Two-phase Particle Image Velocimetry Research Articles

Two-Phase Particle Image Velocimetry Visualization of Rewetting Flow on the Micropillar Interfacial Surface.

Boiling heat transfer has a high thermal efficiency by latent heat absorption, which makes it an attractive process for cooling electronic device chips. Critical heat flux (CHF), the maximum heat flux, is a crucial factor determining the operating range of the boiling applications. The CHF can be enhanced by improving the fluid supply to the boiling surface. Herein, micropillar interfacial surfaces have been proposed to increase the CHF by increasing the rewetting flow, which determines the fluid-supply capacity near the bubble contact line. A state-of-art two-phase particle image velocimetry (two-phase PIV) technique is introduced for rewetting flow measurement on micropillar structures (MPSs) to analyze the CHF-enhancement mechanism. The two-phase PIV visualization setup offers high spatial (∼120 μm) and temporal (∼2000 Hz) resolutions for measuring rewetting flow during bubble growth. The MPS samples exhibit enhanced CHF and rewetting flows compared to those on a plain surface. The roughest case, D04G10 sample, had a CHF of 164 W/cm2, 1.84 times higher than that of the plain surface. The D04G10 sample also recorded the highest rewetting velocity of 0.311 m/s, 4.7 times higher than that of the plain surface. The comparison between the rewetting flow and wicking performance shows that wicking-induced flow accounted for a substantial part (∼17%) of the rewetting flow and contributed significantly to the CHF enhancement owing to large rewetting flow by delaying vapor-film formation. Based on these findings, a new CHF model suggested by introducing the rewetting parameter shows a high CHF prediction accuracy of 94%.

EXPERIMENTAL STUDY OF TWO-PHASE TURBULENT WAKE USING PIV TECHNIQUE

Near-wake regions of the turbulent wake flows behind a long cylinder at two Reynolds numbers (Re<sub>d</sub> = 3860 and 9650) and laden with the particles with mean size of 55 μm are studied using a two-phase particle image velocimetry (PIV) technique. Four mass-loading ratios (α) of 0 (equivalent to the single-phase case), 1, 3, and 5% corresponding to the dispersed-phase volume fractions of 1.248 × 10<sup>-6</sup>, 3.743 × 10<sup>-6</sup>, and 6.241 × 10<sup>-5</sup> for the last three cases, respectively, are investigated. In the particle-laden cases, complex variations of turbulence intensity are discovered; in the near-wake regions, turbulent intensity is either suppressed or enhanced locally compared with the single-phase case. No correlation between the turbulence-intensity modulation and the parameter of either α or Re<sub>d</sub> is found to exist. However, the turbulent mixing capability is attenuated for the cases with particles laden in the wakes. This attenuation is increased with α in both Re<sub>d</sub> cases. In contrast to the turbulence modulation, the turbulent dispersion of the laden particles increases as either α or Re<sub>d</sub> is increased.

Experimental study on turbulent characteristics of dispersed bubbly flow in a narrow rectangular channel by two-phase PIV

Narrow rectangular channels are widely used in high heat flux applications, due to the high heat transfer efficiency and compact structure. However, a rare understanding was known about the two-phase turbulent characteristics. In this study, an experimental investigation of bubble and turbulent characteristics of dispersed bubbly flow in the narrow rectangular channel was carried out by using the high-speed camera and two-phase particle image velocimetry. The bubble size effect was studied independently using the continuous spectrum bubble generator. According to the results, the bubble size distribution conforms to a Weibull distribution, and the wall-peak void fraction distributions were found. Both turbulent enhancement and suppression were observed in current conditions. Large bubbles enhance turbulence intensity, while small bubbles suppress it. The eddy viscosity ratio ϕ characterizes the modification of turbulence intensity by bubbles, with a critical value of ϕ=1.

Bubble-induced oscillating flow in microbubble emission boiling under highly subcooled conditions

Microbubble emission boiling (MEB), a phenomenon that occurs under highly subcooled boiling conditions, can achieve a high heat flux beyond critical heat flux. MEB has been observed to differ from nucleate boiling and is always accompanied by the emission of microbubbles from an oscillating bubble. The heat transfer mechanism of MEB differs from that of nucleate boiling and remains elusive. In this study, we measure the behaviour of a vapour bubble and its induced liquid flow simultaneously during subcooled boiling using the high-speed two-phase particle image velocimetry method. Different from nucleate boiling, we observe a rapid oscillating flow outside the bubble that constantly attaches on the heating surface in MEB. The spatially and temporally averaged velocity magnitude values under this oscillating flow, and their dependency on liquid subcooling and heat flux, are quantified. Then we derive a scaling law for the heat transfer of MEB as a function of Péclet and Jacob numbers. With such correlations, we suggest that the oscillating flow induced by bubble oscillations is important to MEB, thus elucidating a different heat transport process from nucleate boiling.

