Flames speed

Fuel Quantity of fuel per Gf" Flammability limit ia air, vol % gas Lower Higher Maximum flame speed, cm/s Spontaneous ignition temperature, °C Ignition d -jC energy, m ... 

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). 

Turbulent Diffusion FDmes. Laminar diffusion flames become turbulent with increasing Reynolds number (1,2). Some of the parameters that are affected by turbulence include flame speed, minimum ignition energy, flame stabilization, and rates of pollutant formation. Changes in flame stmcture are beHeved to be controlled entirely by fluid mechanics and physical transport processes (1,2,9). 

Deflagration Arresters The two types of deflagration arrester normally considered are the end-of-line arrester (Figs. 26-23 and 26-24) and the tank vent deflagration arrester Neither type of arrester is designed to stop detonations. If mounted sufficiently far from the atmospheric outlet of a piping system, which constitutes the unpro-tec tea side of the arrester, the flame can accelerate sufficiently to cause these arresters to fail. Failure can occur at high flame speeds even without a run-up to detonation. 

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. 

Flashback into a pre-mix duct occurs when the local flame speed is faster than the velocity of the fuel/air mixture leaving the duct. 

The conseqnence is that the rate of prodnction of volnme of hnrned prodncts is greater dne to the density decrease resnlting from the reaction. As the prodncts expand this canses the nnhnrned mixtnre to move as well. The flame is then seen to move forward with a higher apparent velocity, Vf, the snm of the mean nnhnrned gas velocity, u, and the tnrhnlent hnrning velocity, S. Vf is called the flame velocity (flame speed). 

Pipe diameter has an effect on flame propagadon. It is minimal in the range of L/D from 1 to —50. In this secdon of the pipe, the flame velocity is not affected by the diameter. Beyond an L/D of 50, flame speed increases with pipe diameter. 

Flame speed is proportional to the fnndamental bnrning velocity that is, as the fnndamental bnrning velocity increases, the flame speed increases. 

Broschka et al. (1983) report results of experimental tests on parallel plate flame arresters in piping systems. Tests were conducted on 3-inch and 6-inch diameter parallel plate flame arresters installed in 3-inch and 6-inch diameter piping sections using butane-air mixtures to generate a flame. The ignition source was varied from 3 to 43 feet from the flame arrester. The flame speed varied between 0 to 20 ft/s, and when the flame speed was 20 ft/s, the flame passed through the arrester (flame arrester failure). 

The dual 20-mesh stainless steel arrester was effective in arresting flashback from all eight fnel-air mixtures tested except in some ethylene-air tests. It failed in three out of three tests where the flame speed was 4.86 m/s (15.94 ft/s) or greater. 

Hydranlic flame arresters may fail to stop high flame speed gas mixtures nnder certain conditions. Fundamental test work (Overhoff et al. 1989) demonstrates mechanisms whereby liqnid seal arresters may fail to... 

It has been shown by Palmer at the Fire Research Station (FRS) that a crucial variable governing the performance of a flame arrester is the flame speed incident on the arrester. The critical flame speed (minimum speed at which the flame can pass through the arrester) is discussed by Phillips and Pritchard (1986), drawing largely on the FRS work on propane-air mixtures at atmospheric pressure. A simple model based on heat removal from the flame yields the following relation for deflagration flame arresters ... 

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 ... 

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