Flame radiation

Radiation from flames and combustion products involve complex processes, and its determination depends on knowing the temporal and spatial distributions of temperature, soot size distribution and concentration, and emitting and absorbing gas species concentrations. While, in principle, it is possible to compute radiative heat transfer if 

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Microbumer Studies of Flame Radiation as Related to Hydrocarbon Structure, Report 3752-64R, Navy Buweps Contract NOw 63-0406d, Phillips Petroleum Co., Bartiesville, OHa., May 1964. 

Step 6 Calculate the location of the flame center, which is treated as the source of all radiation from the flame. Only flames bent over by the wind are considered, since for nearly vertical flames (calm air) the effective center of flame radiation is higher off the ground and therefore not limiting for spacing purposes. 

Radiation heat flux is graphically represented as a function of time in Figure 8.3. The total amount of radiation heat from a surface can be found by integration of the radiation heat flux over the time of flame propagation, that is, the area under the curve. This result is probably an overstatement of realistic values, because the flame will probably not bum as a closed front. Instead, it will consist of several plumes which might reach heights in excess of those assumed in the model but will nevertheless probably produce less flame radiation. Moreover, the flame will not bum as a plane surface but more in the shape of a horseshoe. Finally, wind will have a considerable influence on flame shape and cloud position. None of these eflects has been taken into account. 

Ground Distance (m) View Factor Solid Flame Radiation (kW/m ) Point Source Radiation (Hymes) (kW/ni )... 

The flame radiation at a distance r from tlie pool center is given by... 

For the quantification of fire propagation behavior of the FRC materials, 0.10 m wide and 0.61 m long vertical sheets with thickness varying from 3 mm to 5 mm were used. The bottom 0.15 m of the sheet was exposed to 50 kW/m2 of external heat flux in the presence of a 0.01 m long pilot flame to initiate fire propagation. For the simulation of large-scale flame radiation, experiments were performed in k0% oxygen concentration. 

Delichatsios, M. A., "Flame Heights in Turbulent Wall Fires with Significant Flame Radiation", Comb. Sci. Tech., 39, p. 195, 1984. 

A flame radiates 40 % of its energy. The fuel supply is 100 g/s and its heat of combustion is 30kJ/g. A thin drapery is 3 m from the flame. Assume piloted ignition. When will the drapery ignite The ambient temperature is 20 °C and the heat transfer coefficient of the drapery is... 

The incident flame heat flux can be estimated as XrQj(4ttr1), where Xr is the flame radiation fraction and r is the distance from the flame. 

The asymptotic burning rate behavior under saturated flame radiation conditions is a useful fact. It provides an upper limit, at pool-like fuel configurations of typically 1-2 m diameter, for the burning flux. Experimental values exist in the burning literature for liquids as well as solids. They should be thoughtfully used for design and analysis purposes. Some maximum values are listed in Table 9.3. 

Equation (9.73) can be used to explain the burning behavior with respect to the roles of flame radiation and convection. It can also explain the effects of oxygen and the addition of external radiant heat flux. These effects are vividly shown by the correlation offered from data of Tew arson and Pion [18] of irradiated horizontal small square sheets of burning PMMA in flows of varying oxygen mole fractions. The set of results for steady burning are described by a linear correlation in q" and Xq2 for L= 1.62 kJ/g in Figure 9.16. This follows from Equation (9.73) ... 

In this case, all of the radiation loss from the flame is accounted for by XT, including that received by the surface of the condensed phase. Therefore flame radiation heat flux does not show up here. 

The flame radiation is taken as zero since the flame size diminishes. 

Assume steady burning with the sample originally at 25 °C with a perfectly insulated bottom. At extinction you can ignore the flame radiation. Assume that all of the flakes hit the surface and ignore the gas phase effects of the extinguishment agents. Use thermodynamic properties of the C02 and H20, and the property data of PMMA from Table 9.2. 

Later we shall include combustion and flame radiation effects, but we will still maintain all of assumptions 2 to 5 above. The top-hat profile and Boussinesq assumptions serve only to simplify our mathematics, while retaining the basic physics of the problem. However, since the theory can only be taken so far before experimental data must be relied on for its missing pieces, the degree of these simplifications should not reduce the generality of the results. We shall use the following conservation equations in control volume form for a fixed CV and for steady state conditions ... 

