[论文解读] Bubble deformability is crucial for strong drag reduction in bubbly turbulent Taylor-Couette flow

Dennis P. M. van Gils, Daniela Narezo Guzmán+2 more|arXiv (Cornell University)|Nov 29, 2011

Fluid Dynamics and Turbulent Flows被引用 2

一句话总结

本研究证明，在湍流泰勒-库埃特流中，气泡可变形性是实现强阻力降低（DR）的关键机制，当雷诺数 Re = 2.0 × 10^6 且气相体积分数为 4% 时，DR 超过 40%。当局部气泡韦伯数（We）超过约 1 时，阻力降低由中等（≤7%）过渡为强DR，表明靠近内壁的气泡形变通过改变局部流场动力学增强了DR。

ABSTRACT

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).

研究动机与目标

• 研究在不同雷诺数下，气泡湍流泰勒-库埃特（TC）流中阻力降低（DR）的机制。
• 确定气泡可变形性在实现强DR中的作用，特别是与低雷诺数下中等DR的对比。
• 量化局部流动和气泡统计量，包括径向气相浓度和粒径分布，以识别关键的过渡条件。

提出的方法

• 通过全局扭矩测量计算阻力降低（DR）随全局气相体积分数（a_global）的变化，覆盖 Re = 5×10^5 至 2×10^6 的范围。
• 利用高分辨率诊断手段，在靠近内壁的TC间隙中测量局部液相速度和气泡统计量。
• 定义并测量了局部气泡韦伯数（We），以评估靠近内壁的气泡形变程度。
• 分析径向气相浓度分布和气泡粒径分布，以比较中等DR与强DR状态下的差异。
• 将实验结果与先前的全局实验（van den Berg et al., 2005）和数值模拟（Lu, Fernandez & Tryggvason, 2005）进行对比，以验证研究发现。

实验结果

研究问题

• RQ1 气泡可变形性在气泡湍流泰勒-库埃特流中实现强阻力降低的过程中起什么作用？
• RQ2 局部气泡韦伯数（We）达到何种临界值时，阻力降低由中等过渡为强DR？
• RQ3 在中等DR与强DR状态下，局部气泡统计量（如径向气相浓度和粒径分布）有何差异？
• RQ4 为何在更高雷诺数下阻力降低更强，尽管全局气相体积分数相似？

主要发现

• 在 Re = 5.1 × 10^5 且 a_global = 4% 时，阻力降低（DR）达到最高 7%，表明在低雷诺数区域为中等DR。
• 在 Re = 1.0 × 10^6 且 a_global = 4% 时，DR 超过 40%，表明发生了显著的向强DR的过渡。
• 中等DR向强DR的过渡与局部气泡韦伯数（We）约为 1 相吻合，标志着显著气泡形变的开始。
• 在强DR状态（Re = 1.0 × 10^6）下，靠近内壁的 We 达到 9 (+7, -2)，表明气泡高度形变。
• 在中等DR状态（Re = 5.1 × 10^5）下，尽管平均气泡直径（1.2 mm）大于高Re下的 0.9 mm，We ≈ 1，表明气泡行为更刚性。
• 气泡可变形性被确定为强DR的关键机制，靠近内壁的局部 We > 1 使流场调制能力显著增强。