Diffusion flame studies

The diffusion flames of methanol [27], ethanol [27], n- and iso-propanol [27], and the four isomeric butanols [28] have been investigated using a quartz probe technique. The alcohol flames were burned on a pyrex wool wick and samples for analysis were taken from various positions in the flame using a quartz microprobe. Thermocouple measurements showed that the temperature varied from 200 °C at the wick to around 1400 °C at the tip and edges of the flame. 

The products were extremely complex, but show that pyrolysis of the fuel occurred at the centre of the flame, followed by oxidation of the pyrolysis products in the outer zone. 

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The alkali metals are easily vaporized at temperatures of 300—500"C, and most studies of this group have been reviewed in Sections 2 and 3. The reactivity of alkali metals in co-condensation reactions is high, but little different from that in diffusion flame studies. However, the alkali metals have been used in a number of low-temperature reactions, largely to produce radicals or intermediates of spectroscopic interest. For example, the trichloromethyl radical has been produced in a solid argon matrix by reaction of lithium atoms with carbon tetrachloride [294]. A similar technique has been used to produce the CBr2H radical from bromoform [295], the CCljH radical from chloroform [296], and the methyl radical from methyl iodide and methyl bromide [297]. In all these cases the corresponding lithium halide is produced. 

Apart from the alkali metals, the reactions of which have been reviewed in Section 2, very few metals are sufficiently volatile to enable gas-phase diffusion flame studies to be undertaken. The high temperatures required for vaporization would destroy organic materials, so only reaction with inorganic gases can be considered. The combustion of some metals has been studied, as they are of importance as possible rocket propellant systems. The kinetics, however, are complex. Metals burn predominantly, and in some cases exclusively, by heterogeneous reactions [305], since both the fuel and the products are usually in the condensed state. In metal combustion, transport processes exert at least a partially controlling influence, and so information on reaction kinetics is difficult to obtain. The reaction may occur at the surface of the metal, or on the surface of... 

Validation and Application. VaUdated CFD examples are emerging (30) as are examples of limitations and misappHcations (31). ReaUsm depends on the adequacy of the physical and chemical representations, the scale of resolution for the appHcation, numerical accuracy of the solution algorithms, and skills appHed in execution. Data are available on performance characteristics of industrial furnaces and gas turbines systems operating with turbulent diffusion flames have been studied for simple two-dimensional geometries and selected conditions (32). Turbulent diffusion flames are produced when fuel and air are injected separately into the reactor. Second-order and infinitely fast reactions coupled with mixing have been analyzed with the k—Z model to describe the macromixing process. 

Many different configurations of diffusion flames exist in practice (Fig. 4). Laminar jets of fuel and oxidant are the simplest and most well understood diffusion flames. They have been studied exclusively in the laboratory, although a complete description of both the transport and chemical processes does not yet exist (2). 

The various studies attempting to increase our understanding of turbulent flows comprise five classes moment methods disregarding probabiUty density functions, approximation of probabiUty density functions using moments, calculation of evolution of probabiUty density functions, perturbation methods beginning with known stmctures, and methods identifying coherent stmctures. For a thorough review of turbulent diffusion flames see References 41—48. 

Singhal J.S. T ien, Flammability Study of Polymer Fuels Using Opposed-Jet Diffusion Flame Technique, Rept No SQUID-TR-CWRU-3-PU, Contract N00014-67-A-0226-0005, Purdue Univ, Lafayette (1975)... 

F. Takahashi and V. R. Katta, Further studies of the reaction kernel structure and stabilization of jet diffusion flames, Proc. Combust. Inst. 30 383-390, 2005. 

Z3.2.2.2 Diffusion Flame in a Temporal Mixing-Layer Together with HIT, the temporal mixing-layer (TML) is a useful configuration for the numerical study of turbulent flows. The TML configuration can be thought... 

