Turbulent Flame Quenching

In mixtures with lower fuel concentration, the maximum turbulent flame velocity values decrease as is shown in Fig. 3.16. In a 10% (H2 + CO) binary fuel mixture with H2 concentration twice as high as CO, the St curvature is distinctly seen. For the aforementioned mixtures the initial pressure effect has not been observed. 

According to the turbulent flame velocity diagrams, in mixtures diluted with an incombustible component or in near limiting mixtures, flame extinction is expected at some value of the turbulence intensity. 

The turbulence intensity growth leads to acceleration of combustion products mixing with the initial mixture. At high u the bum rate does not grow with increasing i/. The resultant conditions allow the initial source of the flame to be diffused by turbulence faster than new combustion products are generated. This phenomenon corresponds to the combustion limit of a turbulent combustible mixture. 

The suggested approximation differs to a small degree from the more complicated general relation found using the least-squares method and denoted by the dashed line. 

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The article hy Wilson and Flessner gives the dividing line as roughly 50 ft/s between slow flames that can be simply quenched and fast flames that must also be decelerated. Fast flames described in the article have speeds above 60 ft/s as opposed to turbulent flames, which are described as having speeds from 5 to 100 m/s in most venting systems. The test rig described in the article was composed of 6-inch diameter pipe. 

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]. 

Flame Quenching by Turbulence Criteria of Flame Quenching.110... 

The concept of turbulent flame stretch was introduced by Karlovitz long ago in [15]. The turbulent Karlovitz number (Ka) can be defined as the ratio of a turbulent strain rate (s) to a characteristic reaction rate (to), which has been commonly used as a key nondimensional parameter to describe the flame propagation rates and flame quenching by turbulence. For turbulence s >/ />, where the dissipation rate e and u, L and v... 

Figure 6.2.5 reveals fhe criferia of flame quenching in fhe Ka- plof obfained from three different turbulent... 

Chomiak, J. and Jarosinski, J., Flame quenching by turbulence. Combust. Flame, 48, 241, 1982 Jarosinski, J., Strehlow, R.A., and Azarbarzin, A., The mechanisms of lean limit extinguishment of an upward and downward propagating flame in a standard flammability tube, Proc. Combust. Inst., 19,1549,1982. 

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... 

Chapter 6.2, contributed by S.S. Shy, is devoted to the problem of flame quenching by turbulence, which is important from the point of view of combustion fundamentals as well as for practical reasons. Effecfs of turbulence straining, equivalence ratio, and heat loss on global quenching of premixed furbulenf flames are discussed. 

The values of laminar flame speeds for hydrocarbon fuels in air are rarely greater than 45cm/s. Hydrogen is unique in its flame velocity, which approaches 240cm/s. If one could attribute a turbulent flame speed to hydrocarbon mixtures, it would be at most a few hundred centimeters per second. However, in many practical devices, such as ramjet and turbojet combustors in which high volumetric heat release rates are necessary, the flow velocities of the fuel-air mixture are of the order of 50m/s. Furthermore, for such velocities, the boundary layers are too thin in comparison to the quenching distance for stabilization to occur by the same means as that obtained in Bunsen burners. Thus, some other means for stabilization is necessary. In practice, stabilization... 

The other definition of the PDF was also applied, effectively accounting for flame quenching under high-turbulence intensities ... 

The relatively simple structure of laminar flames has made it possible to characterize this class of flames rather completely. Except for the pressure-dependence of burning velocity, the effects of most variables on laminar burning velocity and on wallquenching have been established. Turbulent flames, on the other hand, have a complex structure which has not yet been elucidated. Therefore, turbulent burning velocities can only be measured with reference to arbitrarily chosen flame surfaces. Another phenomenon connected with turbulent flames is gas-phase quenching study of this problem has hardly begun. 

A candle flame is a miniature example of a type F long, luminous, laminar flame. Author Reed has often demonstrated some of the features of type F flames with a candle—polymerization soot formation, flame quenching, flame holders, starved air incineration, natural convection, particulate emission, streams in laminar, transition, and turbulent flows, aeration (by exhaling through a tiny straw across the blue base of the candle flame) changes it to a compact, all-blue flame that demonstrates combustion roar. Some of these demonstrations were recently found to have been alluded to in Professor Michael Faraday s famous candle lectures of the 1850s r erence 19). 

The diagrams illustrate the comparison between the turbulent velocities in undiluted mixtures and mixtures with various content of the diluent. It is seen that the increase in the diluent content leads to a reduction in the maximum turbulent velocity value and the top pulsating speed which cause flame quenching. In the mixtures diluted with CO2 the pressure effect has not been observed. 

The general behavior of turbulent flame velocity growth with increasing turbulence intensity is reproduced the water steam additive reduces S - Flame quenching occurs at a turbulence intensity that is close to 4-5 m/s. As can be expected, the leaner the initial mixture, the less water steam is required to extinguish the flame. 

Characteristic quenching scale (CQS) Critical dimensions of a channel/tube where laminar fiame can be quenched. Minimum channel height at which hydrocarbon + air mixture (components ratio in the range between 1 and 2) can burn at normal conditions is close to 1.6 mm. Minimum tube diameter at the same conditions is close to 2 mm. CQS is inversely proportional to the initial pressure and is less for quick-burning mixtures. Turbulent flame breakthrough is possible when a tube dimension is equal to CQS. For safety purposes (completely excluding the blast transition) it is reasonable to use MSSO. 

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