Pulse picking of a fiber laser enables velocimetry of biomass-laden jets at low and ultra-high repetition rates

The introduction of fibrous aspherical biomasses for the substitution of coal as a sustainable solid fuel causes complex particle-turbulence interactions, which need to be captured using minimally invasive laser-based diagnostics. High-resolution two-phase particle image velocimetry (PIV) of high-velocity biomass-laden turbulent flows require laser inter-pulse times of only a few microseconds, which is usually achieved with separate laser systems for low-speed statistical and time-resolved measurements. In this work, we introduce a novel approach to achieve low-speed dual-pulse and ultra-high-speed time-resolved two-phase PIV using a single laser system. The flexible repetition rate laser output is realized by coupling a continuously pulsing fiber laser to an acousto-optic deflector (AOD) for pulse picking. The selective deflection of two adjacent laser pulses into the measurement volume is presented at a camera repetition rate of 25 Hz and laser repetition rate of 312.5 kHz resulting in an inter-pulse time of 3.2 µs. Temporal limitations of the AOD pulse picking system are evaluated beyond the access time limit for partially overlapping pulse pairs. To demonstrate the feasibility of the diagnostic system, low-speed measurements and additional ultra-high-speed measurements at 200 kHz are performed at high spatial resolutions to resolve individual biomass particles and their surrounding flow field in a non-reacting turbulent round jet. Simultaneous diffuse back-illumination measurements using a pulsed LED for particle size measurements of a high-aspect-ratio biomass (miscanthus) are conducted. The diagnostic system offers the capability of determining the slip velocity field and projected particle size, enabling a direct measurement of the local particle Reynolds number under high-velocity turbulent conditions at low and ultra-high repetition rates.

Flow induced by the single-bubble chain depending on the bubble release frequency

In the present study, we experimentally investigated the flow induced by the in-chain bubbles (2.4–2.8 mm in size) while varying the bubble release frequency (fb=0–31 Hz), using high-speed two-phase particle image velocimetry and shadowgraphy. Along the streamwise (vertical) direction, we identified the transition of in-chain bubble dynamics and associated liquid flow. That is, the released bubbles tend to follow the straight path initially but transition to the oscillating (zigzag or spiral) trajectories. In particular, in the developing regime (near the nozzle), the coherent behavior of bubble chain generates the rib-like oblique jet flow, deviating from the rise path, even after the onset of the path instability of each bubble. As the flow develops, the scattering of rising path along the lateral direction becomes significant, and the liquid flow shows the similarity to the typical single-phase jet flow, which becomes more obvious at fb > 8 Hz. In this regime, with increasing the fb, the oscillating motion of bubbles changes to the flattened spiral one, and the deviation (fluctuation) of the lateral amplitude increases resulting from the enhanced flow-induced wobbling of bubble shape. Finally, we developed an analytical model to predict the evolution of time-averaged vertical velocity profile of the liquid, which considers the contributions of (i) local void fraction and (ii) evolution of upstream flow, and validated successfully with the measurement.

Quantification of coflow effects on primary atomization of pressure swirl atomizers

Extension of high momentum jet stabilised combustors towards liquid fuel operation has shown significant injector shape and injection strategy impact on the fuel air mixing behaviour, the flame position and stability and ultimately pollutants. This can be attributed to the lack of optimised injection concepts as well as fundamental understanding of turbulence–spray interactions that dictate atomization and fuel placement in high turbulence gas flow conditions. The current work uses a canonical flow channel to investigate the bilateral interactions between a highly turbulent coaxial air flow and the primary atomization process of a liquid spray generated by pressure-swirl atomizers. A novel two-phase particle image velocimetry (PIV) method with phosphor-particles is developed to enable flow field measurements near the gas–liquid interface as well as in the dilute spray region. In addition, a shadowgraphy technique is applied to detect the disturbances on the sheet surface. Liquid fuel is replaced by deionised water due to safety concerns and the coaxial air flow velocity is varied from 0 to 80 m/s. The results show a significant influence of the aerodynamic forces on the growth rate of sheet disturbances with increasing coflow velocities. This induces an advanced breakup of comparable thick liquid sheets at large Weber numbers. The novel PIV method enabled the quantification of the turbulent kinetic energy transfer between the phases that is strongly dependent on the slip velocity at the gas–liquid interface. These findings assist the ongoing development of liquid fuel injection systems for high momentum jet based combustors and provide validation data for numerical simulations of primary atomization.

