Does Density Ratio Significantly Affect Turbulent Flame Speed?

作者：Andrei N. Lipatnikov, Wenya Li, Long Jiang, S.S. Shy · 发表于：Flow Turbulence and Combustion · 年份：2017 · DOI：10.1007/s10494-017-9801-6 · 被引用次数：25 · 研究领域：Combustion and flame dynamics、Combustion and Detonation Processes、Fire dynamics and safety research

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 R ̄ f t $\bar {R}_{f}\left (t \right )$ and flame displacement speed S t = σ − 1 d R ̄ f dt $S_{t}=\sigma ^{-1}{d\bar {R}_{f}} \left / \right . {dt}$ 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 ′ $u^{\prime }$ by changing the fan frequency. In this study, u ′ $u^{\prime }$ 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 ′ ) $u^{\prime })$ , five (respectively, three) statistically equivalent runs were perfo...