Showing papers on "Premixed flame published in 1975"

The structure and propagation of turbulent flames

Abstract: The influence of turbulence intensity, scale and vorticity on burning velocity and flame structure is examined by using premixed propane-air mixtures supplied at atmospheric pressure to a combustion chamber 31cm long and lOcmx 10 cm cross-section. The chamber is fitted with transparent side walls to permit flame observations and schlieren photography. Control over the turbulence level is achieved by means of grids located upstream of the combustion zone. By suitable modifications to grid geometry and flow velocity, it is possible to vary turbulence intensity and scale independently within the combustion zone in such a manner that their separate effects on burning velocity and flame structure are readily distinguished. From analysis of the results obtained three distinct regions may be identified, each having different characteristics in regard to the effect of scale on turbulent burning velocity. For each region a mechanism of turbulent flame propagation is proposed which describes the separate influences on burning velocity of turbulence intensity, turbulence scale, laminar flame speed and flame thickness. The arguments presented in support of this 3-region model are substantiated by the experimental data and by the pictorial evidence on flame structure provided by the schlieren photographs. This model also sheds light on some of the characteristics which turbulent flames have in common with laminar flames when the latter are subjected to pressure and velocity fluctuations. Finally the important role of vorticity is examined and it is found that turbulent flame speed is highest when the rate of production of vorticity is equal to about half the rate of viscous dissipation.

Structure of Bunsen flames

Abstract: The structure of Bunsen cone flame tops is studied using the thermal diffusion flame model. Assuming a strong temperature dependence of the reaction rate, the shape of the flame front as well as the temperature distribution at the front are found. It is shown that for the case in which the Lewis number of the limiting component is greater than unity, the front is a smooth surface with a radius of curvature at the flame top substantially exceeding the width of the preheating zone. Towards the flame top, the temperature in the reaction zone increases along with the velocity of the flame relative to the gas. When the Lewis number of the limiting component is less than unity, the temperature at the front decreases towards the flame top, the velocity of the flame relative to the gas drops, and the flame is extinguished at distances close to the top.

Flame propagation in fuel droplet-vapor-air mixtures

Abstract: As a simple model of spray combustion, a mono-sized ethanol droplet-vapor-air system has been realized using an expansion apparatus based on the principle of the Wilson cloud chamber, in which the mean droplet diameter can be varied up to 7 microns at its maximum. The burning time, peak pressure, flame speed and burning velocity have been measured for mixtures containing droplets, as well as for homogeneous vapor-air mixtures. The effects of fuel droplets on the burning characteristics are more significant for fast burning mixtures than for slow burning mixtures, suggesting that ethanol droplets of several microns in diameter are not completely evaporated in the preheating zone. Microphotographic studies have also been made for mixtures containing larger droplets (about 20 microns in diameter) to observe the fine structure of the flame propagating in a droplet-vapor-air mixture. Shadow photographs of droplets suspended in the chamber and schlieren photographs of the flame have been taken by means of a high-magnification camera. Many spots probably due to the vapor from droplets are noticeable ahead of the flame boundary, which is rough and difficult to determine precisely.

Studies on the structure of a spray combustion flame

Abstract: To clarify the flame structure of a spray burner, the following experiments and analysis were carried out. (1) Droplet and temperature distributions, flow velocity, and gas composition were measured in the flame of an air-atomizing burner. It was found that the region where the droplets exist is limited to a small area above the burner nozzle. From the correlation between the above various distributions, it was concluded that most of the droplets in the flame do not burn individually, but that fuel vapor from the droplets concentrates and burns like a gas diffusion flame. (2) Various measurements were then made on a spray combustion flame and a turbulent gas diffusion flame under the same conditions. Comparing the two sets of data, it was found that the flames are similar in structure. (3) Assuming that the droplets evaporate in the flame, their behavior was analyzed by making use of the knowledge which has been obtained for a single droplet. The calculated results were in fairly close agreement with the experimental results. The above facts suggest the possibility that the spray combustion flame could be treated theoretically by applying the information for a single droplet and for a turbulent gas diffusion flame.