Bubble dynamics and induced flow in a subcooled nucleate pool boiling with varying subcooled temperature

We experimentally investigate the dynamics of a vapor bubble and subsequent flow induced in a bulk liquid, generated by a subcooled nucleate pool boiling. We focus on the dependency of bubble lifetime (e.g., growth, departure, rise and collapse) on the subcooled temperature. While varying the subcooled temperature (ΔTsub) as 2−15∘C, the temporal variation of bubble dynamics and liquid flow induced in its vicinity are measured simultaneously using the high-speed two-phase particle image velocimetry. In general, the bubble departure diameter and growth time tend to decay exponentially with increasing ΔTsub, and it is possible to classify the regimes further. In a highly subcooled condition (ΔTsub≥8∘C), the bubble starts to shrink (condense) before departing from the wall, while it grows without shrinkage in a less subcooled condition (ΔTsub<8∘C). We modify and combine the existing bubble growth models to predict this phenomenon, which shows a good agreement with the measured data. When ΔTsub<8∘C, the departing bubble (2.5−3.1 mm in size) rises vertically with a significant deformation driven by the liquid inertia. However, the smaller bubble (1.5−2.2 mm) in a highly subcooled condition dissipates rapidly (much shorter lifetime) with a less deformation, owing to the enhanced condensation rate and surface tension. The rising velocity and aspect ratio of the bubble in a less subcooled condition fluctuate following the liquid flow induced by the bubble deformation. Counter-rotating vortical structure around a rising bubble develops larger for the less subcooled condition, which convects further from the wall than the cases of a highly subcooled condition.

An effective convolutional neural network for liquid phase extraction in two-phase flow PIV experiment of an object entering water

In the two-phase flow particle image velocimetry (PIV) experiment of an object entering water, the accurate extraction of the liquid phase region is an important step. In this paper, we elaborately design an effective convolutional neural network (CNN) called LTPNet to solve the problem of two-phase-flow boundary segmentation. Considering the supervised learning strategy, we make a dataset which is based on the two-phase flow PIV experiment of an object entering the water. The experimental results show that LTPNet can achieve high segmentation precision (above 0.98 DSC) on the test images. Meanwhile, our approach can have high computational efficiency with only 2.61M parameters and a speed of 17.34 ms on a single GTX 1080Ti card. Furthermore, the cross-correlation algorithm WIDIM is used to process the images segmented by LTPNet. The results show that our method can efficiently achieve the segmentation of two-phase-flow images and reduce the error vector of the phase edge.

Analytical Model Quantifying the Net Coolant Flow Rate to a Microstructured Copper Surface Validated With Two-Phase Particle Image Velocimetry Measurements

Abstract Using particle image velocimetry (PIV), the amount of fluid required to sustain nucleate boiling was quantified to a microstructured copper circular disk. Having prepared the disk with preferential nucleation sites, an analytical model of the net coolant flow rate requirements to a single site has been produced and validated against experimental data. The model assumes that there are three primary phenomena contributing to the coolant flow rate requirements at the boiling surface; radial growth of vapor throughout incipience to departure, bubble rise, and natural convection around the periphery. The total mass flow rate is the sum of these contributing portions. The model accurately predicts the quenching fluid flow rate at low and high heat fluxes with 4% and 30% error of the measured value, respectively. For the microstructured surface examined in this study, coolant flow rate requirements ranged from 0.1 to 0.16 kg/s for a range of heat fluxes from 5.5 to 11.0 W/cm2. Under subcooled conditions, the coolant flow rate requirements plummeted to a nearly negligible value due to domination of transient conduction as the primary heat transfer mechanism at the liquid/vapor/surface interface. PIV and the validated analytical model could be used as a test standard where the amount of coolant the surface needs in relation to its heat transfer coefficient or thermal resistance is a benchmark for the efficacy of a standard surface or boiling enhancement coating/surface structure.

