Flame speed adiabatic

As in the soap bubble method, only fast flames can be used because the adiabatic compression of the unbumed gases must be measured in order to calculate the flame speed. Also, the gas into which the flame is moving is always changing consequently, both the burning velocity and flame speed vary throughout the explosion. These features make the treatment complicated and, to a considerable extent, uncertain. 

The flame speed for a combustible hydrocarbon-air mixture is known to be 30cm/s. The activation energy of such hydrocarbon reactions is generally assumed to be 160kJ/mol. The true adiabatic flame temperature for this mixture is known to be 1600 K. An inert diluent is added to the mixture to lower the flame temperature to 1450 K. Since the reaction is of second-order, the addition of the inert can be considered to have no other effect on any property of the system. Estimate the flame speed after the diluent is added. 

Original studies of gaseous detonations have shown no single sequence of events due primarily to what is now known as the complex cellular structure of a detonation wave. The primary result of an ordinary thermal initiation always appears to be a flame that propagates with subsonic speed. When conditions are such that the flame causes adiabatic compression of the still unreacted mixture ahead of it, the flame velocity increases. According to some early observations,... 

Use GRI-Mech (GRIM30. mec) and a laminar premixed flame code to calculate the burning velocity of a methane-air mixture at 1.0 atm. Repeat the calculation, replacing the nitrogen in the combustion air with helium. Compare flame speeds and adiabatic flame temperatures. 

M.D. Smooke, J.A. Miller, and RJ. Kee. Determination of Adiabatic Flame Speeds by Boundary Value Methods. Comb. Sci. Techn., 34 79-89,1983. 

The dependence of the calculated flame speed on the initial reactant concentrations arises primarily through the influence of the concentrations on the adiabatic flame temperature T f. If the reactant concentration is... 

Fuel Formula Maximum Flame Speed (m/s) Adiabatic Flame Temperature (K) Expansion Factor Autoignition Temperature (°C)... 

At large values of the Zel dovich number, the chemical reaction is confined to a thin sheet in the flow. For all purposes except the analysis of the sheet structure, the sheet may be treated as a surface—for example, G(x, t) = 0—which in terms of the Cartesian coordinates (x, y, z) may be written locally as x = F y, z, t) if the x coordinate is not parallel to the sheet in its local orientation. When analyses are pursued in outer-scale variables, it is convenient to work in a coordinate system that moves with the sheet. For the undisturbed flow, let the x coordinate be normal to the planar flame, with the unburnt gas extending to x = — oo and the burnt gas to x = -1- oo. Employ the steady, adiabatic, laminar flame speed, measured in the burnt gas, = po Vq/p with Vq given by equation (5-78), and the thermal diffu-... 

NO doped Methane Flames. Most of the NO passes through the flame unreacted, and the NO primarily acts as an inert diluent. Diluting the flames with 1.5% NO causes the adiabatic flame temperatures to drop by about 100 K and the calculated flame speeds to decrease by about 10%. The NO fraction decreases by less than 10% through the flame front for these flames which were doped far above the equilibrium NO concentrations (420-3700 ppm depending on stoichiometry). In the experiments there was no post flame decay in the NO concencentrations ( 5%), while the calculations do show some decay (<2%/mm) which depends upon stoichiometry. The calculated post flame conversion rate of NO to Nj decreases as the super-equilibrium radicals (e.g. 0 and OH) decay. 

In the Spalding burner, a flat plane flame stands off the surface of a cooled porous matrix. Data are compiled to give Su as measured by the speed in the burner supply, to maintain a stable flame, for a given measured cooling rate. By plotting these data so as to extrapolate to a zero cooling condition yields, S U under nearly adiabatic conditions. 

In both of these experimental arrangements, for a given mixture, there is a unique duct velocity (vu) that matches the burning velocity. In the Spalding burner, this is the adiabatic burning velocity (or the true, S U). If vu > Su the condition is not stable and the flame will blow off or move away from the exit of the duct until a reduced upstream velocity matches Su. If vu <, S U, the flame will propagate into the duct at a speed where the flame velocity is, S U vu. This phenomenon of upstream propagation is known as... 

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