Does Density Ratio Significantly Affect Turbulent Flame Speed?

In order to experimentally study whether or not the density ratio σ substantially affects flame displacement speed at low and moderate turbulent intensities, two stoichiometric methane/oxygen/nitrogen mixtures characterized by the same laminar flame speed SL = 0.36 m/s, but substantially different σ were designed using (i) preheating from Tu = 298 to 423 K in order to increase SL , but to decrease σ, and (ii) dilution with nitrogen in order to further decrease σ and to reduce SL back to the initial value. As a result, the density ratio was reduced from 7.52 to 4.95. In both reference and preheated/diluted cases, direct images of statistically spherical laminar and turbulent flames that expanded after spark ignition in the center of a large 3D cruciform burner were recorded and processed in order to evaluate the mean flame radius R¯ft and flame displacement speed St=σ-1dR¯fdt with respect to unburned gas. The use of two counter-rotating fans and perforated plates for near-isotropic turbulence generation allowed us to vary the rms turbulent velocity u' by changing the fan frequency. In this study, u' was varied from 0.14 to 1.39 m/s. For each set of initial conditions (two different mixture compositions, two different temperatures Tu , and six different u') , five (respectively, three) statistically equivalent runs were performed in turbulent (respectively, laminar) environment. The obtained experimental data do not show any significant effect of the density ratio on St . Moreover, the flame displacement speeds measured at u'/SL = 0.4 are close to the laminar flame speeds in all investigated cases. These results imply, in particular, a minor effect of the density ratio on flame displacement speed in spark ignition engines and support simulations of the engine combustion using models that (i) do not allow for effects of the density ratio on St and (ii) have been validated against experimental data obtained under the room conditions, i.e. at higher σ.