Scale-wise analysis of upward turbulent bubbly flows: An experimental study

In this work, we investigate the upward turbulent bubbly pipe flow according to relevant length scales, to understand the bubble-induced turbulence modulation of the liquid-phase flow. Using two-phase particle image velocimetry, the bubbly flow fields in a vertical pipe (diameter of 40 mm) were obtained for Reynolds numbers (ReD) of 5300 and 44 000, while increasing the volume void fraction (bubble size of 2.5–4.5 mm) to 1.8% and 1.5%, respectively. The turbulent bubbly flow is visualized and further quantified from the decomposed flow fields (according to the length scales) using the discrete wavelet transform. At ReD = 5300, flow structures of the energy-containing scales (larger than the Taylor microscale) are energized by the disturbance from bubble surface deformation and near wake (whose scales are located close to the integral scale), resulting in the turbulence enhancement across the entire pipe. The interaction between wake vortices, sized between the integral scale and Taylor microscale, apart from the near wake also contributes to the turbulence enhancement. At higher ReD, despite the similar bubble size and void fraction, the scales of flow agitation owing to the bubble surface and the shed wake vortices become smaller, having a negligible effect on turbulence modification. However, the scale of the near wake is still large enough to enhance the turbulence at the pipe center (although it is significantly reduced toward the wall). With the highest void fraction among tested, on the other hand, the mean liquid velocity gradient becomes milder near the pipe wall, which causes a turbulence suppression locally near the wall.

An experimental study on the heat transfer by a single bubble wake rising near a vertical heated wall

We experimentally investigate the effect of a single bubble wake on the convective heat transfer (cooling) on a vertical heated wall in a quiescent water (room temperature) tank. The wall temperature is measured with an infrared camera, and gas-liquid velocity fields are also measured simultaneously using a two-phase particle image velocimetry. We consider two conditions of bubble that rises along two-dimensional zigzagging and straight paths, while varying the bubble-wall distance. When it is sufficiently close to the wall for the zigzagging bubble to impact on the wall, the local heat transfer increases up to eight times compared to that by natural convection. As the bubble-wall distance increases, the amount of heat transfer enhancement decreases, while there is a time delay between the instants of maximum Nusselt number and closest bubble-wall distance. This indicates that the bubble-induced liquid flow is the main source of heat transfer, rather than the physical collision to the wall, confirmed by the strongest correlation between the wall-normal liquid velocity and Nusselt number variation. Although the instantaneous maximum rise is smaller, the total augmentation of heat transfer from the wall is maximized at an intermediate distance (about 2.5 times of bubble size), which is determined by the effective time span and area under the effect of bubble wake. When the bubble rises in a straight path, however, the bubble is not effective for heat transfer. Even for the smallest bubble-wall distance, at which the bubble is attracted (and collides) toward the wall, the variation of Nusselt number is quite below 10%. We find that the linearly rising bubble wake exists inside the thermal-boundary layer, while the zigzagging bubble wake significantly disturbs it. Therefore, the main mechanism of the surface cooling is the disruption of a thermal-boundary layer with the entrained bulk liquid by the bubble wake.

Bubble dynamics and bubble-induced agitation in the homogeneous bubble-swarm past a circular cylinder at small to moderate void fractions

An investigation into how the presence of a circular cylinder disturbs a buoyancy-driven homogeneous bubble-swarm using a novel two-phase particle-image velocimetry is presented. Depending on the void fraction and ratio of bubble size to cylinder diameter, two regimes of bubble distribution in the cylinder wake are classified and corresponding mechanisms are explained. Also, the liquid-phase flow forms an interesting two-phase wake behind a cylinder. To analyze this, a novel model for bubble-induced agitation (Reynolds stress) that agrees well with the measurements is suggested.

Characterization of net coolant flow rate to copper boiling surfaces using two-phase particle image Velocimetry and dielectric fluid

