Quenching distances

A flame is quenched in a tube when the two mechanisms that permit flame propagation—diffusion of species and of heat—are affected. Tube walls extract heat the smaller the tube, the greater is the surface area to volume ratio within the tube and hence the greater is the volumetric heat loss. Similarly, the smaller the tube, the greater the number of collisions of the active radical species that are destroyed. Since the condition and the material composition of the tube wall affect the rate of destruction of the active species [5], a specific analytical determination of the quenching distance is not feasible. 

Intuition would suggest that an inverse correlation would be obtained between flame speed and quenching diameter. Since flame speed SL varies with equivalence ratio cf , so should dT vary with however, the curve of dT would be inverted compared to that of SL, as shown in Fig. 4.27. 

One would also expect, and it is found experimentally, that increasing the temperature would decrease the quenching distance. This trend arises because 

FIGURE 4.27 Variation of quenching diameter dT as a function of equivalence ratio if and trend with initial temperature. 

It has been concretely established and derived theoretically [30] that quenching distance increases as pressure decreases in fact, the correlation is almost exactly 

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Fig. 3. Quenching distance as function of equivalence ratio for hydrocarbon mixtures with air (1), where x = methane, = propane, A = propylene, and...

If the tube diameter is appreciably larger than the quenching distance, S will exceed p in some parts of the flowing mixture due to a lack of quenching, and the flame will then propagate down the tube as far as there is mixture to consume. This undesirable condition is referred to as flashback. If, on the other hand, p exceeds S in the mixture flow, the flame lifts from the port and blows off. This condition is referred to as blowoff and like flashback should be avoided (Fig. 8). The velocity gradient at the wall, is defined as... 

There is a relationship between the MIL of a gas or vapor and the quenching distance, as shown in Figure 4-5 (van Dolah and Burgess 1974). These data are for a large variety of chemicals with oxygen varying between 21 and 100 volume percent and pressure between 0.1 to 2 atmospheres. 

FIGURE 4-5. The relationship between minimum ignition energy (MIE) and quenching distance... 

It has been found experimentally that increasing the temperature reduces the quenching distance. However, sufficient data are not available to develop any specific correlation (Classman 1996). 

It has been established that the quenching distance increases as pressure decreases as shown in the following equation (Classman 1996) ... 

Quenching Distance The distance between two parallel plates (flat walls) that will jnst qnench (extingnish) the flame front of a particnlar fnel-oxidant mixtnre. It is smaller than the qnenching diameter in cylindrical tnhes. 

Liftoff height with jet velocity in free jet [10] (H attached flame length, quenching distance, Hp liftoff height, L premixed flame length). 

As the quenching distance for flames propagating in different mixtures was often important for industrial applications, experimental methods to measure this quantity were developed. The most frequently used ones were... 

Some of the early studies were devoted to assessing the effect of walls on the quenching distance. It was found that the nature of the wall material hardly affected the quenching distance [7,8,16]. 

This is because the heat capacity of a wall of finite thickness is several orders of magnitude higher than that of the hot combustion products. However, some researchers did observe a small effect of the properties of the wall [17] on the quenching distance. This was interpreted in terms of some residual catalytic activity of the wall surface, poisoned by the combustion products from the preceding experiments [18]. With respect to this explanation, the surface of any material moistened through the condensation of the water vapor produced in the reaction is supposed to have very similar, low activity. 

The quenching distance is a very important parameter, characterizing a laminar flame in contact with solid... 

Definitions of flame parameters in channels. D, distance between channel walls effective in flame quenching (quenching distance). D, flame width dead space R, radius of curvature of the flame. 

To understand the mechanism of flame quenching in narrow channels in detail, one should first examine the data of flames in mixtures of constant composition, but in charmels of different sizes (Figure 6.1.2). The measured propagation velocities in stoichiometric propane/ air mixture are shown in Figure 6.1.2a. For channel widths slightly larger than the quenching distance, the... 

Measured quenching distance as a function of equivalence ratio for propane/air mixture (top), and pictures of (a) downward and (b) upward propagating flames in channels, close to quenching. Channel widths as in the graph. Frame numbers correspond to the numbers of experimental points. 

Length of the high-temperature zone behind a flame as a function of quenching distance. Downward propagation in lean propane/air mixture. 

The radius of curvature of flame is shown in Figure 6.1.7 as a function of the quenching distance (Figure 6.1.7a) and of the equivalence ratio (Figure 6.1.7b). The radius was determined from the flame pictures. For lean mixtures, the radius increases linearly with the channel width, both for the downward and upward propagating flames. For rich mixtures and downward propagation, the increase is linear for quenching distances up to Dq = 7 mm, but the increase is not as steep as that of lean mixtures. However, the increase accelerates. For rich... 

Dead space as a function of (a) quenching distance and (b) equivalence ratio. 

For propane flames, fhe quenching distance (for downward propagating flames) is limited by the distance between the walls of about 10 mm. In larger channels, the flame is quenched at the flammability limits. 

In Chapter 6.1, A. Gutkowski and J. Jarosinski present results of an experimental and numerical study of flame propagation in narrow channels and the mechanism of quenching due to heat losses. This work takes up again classical studies of the quenching distance. The most characteristic features of limit flames are determined experimentally. 

Two other useful parameters related to capacitive sparks and dust ignitions are (i) the optimum sparking distance is 10 mm and (ii) the quenching distance is 7 mm. Refer to Sec. 4.1 and Table 5, where MIE is discussed in more detail. 

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