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Methane/coal dust explosions pose a serious threat to the safety of coal mining. Since the flow field in the tunnel is unsteady, the effect of turbulence on flame propagation is essential for the disaster risk assessment and safety protection. In this study, the effect of turbulence intensity (u′) on flame propagation characteristics and extinction limits of methane/coal dust explosions is investigated. The turbulence parameters of the flow field and the turbulence-flame interaction are estimated by Particle Image Velocimetry. Luminous flame (raw image) and reaction front (OH radical image) of methane/coal dust mixture are captured. The results reveal that the flame propagation velocity increases by 78–200%, when the u′ increases from 1.86 to 2.66 m/s. The acceleration of flame propagation velocity is due to the increase of the release of volatile matter and the flame folded regions caused by the turbulence. When the characteristic time of the turbulence disturbance becomes shorter than that of the chemical reactions, the heat sink effect of turbulence will dominate the combustion process. In this case, the extinction limits of CH4/coal flames increase with the u′. When the u′ increases from 1.86 to 2.66 m/s, the extinction limits of CH4/coal flames increase by 14–60%.

To reveal clearly the motion behaviors and velocity distributions of the unburned particles ahead of flame front in hexadecanol dust explosion, the flame propagation process was recorded by a high speed camera combining the particle image velocimetry (PIV) techniques. Flame region was recognized from the direct-light-emission photographs by the color model; Otsu's automatic threshold segmentation model and Fast Fourier Transform were adopted to set the threshold and to measure the flow field, respectively. It was observed that the propagating flame consisted of blue-spots of flame at the leading zone and luminous flame behind them. The flame firstly propagated towards the small particles nearby, when the small particles were completely pyrolyzed, the local pre-mixing flame would continue to heat the larger particles, establishing the local diffusion flame, and then the isolated blue luminous spots. Single dust particle velocity changed with the distance to the flame front. There existed a stagnation region, in which the particle velocities approached zero. Beyond this region, the particles settled down at a constant velocity. Vortex structures appeared in the regions where the settlement particles encountered with the outward movement particles.

The influences of fuel dilution, inlet velocity, and gravity on the shape and structure of laminar coflow CH4-air diffusion flames were investigated computationally and experimentally. A series of nitrogen-diluted flames measured in the Structure and Liftoff in Combustion Experiment (SLICE) on board the International Space Station was assessed numerically under microgravity (mu g) and normal gravity (1g) conditions with CH4 mole fraction ranging from 0.4 to 1.0 and average inlet velocity ranging from 23 to 90 cm/s. Computationally, the MC-Smooth vorticity-velocity formulation was employed to describe the reactive gaseous mixture, and soot evolution was modeled by sectional aerosol equations. The governing equations and boundary conditions were discretized on a two-dimensional computational domain by finite differences, and the resulting set of fully coupled, strongly nonlinear equations was solved simultaneously at all points using a damped, modified Newton's method. Experimentally, flame shape and soot temperature were determined by flame emission images recorded by a digital color camera. Very good agreement between computation and measurement was obtained, and the conclusions were as follows. (1) Buoyant and nonbuoyant luminous flame lengths are proportional to the mass flow rate of the fuel mixture; computed and measured nonbuoyant flames are noticeably longer than their 1g counterparts; the effect of fuel dilution on flame shape (i.e., flame length and flame radius) is negligible when the flame shape is normalized by the methane flow rate. (2) Buoyancy-induced reduction of the flame radius through radially inward convection near the flame front is demonstrated. (3) Buoyant and nonbuoyant flame structure is mainly controlled by the fuel mass flow rate, and the effects from fuel dilution and inlet velocity are secondary.

To reveal the characteristics of flame propagation and pressure in organic dust explosions during pneumatic transportation, 30 μm polymethyl methacrylate (PMMA) dust explosions with different airflow velocities were experimentally conducted in a horizontal pipe. The results showed that dust flame transited from the luminous flame with a continuous structure to discrete flames as flame propagating. After the appearance of the discrete flames, flame propagation velocity increased sharply owing to the positive feedback coupling between combustion and expansion. With the increase of dust concentration, the average flame propagation velocity, maximum flame temperature, temperature rising rate, maximum pressure, and maximum rate of pressure rise increased and then decreased. These explosion characteristics (except for the maximum flame temperature) increased significantly with the increase of airflow velocity. When airflow velocity increased from 6.39 m/s to 13.3 m/s, the peaks of average flame propagation velocity increased from 23.9 m/s to 38.2 m/s; the peaks of maximum pressure and maximum rate of pressure rise were increased by 1.9 and 6.1 times respectively. In addition, it was found that the optimum dust concentration decreased as airflow velocity increased. Furthermore, the relationship among flame propagation, temperature, and pressure was discussed in detail.

The combustion characteristics of lean non-premixed Thai synthetic natural gas on improved inverse diffusion flame (IDF) burner were investigated. Air and fuel nozzles were improved from normal diffusion flame burner (NDF) and set as inside and outside exits on burner, respectively. The combustion flames were studied with variations of fuel and air velocity. The luminous flame length was lower and premixed flame length was higher by designed IDF burner due to enhance of mixing between inside air and ambient air. When the equivalence ratio decreased, lower flame length and flame temperature were obtained. Moreover, the highest premixed combustion flames and the maximum temperature were observed equivalence ratio between Φ = 0.76-0.80 on lean non-premixed synthetic Thai natural gas flames with designed IDF burner.

The structure and dynamics of swirled, strongly pulsed, turbulent jet diffusion flames were examined experimentally in a co-flow swirl combustor. The dynamics of the large-scale flame structures, including variations in flame dimensions, the degree of turbulent flame puff interaction, and the turbulent flame puff celerity were determined from high-speed imaging of the luminous flame. All of the tests presented here were conducted with a fixed fuel injection velocity at a Reynolds number of 5000. The flame dimensions were generally found to be more impacted by swirl for the cases of longer injection time and faster co-flow flow rate. Flames with swirl exhibited a flame length up to 34% shorter compared to nonswirled flames. Both the turbulent flame puff separation and the flame puff celerity generally decreased when swirl was imposed. The decreased flame length, flame puff separation, and flame puff celerity are consistent with a greater momentum exchange between the flame and the surrounding co-flow, resu...

