Bubbly turbulent Taylor-Couette (TC) flow is globally and locally studied at Reynolds numbers of Re = 5 × 10 5 to 2 × 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 α global over the range 0-4%. We observe a moderate DR of 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 at Re = 2.0 × 10 6 and α 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 × 10 5 in the moderate DR regime and Re = 1.0 × 10 6 in the strong DR regime, both at α global = 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 × 10 6 is ≈ 2.3 times lower than that for Re = 5.1 × 10 5 , 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 (W e) 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 W e ∼ 1. In the strong DR regime at Re = 1.0×10 6 we find W e > 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 × 10 5 we find W e ≈ 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 × 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).

We report on the enhancement of turbulent convective heat transport due to vapour-bubble nucleation at the bottom plate of a cylindrical Rayleigh-Bénard sample (aspect ratio 1.00, diameter 8.8 cm) filled with liquid. Microcavities acted as nucleation sites, allowing for well-controlled bubble nucleation. Only the central part of the bottom plate with a triangular array of microcavities (etched over an area with diameter of 2.5 cm) was heated. We studied the influence of the cavity density and of the superheat T b − T on (T b is the bottom-plate temperature and T on is the value of T b below which no nucleation occurred). The effective thermal conductivity, as expressed by the Nusselt number Nu, was measured as a function of the superheat by varying T b and keeping a fixed difference T b − T t 16 K (T t is the top-plate temperature). Initially T b was much larger than T on (large superheat), and the cavities vigorously nucleated vapour bubbles, resulting in two-phase flow. Reducing T b in steps until it was below T on resulted in cavity deactivation, i.e. in one-phase flow. Once all cavities were inactive, T b was increased again, but they did not reactivate. This led to one-phase flow for positive superheat. The heat transport of both one-and two-phase flow under nominally the same thermal forcing and degree of superheat was measured. The Nusselt number of the two-phase flow was enhanced relative to the one-phase system by an amount that increased with increasing T b . Varying the cavity density (69, 32, 3.2, 1.2 and 0.3 mm −2 ) had only a small effect on the global Nu enhancement; it was found that Nu per active site decreased as the cavity density increased. The heat-flux enhancement of an isolated nucleating site was found to be limited by the rate at which the cavity could generate bubbles. Local bulk temperatures of one-and two-phase flows were measured at two positions along the vertical centreline. Bubbles increased the liquid temperature (compared to one-phase † Email address for correspondence: daniela.narezo@gmail.com 332 D. Narezo Guzman and others flow) as they rose. The increase was correlated with the heat-flux enhancement. The temperature fluctuations, as well as local thermal gradients, were reduced (relative to one-phase flow) by the vapour bubbles. Blocking the large-scale circulation around the nucleating area, as well as increasing the effective buoyancy of the two-phase flow by thermally isolating the liquid column above the heated area, increased the heat-flux enhancement.

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We report on an experimental study of the large-scale flow (LSF) and Reynolds numbers in turbulent convection in a cylindrical sample with height equal to its diameter and heated locally around the center of its bottom plate (locally heated convection). The sample size and shape are the same as those of Narezo Guzman et al. [D. Narezo Guzman et al., J. Fluid Mech. 787, 331 (2015); 795, 60 (2016)]. Measurements are made at a nearly constant Rayleigh number as a function of the mean temperature, both in the presence of controlled boiling (two-phase flow) and for the superheated fluid (one-phase flow). Superheat values T-b - T-on up to about 11 K (T-b is the bottom-plate temperature and T-on is the lowest Tb at which boiling is observed) are used. The LSF is less organized than it is in (uniformly heated) Rayleigh-Bnard convection (RBC), where it takes the form of a single convection roll. Large-scale-flow-induced sinusoidal azimuthal temperature variations (like those found for RBC) could be detected only in the lower portion of the sample, indicating a less organized flow in the upper portions. Reynolds numbers are determined using the elliptic model (EM) of He and Zhang [G.-W. He and J.-B. Zhang, Phys. Rev. E 73, 055303(R) (2006)]. We found that for our system the EM is applicable over a wide range of space and time displacements, as long as these displacements are within the inertial range of the temporal and spatial spectrum. At three locations in the sample the results show that the vertical mean-flow velocity component is reduced while the fluctuation velocity is enhanced by the bubbles of the two-phase flow. Enhancements of velocity fluctuations up to about 60% are found at the largest superheat values. Local temperature measurements within the sample reveal temperature oscillations that also used to determine a Reynolds number. These results are generally consistent with the mean-flow EM results and show a two-phase-flow enhancement of up to about 30%