Flame stabilization

In summary, the bad features of partial combustion processes are the cost of oxygen and the dilution of the cracked gases with combustion products. Flame stability is always a potential problem. These features are more than offset by the inherent simplicity of the operation, which is the reason that partial combustion is the predominant process for manufacturing acetylene from hydrocarbons. 

Turbulent Diffusion FDmes. Laminar diffusion flames become turbulent with increasing Reynolds number (1,2). Some of the parameters that are affected by turbulence include flame speed, minimum ignition energy, flame stabilization, and rates of pollutant formation. Changes in flame stmcture are beHeved to be controlled entirely by fluid mechanics and physical transport processes (1,2,9). 

The maximum velocity at the axis is twice the average, whereas the velocity at the wall is zero. The effect of the burner wall is to cool the flame locally and decrease the burning velocity of the mixture. This results in flame stabilization. However, if the heat-transfer processes (conduction, convection, and radiation) involved in cooling the flame are somehow impeded, the rate of heat loss is decreased and the local reduction in burning velocity may no longer take place. This could result in upstream propagation of the flame. 

Endurance Burn Under certain cou(itious, a successfully arrested flame may stabilize on the unprotected side of an arrester element. Should this condition not be corrected, the flame will eventually penetrate the arrester as the channels become hot. An endurance burn time can be determined by testing, which specifies that the arrester has withstood a stabilized flame without penetration for a given period. The test should address either the actual or worst-case geometry, since heat transfer to the element will depend on whether the flame stabilizes on the top, bottom, or horizontal face. In general, the endurance burn time identified by test should not be regarded as an accurate measure of the time available to take remedial action, since test conditions will not necessarily approximate the worst possible practical case. Temperature sensors may be incorporated at the arrester to indicate a stabilized flame condition and either alarm or initiate appropriate action, such as valve closure. 

Gas Burners Gas burners may be classified as premixed or non-premixed. Many types of flame stabilizer are employed in gas burners (see Fig. 27-32). Bluff body, swirl, and combinations thereof are the predominant stabilization mechanisms. 

Figure 10-2. Baffle added to straight-walled duet to ereate flame stabilization zone.

Figure 10-4. Flame stabilization created by impinging jets and general airflow pattern. (C Rolls-Royce Limited.)...

The air enters the annular spaee between the liner and easing, and is admitted into the spaee within the liner through holes and slots beeause of the pressure differenee. The design of these holes and slots divides the liner into distinet zones for flame stabilization, eombustion, dilution, and provides film eooling of the liner. 

Length. Combustor length must be sufficient to provide for flame stabilization, combustion, and mixing with dilution air. The typical value of the length-to-diameter ratio for liners ranges from three to six. Ratios for casing range from two to four. 

Flare stack sizing and pressure drop is included with considerations of pressure drop through the safety valve headers, blowdown drums, flare headers, seal drum, etc. Elevated flare tips incorporating various steam injection nozzle configurations are normally sized for a velocity of 120 m/s at maximum flow, as limited by excessive noise and the ability of manufacturers to design tips which will insure flame stability. This velocity is based on the inclusion of steam flow if injected internally, but the steam is not included if added through jets external to the main tip. 

In this case ignition occnrs on the downstream side of the flame arrester. If the velocity of the gas is less than the flame speed, the flame stabilizes on the flame arrester and continnes to heat it nntil the gas flow is stopped or the flame is qnenched by other means. Continned heating conld canse ignition of the gas on the opposite side of the flame arrester. 

In these cases, it is assumed that die flame stabilizes on die upstream side of the flame arrester and hot gas flows throngh the arrester, heats it, and... 

Stabilized Burning Steady burning of a flame, stabilized at, or close to die flame arrester element. 

Utilizing a forced-draft fan, the burner has a gas head arranged to mix the fuel and air in a blast tube which controls the stability and shape of the flame. Gas exits from nozzles or holes in the head and is mixed partly in the high-velocity air stream and partly allowed to exit into an area downstream of a bluff body. Behind the bluff body, a relatively quiescent zone forms which provides a means for flame stability. Many configurations exist, but the most... 

Role of Edge Flames for Lifted Flame Stabilization.61... 

Laminar flame speed is one of the fundamental properties characterizing the global combustion rate of a fuel/ oxidizer mixture. Therefore, it frequently serves as the reference quantity in the study of the phenomena involving premixed flames, such as flammability limits, flame stabilization, blowoff, blowout, extinction, and turbulent combustion. Furthermore, it contains the information on the reaction mechanism in the high-temperature regime, in the presence of diffusive transport. Hence, at the global level, laminar flame-speed data have been widely used to validate a proposed chemical reaction mechanism. 

The best fit of velocity exponent n in Hp °c ug (Figure 4.3.11) for pure propane (n-butane) is n = 4.733 (3.638), corresponding to Sc = 1.37 (1.61) from n = (2Sc-l)/ (Sc -1), which agreed well with the suggested value of Sc = 1.376 (1.524). The experimental liftoff height data are shown in Figure 4.3.12 for various nozzle diameters and partial air dilutions to fuel [53]. It can be observed that the air dilution to fuel does not alter Ypst and S° sf The results substantiated the role of tri-brachial flames on flame stabilization in laminar jets. As mentioned previously. Equation 4.3.5 limits the maximum velocity Ug for Sc > 1, which corresponds to blowout condition. 

Based on the flame-hole dynamics [59], dynamic evolutions of flame holes were simulated to yield the statistical chance to determine the reacting or quenched flame surface under the randomly fluctuating 2D strain-rate field. The flame-hole d5mamics have also been applied to turbulent flame stabilization by considering the realistic turbulence effects by introducing fluctuating 2D strain-rate field [22] and adopting the level-set method [60]. 

D. Veynante, L. Vervisch, T. Poinsot, A. Linan, and G. R. Ruetsch, Triple flame structure and diffusion flame stabilization, Proceedings of the Summer Program, Center for Turbulent Research 55-73,1994. 

T. Echekki, J. -Y. Ghen, and U. Hedge, Numerical investigation of buoyancy effects on triple flame stability. Combust. Sci. Technol. 176(3) 381-407, 2004. 

T. Schuller, D. Durox, and S. Candel. Self-induced combustion oscillations of laminar premixed flames stabilized on annular burners. Combustion and Flame, 135 525-537, 2003. 

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

Muniz, L. and Mungal, M. G., Instantaneous flame-stabilization velocities in Ufted-jet diffusion flames. Combust. Flame, 111, 16,1997. 

Alsairafi, A., Lee, S.T., and T ien, J.S., Modeling gravity effects on diffusion flames stabilized around a cylindrical wick saturated with liquid fuel. Combust. Sci. Technol., 176, 2165, 2004. 

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