Flame Volume Research Articles

Imaging the active flame volume for pulsed laser-enhanced ionization spectroscopy

A technique has been developed which can determine where within a flame laser-enhanced ionization (LEI) actually takes place, thus forming an image of the LEI active volume. The method is based upon spatial control of the flame volume sampled by the ionization detection electrodes by variation of the applied voltage. As and example, this procedure has been used to observe anomalous contributions to LEI caused by scattering or fluorescence from the laser beam to other regions in the flame

Influence of an external radiant flux on a 15-cm-diameter kerosene pool fire

An experimental study is presented of the influence of an externally applied thermal radiant flux on the evaporation, or burning rate of a 15-cm-diameter kerosene pool fire, and on the structure of the buoyant flame developed over the liquid fuel surface. The results show that the burning rate increases with the external radiation, but as the radiant flux increases its influence is reduced. This seems to be due primarily to the related increase in the flame volume and opacity that attenuate the radiation that actually reaches the liquid surface. Detailed measurements of the temperature, CO 2, CO, and O 2 concentrations, and monochromatic absorption coefficient—or equivalently soot—provide information about the flame structure and its dependence on the applied radiant flux. Increasing the radiant flux causes an increase in the flame volume and consequently a widening of the spatial distribution of the flame structure, although the overall flame characteristics and maximum temperatures and concentration levels remain unchanged.

Laser soot-scattering imaging of a large buoyant diffusion flame

A novel diagnostic technique, which makes use of laser light scattered by soot particles, was used in an effort to identify the flame sheets within a natural gas diffusion flame. Soot particles, inherently created and consumed in the flame, were used as the scattering medium, which obviated the need for externally supplied seed material. Since no foreign material was added to the flame, the current technique can be considered truly nonintrusive. The soot distribution within a large buoyant natural gas diffusion flame is argued to be a reasonable marker of the flame sheets. Measurements made in 47.4–190 kW natural gas flames stabilized on a 0.5 m diameter burner show that the flame sheets are highly wrinkled and convoluted surfaces. The flame sheets are distributed fairly uniformly within the instantaneous volume of the flame, based on images of the associated soot, and the instantaneous flame volume is devoid of soot for 40–60% of the time. When soot is present, it is observed as thin sheets which become narrower in regions where the average strain rate is estimated to be greater.

Three-dimensional flame development in a closed vessel. 1st report Theory and numerical analysis.

In this report, a mathematical model for expressing three-dimensional development of a laminar flame in a closed vessel has been proposed, and the numerical calculation method, which the author termed a space difference one, is derived. This made possible the evaluation of the changes of properties, such as burning time, flame volume, flame- and flammable-surface area, mass burning rate, and mass burnt fraction, and to illustrate the three-dimensionally expanding flame. The characteristics of this method are also examined. by the use of a cylindrical vessel and a stoichiometric propane/air mixture, the effects of the location of ignition points on the changing processes of those properties are evaluated.

Three-dimensional Flame Development in a Closed Vessel : 1st Report, Theory and Numerical Analysis

In this report, a mathematical model for expressing three-dimensional development of a laminar flame in a closed vessel has been proposed, and a numerical calculation method, which the author termed a space difference one, is derived. This makes it possible to evaluate the changes of properties, such as burning time, flame volume, flame- and flammable-surface area, mass burning rate, and mass burnt fraction, and to illustrate the three-dimensionally expanding flame. The characteristics of this method are also examined. Using a cylindrical vessel and a stoichiometric propane/air mixture, the effects of the location of ignition points on the changing processes of those properties are evaluated.

Measurements of spatial flame propagation and flow velocities in a spark ignition engine

Knowledge of flame propagation and flow characteristics enables a more detailed description of in-cylinder processes as well as assessing the influences of engine operating conditions and design parameters, particularly in regard to fuel economy and emission control. Spatial flame propagation in a single-cylinder spark-ignition engine is studied by a measuring technique that uses optical fibers coupled with photomultipliers. The propagation process is monitored through a large number of optical fibers arranged in a matrix in the combustion chamber wall. The spatial flame front shape, the flame volume, and the flame front velocity as a function of time are derived from these measurements. A dual beam laser-doppler-velocimeter (LDV), operating in forward scatter mode, is used to measure flow velocities of fluid motion in a motored and fired engine. Measurements are performed at several points within the combustion chamber. Signal processing is done by a counter. By averaging measurements over several engine cycles, mean flow velocity and RMS values are obtained. It was found that increasing the air-fuel ratio decreases the flame propagation rate and the burning velocity, whereas mean flow velocities of fluid motion are hardly affected. Swirl flow, generated by a shrouded inlet valve, leads to higher mean flow velocities, higher flame front velocities, and higher burning velocities, and the flame front is deformed.

