Turbulent flames confined

Spalding, D. B. 1967. The spread of turbulent flames confined in ducts. 11th Symposium (International) on Combustion Proceedings. Pittsburgh, PA The Combustion Institute. 807-15. 

If the release forms a vapor that mixes sufficiently with air to create a flammable mixture, and upon ignition there is not sufficient turbulence or confinement to accelerate the flame and produce a blast, a flash fire results. Damage is again caused by radiant heat and direct flame contact, but the affected area may be much larger than for a pool or jet fire. 

CONFINED TURBULENT FLAMES STABILIZED ON BLUFF BODIES... 

Gore et al. [8] reported, for the first time, the existence of an optimum level of partial premixing for minimum NO emissions from turbulent jet flames. The optimum equivalence ratio ( 5) for a minimum emission index was found to be f.5, which is less than that found for the laminar flames discussed above. Lyle et al. [f5] confirmed the existence of an optimum level of partial premixing for both confined and unconfined turbulent flames. Lyle et al. [15] established that changes in thermal NO production do not control the emission behavior of partially premixed turbulent flames. More recently, Kemal et al. [10] have shown that a minimum in NO emissions can also be obtained for sudden dump-stabilized turbulent partially premixed flames. 

An alternative view [36] is that if l/d becomes too small, then extinctions of laminar flamelets are reflected in extinction of the turbulent flame. According to this idea, there is a region to the upper left in Figure 10.5 in which turbulent flame propagation cannot occur. It seems physically that phenomena of this type may pertain to confined turbulent flows in reactors of small volume, where they would reflect influences of turbulence properties... 

Spalding, D.B. (1970), Mixing and chemical reaction in steady confined turbulent flames, in 13th Symposium on Combustion, Salt Lake city, UT, 649. 

More ICT tests were conducted under conditions of partial confinement (two parallel walls forming al0x3x3m lane) and with rich (36 - 41 vol%) H2-air mixtures. With full fan-induced turbulence, flame acceleration was observed until DDT occurred resulting in pressures and velocities exceeding the Chapman-Joguet values (see next section) [35]. Flame and detonation front profiles for one of the tests are plotted in Fig. 8-11 clearly showing the sudden transition to detonation shortly past the fan. 

In a deflagration the flammable mixture burns relatively slowly. Flame propagation is mainly determined by molecular diffusion and turbulent transport processes. Mixtures of hydrocarbons and air burn in the absence of turbulence, i.e. under laminar or almost laminar conditions, with flame speeds of the order of 5-30 m/s. If there is no confinement this is too slow to produce tangible overpressures and only a flash fire is produced. That is why there is always turbulence involved in a vapour cloud explosion (turbulent flame speeds 100-300 m/s), which increases the rate of combustion and hence the overpressure [12]. 

Vapor cloud explosions in partially confined areas with obstacles exhibit a further increase of the maximum explosion pressure compared to unconfined explosions due to additional turbulent flame acceleration at obstacles. In experiments we found an increase up to a factor of four and a scaling behavior similar to that of the unconfined case. 

The turbulent flame acceleration by obstacles in partly confined situations. 

Spalding, D.B., Mixing and Chemical Reaction in Steady Confined Turbulent Flames, Imperial College of Science and Technology, London, 649 (1972). 

Spalding, D. B (1971). Mixing and chemical reaction in confined turbulent flames, presented at the 13th International Symposium on Combustion, Combustion Institute, Pittsburgh, PA, pp. 649-657. 

In elongated confined vessels, with one end closed and the opposite end open or removable, when an explosion begins at or near the closed end, the rapid movement of the flame front caused by the high volume from combustion wall cause displacement of the unburnt mixture ahead of it. Apparently this characteristic is independent of the nature of the combustible material [54], and the velocity can reach 80%-90% of the flame velocity, in part due to the high turbulence generated in the unburnt mixtures. 

The inherent properties of Class 1B liquids, under the storage and release conditions specified (lack of confinement, congestion, and release of material at low pressure), preclude formation of a well-mixed, turbulent vapor cloud that can support rapid flame propagation. Thus, the potential for VCE is low. 

These studies have found that increased confinement leads to flame acceleration and increased damage. The flame acceleration is caused by increased turbulence which stretches and tears the flame front, resulting in a larger flame front surface and an increased combustion rate. The turbulence is caused by two phenomena. First, the unburned gases are pushed and accelerated by the combustion products behind the reaction front. Second, turbulence is caused by the interaction of the gases with obstacles. The increased combustion rate results in additional turbulence and additional acceleration, providing a feedback mechanism for even more turbulence. 

The speed of the flame propagation must accelerate as the vapor cloud burns. This acceleration can be due to turbulence, as discussed in the section on confined explosions. Without this acceleration, only a flash fire will result. 

The amount of explosion overpressure is determined by the flame speed of the explosion. Flame speed is a function of the turbulence created within the vapor cloud that is released and the level of fuel mixture within the combustible limits. Maximum flame velocities in test conditions are usually obtained in mixtures that contain slightly more fuel than is required for stoichiometric combustion. Turbulence is created by the confinement and congestion within the particular area. Modem open air explosion theories suggest that all onshore hydrocarbon process plants have enough congestion and confinement to produce vapor cloud explosions. Certainly confinement and congestion are available on most offshore production platforms to some degree. 

Finally, there must be a flame acceleration mechanism, such as congested areas, within the flammable portion of the vapor cloud. The overpressures produced by a vapor cloud explosion are determined by the speed of flame propagation through the cloud. Objects in the flame pathway (such as congested areas of piping, process equipment, etc.) enhance vapor and flame turbulence. This turbulence results in a much faster flame speed which, in turn, can produce significant overpressures. Confinement that limits flame expansion, such as solid decks in tnulti-lcvc process structures, also increases flame speed. Without flame acceleration, a large fireball or flash fire can result, but not an explosion. 

The behavior of confined flames differs considerably from that of unconfined flames. Acceleration of the gases, caused by confinement, results in the generation of shear stresses and turbulent motions, which decrease the influence of approach stream turbulence and the effect of chemical kinetic factors. How the implementation of the ABC and the PPDF method helps to obtain the experimentally observed flow patterns and to understand the mechanism of flame stabilization and blow-off is demonstrated in this section. 

Solntsev, V. P. 1961. Influence of turbulence parameters on the combustion process of homogeneous gasoline-air mixture behind a stabilizer under conditions of confined flow. In Flame stabilization and the development of combustion in turbulent How. Ed. G. N. Gorbunov. Moscow Oborongiz. 75. 

The front jet produces a countercurrent shear environment which entrains reactants and pumps them radially inward toward the flame as depicted in the sketch of Fig. 17.3a. Since the reactants are premixed, the flame sheet must reside at a position where the flame speed (determined by the reactant equivalence ratio and the local turbulence intensity) matches the radial inward velocity of reactants. Since the radial velocity must be zero at the cylinder axis and wall, a maximum must exist (as indicated in Fig. 17.36). As long as the flow rates are maintained at high enough rates, this maximum is greater than the local flame speed and the flame remains confined. If the flame speed exceeds the maximum radial velocity anywhere along the axis of the cylinder, the flame changes modes and flashes out to the cylinder wall. 

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