Cylindrical flames

For a line spark source, the flame volume is initially cylindrical with the cylinder length equal to the separation distance between the electrodes. Thus, for a cylindrical flame, = e, and the critical ignition volumes are equation 7 for a spherical flame and equation 8 for a cylindrical flame where = critical ignition volume, m /kg e = thickness of flame front, m and d = flame height, m. 

The problem focuses on determination of the geometric view factor, which can be read from tables and graphs in Appendix A. View factors for cylindrical flames... 

After the establishment of a cylindrical flame, continuous reduction of the flame speed relative to an external frame of reference was observed. This reduction was linear in time and quenching occurred in the system when the flame speed was close to zero. The declining flame speed was attributed to heat losses to the walls (reducing the effective expansion ratio in the flame) and quenching was interpreted in terms of the following criterion ... 

The cylindrical flames should be distinguished from the so-called tubular flames generated by the tangential supply of mixture into a cylinder [2], as the latter are subjected to strong axial stretch controlling their behavior and do not interact with the walls. 

A close examination of the cylindrical flame as it exits the exhaust nozzle will reveal that there is a thin annular flow of unburned reactants near the front of the burner which never passes through the flame. The authors know this... 

Figure 17.7 (a) Cylindrical flame sheet area as a function of gray scale threshold (6) corresponding flame image and (c) image contours at threshold values of 0.34, 0.54, and 0.69... 

It is useful to examine the time scales in flames a little more closely. The exact scale is a function of the particular mixture being burnt, and of the burner used, but the best burners are those designed to give a cylindrical flame, laminar rather than brush-shaped, with many small cones of reaction. These Meker type burners are composed of a bundle of capillary (hypodermic) tubes. To obviate pressure drop these should have the greatest possible diameter consistent with the extinction distance of the flame mixture used. The burnt gas is hot, so its volume Fjg is greater than that of the unbumt gas, F . The tubes are spaced out... 

The surface emissive power (SEP) for a cylindrical flame is calculated as follows... 

D expansion means that a cylindrical flame expands between two limiting surfaces. 

There exists a strong correlation between flame speed and induced pressure wave, the maximum value for the presented experiments being higher than 800 mbars. In addition, the pressure curves calculated by Kuhl et al. are shown for one-dimensional (planar) and two-dimensional (cylindrical) flame propagation. The planar wave line appears as a fair representation of the measuring data. 

In line-symmetry, that is, a cylindrical flame between two plates, the overall flame surface area is proportional to the distance from the ignition point. Conse-quendy, deformation of the flame surface will have a stronger effect than in the point-symmetry case. 

Electrothermal Atomizers A significant improvement in sensitivity is achieved by using resistive heating in place of a flame. A typical electrothermal atomizer, also known as a graphite furnace, consists of a cylindrical graphite tube approximately... 

Schematic diagram of a flame ionization detector. Ions and electrons formed in the flame provide an electrically conducting path between the flame at earth potential and an insulated cylindrical metal electrode at high potential. surrounding the flame the flow of current is monitored, amplified, and passed to the recording system.

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. 

The view factor depends on the shape of the emitting surface (plane, cylindrical, spherical, or hemispherical), the distance between emitting and receiving surfaces, and the orientation of these surfaces with respect to each other. In general, the view factor from a differential plane dAj) to a flame front (area A,) on a distance L is determined (Figure 3.10) by ... 

For a flash fire, the flame can be represented as a plane surface. Appendix A contains equations and tables of view factors for a variety of configurations, including spherical, cylindrical, and planar geometries. 

Fuel-pair mixtures, in soap bubbles ranging from 4 to 40 cm diameter and with no internal obstacles, produced flame speeds very close to laminar flame speeds. Cylindrical bubbles of various aspect ratios produced even lower flame speeds. For example, maximum flame speeds for ethylene of 4.2 m/s and 5.5 m/s were found in cylindrical and hemispherical bubbles, respectively (Table 4.1a). This phenomenon is attributed to reduced driving forces due to the top relief of combustion products. 

Obstacles introduced in unconfined cylindrical bubbles resulted only in local flame acceleration. Pressures measured at some distance from the cylindrical bubble were, in general, two to three times the pressure measured in the absence of obstacles. 

Figure 4.8. Experimental setup to study flame propagation In a cylindrical geometry.

Van Wingerden and Zeeuwen (1983) demonstrated increases in flame speeds of methane, propane, ethylene, and acetylene by deploying an array of cylindrical obstacles between two plates (Table 4.3). They showed that laminar flame speed can be used as a scaling parameter for reactivity. Van Wingerden (1984) further investigated the effect of pipe-rack obstacle arrays between two plates. Ignition of an ethylene-air mixture at one edge of the apparatus resulted in a flame speed of 420 m/s and a maximum pressure of 0.7 bar. 

Leyer, J. C. 1982. An experimental study of pressure fields by exploding cylindrical clouds. Combustion and Flame. 48 251-263. 

Combustion behavior differed in some respects between continuous and instantaneous spills, and also between LNG and refrigerated liquid propane. For continuous spills, a short period of premixed burning occurred immediately after ignition. This was characterized by a weakly luminous flame, and was followed by combustion of the fuel-rich portions of the plume, which burned with a rather low, bright yellow flame. Hame height increased markedly as soon as the fire burned back to the liquid pool at the spill point, and assumed the tilted, cylindrical shape that is characteristic of a pool fire. 

Figure 5.3 shows a moment of flame propagation in an unconfined propane cloud. On the left side, a flame is propagating through a premixed portion of the cloud its flame is characteristically weakly luminous. In the middle of the photograph, fuel-rich portions of the cloud are burning with characteristically higher flames in a more-or-less cylindrical, somewhat tilted, flame shape. 

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