Flame character

For burning in a reactive atmosphere, conditions may be encountered for which the analysis of the diffusion-flame regime is relevant. Under suitable conditions there are two thin reaction sheets, an inner one at which premixed decomposition of the monopropellant occurs and an outer one having a diffusion-flame character. Categorizations are available for potential limiting behaviors at large Damkohler numbers in reactive environments [206]. There are many flame-structure possibilities, not all of which have been analyzed thoroughly. 

It has been demonstrated that CFD has the capacity to predict the flame character and the temperature profiles observed by experience. Although intrinsic details of the combustion and flame... 

The amount and physical character of the char from rigid urethane foams is found to be affected by the retardant (20—23) (see Foams Urethane polymers). The presence of a phosphoms-containing flame retardant causes a rigid urethane foam to form a more coherent char, possibly serving as a physical barrier to the combustion process. There is evidence that a substantial fraction of the phosphoms may be retained in the char. Chars from phenohc resins (qv) were shown to be much better barriers to pyrolysate vapors and air when ammonium phosphate was present in the original resin (24). This barrier action may at least partly explain the inhibition of glowing combustion of char by phosphoms compounds. 

What are the mechanisms by which slow, laminar combustion can be transformed into an intense, blast-generating process This transformation is most strongly influenced by turbulence, and secondarily by combustion instabilities. A laminar-flame front propagating into a turbulent mixture is strongly affected by the turbulence. Low-intensity turbulence will only wrinkle the flame front and enlarge its surface area. With increasing turbulence intensity, the flame front loses its more-or-less smooth, laminar character and breaks up into a combustion zone. In an intensely turbulent mixture, combustion takes place in an extended zone in which... 

As mentioned above, the numerical solution of exact equations breaks down for low flame speeds, where the strength of the leading shock approaches zero. To complete the entire range of flame speeds, Kuhl et al. (1973) suggested using the acoustic solutions by Taylor (1946) as presented earlier in this section. Taylor (1946) already noted that his acoustic approach is not fully compatible with the exact solution, in the sense that they do not shade into one another smoothly. In particular, the near-piston and the near-shock areas in the flow field, where nonlinear effects play a part, are poorly described by acoustic methods. In addition to these imperfections, the numerical character of Kuhl etal. (1973) method inspired various authors to design approximate solutions. These solutions are briefly reviewed. 

The pc-Au/propylene carbonate (PC) + NaC104 interface has been studied by Nguyen Van Huong.481 A flame-annealed (02 + H2) pc-Au sphere was used. Before each experiment the pc-Au electrode was cleaned in an NaC104 aqueous solution by a few potential cycles involving oxidation-reduction of the surface until the i,E and C,E curves exhibited stable character. The C,E curves were recorded in the interval 15 150... 

The catalytic experiments were performed at the stationnary state and at atmospheric pressure, in a gas flow microreactor. The gas composition (NO, CO, O2, C3H, CO2 and H2O diluted with He) is representative of the composition of exhaust gases. The analysis, performed by gas chromatography (TCD detector for CO2, N2O, O2, N2, CO and flame ionisation detector for C3H6) and by on line IR spectrometry (NO and NO2) has been previously described (1). A small amount of the sample (10 mg diluted with 40 mg of inactive a AI2O3 ) was used in order to prevent mass and heat transfer limitations, at least at low conversion. The hourly space velocity varied between 120 000 and 220 000 h T The reaction was studied at increasing and decreasing temperatures (2 K/min) between 423 and 773 K. The redox character of the feedstream is defined by the number "s" equal to 2[02]+[N0] / [C0]+9[C3H6]. ... 

Certain catalytic modes have been well exploited in flame retardant systems, namely the dehydrating action of compounds which yield strong acids under flaming or smoldering conditions. Friedel-Crafts and other acid catalyzed condensation reactions have been exploited to increase char. These mechanisms don t work very well for polymers of mainly hydrocarbon character. Are there other modes of catalysis which might work better ... 

The inclusion of radiative heat transfer effects can be accommodated by the stagnant layer model. However, this can only be done if a priori we can prescribe or calculate these effects. The complications of radiative heat transfer in flames is illustrated in Figure 9.12. This illustration is only schematic and does not represent the spectral and continuum effects fully. A more complete overview on radiative heat transfer in flame can be found in Tien, Lee and Stretton [12]. In Figure 9.12, the heat fluxes are presented as incident (to a sensor at T,, ) and absorbed (at TV) at the surface. Any attempt to discriminate further for the radiant heating would prove tedious and pedantic. It should be clear from heat transfer principles that we have effects of surface and gas phase radiative emittance, reflectance, absorptance and transmittance. These are complicated by the spectral character of the radiation, the soot and combustion product temperature and concentration distributions, and the decomposition of the surface. Reasonable approximations that serve to simplify are ... 

