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 R̄ft$\bar {R}_{f}\left (t \right )$ and flame displacement speed St=σ−1dR̄fdt$S_{t}=\sigma ^{-1}{d\bar {R}_{f}} \left / \right . {dt}$ 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 Tu, and six different u′)$u^{\prime })$, 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.

中文翻译：

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密度比是否显着影响湍流火焰速度？

为了通过实验研究密度比 σ 是否在低和中等湍流强度下显着影响火焰位移速度，两种化学计量的甲烷/氧气/氮气混合物具有相同的层流火焰速度 SL = 0.36 m/s，但 σ 有显着差异设计使用（i）从 Tu = 298 预热到 423 K 以增加 SL，但降低 σ，以及（ii）用氮气稀释以进一步降低 σ 并将 SL 降低回初始值。结果，密度比从 7.52 降低到 4.95。在参考和预热/稀释的情况下，记录并处理在大型 3D 十字形燃烧器中心进行火花点火后膨胀的统计球形层流和湍流火焰的直接图像，以评估平均火焰半径 R̄ft$\bar {R}_{f}\left (t \right )$ 和火焰位移速度 St=σ−1dR̄fdt$S_{t}=\sigma ^{-1}{d\bar {R}_{f}} \left / \right 。{dt}$ 相对于未燃烧的气体。使用两个反向旋转的风扇和穿孔板来产生近乎各向同性的湍流使我们能够通过改变风扇频率来改变均方根湍流速度 u′$u^{\prime }$。在本研究中，u′$u^{\prime }$ 从 0.14 到 1.39 m/s 变化。对于每组初始条件（两种不同的混合物成分，两种不同的温度 Tu 和六种不同的 u'）$u^{\prime })$，五个（分别为，三）在湍流（分别为层流）环境中进行统计等效运行。获得的实验数据没有显示密度比对 St 的任何显着影响。此外，在所有研究情况下，在 u'/SL = 0.4 下测量的火焰位移速度接近层流火焰速度。这些结果特别意味着，密度比对火花点火发动机中火焰位移速度的影响很小，并支持使用 (i) 不允许密度比对 St 和 (ii) 影响的模型的发动机燃烧已经针对在房间条件下获得的实验数据进行了验证，即在较高的 σ 下。在 u'/SL = 0.4 下测量的火焰位移速度在所有研究的情况下都接近层流火焰速度。这些结果特别意味着，密度比对火花点火发动机中火焰位移速度的影响很小，并支持使用 (i) 不允许密度比对 St 和 (ii) 影响的模型的发动机燃烧已经针对在房间条件下获得的实验数据进行了验证，即在较高的 σ 下。在 u'/SL = 0.4 下测量的火焰位移速度在所有研究的情况下都接近层流火焰速度。这些结果特别意味着密度比对火花点火发动机中火焰位移速度的影响很小，并支持使用 (i) 不允许密度比对 St 和 (ii) 影响的模型的发动机燃烧已经针对在房间条件下获得的实验数据进行了验证，即在较高的 σ 下。