Session A36: Bubbly Flows I

Dynamics of bubble swarms in pipe flows at high Reynolds numbers

Two-phase flows in pipes are paramount in several industries, including oil and gas, chemical processing, and nuclear engineering, where efficient transport and mixing of multiphase fluids are critical. For this reason, they have been widely studied across many bubbly regimes and void fraction profiles (e.g., core peaking, edge peaking…) in extensive parameter space. However, many questions remain unanswered, and it remains challenging to establish links with fundamental knowledge either of isolated bubbles in turbulence or of swarms of bubbles at low Reynolds numbers. 

We study bubble dynamics in turbulent pipe flows at various bubbly regimes using high-resolution high-speed imaging. Upward, downward, and horizontal configurations and the role of different means of injections are considered. The Reynolds number varies from 5000 to 150 000 and the void fraction from a fraction of a percent to a few percents. Image processing tools and particle tracking velocimetry were used to characterize the size and shape distribution of the bubbles, and the bubble dynamics, and draw quantitative regime maps. We employed particle tracking velocimetry to study the dynamics of the dispersed phase. Additionally, qualitative work is attempted on tumbling and break-up.   

Interaction of a thin vortex ring with a deforming and non-deforming bubble: Similarities and differences

In bubbly turbulent flows, the deformation of the bubble is known to play an important role in the interaction between the carrier phase and the dispersed phase. In order to understand the role of bubble’s deformability on these complex interactions, we experimentally study an idealization, namely, the interaction of a deforming air bubble (ρ_bubble/ρ_water ≈ 0.001) and a rigid buoyant particle (ρ_particle/ρ_water≈0.008, a rigid bubble), with a single water vortex ring (Re=6000–67300), with the main difference being the distinct difference in their deformability. The rigid and the deforming bubbles are directly engulfed into the vortex core. The deforming bubble undergoes elongation outside the ring during its capture, followed by elongation (azimuthally) and breakup inside the ring. In contrast, the rigid bubble remains spherical during capture, and stays more localized within the ring. We capture the deformability effects on the bubble dynamics and in modifications to the ring core's vorticity, the former measured using high speed imaging, and the latter with time-resolved PIV. The differences in deformability lead to distinct differences in the ring’s convection speed, azimuthal vorticity, and enstrophy. The details of these interactions, including both differences and similarities across the deforming and rigid bubble cases at low and large ring Reynolds numbers, will be presented at the conference.    

Mechanism of reattachment length reduction in a bubbly flow in a pipe with sudden expansion

The reduction of the reattachment length has been targeted in controlling the flow over a backward-facing step because of its relation to the mixing enhancement. In this study, we investigate the bubble-assisted reattachment length reduction in the upward bubbly flow through expansion square pipes with area expansion ratios (ER) 4.0 to 9.0. When air bubbles are injected into the water flow, the liquid-phase turbulence is augmented significantly, which is known to one of the mechanisms of reattachment length reduction in single-phase backstep flow. For the present two-phase flows, however, it is dependent on more than just the global increase of turbulence due to the bubble-induced agitation; its local distribution is also important. Using the two-phase particle image velocimetry, we measure the effects of ER and volume void fraction (0.12-2.80% with the bubble size of 1.6-3.6 mm) on the liquid-phase turbulence with two Reynolds numbers of 420 (laminar) and 6000 (turbulent), based the single-phase flow statistics. We will discuss how the bubble dynamics, origin of bubble-induced turbulence, would affect differently the reattachment length of the flow depending on laminar and turbulent background flows.   

Effect of Superhydrophobic Surfaces on Rod Bundle Flow Dynamics

Pressurized water reactors are designed to operate in a single-phase flow. However, during a flow loss or other off-design conditions liquid temperature

may exceed saturation temperature and, if a continuous film of gas forms, a boiling casualty may result. If superhydrophobic surfaces are introduced

among the fuel cell assembly, vapor bubbles show an affinity to these surfaces and gas may coalesce to escape faster resulting in a larger margin to reach

Critical Heat Flux. In the present study we consider air and liquid water mixture examining the overall flow dynamics in a case with no bulk liquid

flow, reminiscent of a case with coolant pump failure. As the flow with high gas volume fraction becomes optically opaque and is sensitive to intrusive

instrumentation, a custom build photon-counting dual energy threshold X-ray computed tomography system is employed for measurement of the time

average phase fraction within the bundle non-intrusively. Two dual-plane wire mesh sensors upstream and downstream of the rod bundle are employed

to obtain comparison phase fraction, velocity profiles and bubble size distributions. Additionally, traditional pressure-based gas holdup measurements

are employed to calculate time and volume averaged phase fraction. The data shows that a superhydrophobic surface being present in the rod bundle

results in a significantly lower gas volume fraction compared to those in a similar rod bundle without superhydrophobic internals.   

