气泡变形能力对Taylor-Couette泡状湍流强减阻的重要性
The importance of bubble deformability for strong drag reduction in bubbly turbulent Taylor-Couette flow
摘要：
在雷诺数Re = 5 × 10^5 ~ 2 × 10^6范围内，采用静止外圆柱和平均气泡直径约为1 mm的气泡湍流Taylor-Couette（TC）流动，对整体和局部进行了研究。在0%~ 4%的范围内，我们测量了基于整体尺度扭矩作为整体气体体积分数a_global的函数的减阻（DR）。当Re = 5.1 x 10^5时，我们观察到高达7%的中等DR。当Re = 1.0 x 10^6和2.0 x 10^6时，DR明显更强，当Re = 2.0 x 10^6和a_global = 4%时，DR明显超过40%。为了揭示两个明显不同的制度，中度DR和强DR，我们调查的局部液体流动速度和局部气泡的统计，特别是径向气体浓度分布和气泡尺寸分布，为两种不同的情况;在中度DR区域中Re = 5.1 x 10^5，在强DR区域中Re = 1.0 x 10^6，两者均在a_global = 3 +/-0.5%。通过定义和测量局部气泡韦伯数（We）在TC间隙接近IC壁，我们观察到，从中度到强DR政权的交叉大致发生在We ~ 1的交叉。在Re = 1.0 x 10^6的强DR区域中，我们发现We > 1，当接近内壁时达到9（+7，-2）的值，这表明气泡在靠近内壁时会逐渐变形。在Re = 5.1 x 10^5的中等DR区域，我们发现We ~ 1，表明气泡更硬，即使平均气泡直径更大，即1.2（+0.7，-0.1）mm，与Re = 1.0 x 10^6的情况相比，其为0.9（+0.6，我们得出结论，气泡变形性是所观察到的强DR背后的相关机制。这些局部结果匹配并扩展了van den Berg等人（2005）从全局流动实验中发现的结论。以及Lu，Fernandez & Tryggvason（2005）的数值模拟。
英文摘要：
Bubbly turbulent Taylor-Couette (TC) flow is globally and locally studied at Reynolds numbers of Re = 5 x 10^5 to 2 x 10^6 with a stationary outer cylinder and a mean bubble diameter around 1 mm. We measure the drag reduction (DR) based on the global dimensional torque as a function of the global gas volume fraction a_global over the range 0% to 4%. We observe a moderate DR of up to 7% for Re = 5.1 x 10^5. Significantly stronger DR is achieved for Re = 1.0 x 10^6 and 2.0 x 10^6 with, remarkably, more than 40% of DR at Re = 2.0 x 10^6 and a_global = 4%. To shed light on the two apparently different regimes of moderate DR and strong DR, we investigate the local liquid flow velocity and the local bubble statistics, in particular the radial gas concentration profiles and the bubble size distribution, for the two different cases; Re = 5.1 x 10^5 in the moderate DR regime and Re = 1.0 x 10^6 in the strong DR regime, both at a_global = 3 +/- 0.5%. By defining and measuring a local bubble Weber number (We) in the TC gap close to the IC wall, we observe that the crossover from the moderate to the strong DR regime occurs roughly at the crossover of We ~ 1. In the strong DR regime at Re = 1.0 x 10^6 we find We > 1, reaching a value of 9 (+7, -2) when approaching the inner wall, indicating that the bubbles increasingly deform as they draw near the inner wall. In the moderate DR regime at Re = 5.1 x 10^5 we find We ~ 1, indicating more rigid bubbles, even though the mean bubble diameter is larger, namely 1.2 (+0.7, -0.1) mm, as compared to the Re = 1.0 x 10^6 case, where it is 0.9 (+0.6, -0.1) mm. We conclude that bubble deformability is a relevant mechanism behind the observed strong DR. These local results match and extend the conclusions from the global flow experiments as found by van den Berg et al. (2005) and from the numerical simulations by Lu, Fernandez & Tryggvason (2005).
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