We investigate the radiative heat transfer in a co-flowing turbulent nonpremixed propane-air flame inside a three-dimensional cylindrical combustion chamber. The radiation from the luminous flame, which is due to the appearance of soot particles in the flame, is studied here, through the balance equation of radiative transfer which is solved by the Discrete Ordinates Method (DOM) coupling with a Large Eddy Simulation (LES) of the flow, temperature, combustion species and soot formation. The effect of scattering is ignored as it is found that the absorption dominates the radiating medium. Assessments of the various orders of DOM are also made and we find that the results of the incident radiation predicted by the higher order approximations of the DOM are in good agreement.

Experimental studies of the length of vertical isolated burning flames with hydrogen and methane outflow from cylindrical nozzles of diameter 2, 3, 5 and 10 mm into the atmosphere at Re 0 = 100-6800 have been made. The data obtained have been approximated by simple calculated dependences. The application of various methods for recording the length of a luminous flame (visual, black-and-white photography, video and digital cameras) has made it possible to not only explain the discrepancies between sizes but also record (at short exposures) elementary bending bright cylindrical jets creating, when superimposed on the another, one known spindle-shaped form of the flame.

Investigations of hydrothermal flames were reviewed with three primary objectives: (1) to classify and describe experiments performed to date, (2) to assess the state of the art of experimental instruments and measurement techniques, and (3) to characterize current understanding and outline future research needs of hydrothermal flame processes. For this review, hydrothermal flames are defined as rapid oxidation reactions occurring in a supercritical aqueous environment at sufficient temperature (∼1000 °C and higher) and rate to produce a luminous flame. Under these conditions, chemical kinetics are very fast, with reactions going to completion within 10–100 ms. Hydrothermal flame experiments were divided into two categories: laminar inverse diffusion flames in semi-batch reactor systems and turbulent diffusion flames in continuous reactor systems. Laminar diffusion flame studies have demonstrated the range of temperatures, pressures, and fuel and oxidant types and concentrations from which hydrothermal flames can be produced, while turbulent diffusion flame studies have focused on the application of hydrothermal flames to the remediation of aqueous waste streams. Experimental apparatuses, instrumentation, and techniques used in hydrothermal flame investigations are presented in some detail. Models developed to describe hydrothermal flame processes, especially flow fields in turbulent diffusion flame studies, are presented as well.

The effect of non-luminous thermal radiation on the combustion of a suspended methanol droplet in a low temperature (300 K) and low pressure (1 atm) environment is discussed in detail. Numerical results are obtained using a predictive, transient, two-phase, axisymmetric numerical model that includes surface tension effects. Radiation is modeled using the optically thin approximation with the product species CO2 and H2O as the radiating species. Results for combustion in a nearly quiescent atmosphere (initial Reynolds number 0.01) and initial droplet diameters in the range of 0.43 mm to 3 mm are presented. In agreement with the reported literature, it is shown that the effect of flame radiation is negligible when the initial droplet diameter is less than approximately 1 mm and becomes increasingly important for larger droplets. As a result, the average evaporation constant decreases with the initial droplet diameter. Radiation and surface tension combined as well as surface tension alone have a significant...

In order to clarify the mechanism of radiant heat transfer in the combustion chamber of DI diesel engines, radiant heat fluxes at various points on the piston head of a rapid compression and expansion machine were measured by using thin-film thermocouples. The ambient oxygen volume fraction, the swirl ratio, and the diameter and the number of the fuel injection nozzle hole were varied as experimental parameters. Measured radiant heat fluxes were compared with the total heat fluxes measured at the similar measurement locations to the radiant heat fluxes, the flame temperature and KL-factor obtained by the two-color method. In addition, in order to investigate the effects of the flame distribution and the reflectivity of the wall on the spatial distribution of the radiant heat loss, numerical simulations of the radiant heat transfer between the luminous flame and the chamber wall were also carried out via Monte-Carlo method. The results showed that the magnitude of the radiant heat flux is smaller than that of the total heat flux by one order, regardless of the changes in the oxygen volume fraction, swirl ratio and fuel injection nozzles. It is also shown that the spatial distribution of the radiant heat loss inside the piston cavity is uniform due to high reflectivity of the chamber wall made of aluminum alloy.

In this study, we have examined co-flowing diffusion flames formed in a triple port burner. The coannual burner consists of three concentric tubes. Air flows in both inner (central) and outer tubes, and fuel flows in the annulus between these air tubes. These velocities are defined as U1A, U2F, and U3A (or U3N), respectively. The fuel is propane. Two diffusion flames are formed in the boundary of fuel and air. For comparison, nitrogen is ejected in the outer tube to examine the mutual effect of two flames. The luminous flame height was obtained by direct photographs. A laser induced incandescence (LII) technique was applied to examine instantaneous soot concentration. To discuss the mixing of fuel and air, a NO-PLIF technique was used, where NO is added in the fuel flow. Results show that, as the fuel velocity, U2F, is increased, the flame height becomes larger. When nitrogen is ejected in the outer tube, the flame height is larger. However, the flame temperature for both cases is almost the same. The LII measurement shows that there are two soot regions in two luminous flame zones, which are emerged in the downstream. The soot concentration in the inner flame is higher than that in outer flame when U1A U3A. The minimum soot concentration is achieved when U1A and U3A are the same.

This paper shows the results of n-octane two droplets combustion in DC electric field following our previous study. Fiber-supported two droplets are burned under microgravity in air at atmosphere pressure and room temperature. Droplets are arrayed in the directions of electric field with 10 mm distance between droplets and in this condition both flames are individual and have very little interaction between them without an electric field. The equivalent applied elctric field is around 0 to 120 kV/m. The results show that flames are principally deformed to cathode with electric field, however, luminous flame at anode-side end is observed for anode-side flame while blue flame is observed at anode-side end of cathode-side flame. In addition, the increase in burning rate constants for both flames are observed and the increase in burning rate constant for cathode-side flame is larger than that for anode-side flame. The increase in burning rate constant is consistent to information from flame intensities. For cathode-side flame, an analogy to droplet combustion in force convection is discussed in terms of flame deformation and burning rate constants and a relation between them shows good agreement to that under forced convection quantitatively. The increase for anode-side flame is also mentioned from flame deformations.

