10.2.2 Laminar Flame Speed

The premixed models require the laminar flame speed (see Equation  9.2-4), which depends strongly on the composition, temperature, and pressure of the unburnt mixture. For adiabatic perfectly premixed systems as in Chapter  9, the reactant stream has one composition, and the laminar flame speed is 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. ANSYS FLUENT uses fitted curves obtained from numerical simulations of the laminar flame speed [ 114]. 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.

ANSYS 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 laminar flame speed at ten mixture fraction ( $\overline{f}$) points. The first (minimum) and last (maximum) $\overline{f}$ inputs are the flammability limits of the mixture and the laminar flame speed is zero outside these value.

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, or if the unburnt temperature or pressure is outside the range of validity, then the curve fits will be incorrect. Although ANSYS FLUENT defaults to a methane-air mixture, the laminar flame speed polynomial and the rich and lean limits are most likely incorrect for your specified fuel/oxidizer and unburnt temperature/pressure conditions. 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 ANSYS FLUENT.