducted to characterize the monopropellant droplet combustion of pure and blended isopropyl nitrate (IPN), suspended on quartz fibers in a quiescent atmosphere. The blends were prepared by mixing varying percentages by weight of IPN with less viscous n-heptane, as well as highly viscous desensitizer dibutyl sebacate (DBS). Ignition was achieved by using a heated 60μm Nichrome wire. The dependence of the burning rate constant of pure IPN on initial droplet diameter was investigated in the droplet size range of 0.79–1.97mm. The blended IPN studies were carried out with initial droplet diameters of 2 and 1.5mm for IPN-n-heptane and IPN-DBS blends respectively, to characterize the effect of gravimetric composition. The experiments revealed a strong dependence of IPN burning rate on droplet size. The IPN-DBS blends were characterized by severe micro-explosions, further atomizing the droplet, governed by the preferential evaporation of IPN over DBS. However, micro-explosions were conspicuously absent in case of IPN-n-heptane blends due to simultaneous gasification of both components.

Experimental studies the burning process of gelled unsymmetrical dimethylhydrazine droplets under oxidant convective conditions

Gelled hypergolic propellants offer potential safety and performance improvements over conventional liquid and solid propellants. Understanding the combustion process of single gelled droplets is the first basic step to predict their behavior in the future combustion chambers. Experimental studies were performed to investigate the burning behavior of gelled unsymmetrical dimethylhydrazine (UDMH) droplets under oxidant convective conditions. The effects of oxidant convective conditions including velocity and temperature on the burning behavior were analyzed. The burning process was broken down into four stages: heating and swelling period, initial combustion period, violent combustion period, and stable and extinguished combustion period. Sometimes the droplet inside seemed to be porous or botryoidal. The microexplosion period lasted for a long time sometimes exceeding about 70% of the burning lifetime, and the phenomenon of gas jet combustion due to the burst steam from microexplosions was founded. The conversion of burning flame from a layered and enveloped flame structure to an escaped flame structure with increase in the convective velocity was observed. When the enveloped flame appeared, it was more helpful to the burning process, and the intensity of microexplosions and the burning rate constant were found to increase with the convective temperature. When the escaped flame appeared, it was disadvantageous to the burning process, and the intensity of microexplosions decreases with rise in convective velocity. Compared with the escaped flame, the intensity of microexplosions of the enveloped flame was lower, but the frequency was higher.

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.

Modeling droplet vaporization and combustion with the volume of fluid method at a small Reynolds number

The volume of fluid (VOF) formulation is applied to model the combustion process of a single droplet in a high-temperature convective air free stream environment. The calculations solve the flow field for both phases, and consider the droplet deformation based on an axisymmetrical model. The chemical reaction is modeled with one-step finite-rate mechanism and the thermo-physical properties for the gas mixture are species and temperature dependence. A mass transfer model applicable to the VOF calculations due to vaporization of the liquid phases is developed in consideration with the fluctuation of the liquid surface. The model is validated by examining the burning rate constants at different convective air temperatures, which accord well with experimental data of previous studies. Other phenomena from the simulations, such as the transient history of droplet deformation and flame structure, are also qualitatively accordant with the descriptions of other numerical results. However, a different droplet deformation mechanism for the low Reynolds number is explained compared with that for the high Reynolds number. The calculations verified the feasibility of the VOF computational fluid dynamics (CFD) formulation as well as the mass transfer model due to vaporization.

Effects of initial droplet diameter and pressure on burning of ATF gel propellant droplets

The increasing demand for higher energy density fuels and the ever-increasing concern for their safety have propelled research in the field of gel propellants. For studying the fundamental parameters without the interference of neighbouring droplets, isolated droplet burning of organic ATF (aviation turbine fuel) gel propellants was chosen to investigate experimentally the effects of initial droplet diameter and chamber pressure on the burning under normal gravity conditions at room temperature. Under ambient pressure condition, an increase in the burning rate constant was observed with increase in initial droplet diameter. For a given range of diameters, the burning rate constant also continued to increase with pressure. Experiments were also carried out to study how these variations were affected by initial droplet diameter and chamber pressure changes, respectively. The intensity of microexplosions was observed to decrease with increase in chamber pressure. A balance between the heat loss by the droplet to the surroundings and the heat gain by the droplet has been put forward to explain the variations of burning rate constant with varying pressures as well as varying initial droplet diameters.