51st AIAA/SAE/ASEE Joint Propulsion Conference (July 27-29, 2015.Hilton Orlando) Orlando, Florida, USA., The pintle injector is a promising candidate of the propellant injection systems for a rocket engine with deep throttling capability which is essential for future space transportation missions. However, studies focusing on combustion phenomena in a rocket engine with a pintle injector is rather limited. In this study, optical measurements inside an ethanol/liquid oxygen rocket engine combustor with a planar pintle injector are conducted to clarify spray and flame structures in the pintle injector. Combustion tests where the chamber pressure is 0.36 to 0.40MPa and O/F is 1.15 to 1.40 are conducted. Effects of the injection configuration on spray structures are also evaluated. High speed imaging techniques are used to observe the flame and spray structures under hot fire conditions. Strong chemiluminescence of CH is observed in the vicinity of the impinging point of two propellants. Luminous flame is observed intermittently in the vicinity of the faceplate and the upper wall of the combustor with the direct observation of the flame. Characteristic exhaust velocity efficiency with oxidizercentered configuration is lower than fuel-centered configuration due to the large amount of propellant impinging on the upper wall of the combustor. Periodical phenomena with the frequency of approximately 300Hz which can be related to atomization processes are also observed., 形態: カラー図版あり, Physical characteristics: Original contains color illustrations, 資料番号: AC1600102000, レポート番号: AIAA 2015-3845

A response surface model of the luminous flame emissivity of sodium pool fire has been proposed for use in safety analysis computer codes of a liquid metal fast reactor. The liquid sodium burns in air resulting in not only heat generation but also release of sodium oxide aerosols of sub-micron diameters. Aerosols levitating in air are radiative and they influence the allocation of combustion heat from the flame to atmospheric gas or sodium pool. The emissivity of the flame needs to be quantified, as it is one of user-specified parameters of the computer codes for the sodium fire analysis. The response surface model of the flame emissivity is developed based on numerical experiments on the physics of mass and heat transfer and behavior of the aerosol. Thermal-hydraulic equations have been solved coupled with aerosol dynamics and chemical reaction. Three influential variables on the emissivity are identified as pool temperature, gas temperature and oxygen molar fraction in the air. It has been found that the emissivity is calculated reasonably as a function of the three variables. The proposed response surface model can be easily employed in the sodium fire analysis codes because it is a simple quadratic expression. For the safety evaluation of the sodium fire, combined use is recommended of the proposed model and the lumped-mass zone model code.

The flame jet width and flame jet velocity of the burnt gases of a premixed Bunsen-type flame are important parameters for quantifying the heat-transfer rates of these flames. In this paper a simple expression is derived to estimate the flame jet width and flame jet velocity of burnt gases of a free flame after expansion over the flame front for the special case of plug flow in and above the burner. The results of particle image velocimetry experiments on three different flame types having different oxygen concentrations are presented. The model is validated with these measurements and shows good agreement. Deviations occur, however, when the curved flame front area of the flame tip is not negligible compared to the total flame area.

Ignition and combustion characteristics of diesel spray impinging on a wall were experimentally investigated. Ignition position and appearance position of the luminous flame kernel were stereoscopically observed using a two-way fiber optical system. Flat impingement wall was fixed in a high temperature, high pressure combustion chamber. Inclined angle of the flat wall was set at 30 degees or normal against the center axis of the injection spray. Distance from nozzle tip to the impingement point on the wall was set at 50 mm. Effects of injection pressure and fuel quantity on ignition position were investigated. As the result, ignition positions were observed near the spray periphery. And the radial position of ignition was shifted to the larger radial position when the injection pressure increased. Luminous flame appeared near the wall surface in the both cases of 30 degrees and normal impingements. And the region of luminous flame appearance was not changed significantly with an increase of injection pressure.

In nonpremixed combustion, edge flames can form as a region of flame propagation or flame recession. Forwardly propagating edge flames, as occur in lifted flames, have a local gas velocity at the flame edge that is from unburned partially premixed fuel and air into the flame. These flames represent an ignition process, and permit the flame itself to either stabilize against an incoming gas stream or propagate into unburned fuel and air. Negative edge flames represent the opposite case of a local gas velocity from burned products through the flame edge. The negative edge flame represents a local extinction process, and occurs, for example, during vortex-induced extinction of a nonpremixed flame sheet. A technique for generating steady negative edge flames in a standard counterflow burner is presented, which permits detailed examination of their properties. A coannular counterflow burner is used to create a strain gradient that quenches a central diffusion flame. Unlike previous research on strain-induced flame edges, the axisymmetric flow field ensures gas flow from products through the edge. Measurements of the edge flame's sensitivity to global strain rates and fuel mixtures are presented, along with measurements of the edge flame structure using OH fluorescence and CH emission imaging.

An attempt has been made to examine the characteristics of ion current obtained from Low-Oxygen, High-Temperature Air Combustion. Non-luminous zone is seen in the upstream region and luminous zone is seen in the downstream region in low-oxygen, high-temperature air flame, while only the luminous region is seen in the high-temperature air flame. The sharp spike like signals are seen on the ion current obtained from the luminous zone of both high-temperature air and low-oxygen, high-temperature air flames. The spike like signal is not seen on the ion current obtained from the non-luminous zone of low-oxygen, high-temperature air flame. The ion current looks quite similar with that obtained from turbulent premixed flames in the reaction sheet regime. It implies that the local structure of low-oxygen, high-temperature air flame is not the flameless combustion but shmilar to that of turbulent premixed flames in the reaction sheet regime, in which the local reaction zone is separately defined from burnt and unburnt gases.

