The importance of bubble deformability for strong drag reduction in bubbly turbulent Taylor-Couette flow

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Abstract

Bubbly turbulent Taylor–Couette (TC) flow is globally and locally studied at Reynolds numbers of Re=5×105 to 2×106 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 αglobal over the range 0–4 %. We observe a moderate DR of up to 7 % for Re=5.1×105. Significantly stronger DR is achieved for Re=1.0×106 and 2.0×106 with, remarkably, more than 40% of DR at Re=2.0×106 and αglobal=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=5.1×105 in the moderate DR regime and Re=1.0×106 in the strong DR regime, both at αglobal=3±0.5%. In both cases the bubbles mostly accumulate close to the inner cylinder (IC). Surprisingly, the maximum local gas concentration near the IC for Re=1.0×106 is ≈2.3 times lower than that for Re=5.1×105, in spite of the stronger DR. Evidently, a higher local gas concentration near the inner wall does not guarantee a larger DR. By defining and measuring a local bubble Weber number (We) in the TC gap close to the IC wall, we observe that the cross-over from the moderate to the strong DR regime occurs roughly at the cross-over of We∼1. In the strong DR regime at Re=1.0×106 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=5.1×105 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 with the Re=1.0×106 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. (Phys. Rev. Lett., vol. 94, 2005, p. 044501) and from the numerical simulations by Lu, Fernandez & Tryggvason
Original languageEnglish
Pages (from-to)317-347
Number of pages31
JournalJournal of fluid mechanics
Volume722
DOIs
Publication statusPublished - 2013

Keywords

  • METIS-295431
  • IR-89905

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