To examine the effect of turbulence on flames, and hence the mass consumption rate of the fuel mixture, it is best to first recall the tacit assumption that in laminar flames the flow conditions alter neither the chemical mechanism nor the associated chemical energy release rate. Now one must acknowledge that, in many flow configurations, there can be an interaction between the character of the flow and the reaction chemistry. When a flow becomes turbulent, there are fluctuating components of velocity, temperature, density, pressure, and concentration. The degree to which such components affect the chemical reactions, heat release rate, and flame structure in a combustion system depends upon the relative characteristic times associated with each of these individual parameters. In a general sense, if the characteristic time (r0) of the chemical reaction is much shorter than a characteristic time (rm) associated with the fluid-mechanical fluctuations, the chemistry is essentially unaffected by the flow field. But if the contra condition (rc > rm) is true, the fluid mechanics could influence the chemical reaction rate, energy release rates, and flame structure. 

The general data representation in Fig. 4.47 does show a rapid rise of (ST/SL) for values of (U ISL) greater than 1, the character of the turbulent flame varies and under the appropriate turbulent variables, it can change as depicted in Fig. 4.48, which essentially comes from Borghi [60] as presented by Abdel-Gayed etal. [61],... 

FIGURE 6.10 Variation of the character (height) of a gaseous diffusion flame as a function of fuel jet velocity showing experimental flame liftoff (after Linan and Williams [13] and Hawthorne etal. [14]). 

Both types of surface oxides are found on technical products. Rubber grade carbon blacks are produced in different processes. Channel blacks are made by cooling a flame on iron plates, the so-called channels. The resulting carbon blacks are acidic in character because an excess of air is present (25). In the production of furnace blacks, the fuel, mostly oil or natural gas, is burned with a limited supply of air. Thermal blacks are obtained by thermal cracking of the gas, which sometimes is diluted with hydrogen. In consequence, both types show weakly basic reaction in aqueous suspension. 

It could be expected, that combustion reactions and possibly flames can be produced in such dense supercritical mixtures. Technical aspects of hydrothermal oxydation at moderate pressures have already been tested and discussed [7,8]. The study of combustion and flames in supercritical phases offers several possibilities 1. The variation of pressure over wide ranges should influence reaction mechanisms and flame characteristics because the density can be changed from low, gas-like, to high, liquid-like, values. 2. The variable temperature of the dense, fluid environment can have an influence on reactions and flames. 3. The chemical and physical character of this environment can be varied considerably, for example by using supercritical water as the major component, as in the present experiments. Certainly, the knowledge of transport coefficients of gases involved is desirable. For water the viscosity has been determined to... 

Bachelard writes of the athanor as an enclosure of volatile desire What may at first sight hide its psychological character is the fact that alchemy quickly took on an abstract aspect. The alchemists worked with the enclosedfire, the fire confined in a furnace. The images which are created so lavishly by open flames and which lead to a more free and winged kind of reverie, were now reduced and decolorized to the benefit of a more precise and concentrated dream. And he goes onto say that several of the furnaces and retorts used by the alchemists had undeniable sexual shapes (Bachelard 51). 

Under these circumstances [said Davy] a vivid action was soon observed to take place. The potash began to fuse at both its points of electrization. There was a violent effervescence at the upper surface at the lower, or negative, surface, there was no liberation of elastic fluid but small globules having a high metallic lustre, and being precisely similar in visible characters to quicksilver, appeared, some of which burnt with explosion and bright flame, as soon as they were formed, and others remained, and were merely tarnished, and finally covered by a white film which formed on their surfaces. 

The treatment of the complete two-stage GDF theory will be based on extrapolations of the functional relationships Ti,n(p) to low pressures. Similar extrapolation cannot be carried out indefinitely to high pressures for tu because Lu would become small and soon approach the effective roughness dimension of the propellant surface. The one-dimensional character of the O/F flame would then be lost. 

By local heating of an explosive by a heat source in the form of a flame, spark, etc. A localized thermal impulse creates a local zone in which reactions start to occur and then propagate through the rest of the explosive due to their self-sustaining character. 

The extraordinary resistance of wax to penetration by water has made it unusually useful. The clean, nonsmoky character of its flame makes it an ideal candle ingredient, although it is necessary in their making to include high melting, debrittle waxes or stearic acid (22,31). 

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