A premixed flame stabilized on a circular porous plug burner produces a uniform, steady luminous flame front. Throughout much of the parameter range hydrocarbon-oxygen mixtures form spiral-shaped fronts. In methane-oxygen flames at low pressure, the flame exhibits a sequence of states as a control parameter is decreased. These states include periodic rotation of a spiral front; precession of the spiral front in a direction opposite to its rotation, corresponding to doubly periodic petals-out meandering; and nonperiodic states with intermittent jumps associated with linear excursions of the tip, which occur after the spiral front has reached the boundary of the circular burner. We use Karhunen-Loeve (KL) analysis to find the coefficients of the dominant KL spatial eigenfunctions. Their phase space portraits and power spectra provide a description of the dynamics as flow rates are reduced and the system destabilizes. We discuss how these experimental results relate to previous theoretical studies that assume Euclidean symmetry for the experimental configuration.

This paper reports a PIV measurement technique for application to the combusting flow with a luminous flame. The experiments are conducted on the velocity field and the flame structure of spray combustion in a combustion chamber model. The results with and without combustion indicate that the oncoming flow velocity from the burner is enhanced by the chemical reaction to promote the re-circulating flow inside the chamber model. The simultaneous visualization of flame structure showed the distribution of luminous flame, the unburned droplets, and the soot structure generated in a flame.

This article reports observations of structures consistent with triple flame reaction zones in the stabilization region of turbulent nonpremixed jet flames. Previous studies of laminar mixing layers and nonpremixed jets show Hi brachial structures, however reports of triple flame structures in turbulent flowfields are sparse, The asymmetric coflow, which facilitates visualizing the luminous flame structure, is described and the observed double and triple flame structures are discussed. Flame luminosity data is presented and the relevance to CH planar laser-induced fluorescence (PLIF) studies of leading edge flame structures is discussed. Furthermore, connection is drawn to select theoretical studies of triple flames and partially premixed combustion.

A comprehensive numerical analysis has been conducted to study the combustion of a double-base homogeneous propellant in a rocket motor. Emphasis was placed on the motor internal flow development and its influence on propellant combustion. The formulation is based on the Favre-averaged, filtered equations for the conservation laws and takes into account finite-rate chemical kinetics and variable thermophysical properties. Turbulence closure is obtained using the large-eddy-simulation technique. The contribution of large energy-carrying structures to mass, momentum, and energy transfer is computed explicitly, and the effect of small scales of turbulence is modeled. The governing equations and associated boundary conditions are solved using a time-accurate, semi-implicit Runge-Kutta scheme coupled with a fourth-order central difference algorithm for spatial discretization. The motor internal flowfield is basically determined by the balance between the inertia force and the pressure gradient arising from the mass injection at the propellant surface. The temporal evolution of the vorticity field shows a laminar upstream region, a transition zone in the midsection of the chamber, and a fully developed turbulent regime further downstream. The turbulent mixing proceeds at a rate faster than chemical reactions, and the flame stretch is strong enough to regard propellant combustion as a well-stirred reactor. The combustion wave in the laminar region exhibits a two-stage structure consisting of a primary flame, a dark zone, and a secondary luminous flame. The enhanced energy and mass transport in the turbulent region partially merges the primary and secondary flames, thereby raising the temperatures in the dark zone. In the present study, the smoother axial velocity gradient and vertical flow convection prevent turbulence from deeply penetrating into the primary flame zone. The turbulence energy spectra indicate dominant harmonics in a frequency range capable of triggering combustion instabilities.

The shapes (luminous flame boundaries) of round luminous nonbuoyant soot-containing hydrocarbon/air laminar jet diffusion flames at microgravity were found from color video images obtained on orbit in the Space Shuttle Columbia. Test conditions included ethylene- and propane-fueled flames burning in still air at an ambient temperature of 300 K, ambient pressures of 35-130 kPa, initial jet diameters of 1.6 and 2.7 mm, and jet exit Reynolds numbers of 45-170. Present test times were 100-200 s and yielded steady axisymmetric flames that were close to the laminar smoke point (including flames both emitting and not emitting soot) with luminous flame lengths of 15-63 mm. The present soot-containing flames had larger luminous flame lengths than earlier ground-based observations having similar burner configurations: 40% larger than the luminous flame lengths of soot-containing low gravity flames observed using an aircraft (KC-135) facility due to reduced effects of accelerative disturbances and unsteadiness; roughly twice as large as the luminous flame lengths of soot-containing normal gravity flames due to the absence of effects of buoyant mixing and roughly twice as large as the luminous flame lengths of soot-free low gravity flames observed using drop tower facilities due to the presence of soot luminosity and possible reduced effects of unsteadiness. Simplified expressions to estimate the luminous flame boundaries of round nonbuoyant laminar jet diffusion flames were obtained from the classical analysis of Spalding (1979); this approach provided Successful Correlations of flame shapes for both soot-free and soot-containing flames, except when the soot-containing flames were in the opened-tip configuration that is reached at fuel flow rates near and greater than the laminar smoke point fuel flow rate.

The purpose of the present research is to investigate the aerodynamic characteristics of evaporating droplet near the burner where flame is hold by the recirculating flow and to characterize droplet aerodynamic response regarding follow or penetration and turbulent interaction with the surrounding air by classifing droplets. Aerodynamic characteristics of combusting spray were measured in a small industrial oil furnace by a phase Doppler anemometer. The size-classified technique was used together with the relative Reynolds number and the recombined two dimensional size-classified droplet velocity. The results show that the two dimensional behavior of a spray flame can be represeated, The size-classified droplet technique can provide very useful information on droplet aerodynamics and dispersion. The follow/penetration characteristics of spray can be understood very well in consideration of the features of classified droplet in combustion condition. Larger droplets had a large mass and inertia, and thus could penetrate through the recirculation flow region. Consequently larger droplets form large luminous flame. On the other hand, smaller droplets were entrained by the turbulent air flow and played a role of flame-holder. In addition, the interaction between fuel droplets and air flow is a process that significantly affects flame holding mechanism of spray flames