Particle Image Velocimetry (PIV) has been widely used in fluid mechanics for various single and two-phase flow visualization studies. Using PIV, the net coolant flow rate required for quenching a copper circular heated surface under high heat flux pool boiling applications was quantified. The flow rate was calculated using a control surface analysis of all the periphery vectors surrounding the pool boiling activity. The vectors were deconstructed into their relevant Cartesian components using a custom Matlab code. The acquired data may be useful for empirical modeling of many practical high-performance electronic systems including thermosiphons and multi-component liquid immersion cooled servers. Boiling heat transfer fluxes ranging from 5.5 W/cm2 to 11 W/cm2 yielded quenching fluid flow rate requirements ranging from 1.7 g/s to 93.6 g/s. The highest heat flux tested, 11 W/cm2 is near the Critical Heat Flux (CHF) for the PF-5060 working fluid with a resulting maximum heat transfer coefficient of 8.8 kW/(m2K). With subcooled boiling conditions, it was found that as little as 2.5 °C reduction in pool temperature below the saturation point resulted in a 42% reduction in required coolant flow rate. A further reduction in the cooling flow rate requirement, 87% below the saturated pool condition, was found when the subcooling level was increased to 5 °C. When subcooling is introduced, transient conduction near the microlayer vapor/surface/liquid interface of bubble formation begins to be the dominant heat transfer mechanism.

Upward bubbly flows in a square pipe with a sudden expansion: Bubble dispersion and reattachment length

We investigate the bubble dynamics and subsequent changes in the liquid-phase flow characteristics of upward bubbly flows in a square pipe with a sudden expansion (expansion ratio of 4.0). The experiments are conducted under three liquid-phase Reynolds numbers of 0 (stationary), 420 (laminar) and 6000 (turbulent). The inlet volume void fraction ranges about 0−1.0%, and we use a high-speed two-phase particle image velocimetry and shadowgraphy to measure two phases simultaneously. It is observed that after the expansion, smaller bubbles tend to migrate toward the wall and larger bubbles rise in a core region, which is more encouraged by the steeper velocity gradient with increasing the Reynolds number. This bubble distribution is further analyzed by estimating the interfacial forces acting on rising bubbles. Affected by this bubble distribution, the enhanced turbulence in the inlet flow energizes the separating shear layer, resulting in the reduction of reattachment length behind the edge. Finally, we suggest an empirical relation for a two-phase flow reattachment length in terms of Reynolds number and mean void fraction, which explains the contribution of added dispersed phase on the mixing enhancement in the backstep flows.

Wake structures behind an oscillating bubble rising close to a vertical wall

In the present study, the wake structures behind an oscillating (zigzagging in a plane) air bubble, rising in a close vicinity of a vertical wall are experimentally investigated using a high-speed two-phase particle image velocimetry. While varying the distance between the rising bubble and the wall, the spatial and temporal variations in the spanwise and streamwise vorticity components contained in the wake vortices, in addition to the bubble trajectory, are measured in a tank filled with water. In particular, the Lagrangian streamwise vorticity fields in the bubble wake have been reconstructed and investigated in detail with different conditions. Without the wall, it is confirmed that there exist counter-rotating streamwise vortex tubes in the bubble wake, agreeing with the case of a two-dimensional zigzagging bubble, as reported in the literatures. It is also found that the hairpin vortex chain structures, initially attached to the bubble rear, evolve to detached vortex ring structures as the bubble rises in an oscillating path. While the detailed vortex structures show up quite differently from the reference case depending on the distance to the wall (e.g., actual bubble-wall collision), in general the wake behind the bubble as it moves toward and away from the wall can be summarized as: (i) transition to the detached vortex ring structures is accelerated; (ii) streamwise length of vortex tubes is shortened (evolution is interfered); (iii) counter-rotating vortex tubes approaching the wall tend to slightly bounce off and slide away (being dissipated fast) from each other on the wall; and (iv) boundary-layer like secondary flow structures are induced on the wall due to additional viscous effects. These wall-induced wake modification indicates that more fluid energy is wasted due to the wall interference, rather than being used to force the lateral movement of the bubble, which agrees with the reduced amplitude and wavelength of the oscillating bubble path on the wall. Finally, this explanation has been further confirmed by estimating the vortex-induced lateral forces acting on the bubble for each case.

Single- and Two-Phase Particle Image Velocimetry Characterization of Fluid Flow Within a Liquid Immersion Cooled Electronics Module

With the growth and acceptance of liquid immersion cooling as a viable thermal management technique for high performance microelectronics, fundamental questions regarding the nature of the flow within the system will need to be addressed. Among these are how the coolant is directed toward components of primary interest as well as how other elements within the electronics package may affect the delivery of fluid to these more critical locations. The proposed study seeks to experimentally address these issues with particle image velocimetry (PIV) measurements of unheated and heated flow within an electronics enclosure. The effectiveness of three flow distribution designs at delivering coolant to elements of importance, in this case 6.45 cm2 (1 in.2) components meant to simulate processor chips, has been examined using the vectors yielded by the PIV experimentation in a control surface analysis around these critical components. While these previous scenarios are unheated, two-phase PIV has also been conducted with FC-72 as the working fluid while boiling is taking place. A control surface analysis around all four heated elements within the enclosure shows an expected roughly monotonic increase in the net liquid flow rate to the boiling elements as the power applied to them is increased. Additionally, discretized mapping of how the fluid is entering the area surrounding these boiling elements has been constructed to offer insight into how passive elements should be placed within an electronics enclosure so as to not obstruct or hinder the vital flow of coolant to the most critical components.

