Does Density Ratio Significantly Affect Turbulent Flame Speed?

Abstract

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 S L = 0.36 m/s, but substantially different σ were designed using (i) preheating from T u = 298 to 423 K in order to increase S L, but to decrease σ, and (ii) dilution with nitrogen in order to further decrease σ and to reduce S L 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 and flame displacement speed 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 by changing the fan frequency. In this study, was varied from 0.14 to 1.39 m/s. For each set of initial conditions (two different mixture compositions, two different temperatures T u, and six different , 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 S t. Moreover, the flame displacement speeds measured at u′/S L = 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 S t and (ii) have been validated against experimental data obtained under the room conditions, i.e. at higher σ.

DNS Study of the Bending Effect Due to Smoothing Mechanism

“…The thermal expansion effects are not taken into account in the present simulations, because the physical mechanisms from Group (C) do not allow for them. We may also note that (i) the vast majority of approximations of experimental data on U T , e.g., see review papers [30,31], do not invoke the density ratio σ, thus, implying a weak influence of σ on U T or S T , (ii) recent target-directed experiments [32], as well as earlier measurements [33], did not reveal a substantial influence of σ on U T either, and (iii) recent DNS studies, e.g., Figures 10 and 11 in [34] or Figure 2a in [35], do not indicate such an influence.…”

Effect of Fuel and Air Dilution on Syngas Combustion in an Optical SI Engine

“…On the other hand, flame-induced turbulence also needs to be considered. Even though it is still a matter of debate [49], it is well established that there are basically three mechanisms determining wrinkling [50]: eddy diffusion associated with turbulence in the unburned gases, which tends to increase it; propagation of the flame into the unburned region, which tends to reduce it; and instability, shear and eddy diffusion resulting from flame generated velocity gradients produced by the pressure drop across the combustion zone due to the density ratio, which tends to increase it. Considering these effects, higher laminar flame speed should be associated with less wrinkled flame fronts; on the other hand, it could increase the influence of the first mechanism, as previously iterated.…”

A DNS study of the physical mechanisms associated with density ratio influence on turbulent burning velocity in premixed flames

“…In spite of avalanche of papers aiming at understanding fundamentals of premixed turbulent combustion, influence of a ratio σ = ρ u /ρ b of the densities of unburned and burned mixtures on turbulent flame speed S t is still a controversial issue. On the one hand, neither approximations of the most extensive experimental databases [1,2,3,4,5] nor a few target-directed experimental studies [6,7] (i.e. studies in that σ was substantially changed by retaining approximately the same laminar flame speed S L ) indicate a significant effect of σ on S t .…”