Jet flames

This volume does not address subjects such as toxic effects, explosions in buildings and vessels, runaway reactions, condensed-phase explosions, pool fires, jet flames, or structural responses of buildings. Furthermore, no attempt is made to cover the frequency or likelihood that a related accident scenario will occur. References to other works are provided for readers interested in these phenomena. 

High-energy ignition of an unobstructed cloud by a jet flame emerging from a partially confined explosion produces a high combustion rate in the jet-flow region. 

Interaction of a jet flame and an obstacle array can result in an increase of flame speed and production of pressures in excess of 700 mbar. 

Orifice flames Pool flames Jet flames Fireball flames Flash fire flames... 

Xg = emissi vity factor dependent on the nature of the combustible material involved (a value of 0.2 is suggested for jet flames), r = distance from the particular point in reference to the flame q = heat flux... 

The flame behavior of a fire is important in determining tlie causes and effects of fires. There are several classificiitions of flames orifice flames, pool flames, fireballs. Jet fimnes, and flash fires. Orifice or pipe flames are characterized as eitlier prenii. ed flame or diffusion flmiies. Pool flames are flames on ground pools and flames on tanks. Fireballs radiate intense heat, wliich can cause fatal bums and can quickly ignite otlier materials. Jet flame or flares also radiate intense heat. 

Jet-flame behavior demonstrating continuous transition from lami-nar-lifted to turbulent-lifted flames. (From Lee, B.J., Kim, J.S., and Chung, S.H., Proc. Combust. Inst., 25,1175,1994.)... 

C. M. Muller, H. Breitbach, and N. Peters, Partially premixed turbulent flame propagation in jet flames, Proc. Combust. Inst. 25 1099-1106,1994. 

D. Durox, T. Schuller, and S. Candel. Self-induced instability of premixed jet flame impinging on a plate. Proceedings of the Combustion Institute, 29 69-75, 2002. 

T. Schuller, D. Durox, and S. Gandel. Dynamics of and noise radiated by a perturbed impinging premixed jet flame. Combust. Flame, 128 88-110, 2002. 

Jet flames provide a simple canonical geomefry for illusfrafing the essential features of furbulenf nonpremixed flames. In Figure 7.2.1, chemiluminescence images, using different camera-exposure times, show the mean and fluctuating structure of a furbulenf... 

Chemiluminescence images of a turbulent CH4/H2/N2 jet flame (Re = 15,200) measured with two different exposure times. The long-exposure image (far left) indicates the mean flame structure, and the six shorter exposures to the right illustrate the instantaneous turbulent structure. 

Chemiluminescence images of a turbulent partially premixed CH4/ air jet flame stabilized by premixed pilot flames. 

Scatter plots of temperature atx/d = 15 in turbulent Cl-14/air jet flames with Reynolds numbers of 13,400 (Flame C) and 44,800 (Flame F). The stoichiometric mixture fraction is = 0.351. The line shows the results of a laminar counterflow-flame calculation with a strain parameter of a = 100 s and is included as a visual guide. (From Barlow, R.S. and Frank, J.H., Proc. Combust. Inst, 27,1087,1998. With permission.)... 

One of the most challenging aspects of modeling turbulent combustion is the accurate prediction of finite-rate chemistry effects. In highly turbulent flames, the local transport rates for the removal of combustion radicals and heat may be comparable to or larger than the production rates of radicals and heat from combustion reactions. As a result, the chemistry cannot keep up with the transport and the flame is quenched. To illustrate these finite-rate chemistry effects, we compare temperature measurements in two piloted, partially premixed CH4/air (1/3 by vol.) jet flames with different turbulence levels. Figure 7.2.4 shows scatter plots of temperature as a function of mixture fraction for a fully burning flame (Flame C) and a flame with significant local extinction (Flame F) at a downstream location of xld = 15 [16]. These scatter plots provide a qualitative indication of the probability of local extinction, which is characterized... 

Figure 7.2.5 provides a visualization of a localized extinction event in a turbulent jet flame, using a temporal sequence of OH planar LIF measurements. The OH-LIF measurements, combined with particle image velocimetry (PIV) reveal that a distinct vortex within the turbulent flow distorts and consequently breaks the OH front. These localized extinction events occur intermittently as the strength of the coupling between the turbulent flow and the flame chemistry fluctuates. The characteristics of the turbulent flame can be significantly altered as the frequency of these events increases. 

Temporal sequence of OH-LIF measurements captures a localized extinction event in a turbulent nonpremixed CH4/H2/N2 jet flame (Re 20,000) as a vortex perturbs the reaction zone. The time between frames is 125 ps. The velocity field from PIV measurements is superimposed on the second frame and has the mean vertical velocity of 9m/s subtracted. (From Hult, J. et al.. Paper No. 26-2, in 10th International Symposium on Applications of Laser Techniques to Fluid Mechanics, Lisbon, 2000. With permission.)... 

Qualitative comparison of the inclined structure of thin layers of high scalar dissipation in a piloted CH4/air jet flame as revealed by (a) mixture fraction imaging, (b) LES with a steady flamelet library (a and b are adapted from Kempf, A. Flemming, F., and Janicka, ]., Proc. Combust. Inst, 30, 557, 2005. With permission.), and (c) LES with unsteady flamelet modeling. (Adapted from Pitsch, H. and Steiner, H., Proc. Combust. Inst., 28, 41, 2000. With permission.)... 

Model of ID dissipation spectrum from Pope [19] (line) and measured, noise-corrected spectrum of the square of the radial gradient of fluctuating temperature in a CH4/I-I2/N2 jet flame (Re = 15,200) (symbols). Each spectrum is normalized by its maximum value. The arrow indicates the 2% level, which corresponds to the normalized wavenumber k = 1 according to the model spectrum. (From Barlow, R.S., Proc. Combust. Inst., 31, 49,2007. With permission.)... 

Frank, J. H., Kaiser, S. A., and Long, M. B., Multiscalar imaging in partially premixed jet flames with argon dilution, Combust. Flame, 143, 507, 2005. 

Barlow, R. S. and Frank, J. H., Effects of turbulence on species mass fractions in methane/air jet flames, Proc. Combust. Inst., 2J, 1087, 1998. 

Volume rendering of scalar dissipation rate in a DNS of a temporally evolving CO/Hj jet flame. Re = 9200 [16]. The highest values of scalar dissipation rate (shown in red) exceed 30,000 S . ... 

Vervisch, L., R. Hauguel, R. Domingo, and M. Rullaud, Three facets of turbulent combustion modelling DNS of premixed V-flame, LES of lifted nonpremixed flame and RANS of jet flame. /. Turbulence, 2004. 5(4) 004. 

Hawkes, E.R., S. Sankaran, J.C. Sutherland, and J.H. Chen, Scalar mixing in direct numerical simulations of temporally-evolving plane jet flames with detailed CO/Hj kinetics. Proc. Combust. Inst., 2007. 31 1633-1640. 

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