Turbulent statistics and scales involved in turbulent kinetic energy budget in bubble plumes

Existence of bubble plumes in nature and their wide range of applications in engineering have driven the advancement of our understanding of this unique multiphase fluid mechanics problem in the past few decades. However, bubble induced turbulence is not fully understood due to the incomplete understanding of turbulent scales involved in the interaction between bubble and water. Specifically, the mechanisms of turbulent kinetic energy (TKE) production, transport, and dissipation in both spatial and spectral spaces, are not adequately characterized. To address this knowledge gap, we conducted a series of experiments using fluorescent particle image velocimetry and measured flow field and turbulent statistics within the bubble plume. To support analysis of spatial scales, shadow-graphic imaging was used to measure bubble size distribution. The data indicate that bubble size plays a significant role in mechanisms of turbulence production and transport. The analysis suggests some similarities but differences in turbulent statistics and TKE budget compared to single-phase jets and plumes. Spectral analysis of the TKE budget elucidates the hypothesis of an inverse energy cascade in the bubble plume, a mechanism to transfer TKE from small to large eddies. This is attributed to the direct injection of TKE by bubble passages across a wide range of scales, in contrast to canonical shear production of TKE in large scales.   

Extension of Inertial Subrange in Turbulent Bubbly Jet at Low Void Fraction

Turbulent bubbly jets at Reynolds number 3,000 with low void fraction (≤ 1%) are measured by using time-resolved three-dimensional Particle Tracking Velocimetry and Eulerian reconstruction techniques. The evolution of the mean flow characteristics shows good agreement with the Eulerian-Eulerian DNS of a bubbly jet at the same condition. As the jet flows downstream, the bubble-populated region resides inner interface of the shear layer. Hence, there is a separation between the bubbles and the shear layer. The phase separation occurs earlier with a lower void fraction. After the phase separation, typical coherent structures appear around the shear layer. The identified vortical structures are correlated with the alternating pattern of the axial liquid accelerations. The alternating pattern gets sparse as the jet has higher axial liquid velocity due to the bubbles and vortical structures are drifted by the slip velocity of the bubbles. The wake instability has a strong correlation with the spanwise (radial and azimuthal) motion of the bubbles. In high-frequncy range, all the spectrums collapse with -5/3 slopes. In the liquid phase energy spectra plotted against the wavenumber, bubbly jet cases have an extended cascading process after the Taylor microscale (λ) of the single-phase jet. The extension of the cascading process is regardless of velocity components but a higher void fraction induced a small hill in high-frequency scale. To identify the effect of the bubbles in terms of scales, premultiplied energy spectra normalized by the axial mean liquid velocity are calculated. Inside the jet core, a plateau after the peak of the band regardless of the initial void fraction. At the shear layer, there are still lifted tails of the energy spectra at high wavenumber region depending on the void fraction, but no distinctive plateau. The interactions extended the energy cascading process beyond the wake instability frequency from bubbles. It is concluded that a small amount of bubbles in the jet shows different turbulent characteristics to buoyancy dominant flow.   

Bubble breakup in homogeneous shear turbulence

In turbulent multiphase flows, many studies have been conducted to study the breakup of bubble in homogeneous turbulence. However, most types of turbulence used in chemical and biological reactors deviate from the canonical type, featuring strong shear. The specific roles played by shear flow to breakup remain unclear. In this study, we present a novel experimental setup comprising a vertical high-speed water channel capable of generating homogeneous shear turbulence. By matching the Weber numbers defined by turbulent dynamic pressure and mean shear rate, the experiments allow for a direct comparison of their respective effects. From this experimental result, we aim to unravel the interplay between mean shear rate and turbulent eddies, providing valuable insights into the actual processes underlying bubbly flow or oil emulsion produced by turbulence.   

Collision, and coalescence of bubbles in turbulence

Bubble coalescence in turbulence plays a pivotal role in determining the bubble size spectrum, holding significant importance in various natural phenomena and industrial applications. Despite numerous proposed models and hypotheses, their experimental validation has been limited due to inherent challenges in measuring the process. This study addresses these challenges by introducing an advanced 3D Lagrangian bubble tracking algorithm. Leveraging this method, we can precisely track closely spaced bubble pairs and acquire sufficient statistical data to characterize the approach velocity at which two bubbles collide. From the results, we also measure the contact time, which directly affects the bubble coalescence efficiency. The experimental results revealed the relationship between the bubble approach velocity with the bubble size, which exhibits a scaling law that deviates from the classical model based solely on Kolmogorov theories. This difference was explained by the biased sampling of bubbles in eddies of similar sizes.