Transient computations of methane, ethane, and propane gas-jet diffusion flames in Ig and Oy have been performed using the numerical code developed by Katta [30,46], with a detailed reaction mechanism [47,48] (33 species and 112 elementary steps) for these fuels and a simple radiation heat-loss model [49], for the high fuel-flow condition. The results for methane and ethane can be obtained from earlier studies [44,45]. For propane. Figure 8.1.5 shows the calculated flame structure in Ig and Og. The variables on the right half include, velocity vectors (v), isotherms (T), total heat-release rate ( j), and the local equivalence ratio (( locai) while on the left half the total molar flux vectors of atomic hydrogen (M ), oxygen mole fraction oxygen consumption rate... 

Smooke, M.D., Lin, P, Lam, J.K., and Long, M.B., Computational and experimental study of a laminar axi-symmetric methane-air diffusion flame, Proc. Combust. Inst., 23,575,1990. 

In Chapter 8.1, F. Takahashi presents candle and laminar jet diffusion flames highlighting fhe physical and chemical mechanism of combustion in a candle and similar laminar coflow diffusion flames in normal gravity and in microgravity. This apparently simple system turns out to be very complex, and thereby its study is of greaf importance for the understanding of diffusion flame fundamentals. 

Although fire mainly involves the study and consequences of diffusion flames, premixed flames are important precursors. In order to initiate a diffusion flame, we must first have a premixed flame. In regions where a diffusion flame is near a cold wall, we are likely to have an intermediary premixed flame. Even in a turbulent diffusion flame, some state of a premixed flame must exist (see Figure 4.1). 

The first expression here is very similar to the Damkohler result for A and B equal to 1. Since the turbulent exchange coefficient (eddy diffusivity) correlates well with IqU for tube flow and, indeed, /0 is essentially constant for the tube flow characteristically used for turbulent premixed flame studies, it follows that... 

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],... 

Recent experimental results on thermodynamic properties of high pressure supercritical fluids have opened up the possibility to study combustion and flames at very high pressures and in unusual environments. Stationary diffusion flames have been produced up to 2000 bar in dense aqueous mixed fluid phases. 

The soot formation and its control was studied in an annular diffusion flame using laser diagnostics and hot wire anemometry [17, 18]. Air and fuel were independently acoustically forced. The forcing altered the mean and turbulent flow field and introduced coherent vortices into the flow. This allowed complete control of fuel injection into the incipient vortex shedding process. The experiments showed that soot formation in the flame was controlled by changing the timing of fuel injection relative to air vortex roll-up. When fuel was injected into a fully developed vortex, islands of unmixed fuel inside the air-vortex core led to... 

Methane-air diffusion flames are selected for the example to be studied here. The temperature T and species mass fractions Yi (for species i) in such flames are functions of the mixture fraction Z, which varies from zero in air to unity in fuel and measures the fraction of the material present that came from the fuel. Figure 25.2 is a schematic illustration of major profiles in the methane-air diffusion flame as functions of Z, obtained by the rate-ratio as3miptotics described above. The work to be reported here adds to this picture the chemistry relevant to the production of oxides of nitrogen. 

Partially premixed flames are formed when a rich mixture of fuel and oxidizer is injected into an oxidizer stream. Below a certain value of the equivalence ratio of the rich mixture, a flame structure involving a premixed flame in the vicinity of a diffusion flame exists. Several experimental studies of NO emission properties of partially premixed laminar [1-4] and turbulent [5-10] flames have been reported in the literature. The results from the most recent studies indicate that using an optimum level of partial premixing can reduce NO emissions. 

Early workers had noted the colours imparted to diffusion flames of alcohol by metallic salts, but detailed study of these colours awaited the development of the premixed air-coal gas flame by Bunsen. In 1859, Kirchhoff showed that these colours arose from line spectra due to elements and not compounds. He also showed that their wavelengths corresponded to those of the Fraunhofer lines. Kirchhoff and Fraunhofer had been observing atomic emission and atomic absorption, respectively. 

The Opposed-Flow Laminar Diffusion Flame Laminar diffusion flames can be more complicated chemically and physically than the corresponding premixed flames. This is especially so for a candlelike co-flowing situation. Because of the difficulties of adequately representing the two- or three-dimensional flow field, together with detailed chemistry, these flames are difficult to use as the basis for chemical-kinetic studies. How-... 

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