Active fuel particles dispersion by synthetic jet in an entrained flow gasifier of biomass: Cold flow

Pulverized fuel (PF) burners play a key role for the performance of PF fired gasification and combustion plants, by minimizing pollutant emission, fuel consumption and hence fuel costs. However, fuel diversity in power generation plants imposes limitations on the performance of existing PF burners, especially when burning solid fuel particles with poor flowability like biomass sawdust. In the present study, a vertically downward laminar flow was laden with biomass particles at different particle mass loading ratios, ranging from 0.47 to 2.67. The particle laden flow was forced by a synthetic jet actuator over a range of forcing amplitudes, 0.35–1.1kPa. Pulverized pine particles with a sieve size range of 63–112μm were used as biomass feedstock. Two-phase particle image velocimetry was applied to measure the velocity of the particles and air flow at the same time. The results showed that the synthetic jet had a large influence on the flow fields of both air and powdered pine particles, via a convective effect induced by vortex rings that propagate in the flow direction. The particle velocity, particle dispersion and hence inter-particle distance increased with increasing forcing amplitude. Moreover, particles accumulated within a specific region of the flow, based on their size. The effect on particle dispersion was more pronounced in the forced flows with low mass loading ratios.

Cold flow experiments in an entrained flow gasification reactor with a swirl-stabilized pulverized biofuel burner

Short particle residence time in entrained flow gasifiers demands the use of pulverized fuel particles to promote mass and heat transfer, resulting high fuel conversion rate. The pulverized biomass particles have a wide range of aspect ratios which can exhibit different dispersion behavior than that of spherical particles in hot product gas flows. This results in spatial and temporal variations in temperature distribution, the composition and the concentration of syngas and soot yield. One way to control the particle dispersion is to impart a swirling motion to the carrier gas phase. This paper investigates the dispersion behavior of biomass fuel particles in swirling flows. A two-phase particle image velocimetry technique was applied to simultaneously measure particle and gas phase velocities in turbulent isothermal flows. Post-processed PIV images showed that a poly-dispersed behavior of biomass particles with a range of particle size of 112–160µm imposed a significant impact on the air flow pattern, causing air flow decelerated in a region of high particle concentration. Moreover, the velocity field, obtained from individually tracked biomass particles showed that the swirling motion of the carrier air flow gives arise a rapid spreading of the particles.

Study of bubble-induced turbulence in upward laminar bubbly pipe flows measured with a two-phase particle image velocimetry

In the present study, focusing on characterizing the bubble-induced agitation (turbulence), spatially varying flow statistics of gas and liquid phases in laminar upward bubbly flows (Reynolds number of 750) with varying mean void fraction are investigated using a two-phase high-speed particle image velocimetry. As the flow develops along the vertical direction, bubbles with small-to-moderate void fractions, which were intentionally distributed asymmetrically at the inlet, migrate fast and show symmetric distributions of wall or intermediate peaking. Meanwhile, the mean liquid velocity saturates relatively slowly to a flat distribution at the core region. Despite small void fractions considered, the bubbles generate a substantial turbulence, which increases with increasing mean void fraction. Interestingly, it is found that the mean vertical velocity, bubble-induced normal stress in radial direction, and Reynolds stress profiles match well with those of a single-phase turbulent flow at a moderate Reynolds number (e.g., 104), indicating the similarity between the bubble-induced turbulence and wall-shear-generated turbulence in a single-phase flow. Previously suggested scaling relations are confirmed such that the mean bubble rise velocity and bubble-induced normal stress (in both vertical and radial directions) scale with mean volume void fraction as a power of −0.1 and 0.4, respectively. Finally, based on the analysis of measured bubble dynamics (rise in an oscillating path), a theoretical model for two-phase turbulent (Reynolds) stress is proposed, which includes the contributions by the non-uniform distributions of local void fraction and relative bubble rise velocity, and is further validated with the present experimental data to show a good agreement with each other.