Flat flame

An Erlenmeycr or other form of Combustion Furnace.— The usual length is 80-90 cm. (31-35 in.), and it is provided with 30 to 35 burners. Flat flame burners are undesirable. 

A refractory quarl is usually an integral part of forced-draft burners. Suitable design of burner and quarl can determine the flame characteristics. Long, short, pencil or even flat flames are possible. 

Dishnct fuel-specific reaction chemistry is also seen in premixed flat flames of the four butanols. Figure 2.9 shows PIE curves for m/z = 72 (C4H8O). The species pool is quite different, with butanal present in the 1-butanol flame, 2-methyl propanal in the i-butanol flame, and 2-butanone in both the 2-butanol and the f-butanol... 

Photographs of a simple coaxial burner and the resulting sample flame images, viewed diagonally from the bottom, (a) A preliminary setup. Coaxial ethylene/air flames are formed under the following conditions (b) U, = 1I and = 0.8—flat flame (c) Uj = and... 

FIGURE 4.19 Cooling effect in flat flame burner apparatus. 

As the important effect of temperature on NO formation is discussed in the following sections, it is useful to remember that flame structure can play a most significant role in determining the overall NOx emitted. For premixed systems like those obtained on Bunsen and flat flame burners and almost obtained in carbureted spark-ignition engines, the temperature, and hence the mixture ratio, is the prime parameter in determining the quantities of NOx formed. Ideally, as in equilibrium systems, the NO formation should peak at the stoichiometric value and decline on both the fuel-rich and fuel-lean sides, just as the temperature does. Actually, because of kinetic (nonequilibrium) effects, the peak is found somewhat on the lean (oxygen-rich) side of stoichiometric. 

Prompt NO mechanisms In dealing with the presentation of prompt NO mechanisms, much can be learned by considering the historical development of the concept of prompt NO. With the development of the Zeldovich mechanism, many investigators followed the concept that in premixed flame systems, NO would form only in the post-flame or burned gas zone. Thus, it was thought possible to experimentally determine thermal NO formation rates and, from these rates, to find the rate constant of Eq. (8.49) by measurement of the NO concentration profiles in the post-flame zone. Such measurements can be performed readily on flat flame burners. Of course, in order to make these determinations, it is necessary to know the O atom concentrations. Since hydrocarbon-air flames were always considered, the nitrogen concentration was always in large excess. As discussed in the preceding subsection, the O atom concentration was taken as the equilibrium concentration at the flame temperature and all other reactions were assumed very fast compared to the Zeldovich mechanism. 

For premixed fuel-air systems, results are reported in various terms that can be related to a critical equivalence ratio at which the onset of some yellow flame luminosity is observed. Premixed combustion studies have been performed primarily with Bunsen-type flames [52, 53], flat flames [54], and stirred reactors [55, 56], The earliest work [57, 58] on diffusion flames dealt mainly with axisymmetric coflow (coannular) systems in which the smoke height or the volumetric or mass flow rate of the fuel at this height was used as the correlating parameter. The smoke height is considered to be a measure of the fuel s particulate formation and growth rates but is controlled by the soot particle bumup. The specific references to this early work and that mentioned in subsequent paragraphs can be found in Ref. [50],... 

The reaction mechanism developed was then used to predict the experimental species profiles obtained in one-dimensional flames (Kee et al, 1985 Karra and Senkan, 1987). For one-dimensional, premixed, laminar flat flames, the energy and mass transport equations are given by the following ... 

The experimental setup for diode-laser sensing of combustion gases using extractive sampling techniques is shown in Fig. 24.8. The measurements were performed in the post-flame region of laminar methane-air flames at atmospheric conditions. A premixed, water-cooled, ducted flat-flame burner with a 6-centimeter diameter served as the combustion test-bed. Methane and air flows were metered with calibrated rotameters, premixed, and injected into the burner. The stoichiometry was varied between equivalence ratios of = 0.67 to... 

Multiplexed diode-laser sensors were applied for measurement and control of gas temperature and species concentrations in a large-scale (50-kilowatt) forced-vortex combustor at NAWC to prove the viability of the techniques and the robustness of the equipment for realistic combustion and process-control applications [11]. The scheme employed was similar to that for measurements and control in the forced combustor and for fast extractive sampling of exhaust gases above a flat-flame burner at Stanford University (described previously). 