Numerical Modeling of Steady Burning Characteristics of Spherical Ethanol Particles in a Spray Environment

A numerical study of steady burning of spherical ethanol particles in a spray environment is presented. A spray environment is modeled as a high temperature oxidizer stream where the major products of combustion such as carbon dioxide and water vapor will be present along with reduced amounts of oxygen and nitrogen. The numerical model, which employs variable thermophysical properties, a global single-step reaction mechanism, and an optically thin radiation model, has been first validated against published experimental results for quasi-steady combustion of spherical ethanol particles. The validated model has been employed to predict the burning behavior of the ethanol particle in high temperature modified oxidizer environment. Results show that based on the amount of oxygen present in the oxidizer the burning rate constant is affected. The ambient temperature affects the burning rate constant only after a sufficient decrease in the oxygen content occurs. In pure air stream, ambient temperature variation does not affect the evaporation constant. Results in terms of burning rates, maximum temperature around the particle, and the evaporation rate constants are presented for all the cases. The variation of normalized Damköhler number is also presented to show the cases where combustion or pure evaporation would occur.

Two-Droplet Combustion of N-Octane in a Direct Current Electric Field Under Microgravity

The combustion experiments of two droplets of n-octane arrayed in the direction of the electric field were carried out under microgravity. Experiments with different electrode distances are carried out and two individual flames were observed. The increase in the burning rate constant of the cathode-side flame was arranged reasonably using the representative electric body force, which was expressed by the product of the square of the applied voltage and the inverse cubic electrode distance from the one-dimensional steady-state analysis. The increase in the burning rate constants is discussed by the convection effect and the experimental results are estimated in an analogy of the effect of Grashof number on droplet combustion. For the anode-side flame, a luminous flame at the positive-side end that emits negatively charged soot particles is observed, which induces the local flow in an antiflow direction, preventing convective heat transfer to the droplet.

Effects of gellant concentration on the burning and flame structure of organic gel propellant droplets

The increasing demand for higher energy density fuels and the ever-increasing concern for their safety have propelled research in the field of gel propellants. For studying the fundamental parameters without the interference of neighbouring droplets, an isolated droplet was chosen to investigate experimentally the combustion process of gel propellants under normal gravity conditions. Phase separation of the gel propellant components leading to bubble nucleation, vapor jetting and microexplosions were found to be the main phenomenon involved during the combustion period. Experiments were comprehensively carried out to study the effect of gellant concentration on the burning rate constant as well as flame structure. The burning rate constant was found to decrease with increase in the gellant concentration. Decrease in the calorific value of the increasing gellant concentrations was proposed as one of the reasons for this variation. The flame exhibited a triple flame structure for all the cases for both C 2 ∗ radicals as well luminous flames. The horizontal and vertical flame standoff distances were observed to decrease with gellant content. Decrease in the content of the base fuel was proposed as the reason for the same.

Simulation of the autoignition and combustion of n-heptane droplets using a detailed kinetic mechanism

The autoignition and combustion of n-heptane droplets are simulated using a detailed kinetic mechanism. A mathematical model, based on first principles, contains no adjustable parameters. The burning rate constants for the combustion of droplets are calculated over a wide range of pressures, temperature, fuel-to-oxidizer equivalence ratios of the gas-droplet suspension, and droplet diameters. The calculated and measured delay times of autoignition of droplets are compared. The calculation results agree well with the available experimental data. The detonability of gas-droplet suspensions with partial pre-evaporation of fuel is estimated.

Ignition and combustion of n-heptane droplets in carbon dioxide enriched environments

The ignition process and burning characteristics of fiber-supported n-heptane fuel droplets in carbon dioxide enriched and varying pressure environments have been studied under normal gravity. Measured values of droplet burning rates, flame dimensions, broad-band radiant emission, and ignition times were compared to droplets burning in standard air conditions. The burning rate constants increased with increasing carbon dioxide concentration or pressure. For 21% ambient oxygen concentration ignition was achieved for carbon dioxide concentrations up to 46% with the remaining being nitrogen. The experimental burning rates were compared to existing theoretical models. A flammability map for n-heptane burning under normal gravity as a function of carbon dioxide concentration and pressure was also developed using these results.