Carbon Particulate in Small Pool Fire Flames

Flame radiation, the dominant heat transfer mechanism in many combustion and fire safety related problems, is primarily controlled by the fraction of flame volume occupied by solid carbon particulate. A multi-wavelength laser transmission technique is used here to measure carbon particulate volume fractions and approximate particle size distributions in ten common solid, cellular and liquid fueled small scale, 0 (10 cm dia), pool fire diffusion flames. The most probable particle radius, rmax, and concentration, N0, are two parameters in the assumed gamma function size distribution form which are determined for each fuel by simultaneously measuring light transmission of two superimposed laser wavelengths. The resulting soot volume fractions range from fv ∼ 4 × 10−6 for cellular polystyrene to fv ∼ 7 × 10−8 for alcohol. Cellular polystyrene has the largest particles, rmax ∼ 60 nm while wood has the smallest, rmax ∼ 20 nm. The carbon particulate optical properties used in the analysis are shown to be representative of actual flame soot and are more accurate than the soot refractive index usually assumed in the literature. Finally, mean particle sizes obtained for all fuels indicate that the small particle absorption limit assumption is a reasonable approximation for infrared flame radiation calculations.

Influence of oxygen depletion on the radiative properties of PMMA flames

This work presents data on the flame properties of polymethyl methacrylate (PMMA) pool fires burning in nitrogen-diluted atmospheres. The measurements were performed on a 30-cm diameter pool fire equipped with a fuel level control device, with which it was possible to maintain constant mass loss rate for the duration of the test. The tests were performed in a water-cooled enclosure. The measured quantities were: Schmidt temperature, emittance, radiative power per unit height along the flame, mass loss rate and flame shape. Values for these properties were measured at four different ambient oxygen concentrations. The results were very repeatable. It was found that mass loss rate, flame emittance, and total radiation all decreased significantly with decreasing ambient oxygen concentration, while the Schmidt flame temperature was much less sensitive. The results may be interpreted as indicating that a drop in the ambient oxygen concentration influences the fire primarily through a decrease in soot concentration. The total heat release per unit flame volume was found to be weakly dependent on oxygen concentration.

Five laser-excited fluorescence methods for measuring spatial flame temperatures 1: Theoretical basis

Five methods for measurement of flame temperatures based upon laser-excited fluorescence are discussed with respect to the derived expressions, the assumptions necessary, the advantages, and the disadvantages. The use of each of the five methods for measuring spatial flame volumes (~<1 mm(3)) and temporal flame temperatures (corresponding to ~<1-microsec time period) is given. The five methods consist of one based upon linearity between the fluorescence signal and the laser spectral irradiance, three based upon saturation, and a fifth, which is not critically dependent upon the laser spectral irradiance.

Particulate volume fractions in diffusion flames

Flame radiation, the dominant heat transfer mechanism in full scale fires, is in turn controlled primarily by the fraction of the flame volume occupied by solid carbon. Particulate volume fractions, f v , are measured in situ in small scale, 0(10 cm), buoyant diffusion flames supported in air by solid polystyrene, PMMA and POM, cellular polystyrene, two cellular polyurethanes and liquid isooctane, acetone and alcohol. Two measurement techniques, based on attenuation of known monochromatic laser radiation by the flame, are described. In the small particle absorption limit, valid in the visible for POM, acetone and alcohol transmittance at a single wavelength suffices to determine f v . For the remaining fuels, scattering becomes significant and multiwavelength transmittance measurements are used to determine an approximate two parameter particle radii distribution, N( r )=N o (27 r 3 /2 r max 4 ) exp (−3 r / r max ) where r max is the most probable radius and N o is the particle concentration. The resulting f v =18.6 N o r max 3 may be used to calculate the infrared emission from solid carbon in the flames considered. Volume fractions rank in the expected order of flame luminosity and smokiness from polystyrene, f v ≈5×10 −6 , to alcohol, f v ≈10 −7 . Within the approximations of flame homogeneity, spherical particles, known optical properties and assumed form for the size distribution, the f v , data are from ±5% to ±15% accurate. Good agreement exists with f v of solid polystyrene and PMMA derived independently from infrared flame transmittance and radiance data and between experimental mass pyrolysing rates and calculated rates obtained using these results in a radiation model.

Decomposition of combustion noise scaling rules by direct flame photography

In order to estimate the scaling rules for combustion noise, a turbulence autocorrelation function of the reaction rate must be estimated. This depends in part upon the active reaction volume in a turbulent flame. This volume may be measured by direct flame photography centering on the radiation of an active radical in the reaction zone. The paper describes experiments on fuel-lean propane, propylene, and ethylene-air turbulent open premixed flames with burners of 0.4″–0.96″ diameter and velocities to 600 fps. The flame volume is measured from photographs of the flame centered on the CH emission. The volume is correlated to the flow velocity, burner diameter, laminar flame speed and fuel mass fraction in the form of a power law by a regression technique. The scaling rule obtained from experiments is shown to follow the law V∝UD 3F 3 S L . The scaling laws generated are compared with estimates previously drawn from Strahle's theory of combustion noise. Implications concerning the turbulence structure in the reaction zone are drawn from the correlations.

