Speed of flame

Fourth, the blast effects produced by vapor cloud explosions can vary greatly and are determined by the speed of flame propagation. In most cases, the mode of flame propagation is deflagration. Under extraordinary conditions, a detonation might occur. 

Finally, there must be a flame acceleration mechanism, such as congested areas, within the flammable portion of the vapor cloud. The overpressures produced by a vapor cloud explosion are determined by the speed of flame propagation through the cloud. Objects in the flame pathway (such as congested areas of piping, process equipment, etc.) enhance vapor and flame turbulence. This turbulence results in a much faster flame speed which, in turn, can produce significant overpressures. Confinement that limits flame expansion, such as solid decks in tnulti-lcvc process structures, also increases flame speed. Without flame acceleration, a large fireball or flash fire can result, but not an explosion. 

Analogous results were obtained by Mallard and Le Chatelier,2 who found the speeds of flame in a mixture of methane and air containing 10-4 per cent, of methane, using tubes of glass of different diameters, to be as follow ... 

Measurements have also been made of the speed of the uniform movement in mixtures of air with each one of the hydrocarbons of the paraffin series up to and including pentane. The determinations were carried out with horizontal glass tubes, 2-5 cm. in diameter,1 and the results are shown diagrammatically in fig. 26. With the exception of methane, the maximum speeds are approximately the same, namely, about 82 cm. per second. The value for methane is rather lower than this, being 67 cm. per second. Owing to the few data available for the thermal constants of the paraffin hydrocarbons, it is not easy to explain this difference. In each instance, the mixture having the maximum speed of flame contains more combustible gas than is required for complete combustion. 

Two velocities are used to express the speed of flame propagation. The first is the velocity of the flame front relative to the unburned gas mixture, which is called burning velocity (m). The second is the velocity of a flame front with respect to a stationary observer, which is called flame speed (5). The relationship between these two velocities is ... 

In the explosion process, the explosive mixtures combust and expand around at the same time. The pressure wave transfers to the air in sphere. In the same volume, the explosion pressure is changeable with different encloser shape because of the heat dissipation area and speeds of flame-proof encloser is different. Since there is no any obstacles, pressure waves impact spherical encloser inner wall almost at the same time. The sphere encloser... 

Burning speed is an important parameter of combustion. There are two ways to show the burning speed linear speed of flame front spreading and mass speed of combusted liquids. The linear speed (w) of flame front spreading along the liquid... 

The propagation of flames proceeds the reaction zone forward till the end of the pipe. The flame fronts isolate the reacted and nonreacted zones. Every moment of flame propagation reaction is in process of the thin layer close to flame fronts. The preheating layer isolates the nonreact zone and flame fronts. The speeds of flame propagation are determined by the chemical properties of liquid explosives. 

Studies on the prevention of chemical accidents (Hazard and Operability studies HAZOP) constitute an important part of the setting up of an operational chemical process. They necessitate the accumulation of a large amount of physico-chemical data the self-inflammation and flammability limits, the delays of self-inflammation, the speeds of flame propagation, the conditions under which combustion or detonation occur. 

All of the dry low-NO, combustors currently available for combustion turbines utilize the lean pre-mix principle of operation, which creates a homogeneous, fuel-lean mixture of fuel and air prior to combustion. This mixture is then introduced to the combustion zone of the combustion chamber at a controlled velocity sufficiently higher than the local speed of flame propagation to prevent flashback into the pre-mix zone. However, the pre-mixture velocity must be low enough to avoid blowing the whole flame downstream (Schreiber, 1991). 

Detonation. In a detonation, the flame front travels as a shock wave, followed closely by a combustion wave, which releases the energy to sustain the shock wave. The detonation front travels with a velocity greater than the speed of sound in the unreacted medium. 

S has been approximated for flames stabili2ed by a steady uniform flow of unbumed gas from porous metal diaphragms or other flow straighteners. However, in practice, S is usually determined less directly from the speed and area of transient flames in tubes, closed vessels, soap bubbles blown with the mixture, and, most commonly, from the shape of steady Bunsen burner flames. The observed speed of a transient flame usually differs markedly from S. For example, it can be calculated that a flame spreads from a central ignition point in an unconfined explosive mixture such as a soap bubble at a speed of (p /in which the density ratio across the flame is typically 5—10. Usually, the expansion of the burning gas imparts a considerable velocity to the unbumed mixture, and the observed speed will be the sum of this velocity and S. ... 

Turbulent flame speed, unlike laminar flame speed, is dependent on the flow field and on both the mean and turbulence characteristics of the flow, which can in turn depend on the experimental configuration. Nonstationary spherical turbulent flames, generated through a grid, have flame speeds of the order of or less than the laminar flame speed. This turbulent flame speed tends to increase proportionally to the intensity of the turbulence. 

In high speed dusted, premixed flows, where flames are stabili2ed in the recirculation 2ones, the turbulent flame speed grows without apparent limit, in approximate proportion to the speed of the unbumed gas flow. In the recirculation 2ones the intensity of the turbulence does not affect the turbulent flame speed (1). 

In the reaction 2one, an increase in the intensity of the turbulence is related to the turbulent flame speed. It has been proposed that flame-generated turbulence results from shear forces within the burning gas (1,28). The existence of flame-generated turbulence is not, however, universally accepted, and in unconfined flames direct measurements of velocity indicate that there is no flame-generated turbulence (1,2). 

The proteetive system is independent of the eontrol system and provides proteetion from over-speed, over-temperature, vibration, loss of flame, and loss of lubrieation. The over-speed proteetion system generally has a trans-dueer mounted on the aeeessory gear or shaft, and trips the gas turbine at approximately 10% of maximum design speed. The over-temperature system has thermoeouples similar to the normal temperature eontrols with a similar redundant system. The flame deteetion system eonsists of at least two ultraviolet flame deteetors to sense a flame in the eombustion eans. 

Optical UV scanners or flame rods should be used because of the speed of response. 

Overdriven detonation is the condition that exists during a DDT before a state of stable detonation is reached. Transition occurs over the length of a few pipe diameters and propagation velocities np to 2000 m/s have been measnred for hydrocarbons in air. This is greater than the speed of sonnd as measnred at the flame front. Overdriven detonations are typically accompanied by side-on pressnre ratios (at the pipe wall) in the range of 50-100. A severe test for detonation flame arresters is to adjust the mn-np distance so that DDT occurs at the arrester, subjecting it to the overdriven detonation impulse. 

Another empirical equation given by Phillips and Pritchard (1986) for the critical flame speed of crimped ribbon, wire gauze, and perforated plate arresters is as follows ... 

Eor a wire gauze element, Eq. (5-8) applies only to a single layer of gauze, and the thickness, y, is twice the wire diameter. An increase in flame speed of about 20% of the original value of V may be obtained for each additional layer up to a maximum of hve, but additional layers provide no further advantage (HSE 1980). 

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