A new property evaluation scheme for mass transfer analysis in fire problems

A simple scheme for the estimation of average thermo-physical properties has been presented. Properties such as thermal conductivity and specific heat have been calculated at suitable temperatures, which are functions of the ambient and the adiabatic flame temperatures. These have been successfully employed in the empirical correlations available in standard combustion text books for the estimation of burning rate constants and flame stand-off distances in the combustion of liquid and solid fuels. The average thermal conductivity is estimated as the thermal conductivity of standard air calculated at a temperature given as one-third of the sum of ambient and the adiabatic flame temperatures. The average specific heat is calculated as the specific heat of standard air at the adiabatic flame temperature itself. Extensive validation of the scheme with the experimental data is presented employing several liquid fuels and a solid fuel.

Microgravity experiments of single droplet combustion in oscillatory flow at elevated pressure

An experimental study for 1-butanol single droplet flames in constant and oscillatory flow fields was conducted under microgravity conditions at elevated pressure. In the constant flow experiments, flow velocities from 0 to 40 cm/s were tested. Using obtained data of d 2, the burning rate constants were evaluated. The burning rate constant in the quiescent condition was also calculated successfully at high pressure by the extrapolation method based on the Frössling relation. In the oscillatory flow experiments, the flow velocities were varied from 0 to 40 cm/s at the frequencies of 2–40 Hz. Results showed that the burning rate constant during the droplet lifetime varied following the quasi-steady relation at 0.1 MPa; however, in the conditions with higher frequencies at 0.4 MPa, the average burning velocity became larger than that for the constant flow case with the velocity equivalent to the maximum velocity in the oscillatory flow. Under the condition where the burning rate constant increased, it was observed that the flame did not sufficiently move back upstream, leading to enhancement of the heat transfer from the flame to the droplet surface. Therefore, the instantaneous burning rate constant increased. To investigate the mechanism of such flame behavior, the ratio of two characteristic times, τ f / τ D ( τ f : flow oscillation characteristic time, τ D : diffusion characteristic time), were compared. As the flow oscillatory frequency increased, τ f / τ D becomes smaller. τ f / τ D also became smaller at high pressure. If τ f / τ D is small due to the small mass diffusion rate, the droplet flame could not move back to the appropriate position for the minimum velocity in steady flow, leading to an increase of the burning rate constant, especially in the case of higher frequency at high pressure.

Important roles of multiphase process in enhancement mechanism of micro plastic particle combustion

Thermal recycling of recovered plastic resin has been investigated by supplying plastic resin powder as an additional fuel to the proposed twin-fuel industrial burner. The results emphasize the necessity of fundamental investigations, such as high-speed microscopic observations of micro plastic particles under abrupt heating. To observe the multiphase process of macro plastic particles under abrupt heating and to elucidate their combustion enhancement mechanism, some original devises are introduced; one for optical imaging and three for abrupt heating and ceasing. The former is a mini optical composite which enables simultaneous photographing of the direct silhouetted and schlieren images. The latter three are reforming of a rugged resin particle into a spherical one, a vertical type circular mini-burner for abrupt heating, and a mini-puff generator for abrupt extinguishment. Three types of fine supports made of tungsten, silicon-carbide, and quartz glass are used. Detailed optical observations clarify many characteristics concerning the multiphase process of plastic resin particles under abrupt heating, such as the occurrence of multiple internal micro bubbling, multiple micro explosions and jets. The burning rate constant is also estimated based on the temporal reduction rate of the particle surface area. It is found that the values obtained for three supporting wires are greater than those of typical liquid fuels, and that the key factor to the prominent enhancement rate of micro PET particle combustion can be attributed to the occurrence of the multiphase process particular to micro plastic particles.

Influences of Uniform Electrical Fields on Burning Rate Constant of Ethanol Droplet Combustion

The burning rate constants of ethanol droplet flame under uniform electrical fields were investigated experimentally and numerically. The droplet was burned between two flat electrodes and 1–7 kV DC Volts were applied. The combustion chamber was dropped from the top of a drop tower and the experiment was carried out under microgravity environment to eliminate the effect of buoyancy. Direct photographs of the droplet flame were captured and the change in droplet diameter was measured to obtain burning rate constants. A two-dimensional numerical simulation was also conducted on the droplet combustion in a uniform electrical field. A chemical reaction model with some elementary reactions of ion species was applied. The deformation of the electrical field was predicted by solving the Poisson equation. The predictions show that the burning rate constant increases and the flame deformation becomes large as the applied voltage increases, which is in qualitatively agreement with the experimental results. The relationship between the burning rate constant and the induced flow velocity by Coulomb force is discussed in the analogy of the burning behavior in a convective flow.