Observations of the structure and soot properties of round, soot-emitting, nonbuoyant, laminar jet diffusion flames are described, based on long-duration (175-230 s) experiments at microgravity carried out on orbit in the Space Shuttle Columbia. Experimental conditions included ethylene-fueled flames burning in still air at nominal pressures of 50 and 100 kPa and an ambient temperature of 300 K with luminous flame lengths of 49-64 mm. Measurements included luminous flame shapes using color video imaging, soot concentration (volume fraction) distributions using deconvoluted laser extinction imaging, soot temperature distributions using deconvoluted multiline emission imaging, gas temperature distributions at fuel-lean (plume) conditions using thermocouple probes, soot structure distributions using thermophoretic sampling and analysis by transmission electron microscopy (TEM) and flame radiation using a radiometer. After an initial 20s flame stabilization period (caused by effects of ignitor disturbances, fuel flow rate adjustments and transient development of flame structure), the flames reached steady-state conditions aside from slow (quasisteady) changes due to pressure increases and ambient oxygen consumption within the test chamber caused by combustion. The present flames were larger, and emitted soot more readily, than comparable flames observed during ground-based microgravity experiments due to closer approach to truly steady conditions resulting from the longer test times and the reduced gravitational disturbances of the space-based experiments. Increasing the pressure from 50 to 100 kPa for soot-emitting flames of similar length caused maximum soot volume fractions to increase from 2 to 32 ppm and average primary soot particle diameters to increase from 24 to 40 nm, showing that soot emissions are the result of the relative rates of soot formation and oxidation and do not correlate closely with peak soot concentrations and primary particle sizes within the flames. In addition, comparable sootemitting buoyant laminar diffusion flames at normal gravity and 100 kPa have significantly smaller maximum primary soot particles (32 nm diameter implying roughly 50 percent less mass) than the nonbuoyant flames. It was also found that the tipopening phenomena associated with nonbuoyant sootemitting flames is caused by extinction of the flame near its tip due to radiative heat losses, which means that emissions of unburned fuel are associated with emissions of soot in the present nonbuoyant flames. Finally, soot production properties (characterized by maximum soot concentrations) are similar for various paths through the 50 kPa flame where effects of radiative extinction and soot particle thermophoresis are small, suggesting potential for a simple state relationship between soot concentrations and mixing level (mixture fraction) at flame conditions representative of many practical applications. This behavior follows because flame residence times are relatively independent of path for nonbuoyant laminar jet diffusion flames, and may help to explain the universality of many properties of soot emitted from practical flames (which generally are relatively nonbuoyant).

Interactions between acoustic waves and the transient combustion response of a double-base homogeneous propellant in a rocket motor have been analyzed numerically. The analysis extends the previous work on gas-phase flame dynamics to include the coupling with condensed-phase processes. Consequently, a more complete description of propellant combustion response to imposed acoustic oscillations can be obtained. Emphasis is placed on the near-surface flame-zone physiochemistry and its coupling with unsteady propellant burning in an oscillatory environment. The formulation treats complete conservation equations and the finite-rate chemical kinetics in both the gas-phase and subsurface regions. The instantaneous propellant burning rate is predicted as part of the solution. Various distinct features of unsteady heat release arising from propellant combustion response in a motor with forced oscillations are studied systematically. As in the pure gas-phase dynamics of the previous case, the dynamic behavior of the luminous flame plays a decisive role in determining the motor stability characteristics. However, the propellant combustion response may qualitatively modify the temporal evolution of heat-release distribution in the luminous flame and as a result exerts a significant influence on the global stability behavior. The primary flame structure adjacent to the propellant surface is usually little affected by flow oscillation. This may be attributed to the large thermal inertial of the condensed phase, which tends to restrain the temperature variation in the near-surface zone in the present study of laminar flows. The situation with a turbulent flow may be drastically different, as turbulence may penetrate directly into the, primary flame and substantially change the local flame dynamics and transport phenomena.

A new way of using thermography for radiative objects like pool flames has been developed by the author. The thermography method, which has already been applied to large pool fires, is applied to small pool flames in this paper. The radiative characteristics and flame structure of small pool flames are considered by examining flame temperature distribution from a radiation point of view. In addition, it is shown that pool flame structure for various fuels becomes clearer using the standard deviation and the coefficient of variation obtained from thermographic data.

The interactions between acoustic waves and gas-phase flame dynamics of a double-base homogeneous propellant in a rocket motor has been studied by means of a comprehensive numerical analysis. The formulation treats the complete conservation equations of mass, momentum, energy, and species concentration, and accounts for finite rate chemical kinetics in the gas phase and subsurface reactions. The model has been implemented to examine the detailed flow structures and heat-release mechanisms in various parts of the motor, including microscale motions near the propellant surface and macroscale motions in the bulk of the chamber. Results indicate that strong interactions between exothermic reactions and acoustic waves occur in regions with steep temperature gradients due to the large activation energy of the associated chemical kinetics. The dynamic behavior of the luminous flame plays a decisive role in determining the motor stability characteristics. Distributed combustion response in the gas phase provides the energy for driving flow oscillations, and can be treated correctly as a combination of monopole and dipole sources based on acoustic theory.

A droplet cloud of liquid fuel produced by an ultrasonic atomizer was ignited by a spark, and the ball of flame propagating outwards was observed in detail in order to elucidate the mechanism of flame propagation and the complicated group combustion behavior of spray flames. For this purpose, the instantaneous images of droplet clusters, OH-radical chemiluminescence and C2-band flame luminosity were taken simultaneously, and the light emission signals in OH- and CH-bands, Mie-scattering signal from droplets, and the size and velocity of droplets were monitored simultaneously in time series. It was found that a nonluminous flame propagated ahead of the luminous flame, that droplets disappeared in the luminous flame zone due to their rapid evaporation, and that a number of small droplet clusters burned in the diffusion combustion mode associated with solid-body emissions inside a ball of flame.

In this paper, the spatial distribution of radiative heat flux from a luminous flame to various positions on the cylinder head of a direct injection diesel engine is modelled and the results compared with some published experimental investigations. The model is primarily based on measured pressure data, which are converted into fuel-burned rate data through a single-zone heat-release rate analysis. Coupled with appropriate soot formation and oxidation models, the fuel-burned rate data are converted into the soot contents in the cylinder. By separating the combustion chamber into a burned zone and an unburned zone, the radiation temperature, the absorption coefficient and the spatial distribution of radiative heat flux to the cylinder walls are calculated.

