Does Density Ratio Significantly Affect Turbulent Flame Speed?

A N Lipatnikov; W Y Li; L J Jiang; S S Shy

doi:10.1007/s10494-017-9801-6

Does Density Ratio Significantly Affect Turbulent Flame Speed?

Flow Turbul Combust. 2017;98(4):1153-1172. doi: 10.1007/s10494-017-9801-6. Epub 2017 Feb 15.

Authors

A N Lipatnikov¹, W Y Li², L J Jiang², S S Shy²

Affiliations

¹ 1Department of Applied Mechanics, Chalmers University of Technology, Gothenburg, Sweden.
² 2Department of Mechanical Engineering, National Central University, Jhong-li, Taiwan.

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 ${\bar{R}}_{f} (t)$ and flame displacement speed $S_{t} = σ^{- 1} d {\bar{R}}_{f} () dt$ 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 T_u , 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 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.

Keywords: Darrieus-Landau instability; Density ratio; Expanding spherical flame; Experiment; Low Turbulent Intensity; Thermal expansion; Turbulent flame speed.