Flame Propagation Through Layered Fuel-Air Mixtures

Abstract: It is observed that a layered, unconfined combustible mixture supports a flame propagation velocity of 4 to 5 times the laminar flame speed of the stoichiometric mixture of fuel and air. This effect which has been observed by some and more recently in this laboratory, has never been adequately explained. It is in contrast to the classical soap bubble experiment, in which the flame propagation velocity is of the order of 7 to 8 times the laminar flame speed, that is, the order of the unburned to burned gas density ratio To explain this new effect two physical models with different degrees of complexity are explored here. The first is a step-wise premixed fuel-air and pure air case in a gallery of infinite length. The second deals with one of finite length. Both have been mathematically defined to give quantitative results so that the effects of environmental factors can be explicitly determined Solutions of the first model show that the maximum flame propagation speed occurs when the gallery is of...

Gas velocity and temperature profiles of a diffusion flame stabilized in the stream over liquid fuel

Abstract: The gas velocity and temperature profiles across the laminar boundary layer with a diffusion flame estblished over methanol or ethanol were measured with the free stream, of air parallel to the liquid-fuel surface. The flame stabilizing mechanism and fuel consumption rate are discussed. The results show that the maximum velocity appearing near the blue-flame zone, where the gas stream is accelerated, increases downstream and exceeds the free-stream velocity at a point about 0.2 cm from the leading edge of the fuel vessel. The temperature at the blue-flame zone is found to increase downstream about 1.5 cm from the leading edge of the fuel vessel and then to decrease slightly still farther downstream. The fuel consumption rate is observed to increase monotonically with the increase of the free-stream velocity. It is shown that in order to elucidate the flame stabilizing mechanism, the velocity profile change due to the flame reaction must be taken into account. The diffusion flame over the liquid fuel can be considered to remain stable until the leading flame edge shifts beyond the leading edge of the fuel vessel due to the increase of the free stream velocity.

Effects of radiation and convection on gas velocity and temperature profiles of flames spreading over paper

Abstract: The effects of radiation and convection on the mechanism of flame spread over a thin combustible solid have been studied. The gas velocity and temperature profiles near flames spreading downward over paper were measured using particle tracer techniques and fine-wire thermocouples. The air stream moving vertically upward was decelerated as it approached the leading edge of a stably spreading flame, and a lower velocity region appeared near the paper surface in front of the leading flame edge. When a low-velocity air stream flowed vertically downward, vortices appeared near the spreading flame. The temperature profiles near a stably spreading flame indicated that a large amount of heat flowed to the unburned material in a narrow region adjacent to the pyrolysis front. When the air flowed vertically downward, hot gas flowed along the paper surface in front of the pyrolysis front. The increase of the flame spread rate with the increase of the radiative heat flux was attributed mainly to the increase of the surface temperature due to radiative heating. The flame spread rate was shown to be closely related to the velocity profile just in front of the leading edge of the spreading flame.

The structure of rich fuel-air flames in the forward stagnation region of a porous cylinder

Abstract: An experimental study was made to investigate the behavior and structure of rich fuel-air flames established in the forward stagnation region of a porous cylinder, from the surface of which premixed methane and air were ejected uniformly into a uniform air stream (the secondary air). The appearance of the flame, the location of the flame and the stagnation point, and the temperature and stable species concentration profiles were examined in detail for flames at atmospheric pressure. The stabilized flame is really a double flame. For the mixture inside the rich limit of flammability (equivalence ratio For mixtures outside the rich limit of flammability (about 1.8 about 3, the flame can be treated essentially as a diffusion flame. It is suggested that this type of flame is most suitable for systematic studies of fundamental flame processes in rich fuel-air mixtures.

Some aspects of ignition by localized sources, and of cylindrical and spherical flames

Abstract: The time dependent conservation equations govering flame propagation in clinidrical and pherical systems have been set up and solved by finite difference methods for the case of a 60% hydrogen-air flame. By this means it is possible (a) numerically to follow the sequence of events following an “ignition” at the axis of a cylinder or the center of a sphere, or (b) to investigate the effect of flame curvature on burning velocity and other flame properties. It was found that the minimum ignition energy depended on the form in which the energy was supplied. For a constant total energy, ignition was facilitated by increasing the proportion supplied as H atoms rather than as thermal energy. The velocities of movement of the freely propagating flames from the ignitions were found to be slightly different from thosed of the inward propagating, cylindrical and spherical stationary flames. The velocities of the latter were independent of the flame diameter. The effect of curvature on the flame properties is shown to be an effect on reaction rate distribution, which also leads to differences in H atom concentrating profiles. Unlike the situation in planar flames, the detailed structure of freely propagating curved flames may not be the same as that of the corresponding stationary flames, and this may lead to the apparent differences in burning velocity.