The laminar smoke point properties of jet diffusion flames -- the luminous flame length, the residence time, and the fuel flow rate, at the onset of soot emission from the flames -- have proven to be useful global measures of the soot properties of nonpremixed flames. These measures provide a means to rate several aspects of sooting properties: the relative tendency of various fuels to emit soot from flames the relative effects of fuel structure, flame temperature, and pressure on the soot properties of flames and the relative levels of continuum radiation from soot in flames. However, recent studies suggest potential for fundamental differences between the laminar smoke point properties of buoyant and non-buoyant flames. Thus, the overall objective of present investigation was to measure the laminar smoke point properties of nonbuoyant flames, due to their relevance to many industrial processes where effects of buoyancy are small. Prior to this work, no experiments have been reported to assess these potential effects of buoyancy on laminar smoke point properties. Thus, the present objective was to measure the laminar smoke point flame lengths and residence times of nonbuoyant flames. The scope of the study was limited to round ethylene and propane jetmore » diffusion flames burning in slightly vitiated air at pressures of 0.5--2.0 atm. A low-gravity test environment was used to obtain nonbuoyant flames at the small flow velocities characteristic of laminar smoke point conditions.« less

A droplet suspension of liquid fuel produced by an ultrasonic atomizer was spark-ignited. The flame ball propagating outwards was visualized in order to elucidate the mechanism of flame propagation and the complicated group structure of spray flames. It was found that a nonluminous flame was propagating continuously through a gas-phase mixture followed by luminous flamelets, so that a number of small-scaled droplet clusters with luminous emissions were observed behind the flame front propagating in premixed combustion mode. This fact implies that the group structure of a spray flame is not simple but complex.

When a column of cetane droplets freely falling from an ultrasonic atomizer was ignited behind a reflected shock, no ignition occurred at a temperature below ca. 1100 K, even if the pressure was as high as ca. 1 MPa. Although a higher temperature condition ensured ignition, no flame so luminous as to be recorded by high-speed photography appeared, and even if a luminous flame lump appeared at an extremely high temperature, it disappeared without spreading over the entire column of droplets. It is known however that, if a fuel is injected into a diesel cylinder or an electric furnace, ignition occurs even at a temperature as low as 650 K with a luminous flame spreading over the entire spray. Supposing that the above difference was caused by the difference in intensity of turbulent mixing of droplets with hot air, turbulence-generating rods were placed on the upstream side of the spray column. As a result, the ignition limit was lowered to ca. 840 K, and the ignition delay was reduced as the intensity or turbulence was increased. In addition, the light emission of the flame was intensified, and normal spray combustion was maintained in the low-temperature atmosphere after the shock tube ceased its operation.

There are two well known scaling laws for lengths of laminar jet diffusion flames on circular burners. The more prevalent of these is a linear relationship between normalized flame length and Reynolds number. The other, invoked in most past studies at elevated gravity, relates these lengths to a function of Reynolds and Froude numbers. The Reynolds scaling indicates stoichiometric flame lengths are independent of gravity level, while the Reynolds-Froude scaling indicates lengths decrease at elevated gravity. The ability of both approaches to correlate flame lengths is examined. Published lengths of laminar hydrogen, methane, ethane, and propane flames in 1–15 times earth gravity are considered. The Reynolds scaling yields better correlations of flame lengths for all fuels except hydrogen. The Reynolds-Froude scaling has a weaker theoretical basis and does not account as accurately for variations in fuel flowrate. Further, it does not admit microgravity flames and past predictions for its limiting behavior at low and high Froude number are not supported. Observed reductions in luminous flame length at elevated gravity are attributed to soot interference. The Reynolds scaling is recommended for flame lengths at microgravity, normal gravity, and elevated gravity.

A detailed analysis of the unburned material heat-up during upward flame spread over small slabs of PMMA is provided using a numerical model. The two-dimensional time-dependent Favre-averaged Navier–Stokes equations coupled with sub-models for turbulence, combustion, soot formation, and radiation are solved for the gas phase. The modelling of condensed phase processes is based on the one-dimensional heat conduction equation and pyrolysis is treated as a phase change using the latent heat approach. Pyrolysis heights, xp, up to 0.6 m are considered. Model results agree with data on the pyrolysis front trajectory and heat release rate per unit width, Q ˙ ′ , as a function of xp from experiments in which the lateral air entrainment is expected to have no significant effect on flame spread. It is shown that the average luminous flame height, xfl, defined by the height at which the total wall heat flux is equal to 10 kW/m2, correlates with Q ˙ ′ 1.25 for Q ˙ ′ 20 kW / m , whereas it correlates with Q ˙ ′ 2 / 3 for Q ˙ ′ 20 kW / m , as deduced from experiments. Appropriate scaling of the total wall heat flux distribution is found in terms of ξ = ( x - x p ) / ( x fl - x p ) . This scaling allows us to locate the continuous flame region between ξ=0 and 0.4 and the plume region above ξ=1.6, the intermittent flame region being located between the two. Both continuous and intermittent flames are found to play an important role in the process of heating the unburned solid fuel whereas the plume contribution is negligible.

When a cetane droplets column freely falling from an ultrasonic atomizer is ignited behind a reflected shock, no ignition occurs at a temperature below ca, 1100K even if the pressure is sufficiently high (∼1 MPa). Although a higher temperature ensures ignition, no flame so luminous as to be recorded by high-speed photography appears, and even if a luminous flame lump appears, it disappears without spreading throughout the droplets column. If, on the other hand, fuel is injected into a diesel cylinder or an electric furnace, ignition occurs even at a temperature as low as 650K with a luminous flame spreading over the entire spray. Supposing that the above difference resulted from the difference in intensity of turbulent mixing of droplets with hot air, turbulence-generating rods were placed on the upstream side of the spray column. As a result, the ignition limit was lowered down to ca. 840K, and the combustion made shifted from a blue-flame mode or locally confined ignition mode into an ordinary spray combustion mode.

