High-velocity flames

FIG. 27-33 Inspirator (gas-jet) mixer feeding a large port premix nozzle of the flame retention type. High-velocity gas emerging from the spud entrains and mixes with air induced in proportion to the gas flow. The mixture velocity is reduced and pressure is recovered in the ventnii section. (F om North Ameiioan Comhnstion Handbook, 3d ed., Notih American Manufacturing Company Cleveland 1996. )... 

Vapor Cloud Explosion (VCE) Explosive oxidation of a vapor cloud in a non-confined space (not in vessels, buildings, etc.). The flame speed may accelerate to high velocities and produce significant blast overpressure. Vapor cloud explosions in plant areas with dense equipment layouts may show acceleration in flame speed and intensification of blast. 

The International Maritime Organization (IMO) published a standard in 1984 for the design, testing and locating of deflagration and detonation flame arresters and high velocity vent valves for cargo tanks in tanker ships (IMO 1984). This was amended in 1988 and reissued as Revision I (IMO 1988). A new revised standard was issued in 1994 (IMO 1994). 

Since gauzes have limited effectiveness in quenching high-velocity flames, they are only used as end-of-line deflagration flame arresters. 

Schampel, K. and Steen, H. 1975. Flame Arresting High Velocity Valves on Cargo Tanks of Tankers for Inflammable Liquids./. Hazardous Materials, 1, 223-235. 

There are test procednres for liqnid prodnct flame arresters, high velocity vent valves, flow controlled apertnres (velocity flame stoppers), and hydranlic flame arresters, as well as specific reqnirements for testing flame arresters nsed with compressors, fans, blowers, and vacnnm pnmps, which are not covered in other standards. 

Test procedures for liquid product detonadon arresters, hydranlic flame arresters, high velocity vent valves, and flow controlled apertures (velocity flame stoppers) allow the rise of these kinds of flame arresters as an alternative to standard static fixed element dry type flame arresters. This corild be especially risefnl in high flow applica-... 

The IMO standard MSC/Circ. 677 (1994) provides testing procedures for end-of-line deflagration and in-line detonation flame arresters, as well as high velocity vent valves for use on cargo tanks in tanker ships. Its development is discussed in Section 2.3.4. 

High Velocity Vent Valve A device to prevent the passage of flame in the reverse direction, consisting of a mechanical valve which adjnsts the opening available for flow in accordance with the pressnre at the inlet of the valve in snch a way that the efflnx (exiting) velocity cannot be less than 30 m/s. 

Vapor cloud explosion The explosion resulting from the ignition of a cloud of flammable vapor, gas, or mist in which flame speeds accelerate to sufficiently high velocities to produce significant overpressure. 

Laminar Versus Turbulent Flames. Premixed and diffusion flames can be either laminar or turbulent gaseous flames. Laminar flames are those in which the gas flow is well behaved in the sense that the flow is unchanging in time at a given point (steady) and smooth without sudden disturbances. Laminar flow is often associated with slow flow from small diameter tubular burners. Turbulent flames are associated with highly time dependent flow patterns, often random, and are often associated with high velocity flows from large diameter tubular burners. Either type of flow—laminar or turbulent—can occur with both premixed and diffusion flames. 

Utilizing a forced-draft fan, the burner has a gas head arranged to mix the fuel and air in a blast tube which controls the stability and shape of the flame. Gas exits from nozzles or holes in the head and is mixed partly in the high-velocity air stream and partly allowed to exit into an area downstream of a bluff body. Behind the bluff body, a relatively quiescent zone forms which provides a means for flame stability. Many configurations exist, but the most... 

P. Magre, P. Moreau, G. Collin, R. Borghi, and M. Pealat 1988, Further studies by CARS of premixed turbulent combustion in high velocity flow. Combust. Flame 71 147-168. 

The high-speed flames propagate in a tube with repeat-able obstacles with the steady-state velocity, which is maintained for the rest of their passage over the obstacles. In some cases, the steady-state flame propagation velocity of the combustion products may approach the... 

Unconfined vapor cloud explosion explosive oxidation of a flammable vapor cloud in a nonconfined space (e.g., not in vessels or buildings) the flame speed may accelerate to high velocities and can produce significant blast overpressures, particularly in densely packed plant areas. 

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