Apparatus for the gas phase combustion of liquid fuels

Abstract: An arrangement to produce high efficiency gas phase combustion of liquid fuels. A liquid fuel, such as conventional heating oil, is pumped through a heat exchanger immersed in a liquid bath whose temperature is maintained at a level which is sufficiently high to cause the fuel to gassify but low enough to preclude undesirable chemical decomposition. The liquid bath is contained within an element which is directly exposed to a flame produced by the combustion of the gasified liquid fuel. The liquid bath temperature is controlled by varying the amount of surface area exposed to the flame or by internal forced circulation of the bath liquid. The gasified fuel is injected into an air or oxidizing gas stream and allowed to premix prior to combustion. Combustion is initiated by a spark or pilot flame and is stabilized by a flameholder. By eliminating liquid phase fuel from the flame zone and premixing with air, the apparatus produces a clean flame, free of solid carbon particles, with extremely low levels of carbon monoxide, unburned hydrocarbons and nitric oxide. In essence, the flame exhibits all the desirable characteristics of premixed gas phase combustion.

The Mutual Influence of Multiple Jet Diffusion Flames

Abstract: In some technical gas burners the gas stream is subdivided in a number of single jets which will be influenced mutual. The paper shows some results of experiments with very simple town gas and natural gas burners with three and five parallel single jets. Measurements of concentration, flame length, and flame width in free and enclosed multiple flames are described. The results show that there is a simple relation between the flame length of multiple burners and the number of jets and the distances between the central and the outer flames. The data of flame length derived from photography and from the axial concentration of CO agree well; so it is possible to measure the length, the width, and the average value of combustion intensity of flames quickly and with fairly good accuracy.

A study of flame spread over a porous material under external radiation fluxes

Abstract: Characteristics of horizontal flame spread over the surface of a porous material, a carpet in this study, are studied experimentally and theoretically under various external radiant fluxes (0.1–0.27 cal/cm2 sec). It is observed that the size of flame is increased significantly by increasing the external radiant flux. This increases the radiative heat feedback from the flame so that it becomes comparable to or greater than the convective heat feedback. The external radiation can also cause an unstable motion of the flame front. This effect is probably due to the production of volatile pyrolysis products ahead of the flame front instead of under it. The theoretical calculation indicates that the thermal emission loss from the heated sample is significant and the internal radiation in the porous material must be included in the model.

Gas burner flame temperature amplifier

Abstract: A method and an apparatus for increasing the combustion flame temperature of a partially-aerated atmospheric gas burner heater and the heat transfer therefrom by the injection of jets of preheated secondary air into the combusting chamber transiting the flow of burning gases thereof

Structure of Counterflow Diffusion Flame of Ethanol

Abstract: The opposed jet assembly has been adapted for stabilizing a counter flow diffusion flame of ethanol. Data on the thermo-aerodynamic structure of the flame, obtained with the help of interferometric and particle track techniques, have been presented and analysed to yield the heat release rate. The maximum flame temperature is found to be close to adiabatic and the net heat release is in agreement with the theoretically expected value. The luminous flame is accompanied by endothermic zones on both the fuel and oxygen sides. Mass spectromctric analysis of the flame gases is found to confirm the presence of ethanol pyrolysis on the fuel side and to point out the possibility of significant chemical reactions on the oxygen side. The observations thus tend to strengthen the earlier findings (of endothermic zones) on gaseous diffusion flames.

The effect of pressure on the flame structure in the wake of a burning hydrocarbon droplet

Abstract: Data are presented on the structure of the flame in the wake of a model (6 mm dia porous sphere) n -heptane droplet burning in air. The following measurements were made: axial and radial temperature profiles, axial and radial composition profiles showing H 2 O, CO 2 , N 2 , O 2 , CO, C 7 H 16 , CH 4 , C 2 H 2 , and C 3 H 8 . Envelope flames were studied at pressures up to 40 atm. Wake flames were studied at 5 atm only. The velocity of transition from the envelope flame to the wake flame was measured up to 25 atm. The results show that the effect of pressure on flame structure can be explained in terms of the effect of pressure on the following processes: diffusion and pyrolysis of fuel in the near-wake zone of the envelope flame, premixed combustion and pyrolysis of fuel in the near-wake zone of the wake flame, combustion and coagulation of soot in the far-wake zone of both flames. As pressure increases, the increased rate of pyrolysis becomes predominant in the near wake. In the far wake, the peak temperature drops with increasing pressure and coagulation of soot becomes important. The data are consistent with the model developed by the authors to explain the effect of pressure on flame length. The velocity of transition from an envelope flame to a wake flame increases approximately as P 1/2 , suggesting overall 3/2 order kinetics for n -heptane and air at the stagnation point.