Flame height is defined by the experimentalists as the average position of the luminous flame and, consequently is not directly linked with a quantitative value of a physical parameter. To determine flame heights from both numerical and theoretical results, a more quantifiable criterion is needed to define flame heights and must be in agreement with the experiments to allow comparisons. For wall flames, steady wall flame experiments revealed that flame height may be defined by a threshold value on the wall heat flux. From steady wall flame measurements, three definitions of flame height from wall heat flux are retained: the first is based on the continuous flame while the two others are based on threshold values of 4 kW/m2 and 10 kW/m2. These definitions are applied to determine flame heights from a two-dimensional time-dependent CFD model used to describe flame spread on a vertical slab of PMMA. Results show that the predicted flame heights are consistent with the available data of the literature. Defining flame height by threshold values on the wall heat flux of 4 and 10 kW/m2 allows the correlation of the wall heat flux in terms of (x-xp)/(xfl-xp), which is the dimensionless characteristic length scale for upward flame spread. It is also found that the continuous flame is not a characteristic length for the heat transfer to the unburnt fuel and is not really appropriate to define flame height in upward flame spread. © 2007 Elsevier Ltd. All rights reserved.

The emissive power (E) of a flame depends on the size of the fire and the type of fuel. In fact, it changes significantly over the flame surface: the zones of luminous flame have high emittance, while those covered by smoke have low E values. The emissive power of each zone (that is, the luminous or clear flame and the non-luminous or smoky flame) and the portion of total flame area they occupy must be assessed when a two-zone model is used. In this study, data obtained from an experimental set-up were used to estimate the emissive power of fires and its behaviour as a function of pool size. The experiments were performed using gasoline and diesel oil as fuel. Five concentric circular pools (1.5, 3, 4, 5 and 6 m in diameter) were used. Appropriate instruments were employed to determine the main features of the fires. By superimposing IR and VHS images it was possible to accurately identify the luminous and non-luminous zones of the fire. Mathematical expressions were obtained that give a more accurate prediction of Elum, Esoot and the average emissive power of a fire as a function of its luminous and smoky zones. These expressions can be used in a two-zone model to obtain a better prediction of the thermal radiation. The value of the radiative fraction was determined from the thermal flux measured with radiometers. An expression is also proposed for estimating the radiative fraction.

The effect of non-luminous thermal radiation on suspended (constant relative velocity) methanol droplet combustion in a low temperature (300 K) and low pressure (1 atm) environment is discussed in detail. Numerical results are obtained using a predictive, transient, two-phase, axisymmetric numerical model that includes surface tension effects. Radiation is modeled using the optically thin approximation with the product species CO2 and H2O as the radiating species. Results for combustion in a quiescent atmosphere (initial Reynolds number 0.01) and initial droplet diameters in the range of 0.43 mm to 3 mm are presented. The results show that the effect of flame radiation is negligible when the initial droplet diameter is less than approximately 1 mm and becomes increasingly important for droplets with initial diameters greater than approximately 1 mm, as reported in previous literature. The average evaporation constant decreases with the initial droplet diameter. Both radiation and surface tension have a significant effect on the predicted extinction diameters of initially larger droplets. The extinction diameter presents a non-linear variation with the initial droplet diameter for initially larger droplets and agreement with experiments is good.

Soot formation within a highly luminous turbulent non-premixed flame burning acetylene in air was investigated by conducting laser scattering and extinction experiments. Mean soot volume fractions, spherule diameters, and aggregate sizes were exclusively characterized based on an optical interpretation that can properly account for the actual particulate morphology. This allowed decoupling of surface growth and oxidation from the unavoidable aggregation process and therefore provided accurate descriptions of turbulent soot dynamics. The present experiments broadened the database in a lightly sooting ethylene flame previously considered so that the effects of fuel type on particle evolution within turbulent flames could be explored. The relatively high particle concentrations in the acetylene flame also offered conditions that are encountered in many practical combustors operating at elevated pressures, e.g. diesel engines. The findings reported here are relevant not only to developing computational models for accurate predictions of radiative transfer but also to controlling pollutant emissions from combustion systems.

A comprehensive analysis and data-acquisition of the time-resolved numerical simulations of combustion dynamics of double-base homogenous propellant in a rocket motor is performed. The physicochemical processes occurring in the flame zone and their influence on the unsteady flow evolution in the combustion chamber are investigated in-depth using large eddy simulation (LES) techniques. A fivestep reduced-reaction mechanism is used to obtain a two-stage flame structure consisting of a primary flame, dark zone, and secondary luminous flame in the gas phase. It is observed that, for homogeneous propellant combustion, the chemical time scale is much higher than the smallest Kolmogorov time scale, rendering a highly stretched and thickened flame. The chemical reactions proceed at a slower rate than turbulent mixing and propellant combustion may be locally treated as a well-stirred reactor. The unsteady flowfield in the chamber indicates three regions of flow evolution: the upstream laminar region, transition zone, and fully developed turbulent regime further downstream. The oscillatory flame modifies the vorticity distribution through strong baroclinic effects causing mismatch of the radial density and axial pressure gradients. Volume dilatation also plays an important role in affecting the vorticity transport. A theoretical formulation exploring the coupling between the oscillatory heat-release, and ensuing entropy and vorticity waves in turbulent flows is established by deriving a generalized wave equation. The work done by Reynolds stresses and the interactions between vorticity and flame dynamics may cause resonance effects and excite eigenmode pressure oscillations leading to self-sustained unstable motions within the chamber. Nomenclature a = speed of sound

The purpose of the present paper is to develop a mathematical model for predicting thermal field and heat transfer inside short length/height ratio, rectangular and nonhomogeneous material pipe connecting a combustion chamber with outdoors, while the gas combustion temperatures are in the range of 2 000 to 3 000 K and the pipe is made of heat-resistant material. The used approach is based on a finite difference simulation and makes certain idealizations and approximations in order to get to an engineering workable and at the same time adequate, solution. Main assumptions are an equivalent temperature cavity to describe radiant heat transfer due to both the combustion chamber luminous flame and outdoors. The proposed model shows a very good agreement with experimental results coming from an experimental set-up used for materials testing, that is embodied in a refractory rectangular pipe, at high temperature.

