Flame fronts characteristics

In elongated confined vessels, with one end closed and the opposite end open or removable, when an explosion begins at or near the closed end, the rapid movement of the flame front caused by the high volume from combustion wall cause displacement of the unburnt mixture ahead of it. Apparently this characteristic is independent of the nature of the combustible material [54], and the velocity can reach 80%-90% of the flame velocity, in part due to the high turbulence generated in the unburnt mixtures. 

O.L. Giilder, G.J. Smallwood, R. Wong, D.R. Snelling, R. Smith, B.M. Deschamps, and J.C. Sautet 2000, Flame front surface characteristics in turbulent premixed propane-air combustion. Combust. Flame 120 476-416. 

The rate of pressure rise is indicative of the flame front propagation rate and thus of the magnitude of the explosion. The pressure rate or slope is computed at the inflection point of the pressure curve, as shown in Figure 6-15. The experiment is repeated at different concentrations. The pressure rate and maximum pressure for each run are plotted versus concentration, as shown in Figure 6-16. The maximum pressure and maximum rate of pressure rise are determined. Typically, the maximum pressure and pressure rates occur somewhere within the range of flammability (but not necessarily at the same concentration). By using this relatively simple set of experiments, the explosive characteristics can be completely established in this example the flammability limits are between 2% and 8%, the maximum pressure is 7.4 bar, and the maximum rate of pressure rise is 360 bar/s. 

The characteristics of a reactive gas (a premixed gas) are dependent not only on the type of reactants, pressure, and temperature, but also on the flow conditions. When the flame front of a combustion wave is flat and one-dimensional in shape, the flame is said to be a laminar flame. When the flame front is composed of a large number of eddies, which are three-dimensional in shape, the flame is said to be a turbulent flame. In contrast to a laminar flame, the combustion wave of a turbulent flame is no longer one-dimensional and the reaction surface of the combustion wave is significantly increased by the eddies induced by the dynamics of the fluid flow. 

D. Rae, Safety in Mines Res Estab, Res Rept 253 (1967) CA 69, 1140 (1968). The main characteristics of slow coal-dust explosions and their relation to the testing of barriers. The main characteristics of slow coal-dust explosions in the new gallery at Buxton are a const initial flame front acceleration from... 

Knock in spark ignition engines may be defined as an abnormally rapid combustion of the unburned fuel-air mixture ahead of the normal flame front. A severe pressure unbalance due to this rapid combustion process sets up shock waves which impinge on the cylinder walls and piston and produce the characteristic metallic knocking noise (43). 

A reaction at the initial temperature changes the characteristics of an explosive mixture before the flame front and introduces an element of nonsteadiness into the process of propagation of the combustion wave. The method proposed in [1] to describe this effect consists in replacing the original non-steady problem by a quasi-steady one with adiabatically increasing initial temperature Ta(f) and an effective source of heat release which takes this increase into account. We test this method below by comparing it directly with the results of a numerical solution of the original non-steady problem. 

The main characteristics of slow coal-dust explosions in the new gallery at Buxton are a const initial flame front acceleration from... 

The Effect of Finite Reaction Rates When the fuel and oxidizer react at a finite rate, the flame front can no longer be considered infinitely thin. The reaction rate is then such that oxidizer and fuel can diffuse through each other and the reaction zone is spread over some distance. However, one must realize that although the reaction rates are considered finite, the characteristic time for the reaction is also considered to be much shorter than the characteristic time for the diffusional processes, particularly the diffusion of heat from the reaction zone to the droplet surface. 

The initiation of a detonation by means of a jet ignition has been demonstrated in experiments for the first time at the end of the 1970s. An essential result was flie rule of thumb according to which the high flame front velocity and the overpressure have decayed within the distance of 10 times the characteristic lengtii of the vessel [113]. 

Gulder O L.Flame front surface characteristics in turbulent premixedpropance /air combustion[J]. Combustion and Flame, 2000, 120 407-416. 

In premixed combustion, the characteristic laminar speed, Uf, is the laminar flame propagation speed and the characteristic length scale, Lp, corresponds to the flame thickness. The laminar flame regimes are confined to regions (1) and (2P), where in the latter the turbulence intensity, u, is less than the laminar flame speed, Mp. Region (1) is defined by Rct< and the flame front is essentially a thin planar... 

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