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17.2.2 Laminar Flame Speed

The premix model requires the laminar flame speed (see Equation  16.2-4), which depends strongly on the composition, temperature, and pressure of the unburnt mixture. For perfectly premixed systems as in Chapter  16, the reactant stream has one composition, and the laminar flame speed is approximately constant throughout the domain. However, in partially premixed systems, the laminar flame speed will change as the reactant composition (equivalence ratio) changes, and this must be taken into account.

Accurate laminar flame speeds are difficult to determine analytically, and are usually measured from experiments or computed from 1D simulations. FLUENT uses fitted curves obtained from numerical simulations of the laminar flame speed [ 126]. These curves were determined for hydrogen (H $_2$), methane (CH $_4$), acetylene (C $_2$H $_2$), ethylene (C $_2$H $_4$), ethane (C $_2$H $_6$), and propane (C $_3$H $_8$) fuels. They are valid for inlet compositions ranging from the lean limit through unity equivalence ratio (stoichiometric), for unburnt temperatures from 298 K to 800 K, and for pressures from 1 bar to 40 bars.

FLUENT fits these curves to a piecewise-linear polynomial. Mixtures leaner than the lean limit or richer than the rich limit will not burn, and have zero flame speed. The required inputs are values for the mean mixture fraction $\overline{f}$ at 10 laminar flame speeds. The minimum and maximum $\overline{f}$ limits for the laminar flame speed are the first and last values of $\overline{f}$ that are input.

figure   

These flame speed fits are accurate for air mixtures with pure fuels of H $_2$, CH $_4$, C $_2$H $_2$, C $_2$H $_4$, C $_2$H $_6$, and C $_3$H $_8$. If an oxidizer other than air or a different fuel is used, if the mixture is rich, or if the unburnt temperature or pressure is outside the range of validity, then the curve fits will be incorrect. Although FLUENT defaults to a methane-air mixture, the laminar flame speed polynomial and the rich and lean limits are most likely incorrect. The laminar flame speed polynomial should be determined from other sources, such as measurements from the relevant literature or detailed 1D simulations, and then input into FLUENT.


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