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From air-sea gas exchange, oil pollution, to bioreactors, the ubiquitous fragmentation of bubbles/drops in turbulence has been modeled by relying on the classical Kolmogorov-Hinze paradigm since the 1950s. This framework hypothesizes that bubbles/drops are broken solely by eddies of the same size, even though turbulence is well known for its wide spectrum of scales. Here, by designing an experiment that can physically and cleanly disentangle eddies of various sizes, we report the experimental evidence to challenge this hypothesis and show that bubbles are preferentially broken by the sub-bubble-scale eddies. Our work also highlights that fragmentation cannot be quantified solely by the stress criterion or the Weber number; The competition between different time scales is equally important. Instead of being elongated slowly and persistently by flows at their own scales, bubbles are fragmented in turbulence by small eddies via a burst of intense local deformation within a short time. The classical framework of bubble fragmentation in turbulence usually only considers eddies of the bubble size. The authors present an experimental evidence that bubbles can be broken by the eddies of various sizes, supported by analytical model that includes the bubble and eddy scales, and the bubble breakup time. [ABSTRACT FROM AUTHOR]

The dispersed phase in liquid–liquid emulsions and air–liquid mixtures can often be fragmented into smaller sizes by the surrounding turbulent carrier phase. The critical parameter that controls this process is the breakup frequency, which is defined from the breakup kernel in the population balance equation. The breakup frequency controls how long it takes for the dispersed phase to reach the terminal size distribution for given turbulence. In this article, we try to summarize the key experimental results and compile the existing datasets under a consistent framework to find out what is the characteristic timescale of the problem and how to account for the inner density and viscosity of the dispersed phase. Furthermore, by pointing out the inconsistency of existing experimental data, the key important unsolved questions and related problems on the breakup frequency of bubbles and droplets are discussed. [Display omitted] • Compile experimental datasets on bubble and droplet breakup frequency in turbulence under a unified framework. • Discuss and compare different relevant timescales for breakup among different datasets. • Propose the mean shear time as a potential timescale for breakup in turbulent flows. [ABSTRACT FROM AUTHOR]

We present simultaneous three-dimensional measurements of deformable finite-sized bubbles and surrounding turbulent flows. The orientations of bubbles are linked to two key mechanisms that drive bubble deformation: the turbulent strain rate and slip velocity between the two phases. The strongest preferential alignment is between the bubbles and slip velocity, indicating the latter plays a dominant role. We also compared our experimental results with the deformation of ideal material elements with no slip velocity or surface tension. Without these, material elements show highly different orientations, further confirming the importance of the slip velocity in the bubble orientation. In addition to deformation, when bubbles begin to break, their relative orientations change significantly. Although the alignment of the severely deformed bubbles with the eigenvectors of the turbulent strain rate becomes much stronger, the bubble semi-major axis becomes aligned with (rather than perpendicular to) the slip velocity through an almost 90◦ turn. This puzzling orientation change occurs because the slip velocity contains the contributions from both the bubble and the background flow. As the bubble experiences strong deformation, the rapid elongation of its semi-major axis leads to a large bubble velocity, which dominates the slip velocity and forces its alignment with the bubble's semi-major axis. The slip velocity thereby switches from a driving mechanism to a driven result as bubbles approach breakup. The results highlight the complex coupling between the bubble orientation and the surrounding flow, which should be included when modelling the bubble deformation and breakup in turbulence. [ABSTRACT FROM AUTHOR]

We present an experimental investigation of the acceleration statistics and the added mass tensor of deformable gas bubbles in turbulence. By simultaneously tracking both bubbles and their surrounding flow in three dimensions, we find two independent ways of estimating the bubble acceleration: either directly measured from three-dimensional bubble trajectories or indirectly calculated from the bubble's equation of motion. When such an equation is projected onto the bubble frame, the added-mass coefficient becomes a diagonal tensor with three elements being linked to the standard deviation of bubble acceleration along three bubble principal axes. This constraint aids in experimentally determining the added mass coefficient tensor. The obtained trend of $C_A$ seems to agree with Lamb's potential flow solutions for spheroids, suggesting that the added-mass force on deformable bubbles can be modelled using spheroids with the same geometry and orientation. In addition, the probability density function of the relative orientation between the semi-major axis of deformed bubbles and the slip acceleration in turbulence is shown. A surprising finding is that the bubble orientation, indicated by the bubble's major axis, is not random in turbulence but rather is preferentially aligned with the slip acceleration. The degree of this alignment increases as bubbles deform more. Because accelerating along the major axis of a more deformed bubble entails reduced added mass, the acceleration standard deviation of deformable bubbles increases as a function of the bubble aspect ratio. [ABSTRACT FROM AUTHOR]

