Microbubbly drag reduction in Taylor–Couette flow in the wavy vortex regime

Abstract

We investigate the effect of microbubbles on Taylor-Couette flow by means of direct numerical simulations. We employ an Eulerian-Lagrangian approach with a gas-fluid coupling based on the point-force approximation. Added mass, drag, lift, and gravity are taken into account in the modeling of the motion of the individual bubble. We find that very dilute suspensions of small non-deformable bubbles (volume void fraction below 1%, zero Weber number and bubble Reynolds number 10) induce a robust statistically steady drag reduction (up to 20%) in the wavy vortex flow regime (Re = 600 -2500). The Reynolds number dependence of the normalized torque (the so-called Torque Reduction Ratio (TRR) which corresponds to the drag reduction) is consistent with a recent series of experimental measurements performed by Murai et al. (J. Phys. Conf. Ser. 14, 143 (2005)). Our analysis suggests that the physical mechanism for the torque reduction in this regime is due to the local axial forcing, induced by rising bubbles, that is able to break the highly dissipative Taylor wavy vortices in the system. We finally show that the lift force acting on the bubble is crucial in this process. When neglecting it, the bubbles preferentially accumulate near the inner cylinder and the bulk flow is less efficiently modified.

Effect of bubble’s arrangement on the viscous torque in bubbly Taylor-Couette flow

“…For bubbles larger than the Kolmogorov length scale, torque reduction is likely to be associated either with a de-structuration of the Taylor vortices by the bubble upward motion in the case of weak turbulent and turbulent Taylor vortex flow 7,8 or associated with the deformation of the bubbles in the case of the high Reynolds numbers (Re>8 10 5 ) [9][10][11][12] . According to Murai et al 7 , there is a Reynolds number range, for which the relative contribution of the Taylor vortices to the global flow and the contribution of the bubble deformation are too small to bring about torque reduction, thus leading on the contrary to a torque increase.…”

“…For very small bubbles, the size of which was of the order of the viscous length scale, it appears that the small scale turbulence can also play a role, by trapping bubbles inside the low shear stress streaks near the inner cylinder. But two-way coupling calculations performed by Sugiyama et al 8 highlighted that numerical results of the bubble dispersion near the wall are very sensitive to the modelling of the lift force coefficient. Overall, the numerical prediction of the bubble accumulation near the inner cylinder wall is over-evaluated, without taking into account bubble-bubble interactions 3 .…”

Intensified and attenuated waves in a microbubble Taylor–Couette flow

“…and the microbubbles should modify the momentum transfer very little or not at all, 13 if the mean void fraction is as small as O(0.1%). The effects of the modification in density, effective viscosity, and extra energy input would be of the same order as the void fraction, and would be negligible in dilute two-phase flows, i.e., α ≤ O(10 −4 ).…”

“…19,20 Because of these advantages, so far, Taylor-Couette flows containing bubbles have been both experimentally and numerically investigated. 13,[21][22][23][24][25][26][27] Relatively large bubbles with O(1 mm) in diameter reduce the frictional drag by 20% by modifying the boundary layer in Taylor-Couette flows when bubbles are deformable. 23,25 Bubble deformation or compressibility is also one of the factors in drag reduction as in spatially developing channel flows.…”

The boiling Twente Taylor-Couette (BTTC) facility: Temperature controlled turbulent flow between independently rotating, coaxial cylinders

“…Though well known in the context of rheology, the Taylor-Couette geometry-a geometry in which fluid is bound between two concentric rotating cylinders-has also been used in many fundamental concepts: the verification of the no-slip boundary condition, hydrodynamic stability [1], higher and lower order bifurcation phenomena and flow structures [2][3][4][5], but also in the field of combustion [6][7][8], drag reduction [9][10][11][12], magnetohydrodynamics in order to study e.g. the MRI [13][14][15][16][17], astrophysics to study Keplerian flow in accretion discs [18][19][20][21], rotating filtration in order to extract plasma from whole blood [22][23][24][25][26], cooling of rotating machinery [27], flows in bearings, the fundamentals of high Reynolds number flows [5,[28][29][30][31][32][33][34][35], and as a catalytic and plasmapheretic reactor [36][37][38].…”