If flame radiation occurs in the mass burning process—or any other radiation is imposed, as is frequently the case in plastic flammability tests—one can obtain a convenient expression for the mass burning rate provided one assumes that only the gasifying surface, and none of the gases between the radiation source and the surface, absorbs radiation. In this case Fineman [32] showed that the stagnant film expression for the burning rate can be approximated by... 

Shokri and Beyler correlated experimental data of flame radiation to external targets in terms of an average effective emissive power of the flame (Shokri and Beyler, 1989). The flame is assumed to be a cylindrical, black body radiator with an average emissive power, diameter (D), and height (T/f), see Figure 5-9. 

An increase in droplet size with axial position is observed for all three gases. However, the relative trend of smallest droplet mean size with steam and largest with normal (unheated) air remains unchanged. As an example, at 50 mm downstream from the nozzle exit at r = 0, droplet mean size for steam, preheated air, and normal air were found to be 69, 86, and 107 pm, respectively see Fig 16.3. The droplet size with steam is also significantly smaller than air at all radial positions see Fig. 16.3. The droplet size with preheated air is somewhat smaller than normal air due to the decreased effect of preheated air at this location and increased effect of combustion. Early ignition of the mixture with preheated air (see Fig. 16.1) must provide a longer droplet residence time which results in a smaller droplet size. In addition, the increased flame radiation with preheated air increased droplet vaporization at greater distances downstream from the nozzle exit. Indeed, the results indicate that the measured droplet sizes with preheated atomization air are smaller than normal air in the center... 

Turns et al. [6] studied turbulent partially premixed flames burning methane, propane, and ethylene with air. The NO emission indices for methane and propane flames first increased and then decreased with increased levels of partial premixing. The NO emission indices for ethylene flames continuously increased at least in the limited range of partial premixing considered in the experiments. The results were qualitatively explained by the opposing effects of flame radiation and residence time on NO emissions. 

With the porous layers installed, flame radiation increased, lowering the peak flame temperatures and extending the reaction zone. The combustor pattern factor improved due to the radial conduction and radiation within the solid matrix. 

Effect of turbulence on flame radiation) (Authors measured the radiation intensity from propane flames and found a decrease in radiation with turbulence. Radiation is not thermal, but appears to be a luminescent phenomenon) 11) T.E. Holland et al, JApplPhys 28, 1217(1957)... 

Often flame radiation may be reflecfed from the back of a rotating sector, which is less than totally reliable because it is mechanical. Most modem instmments therefore modulate the power applied to the source and also use this signal to trigger the amplifier (Fig. 12c). Such instmments may still have a rotating sector (but only for atomic emission) between the flame and the monochromator, or the amplifier may be capable of being reset for DC signals. 

Phase contrast observations in flames) 10) R.R. John M. Summer-field, Jet Propulsion 27, 169—175 178 (1957) (Effect of turbulence on flame radiation) 11) H. Selle, Explosivstoffe 8, 9 195—204(I960) (Investigations on flame... 

Radiation and Ignition Studies. The existence of cool flame radiation prior to the occurrence of autoignition in a motored engine was discovered early by Peletier, van Hoogstraten, Smittenberg, and Koojman (109). Since then there has been a constant effort to define the engine conditions limiting the occurrence and extent of cool flame radiation with various hydrocarbons (25, 26, 37, 71, 72, 77, 107). 

Figure 1. Block diagram of cool flame radiation detection equipment...

Emission of cool flame radiation is associated with an early stage in the preflame reactions of most hydrocarbons, as evidenced by the fact that the appearance of cool flames occurs at the same time as the initial pressure development. At a later time in the cycle a second radiation phenomena, described as a blue flame, has been observed under certain conditions (36, 124). A second cool flame also occurring late in the cycle may be the same phenomenon (81, 103, 131). While the importance of cool flame and blue flame phenomena in the over-all reaction mechanism is not fully understood, their occurrences can be used to mark certain stages in the course of the preflame reactions. This principle has been used extensively in studying the effect of hydrocarbon structure, physical variables, and additives on engine preknock reactions. 

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