Bubble deformability is crucial for strong drag reduction in bubbly turbulent Taylor-Couette flow

Details Bubble deformability is crucial for strong drag reduction in bubbly turbulent Taylor-Couette flow Bubble deformability is crucial for strong drag reduction in bubbly turbulent Taylor-Couette flow

2011-11-29

arxiv.org/abs/1111.6781v1

Physics

Fluid Dynamics

24 pages, 14 figures

Scientific paper

Bubbly turbulent Taylor-Couette (TC) flow at Reynolds numbers of Re = 5.1 * 10^5, 1.0 * 10^6 and 2.0 * 10^6 is globally and locally studied in the case of 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 (\alpha_global) over the range 0% up to 4%. We observe a moderate DR up to 7% for Re = 5.1 * 10^5. Significantly stronger DR is achieved for Re = 1.0 * 10^6 and 2.0 * 10^6 with, remarkably, more than 40% of DR for \alpha_global = 4% at Re = 2.0 * 10^6. To shed light on the two apparently different regimes of moderate DR and strong DR we investigate the statistics of the local liquid flow velocity and the local gas concentration for the two different cases; Re = 5.1 * 10^5 at \alpha_global = 3 \pm 0.5% in the moderate DR regime and Re = 1.0 * 10^6 at \alpha_global = 3 \pm 0.5% in the strong DR regime. By defining and measuring a local bubble Weber number (We) in the TC gap close to the inner cylinder 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 * 10^6 we find We > 2 reaching a value of 8 when approaching the inner wall, indicating that the bubbles increasingly deform when approaching the inner wall. In the moderate DR regime at Re = 5.1 * 10^5 the local We is around or less than 1, indicating nearly spherical bubbles even though the mean bubble diameter is larger (1.3 mm) as compared to the Re = 1.0 * 10^6 case (0.9 mm). Hence, we conclude that bubble deformability is the relevant mechanism behind the observed strong DR. These local results match the conclusion from the global results as found by van den Berg et al. (2005) and from the numerical simulations by Lu, Fernandez & Tryggvason (2005).

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