Aviation Fuels with Improved Fire Safety A Proceedings (1997) / Chapter Skim
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IV. Presented Papers: Characterizing Fuel Fires
IV. Presented Papers: Characterizing Fuel Fires
Pages 79-122
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From page 79... ...
lv PRESENTED PAPERS CHARACTERIZING FUEL FIRES
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From page 81... ...
Liquid fuel/air combustion in both spray flames and pool fires is discussed, considering laminar diffusion flames, buoyant turbulent noncombusting flows, buoyant turbulent diffusion flames, and turbulent sprays and spray flames. INTRODUCTION Post-crash fires caused by the ignition of jet fuel released from damaged fuel systems are an important problem of aircraft fire safety.
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From page 82... ...
These correlations have been termed state relationships, and they are applied to turbulent flames by assuming that turbulent flames correspond to wrinkled laminar flames (Bilger, 1976~. Numerous measurements of state relationships for both liquid and gaseous hydrocarbon-fueled laminar diffusion flames have been reported (see Bilger, 1976; Faeth and Samuelsen,1986; Gore and Faeth, 1988a,1988b; Sivathanu and Faeth, 1990, for typical examples)
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BUOYANT TURBULENT NONCOMBUSTING FLOWS Modeling Buoyant Turbulent Flows Post-crash fires are reasonably represented by liquid-fu eled buoyant turbulent diffusion flames. Given state relation ships to find scalar properties in diffusion flames, methods of predicting mixing levels in liquid-fueled buoyant turbulent diffusion flames must be addressed.
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All other mean properties behaved the same way (Dai and Faeth, 1995~. Although the agreement between recent measurements and turbulence model predictions of mean properties in selfpreserving round buoyant turbulent plumes is encouraging, corresponding predictions of turbulence properties are less satisfactory.
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Some typical approaches to this problem follow; methods of predicting flame radiation properties will be deferred for the present. The most widely used methods of predicting the structure of buoyant turbulent diffusion flames exploit the k-e or k-e-g families of turbulence models (Bilger, 1976; Lockwood and Naguib, 1975~.
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From page 86... ...
Thus, no method of treating the scalar properties of buoyant turbulent diffusion flames provides reliable estimates of soot concentrations, which are crucial for making accurate estimates of flame radiation properties. Turbulent Diffusion Flame Structure Two examples of turbulent diffusion flames will be considered: a buoyant turbulent round jet diffusion flame, which provides a well-defined and readily reproducible flame configuration, and a pool fire in an enclosure, which is more representative of practical fire environments.
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of the chemical energy release are radiated to the surroundings by a continuum of radiation from soot (Faeth et al., 1989~. Current methods of modeling flame radiation are described next before representative measurements and predictions of radiation for buoyant turbulent diffusion flames are considered.
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l ~ 90 _ I I ~' - ~ _ it_ 2 3 4 5 6 Wavelength (,um) FIGURE 12-9 Measured and predicted spectral radiation intensities for horizontal paths through the axis of acetylenefueled round buoyant turbulent diffusion flames burning in air.
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TURBULENT SPRAYS AND SPRAY FLAMES Modeling Sprays Sprays and spray flames are an important aspect of postcrash fires. Aircraft fuel is dispersed during crashes by ruptured fuel lines and fuel tanks, by aerodynamic effects due to the motion of the aircraft or wind relative to the aircraft over liquid-fuel streams created by ruptures, and also by splashing or drop formation due to turbulent primary breakup of liquid fuel spilling from ruptured fuel tanks onto aircraft surfaces or the ground.
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From page 90... ...
As a result, DSF predictions exhibit unphysical laminar-like behavior where drops collect in low velocity regions. The SSF methods involve treatment of gas-phase properties very similar to single-phase turbulent diffusion flames, using either the conserved-scalar formalism and state relationships as a function of gas-phase mixture fractions or eddy-dissipation (or eddy-breakup)
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From page 91... ...
These studies were intended to study practical pressure atomized sprays, but the primary breakup of liquid jets from ruptured fuel lines should be similar. More recent work has involved wall jets, where a turbulent boundary layer grows along the wall, the surface of the wall jet becomes roughened as it is approached by the outer edge of the growing turbulent boundary layer, and drop formation by the turbulent primary breakup mechanism 10' 10° Oh 10-1 10-2 91 begins once the kinetic energy of turbulent eddies near the liquid surface exceeds the surface-tension energy required by drops of comparable size (Dai et al., 1997~.
