Combusting Plumes

We will now examine the combustion aspect of plumes and attempt to demonstrate more realistic results. The preceding approach provides a guide for how we might proceed. Since the complexities of combustion are too great to consider in any fundamental way, we simply adopt the same point source approach with the additional assumption  

Combustion occurs uniformly throughout the plume so long as there is fuel present. 

However, it is not likely that all of the air entrained will react with the fuel. The turbulent structure of the flame does not allow the fuel and oxygen to instantaneously mix. There is a delay, sometimes described as unmixedness . Hence, some amount of 

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Fig. 15.19 Combustion plumes of a VFDR subjected to an SF] test (a) with an optimum air-to-fuel ratio, and (b) with a fuel-lean air-to-fuel ratio.

With increasing f (Al), the burn rate increases as shown in Figure 8.40. This is in line with the deflagration behaviour of powdered nano-Al/PTFE in air described above. At a stoichiometric ratio [Pg.115]

Figure 10.13 Morphology of a flare combustion plume and short exposure photographs (1/8000 s, aperture 16) of MTV. (Photo taken by V. Weiser, E. Roth Fraunhofer ICT, Pfinztal/Berghausen, Germany, 2006, Ref [15].)...

Figure 10.44 MIL Flare combustion plume [71]. (Reproduced with kind permission by Uwe Schaller and Volker Weiser.)...

Ionic liquid fluorocarbon compounds [70] wiU come into play as the oxidizers of choice for advanced signature decoy flares that could create structured combustion patterns [71]. Figure 10.44 depicts the combustion plume of Mg/Ionic Liquid (MIL) with 4-amino-l-methyl-l,2,4-triazolium trifluoromethanesulfonate [71]. 

In order to be appHcable as incendiaries, pyrotechnic compositions must fulfil a number of thermo-chemical requirements such as high enthalpy of reaction, low activation energy, suitable rate of combustion, high radiant efficiency and large radiating combustion plume and/or highly conductive slag. In addition, these... 

Unlike the situation with gaseous contaminants, volume ratio or mass ratio in parts per million (ppm) is not used for aerosols, because two phases are involved and aerosol concentrations are numerically very low when expressed in this way. It is informative, however, to make such calculations for some standard concentrations, as shown in Table 1.2. Note that, on a volume basis, a dense combustion plume is 99.999% pure air. 

Nitrogen Oxides. From the combustion of fuels containing only C, H, and O, the usual ak pollutants or emissions of interest are carbon monoxide, unbumed hydrocarbons, and oxides of nitrogen (NO ). The interaction of the last two in the atmosphere produces photochemical smog. NO, the sum of NO and NO2, is formed almost entkely as NO in the products of flames typically 5 or 10% of it is subsequently converted to NO2 at low temperatures. Occasionally, conditions in a combustion system may lead to a much larger fraction of NO2 and the undeskable visibiUty thereof, ie, a very large exhaust plume. 

VDI Part 1 models the dispersion of vapor plumes with output consisting of vapor ctiriccntration as a function of time and downwind distance and denser-than-air vapor releases. VDI Part 2 determines the downwind distance to the lower flammable limit of a combustible vapor. Part 2 may also be used in conjunction with Part 1 to model a toxic gas emission. 

Combustion behavior differed in some respects between continuous and instantaneous spills, and also between LNG and refrigerated liquid propane. For continuous spills, a short period of premixed burning occurred immediately after ignition. This was characterized by a weakly luminous flame, and was followed by combustion of the fuel-rich portions of the plume, which burned with a rather low, bright yellow flame. Hame height increased markedly as soon as the fire burned back to the liquid pool at the spill point, and assumed the tilted, cylindrical shape that is characteristic of a pool fire. 

The model is a straightforward extension of a pool-fire model developed by Steward (1964), and is, of course, a drastic simplification of reality. Figure 5.4 illustrates the model, consisting of a two-dimensional, turbulent-flame front propagating at a given, constant velocity S into a stagnant mixture of depth d. The flame base of width W is dependent on the combustion process in the buoyant plume above the flame base. This fire plume is fed by an unbumt mixture that flows in with velocity Mq. The model assumes that the combustion process is fully convection-controlled, and therefore, fully determined by entrainment of air into the buoyant fire plume. 

Because the Raj and Emmons (1975) expression for tv cannot be applied in a straightforward manner, the expression given here differs from that recommended by Raj and Emmons (1975). It should be emphasized that w, which represents the inverse of the volumetric expansion due to combustion in the plume, is highly... 

Examples of the need for multimedia models are found in contemporary problem areas. Polynuclear aromatic hydrocarbons and metals are emitted into the atmosphere as trace impurities with the products of coal combustion. The organics have low vapor pressure and partially condense on emitted particulates in a stack plume. The particulates are transferred to the soil by dry deposition, rainout or washout. The metals manifest... 

Kung, H.C. and Stavrianidis, R, Buoyant plumes of large - combustion scale pool fires, Proc. Comb. Inst., 1982, 19, pp. 905-12. 

Steward (1964) [4] and (1970) [5] presented a rigorous but implicit approximate analysis for both axisymmetric and two-dimensional strip fire plumes respectively, including combustion and variable density effects. 

Zukoski, E.E., Properties of fire plumes, in Combustion Fundamentals of Fire (ed. G. Cox), Academic Press, London, 1995. 

Hasemi, Y. and Tokunaga, T., Flame geometry effects on the buoyant plumes from turbulent diffusion flames, Combust. Sci. Technol., 1984, 40, 1-17. 

Figure 12.16 shows the rising fire plume (or falling salt plume) of Tanaka, Fujita and Yamaguchi [24] from Chapter 10 compared to the saltwater data of Strege [25]. The saltwater data correspond to the long-time regime, and follow the combustion data from Tanaka. 

If a combustible gas release is not ignited immediately, a vapor plume will form. This will drift and be dispersed by the ambient winds or natural ventilation. If the gas is ignited at this point, but does not explode, it will result in a flash fire, in which the entire gas cloud bums very rapidly. It is unlikely to cause any fatalities, but will damage steel structures. If the gas release has not be isolated during this time, the flash fire will bum back to a jet fire at the source of the release. A flash fire is represented by its limiting envelope, since no damage is caused beyond it. This envelope is usually taken as the LEL of the gas cloud. 

Smoke is composed of combustion gases, soot (solid carbon particles), and unburnt fuel. For outdoor fires, the impact of smoke is usually a secondary consideration after the heat transfer. In many circumstances, the immediate thermal threat from the fire plume (jet, pool, or flash fire) overwhelms the smoke threat, particularly for personnel in close proximity to the event. There may be circumstances where personnel are in a downwind smoke plume where there is no immediate thermal threat. As a rule-of-thumb, all people within a smoke plume may be immediately or nearly immediately affected and at risk from a life safety standpoint (be it from lack of visibility or by toxic products). 

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