Schoenung, S. M., and R. K. Hanson. 1981. CO and temperature measurements in a flat flame by laser absorption spectroscopy and probe techniques. Combustion Science Technology 24 227-37. 

Warnatz, J. 1978. Calculation of the structure of laminar flat flames. Berichte Bunsenges Physicalische Chemie 82 193-200, 643-50, 834-40. 

Instrumental methods have become more sophisticated to face these challenges. In particular, Westmoreland and Cool have developed a flame-sampling mass spectrometer that has provided several revelations in terms of relevant molecular intermediates in combustion. " Their setup couples a laminar flat-flame burner to a mass spectrometer. This burner can be moved along the axis of the molecular beam to obtain spatial and temporal profiles of common flame intermediates. By using a highly tunable synchrotron radiation source, isomeric information on selected mass peaks can be obtained. This experiment represents a huge step forward in the utility of MS in combustion studies lack of isomer characterization had previously prevented a full accounting of the reaction species and pathways. 

The NTC phenomenon actually varies with pressure and combustion environment it is much different in a jet engine than in a diesel engine, which in turn is much different than in an internal combustion engine, which in turn is much different from a flat-flame burner. For the purposes of this review, we have focused on a simplified case. 

SympCombstn (1953), pp 321—28 (Flame Propagation The Influence of Pressure on the Burning Velocities of Flat Flames)... 

The superiority of the flat flames over the simple round ones, explains an observation which has been made with regard to the argand burners. When the apertures in it are placed so far apart as to form a circle of distinct jets, the effect is one-third weaker, -with the same current of gas, than when the jets— of one-sixth, to one-eighth of an inch—unite into a single flat ring. 

When the burner has a single aperture of the diameter of a bristle, a simple jet is produced in the form of a long, thin, conical flame. The bat a-wing, or flattened flame, which the gae forme when it issues from a narrow slit, instead of s round aperture, is muah more appropriate. A similar and equally good flame is produced by a burner with two apertures close to each other, the channels of which are inclined inwards, eo that both the currents of gas cross each other at the base. They then fora a flat flame spreading out in the form of an inverted triangle, and the burner is called a. fish or swallow-tail ... 

Fig. 1.1 Illustration of a premixed flat-flame burner and an opposed-flow diffusion flame.

Perhaps the most studied laboratory flame is the premixed flat flame. As illustrated in the left-hand panel of Fig. 1.1, a steady flame is established above a porous burner face. Such flames are used widely in combustion laboratories, where a variety of optical and probe-based diagnostics are used to measure species and temperature profiles. Models play an essential role in assisting the interpretation of the data. In addition to the premixed flat... 

As mentioned in the previous section, laminar, premixed, flat flames are used widely in the study of combustion chemistry. The left-hand panel of Fig. 1.1 shows a typical burner setup. The flames themselves are accessible to an array of physical and optical diagnostics, and the computational models can incorporate the details of elementary chemical reactions. 

Fig. 1.2 Illustration of a stagnation-flame configuration for the deposition of a polycrystalline diamond film. The photograph of the flame itself shows a highly luminous flat flame just above the deposition surface.

Flat flames can be made to impinge onto surfaces. Such strained flames can be used for a variety of purposes. On the one hand, these flames can be used in the laboratory to study the effects of strain on flame structure, and thus improve understanding of the fluid-mechanical effects encountered in turbulent flows. It may also be interesting to discover how a cool surface (e.g., an engine or furnace wall) affects flame structure. Even though the stagnation-flow situation is two-dimensional in the sense that there are two velocity components, the problem can be reduced to a one-dimensional model by similarity, as addressed in the book. 

The objective of this problem is to explore the multicomponent diffusive species transport in a chemically reacting flow. Figure 3.18 illustrates the temperature, velocity, and mole-fraction profiles within a laminar, premixed flat flame. These profiles are also represented in an accompanying spreadsheet (premixed h2. air-flame. xls). 

Fig. 3.18 Computed solution to an atmospheric-pressure, freely propagating, stoichiometric, premixed, hydrogen-air, flat flame.

---