Both fluctuations of local velocity and local temperature were measured in a steady turbulent diffusion flame of propane by using the semiconductor laser 2-focus velocimeter and the optical fiber thermometer and the optical fiber thermometer respectively. The flame temperature and the soot particle density were calculated by applying the IR two- color method to the measured radiant energy from the soot particles in the flame. In the analysis of the frequency power spectra of temperature and velocity fluctuations, the correlation-based slotting technique was adopted for those data with the nonuniform time interval. It is shown that the time mean value and the fluctuation of the flame temperature decrease gradually toward downstream in the luminous flame region, and those of the soot density increase due to decay of turbulence along the flame axis. On the other hand, both time mean and fluctuation of the flame temperature increase in the radial direction from the center to the periphery due to the effect of air entrainment marked in the peripheral region of the flame. Furthermore, the power spectrum of the velocity fluctuation is not always the same as that of the temperature fluctuation in the flame center.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

When a column of droplets freely falling from an ultrasonic atomizer was ignited behind a reflected shock, no ignition occurred at a temperature below 1100 K, even if the pressure was as high as IMPa. Although, a higher temperature condition ensured ignition, no luminous flame was observable by high-speed photography, and even if a luminous flame lump appeared at an extremely high temperature, it disappeared without spreading over the entire column of droplets in this case. It is known however that, if a fuel is injected into a diesel cylinder or an electric furnace, ignition occurs even at a temperature as low as 650 K with a luminous flame spreading over the entire spray. These differences could be caused by the effects of turbulent mixing between fuel droplets and hot air, in fact, turbulence-generating rods were placed on the upstream side of the spray column. Experimental results indicates that the ignition limit was lowered to 840 K, and the ignition delay period was decreased by increasing the intensity of turbulence. Furthermore, the light emission of the flame was intensified, and normal spray combustion was maintained in the low-temperature atmosphere after the shock tube ceased its operation.

An endoscope-based image acquisition-and-processing camera system was used for diagnostics of pilot injection combustion in a single-cylinder heavy duty diesel engine. A study of the pilot injection or light load is of interest because the spray breakup, mixing and vaporization processes are less influenced by heat feedback from the flame than in full injection cases. This allows the spray process to be decoupled from the combustion process. The experimental cases were modeled using a version of the KIVA-II code that includes improvements in the turbulence, wall heat transfer, spray, ignition and combustion models. The combustion of the pilot injections was characterized by ignition sites located below the point where the spray impinges on the piston bowl surface for the injector configuration used in this study. Multiple ignition sites were observed and the majority of the combustion occurred at the bottom of the piston bowl, spreading along the bowl edge. As the start-of-injection timing was retarded some evidence of ignition above the impingement point was observed. However, these ignition sites did not develop into major combustion zones. Comparisons of the experimental results, which included pressure traces, heat release rates and the luminous flame images, and the numerical computations were made tomore » assess the performance of current models in the KIVA-II code. Good agreement was obtained for the timing and location of ignition. The penetration of the flame observed in the luminous flame images was also captured in the predictions. A feature of the pilot injection that was not captured by the prediction is the spread of the flame along the edge of the piston bowl. The comparisons indicate that recent improvements in KIVA-II allow better prediction of the combustion of pilot injections.« less

A droplet cloud of liquid fuel produced by an ultrasonic atomizer was ignited by a spark, and the flame ball propagating outward was observed in order to elucidate the mechanism of flame propagation and complicated group combustion behaviors of spray flames. For that purpose, the instantaneous images of droplet clusters, OH-radical chemiluminescence and C2-band flame luminosity were taken simultaneously. Furthermore, the light emission signals in OH- and CH-bands, Mie-scattering signal from droplets, and the size and velocity of droplets were monitored simultaneously in time series. It was found that a nonluminous flame propagated ahead of a luminous flame, and that droplets disappeared in the luminous flame zone due to rapid evaporation, where a number of small-scaled droplet clusters were burning in diffusion combustion mode associated with solid-body emissions.

Soot formation process was examined by high speed photographs, using a single combustion diesel engine with a transparent swirl chamber. Fuel-air mixture and flames, and soot clouds were visualized by the schlieren method and the back-illuminated method, respectively. A three dimensional simulation program with soot formation and oxidation models was developed to clarify diesel soot formation processes. The models consist of several models previously proposed and partly improved in this study. Good agreement was obtained between calculated and experimental results. The following points were clarified through observation and numerical studies: The main soot area is considerably smaller than luminous flame area, especially in the initial soot formation process; the main soot cloud first appears in the tip region of fuel-air mixture, downstream of ignition position a few submilliseconds after the ignition. It is the soot carried down from the ignition position by a gas flow; and temperature is more influential in soot formation, rather than fuel vapor concentration.

A laminar methane-air diffusion flame has been extensively probed for its temperature, velocity, and stable species' concentration profiles. The data obtained were used to validate a theoretical model capable of characterizing flame macrostructure and useful in predicting the effects of flow rate, equivalence ratio, fuel and air preheat, and nozzle size on the thermal and aerodynamic fields established in confined, axisymmetric, laminar diffusion flames. The model utilizes the Burke and Schumann flame sheet concept to locate the stoichiometric fuel-oxygen interface and, hence, the points of heat release. The model eliminates the restrictions of the classical Burke-Schumann model in that allowances are made for natural convection effects and variable thermodynamic and transport properties. The stoichiometric fuel-oxygen interface was found to coincide with the outer edge of the primary reaction zone and represented a surface through which an amount of oxygen in stoichiometric proportion to the amount of fuel fired diffuses. The inner edge of the reaction zone was defined by the profile of the luminous flame core and was found to be well fitted by the theoretical predictions when the ratio of oxygen and fuel diffusing to the flame front was assigned a value 0.88 times the stoichiometric coefficient. The concentration and temperature profiles could be generalized by plotting the results as a function of the local-equivalence ratio.

A shock tube technique was developed to observe the ignition process of sprays free from aerodynamic phenomena, such as atomization and mixing. In this technique, a premixed spray column, falling freely from an ultrasonic atomizer directed downwards, was ignited by a reflected shock. The ignition processes observed by this technique seem to be different in essence from those observed by the usual injection technique, in which fuel is injected into a hot atmoshere. The probability of normal ignition occurring reduces rapidly as the temperature is lowered below 1060 K for cetane and a pressure of 0.85 MPa. The luminosity of the flame varies widely up and down, and the period of appearance of a luminous flame observable by high-speed photography is only a part of the ignition process. The dependences of the ignition delay on the temperature and pressure also differ substantially from those obtained by the injection technique.