The theory of free ambient fires. The convectively mixed combustion of fuel reservoirs

The theory of fuel-reservoir fires (i.e., pool flames) is extended and amplified into a quantitative formulation that includes all the significant physical processes: mass diffusion, heat conduction, convective mixing, convective heat transport, and radiative heat transport. The conservation equations across the Gibbsian surfaces of the pool flame are solved, in finite-difference form, under idealized but nevertheless realistic conditions. This semiempirical solution, which quantifies the consensus of views of many previous researchers, is surprisingly simple considering the number of processes involved and their complexity. The predictions are compared with the data for three fuels (gasoline, liquid hydrogen, and methanol), and the comparison gives reasonable agreement. This steady-state theory idealizes the complexities of turbulent, eddy mixing in terms of a single, average mixing radius, whose magnitude is inferred from the observed flame structures. The radiative transport process from the hot flame volume to the pool reservoir is idealized as a planar surface to surface transfer function. The observed flame thickness is used to calculate both the emissivity of the upper “flame-surface” and its average height above the pool. The theory presented is only a beginning in handling the major problem: the feed-back loop between the combustion source function and the flow mixing of vaporizing fuel with entrained air. Combustion and buoyancy forces are the sources of flow, but it is the flow mixing field itself that controls the strength of these forces.

DC discharges in seeded propane-air flames

Direct-current electrical discharges in seeded and unseeded propane-air flames have been studied for currents between 1 and 10 A. Two types of discharge were observed. At low currents and high seed concentrations the discharge occupies about half of the flame volume and is at a relatively low temperature. At higher currents the discharge contracts to about one third of its previous diameter into a high-temperature arc, which can be maintained without seed. The electrical characteristics of the two discharges are similar. Transition between the two arc modes is accompanied by a large change in anode fall voltage.

Diffuse Electrical Discharges in the Absence of Flames

KARLOVITZ1,2 has proposed and investigated a device in which a high voltage electrical discharge is superimposed on a flame seeded with an alkali metal salt. The distinctive feature of this device is the spreading of the discharge through the flame volume. Usually discharges tend to contract as a result of feedback, causing an increase in the temperature of the discharge, which in turn causes a further increase in conductivity. Addition of an easily ionizable salt (in this case potassium carbonate) overcomes this instability by providing a better conducting path at lower temperatures and reducing the temperature coefficient of electrical conductivity. Because the discharge is dispersed through the flame volume higher voltages are possible than with an arc column and consequently currents used are comparatively small (5–15 A). Most of the subsequent work on such systems has concerned electrical properties and heat-transfer measurements3–5.

Flame in a buoyant methane layer

The flame propagating along the interface between a layer of methane and air has been investigated using two techniques. With a flame propagating along a layer, the flame speed was constant at 6 ft/sec, and the volume of flame depended on the total quantity of methane present and its distribution in the layer. The flame speed has been related to the burning velocity of a stoichiometric premixed flame, by observing the movements of gas immediately ahead of the flame front. Flame speed was constant except when the radius of the advancing flame front was very small. In both series of experiments, the flame zone could be divided into three regions, a premixed flame propagating through the 5% to 15% methane mixture, a diffusion flame separating the methane-rich from the air-rich combustion products, and a convection flame, where the flame shape is controlled by displacement of methane by the buoyant combustion products.

Evaluation of Factors Affecting Heat Transfer in Furnaces

Abstract The problem of correlating the heat-absorption efficiencies of coal-burning equipment has been studied for many years with little more than a modicum of success. It has been consistently pointed out that one of the major problems of correlating data such as these has been the lack of precise furnace data. This paper describes the attempt at using statistics to correlate more reliable furnace data by means of the Broido, Hudson-Orrok, a modified Hudson-Orrok, Wohlenberg, and Hurvich correlations. Results suggest that information about flame temperatures, flame volumes, and emissivities and temperatures of flames and slag-ash-covered boiler-tube walls are necessary before precise correlations can be made.

Furnace Heat Absorption in Paddy’s Run Pulverized-Coal-Fired Steam Generator, Using Turbulent Burners, Louisville, Ky.: Part II—Furnace Heat-Absorption Efficiency as Shown by Temperature and Composition of Gases Leaving the Furnace

Abstract This paper is part of the second of a series of formal reports on investigations of heat absorption and distribution of heat transfer in representative pulverized-coal-fired boiler furnaces, sponsored by the ASME Special Research Committee on Furnace Performance Factors. The results are presented of fifteen determinations of heat absorption in the furnace of a 640,000 lb per hr boiler with horizontal firing, operating at 950 psig and 900 F at the superheater outlet. The effect on furnace heat-absorption efficiency is shown for variation of (a) the heat available in the furnace, (b) the excess air, (c) the setting of the secondary-air vanes of the turbulent burners, and (d) at low loads, the number and location of the burners used. Detailed data are given for the distribution of excess air and gas temperature at the furnace outlet. The heat-absorption data are analyzed in terms of radiation heat transfer, and the significance of flame location, volume, and shape is discussed.