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The surprising feature of the findings is that drop sizes after secondary breakup are strong functions of liquid viscosity but are relatively independent of the surface tension. This AVIATION FUELS WITH IMPROVED FIRE SAFETY: A PROCEEDINGS behavior is the opposite of the behavior of drop sizes after turbulent primary breakup (Figure 12- 11)
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Measurements ofthe structure of self-preserving round buoyant turbulent plumes. Journal of Heat Transfer 118~2~:493~95.
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1991. Carbon monoxide emissions from buoyant turbulent diffusion flames.
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1995. Effects of initial flow conditions on primary breakup of nonturbulent and turbulent round liquid jets.
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First, flame spread above liquid fuels is discussed as a significant danger in accidents that result in liquid fuel forming a pool that can be ignited in various ways. Second, the ignition of gaseous combustible mixtures by hot moving particles is analyzed.
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(1996) showed that many parameter variations besides To can change uniform flame spread to pulsating flame spread or vice versa.
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t-~`OD ~(dl) t=1~20 5 y'= 3 rnm FIGURE 13-3 Pulsation cycle for flame spread above liquid fuel at low initial temperatures (To)
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Ignition and flame propagation each require heating and vaporization of the droplets and mixing of the fuel vapor with air before chemical reaction can occur. Intuition might indicate that ignition of or flame propagation through a spray always occurs at a slower rate when compared to a gaseous mixture at the same mixture ratio.
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From page 102... ...
. A spray flame has a complex structure, with both diffusion flame characteristics and premixed flame characteristics.
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A- do= 100 and 200,um B - d2o C - d32 D - do = 100,um - _ _ - _ _ C D ~~-. _._ 0 1.0 2.0 3.0 4.0 Er FIGURE 13-8 Ignition delay versus equivalence ratio for a polydisperse spray.
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A comparison of Figures 13-9 and 13-10 shows that fuel vapor mass fraction is largest well behind the flame-front (the region of a sharp increase in temperature) so that, to a large extent, burning occurs as a diffusion flame.
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Proceedings of the 26th International Symposium on Combustion, held July 28
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1972. A critical discussion of theories of flame spread across solid and liquid fuels.
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The role of fuel dispersal in the post-crash environment can best be described if the crash environment is viewed as a process, albeit an undesirable one. At the most abstract level, as shown in Figure 14-1, the crash process can be thought of as a set of initial conditions (crash scenarios)
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The actual processes involved in fuel dispersal will be discussed in the next section. Energy dissipation processes create the initial conditions for a fire (discussed with respect to Figure 14-1~.
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| / a, Hazardous mtl and personnel response 1 / r 7 Aircraft response to combustion event l/ r Combustion T / | / Ignition 1 / Fuel dispersal | /Fuel tank rupture| Ignition source created | ~ Initial Aircraft Fire Accident impact motion stopped terminated terminated Time FIGURE 14-2 Processes involved in aircraft crashes as a function of lime. The impact velocity regimes are defined in Figure 14-3.
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. The processes involved in fuel dispersal in the medium impact velocity regime are shown in Figures 14-4 and 14-5.
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The two distinct flow streams in this stage of dispersal are airborne droplets, which result from the primary breakup, and the residual fuel stream, which continues to fall until it reaches the ground. The next stage of dispersal of the airborne droplets involves secondary breakup and interphase momentum exchange with the air.
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Very long duration fires can be sustained this way. Fuel Dispersal Processes in High Velocity Impacts The high velocity impact regime has received less attention than the medium velocity regime.
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Figure 14-7 shows the stages of fuel dispersal in the high impact velocity regime. In many respects, the flow regimes are not qualitatively different from flow regimes in the medium impact velocity range, although there are quantitative differences.
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Commercial hardware vendors include Malvern, Insitec, Dantec, and Aerometrics.1 Available instruments can characterize drop size and, in some cases, velocity. Unfortunately, they are most applicable in flow regimes near the end of the primary breakup stage and into the interphase transfer stage.
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Department of Transportation. However, the environments are so difficult to work in that the quantitative measurement of fuel dispersal characteristics during fire transients has received very little attention.
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CONCLUSIONS Much has been learned about post-crash fuel dispersal processes in the last few decades, both from programs directed at understanding this environment and from fun
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REFERENCES Ahlers, R.H.1970. Investigation of Two Methods forImproving the Crashworthiness of Integral Fuel Tanks.
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From page 119... ...
1995. Effects of initial flow conditions on primary breakup of nonturbulent and turbulent round liquid jets.
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