We experimentally investigate the breakup mechanisms and probability of Hinze-scale bubbles in turbulence. The Hinze scale is defined as the critical bubble size based on the critical mean Weber number, across which the bubble breakup probability was believed to have an abrupt transition from being dominated by turbulence stresses to being suppressed completely by the surface tension. In this work, to quantify the breakup probability of bubbles with sizes close to the Hinze scale and to examine different breakup mechanisms, both bubbles and their surrounding tracer particles were simultaneously tracked. From the experimental results, two Weber numbers, one calculated from the slip velocity between the two phases and the other acquired from local velocity gradients, are separated and fitted with models that can be linked back to turbulence characteristics. Moreover, we also provide an empirical model to link bubble deformation to the two Weber numbers by extending the relationship obtained from potential flow theory. The proposed relationship between bubble aspect ratio and the Weber numbers seems to work consistently well for a range of bubble sizes. Furthermore, the time traces of bubble aspect ratio and the two Weber numbers are connected using the linear forced oscillator model. Finally, having access to the distributions of these two Weber numbers provides a unique way to extract the breakup probability of bubbles with sizes close to the Hinze scale. [ABSTRACT FROM AUTHOR]

We experimentally investigate the rise velocity of finite-sized bubbles in turbulence with a high energy dissipation rate of $\unicode[STIX]{x1D716}\gtrsim 0.5~\text{m}^{2}~\text{s}^{-3}$. In contrast to a 30–40 % reduction in rise velocity previously reported in weak turbulence (the Weber number ($We$) is much smaller than the Eötvös number ($Eo$); $We\ll 1), the bubble rise velocity in intense turbulence shows a surprising new behaviour: an abrupt transition from an order of magnitude slower to a factor of two faster than rising in an otherwise quiescent medium. This transition occurs when $We$ increases from below one to above one, underscoring the key role played by the turbulence-induced deformation. We also formulate a model based on bubble–eddy coupling, and the results show an excellent agreement with not only our data in intense turbulence but also other works on weak turbulence. The model also helps us to extract the lift and drag coefficients of bubbles in intense turbulence for a wide range of $We$ and Reynolds numbers in situ. [ABSTRACT FROM AUTHOR]

A vertical water tunnel facility has been constructed to study the dynamics of turbulent multiphase flow. The new system features several unique designs that allow us to study bubble deformation and breakup in strong turbulence: (i) The mean flow can be adjusted to balance the rising velocity of buoyant bubbles/droplets so that they can stay in the view area for an extended period of time. (ii) Turbulence is generated and controlled using a 3D-printed jet array that can fire 88 random high-speed momentum jets with the individual jet velocity of up to 12 m/s. This component allows us to attain turbulence with a high energy dissipation rate (≥0.1 m2/s3), which is orders-of-magnitude higher than most of the existing turbulent multiphase flow facilities. (iii) Turbulence generated in the test section is nearly homogeneous and isotropic, and the turbulent fluctuations are also decoupled from the mean flow. The resulting turbulence intensity can be varied between 20% and 80% with the speed of the mean flow at around 0.2 m/s. (iv) This system has an octagonal test section that allows six cameras to image and reconstruct the 3D shape of deforming bubbles/droplets in turbulence. The same set of cameras was also used for tracking tracers in the surrounding turbulent flow. Both the reconstruction and particle tracking were completed using our in-house codes that were parallelized to run on high-performance computing clusters efficiently. [ABSTRACT FROM AUTHOR]

