Quenching diameter

The energy balance for this problem can be expressed by Equation (4.45) with Q given in Equation (4.43a) as before and Q / given from Equations (4.47) and (4.48) as 

This seems to qualitatively follow data in the literature, as illustrated in Table 4.5. 

The quenching distance is of practical importance in preventing flashback by means of a flame arrestor. This safety device is simply a porous matrix whose pores are below the quenching distance in size. However, care must be taken to maintain the flame arrestor cool, since by Equation (4.49) an increase in its temperature will reduce the quenching distance. 

Even in a diffusion flame the region near a surface becomes a premixed flame since fuel and oxygen can come together in this quenched region. 

However, if the fuel is in the form of droplets in air, it can be flammable at temperatures below the flashpoint. Droplet size will also play a role in this aerosol region. 

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The hole size used is larger than that necessary to quench a flame in a stagnant flammable gas mixture, i.e., larger than the quenching diameter. Howard recommends that the velocity necessary to prevent flashback be calculated by the following equation ... 

Quenching Diameter, Quenching Length, and Flame Velocity... 

Crimped Metal Ribbon A flame arrester element that is manufactured of alternate layers of thin corrugated metal rihhon and a flat metal rihhon that are wound together on a mandrel to form a cylindrical assembly of many layers to produce a range of different sized triangular cells. The height and width of the triangular cells can he varied to provide the required quenching diameter. 

Quenching Diameter The largest diameter of a cylindrical tnhe that will jnst qnench (extingnish) the flame front of a particnlar fnel-air mix-tnre. 

Anagnostou, E. and Potter, A.E., Jr., Quenching diameters of some fast flames at low pressures, Combust. Flame, 3 453,1959. 

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. 

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

Consider Fig. 4.32, a graph of flame velocity 5L as a function of distance, for a wave inside a tube. In this case, the flame has entered the tube. The distance from the burner wall is called the penetration distance dv (half the quenching diameter dT). If iij is the mean velocity of the gas flow in the tube and the line labeled (7, is the graph of the velocity profile near the tube wall, the local flame velocity is not greater than the local gas velocity at any point therefore, any flame that finds itself inside the tube will then blow out of the tube. At a lower velocity u2, which is just tangent to the SL curve, a stable point is reached. Then u2 is the minimum mean velocity before flashback occurs. The line for the mean velocity % indicates a region where the flame speed is greater than the velocity in the tube represented by in this case,... 

As in consideration of deflagration phenomena, other parameters are of import in detonation research. These parameters—detonation limits, initiation energy, critical tube diameter, quenching diameter, and thickness of the supporting reaction zone—require a knowledge of the wave structure and hence of chemical reaction rates. Lee [6] refers to these parameters as dynamic to distinguish them from the equilibrium static detonation states, which permit the calculation of the detonation velocity by C-J theory. 

FIGURE 7.5 Correlation of the minimum spark ignition energy with quenching diameter (from Calcote et al. [15]). 

Assuming that the flame thickness is small compared to the plate thickness, Turns [412] has put forward an analysis that relates the critical quenching diameter to the flame thickness. In low-pressure situations, however, the flame thickness can be relatively large, with the heat-release zone comparable to the plate thickness. The intent of this exercise is to extend the Turns analysis, developing a somewhat more general means to determine critical quenching diameter, especially for flames that are thick compared to the plate thickness. 

For a plate thickness of L = 6 mm at room temperature, estimate the critical quenching diameter. Assume that the flame is positioned in the hole in such a way that the integrated heat release is maximum. 

H/D Isotope Effect on the Quenching Diameter of Various Alky] Groups b... 

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