The Effects of Charge Dilution on Combustion and Its Improvement-Flame Photograph Study

Abstract: Some methods of analyzing combustion phenomena in a single-cylinder engine (27.1 CID) with charge dilution were tried. Multiple ionization probes roughly indicate the pattern of flame propagation, but details of the flame, especially charge dilution, cannot be measured by this method. Flame photographs, on the other hand, showed clearly the effects of combustion chamber design on flame propagation and its cyclic dispersion. The effects of combustion chamber configuration, ignition energy and spark duration, ignition timing, composition of diluent gas, air-fuel ratio, and mixture homogeneity were examined. Mass burnt fraction was calculated. A combustion chamber having high turbulence and a rich air-fuel ratio promoted the initial flame propagation and flame speed and improved cyclic dispersion. High turbulence design was effective for improvement of car surge caused by EGR, and this was confirmed by actual driving on the road. /GMRL/

Flame photometric detector employing premixed hydrogen and oxygen gases

Abstract: A flame photometric detector including a burner assembly having a first passageway leading to the burner tip so that a mixture of the hydrogen fuel gas and the combustion supporting gas such as oxygen may be delivered via the passageway to the burner tip to produce a hydrogen rich reducing flame. The sample to be analyzed is delivered via a second passageway and directed by a sample guide to the peripheral region of the reducing flame where the sample is burned in a relatively low temperature, hydrogen rich region whereby the interfering light emission from interfering substances is maintained at a low level. SP This is a division of application Ser. No. 389,614 filed on Aug. 20, 1973, now U.S. Pat. No. 3,879,126, which was a continuation of application Ser. No. 232,926 filed on Mar. 8, 1972 abandoned.

Flame reaction process for producing hydrogen bromide

Abstract: A flame reactor burner is provided for reacting hydrogen and bromine to produce hydrogen bromide. Flame stability is enhanced by producing a helical flow of well mixed reactants within the burner, whereby the reactants in intimate contact with each other are propelled in an outward spiral path as they exit from the burner producing a hemispherical flame of high stability.

Improvements in flame emission spectrometry through the use of ultrasonic nebulization into a premixed oxygen-hydrogen flame

Abstract: : A laminar flow premixed oxygen-hydrogen flame when combined with ultrasonic nebulization has improved detection limits approaching three orders of magnitude when compared to a conventional turbulent oxygen-hydrogen flame used in flame emission spectrometry. In a comparison with two other premixed flames, the premixed oxygen-hydrogen flame gave the lowest detection limits for elements with wavelengths longer than 4000 A. The increased sensitivity is attributed to the low flame background emission and reduced flame flicker. (Modified author abstract)

Flame Spreading on Gelled JP-4 Pool

Abstract: For the gelled JP-4 of an oil-in-water emulsion type which includes 96∼99% oil concentration by volume as an internal phase and has the drainage temperature higher than 70°C in confined vessel, the flame spreading aspects with a pool combustion were observed by taking movie, and the measurement of temperatures, on and above the surface of the gel from the time when the flame is coming over to the time when it has passed away, was carried out with thermocouples fixed in place.While the flame is spreading on the gelled JP-4, an advance liquid flow ahead of the flame can be found in case of the initial gel temperatures lower than the apparent flash points (26∼35°C) measured by means of open cup method. This liquid flow invades on to the surface of the virgin gel, issued from the upstream region covered already with luminous flame, and is not formed by radiative and conductive heat transfer from the flame pillar to the virgin gel surface. In view point that the temperatures of foregoing liquid flow is 10∼20°C at the initial level of gel surface and the flash point of the original JP-4 is -23°C, a large part of the low hydrocarbon fractions seem to have burned in the flame zone before streaming out on the virgin gel surface. The behavior of liquid flow was observed in detail with fine aluminum powders sowed in the vicinity of the ignition position. As the result, it is found that in the foregoing liquid flow there is a strong convection due to temperature gradient. Thus, the covering of the liquid flow on the virgin gel plays a role to reduce the drainage rate of the gel, protecting the gel surface from being exposed in air in the place close to hot flame, and the flame spreading velocity can be suppressed by one-tenth to one-twentieth, compared to that of the original JP-4.