We report the first experimental study of Lagrangian acceleration in turbulent Rayleigh–Bénard convection, using particle tracking velocimetry. A method has been developed to quantitatively evaluate and eliminate the uncertainties induced by temperature and refraction index fluctuations caused by the thermal plumes. It is found that the acceleration p.d.f. exhibits a stretched exponential form and that the probability for large magnitude of acceleration in the lateral direction is higher than those in the vertical directions, which can be attributed to the vortical motion of the thermal plumes. The local acceleration variance $\langle {a}^{2} \rangle $ was obtained for various values of the three control parameters: the Rayleigh number $\mathit{Ra}$ ($6\ensuremath{\times} 1{0}^{8} \leq \mathit{Ra}\leq 1\ensuremath{\times} 1{0}^{11} $), the Prandtl number $\mathit{Pr}$ ($\mathit{Pr}= 4. 4, 5. 5$ and 6.1) and the system size $L~(L= 19. 2\text{~and~} 48. 6~\mathrm{cm} )$. These were then compared with the theoretically predicted dependence on these parameters for buoyancy-dominated turbulent flows and for homogeneous and isotropic turbulence, respectively. It is found that $\langle {a}^{2} \rangle $ in the central region is dominated by contributions from the turbulent background rather than from the buoyancy force, and the Heisenberg–Yaglom relation holds in this region. From this, we obtain the first experimental results of the constant ${a}_{0} $ of the acceleration variance in the micro-scale Reynolds number range $20\leq {R}_{\lambda } \leq 120$, which fills a gap in this constant in the lower ${R}_{\lambda } $ end from the experimental side, and provides possible constraints for its high ${R}_{\lambda } $ behaviour if a certain fitting function is attempted. In addition, acceleration correlation functions were obtained for different $\mathit{Ra}$. It is found that the zero crossing time of acceleration correlation functions is at $\tau \approx 2. 2{\tau }_{\eta } $ (${\tau }_{\eta } $ is the Kolmogorov time scale) over the range of $\mathit{Ra}({R}_{\lambda } )$ spanned in our experiments, which is the same as the simulation results in isotropic turbulence, and the exponential decay time ${\tau }_{1/ e} = (1. 12\pm 0. 05){\tau }_{\eta } $, which is larger than $(0. 73\ensuremath{\sim} 0. 80){\tau }_{\eta } $ found experimentally for other types of turbulent flows with larger ${R}_{\lambda } $. [ABSTRACT FROM PUBLISHER]

We report heat transfer and flow dynamics measurements of alumina nanofluid in turbulent convective flow. Under the condition of fixed temperature at the top plate and fixed input heat flux at the bottom plate, it has been found that the convective heat transfer coefficient, h, Nusselt number, Nu, and Rayleigh number, Ra, all decrease with the increasing volume fraction [lowercase_phi_synonym] of the nanoparticle. In contrast, the velocity of the convective flow showed no significant change within experimental uncertainty and over the range of nanoparticle concentration of the measurement (from 0% to 1.08%). Under the condition of constant nanoparticle concentration ([lowercase_phi_synonym]=1.08%), a second set of measurements of the heat transport and flow properties have been made over a broad range of Ra (from 2.6×108 to 7.7×109). For heat transport, a transition near Rac≃2.5×109 has been found. For Ra>Rac, the measured Nu of the nanofluid is roughly the same as that of water in terms of both its magnitude and its scaling relation with Ra, which suggests that the nanofluid can be treated as a single phase fluid in this parameter range. For RaRac. This suggests that the significant decrease of the nanofluid Nu comparing to that of water may be caused by the mass diffusion of nanoparticles. Furthermore, measurements of the flow velocity of the bulk nanofluid showed no significant difference from that of water for Ra either above or below Rac. From estimated thermal boundary layer thickness, we found that the deviations of the nanofluid Nu from that of water for Ra

• Bubble breakup process was studied in a vertical water tunnel that is able to maintain homogeneous and isotropic turbulence over a large region. The facility is equipped with a new 3D shape reconstruction method, which helps to extract the 3D shape of both mother and daughter bubbles during breakup. • In addition to our own experiment, seven other experimental results were compiled to provide constraints to the model. These constraints cover many aspects of the breakup process, spanning from the direct measurement of daughter bubble size distribution to the time evolution of bubble size spectrum. • A new breakup model is proposed to satisfy all the constraints. Complex bubble breakup in turbulence has been studied and modeled extensively by employing the population balance equation. This equation hinges on two quantities, i.e. daughter bubble size distribution and breakup frequency. Since there is no first-principle equation that can be solved to calculate these two quantities, many phenomenological models based on different physical mechanisms have been proposed. A large number of possible mechanisms at play leads to models with drastically different, and even contradictory, predictions. In contrast, experimental measurements of these two quantities, including several previous works and our own results collected in a vertical water tunnel that features a large homogeneous and isotropic region, seem to be consistent with one another. To resolve the difference between models and experiments, rather than following another physical argument, we approach the problem from a different direction by asking how to constrain a model based on experimental results. The specific constraints extracted from eight experimental results include: (i) direct measurements of daughter bubble size distribution; (ii) Super-Hinze-scale bubble size spectrum for constraining breakup frequency; (iii) Sub-Hinze-scale bubble size spectrum for modeling daughter bubble size distribution; (iv) Convergence time to an equilibrium state. Finally, based on these experimental constraints, a new breakup model that incorporates a corrected formulation for breakup frequency as well as a simplified function for daughter bubble distribution is developed to meet all constraints. Although the new model is deliberately not connected to any specific physical arguments for simplification, it appears to be robust and consistent with all experimental constraints mentioned. [ABSTRACT FROM AUTHOR]