Air-water Bubbly Flow Research Articles

Bubble identification from images with machine learning methods

An automated and reliable processing of bubbly flow images is highly needed to analyse large data sets of comprehensive experimental series. A particular difficulty arises due to overlapping bubble projections in recorded images, which highly complicates the identification of individual bubbles. Recent approaches focus on the use of deep learning algorithms for this task and have already proven the high potential of such techniques. The main difficulties are the capability to handle different image conditions, higher gas volume fractions and a proper reconstruction of the hidden segment of a partly occluded bubble. In the present work, we try to tackle these points by testing three different methods based on Convolutional Neural Networks (CNN's) for the two former and two individual approaches that can be used subsequently to address the latter. Our focus is hereby on spherical, ellipsoidal and wobbling bubbles, which are typically encountered in air-water bubbly flows. To validate our methodology, we created test data sets with synthetic images that further demonstrate the capabilities as well as limitations of our combined approach. The generated data, code and trained models are made accessible to facilitate the use as well as further developments in the research field of bubble recognition in experimental images.

Air-Water Bubbly Flow by Multiple Vents on a Hydrofoil in a Steady Free-Stream

Flow features, due to air injection through multiple vents on the surface of a hydrofoil inclined at an angle with respect to the free-stream in a cavitation tunnel, are presented here. The hydrofoil, with a chord length, c, is oriented at the angle of inclination, α = 3.5°. The Froude number, Fn, based on the free-stream velocity, V∞, and air injection vent diameter, dh, is 30.30, 50.51 and 70.71. Air is injected through multiple vents on the hydrofoil at the non-dimensional air injection coefficient, Cq∼16−8917. The air bubble packing per unit area is quantified using spatial density, SD, at various combinations of Fn,Cq based on a high-speed video from the side view. The time-averaged spatial density, <SD>, is observed to increase in a logarithmic manner with an increase in the air injection rate, Q, at various Froude numbers. There is an increase in the mean spatial density of the bubbles with the increase in Cq at all Fn. A power–law relation is shown to exist between the time-averaged spatial density, <SD>, and the non-dimensional flow variables, Reynolds number, Reair, Fn and Cq based on a regression analysis. By tracking individual finite volume bubbles flowing with the free-stream, the bubble dimensions in pixels are quantified using quantities such as the deformation rate, ϵ, and standardization, ϵS, from the side-view videos. It is observed that ϵ and ϵS change with time, even as they become advected with the free-stream. Through high-speed imaging from the top view, we characterize the bubbly flow features’ time-averaged thickness, t, at various combinations of Fn,Cq at α = 3.5°. We obtain a power-law relation between the non-dimensional time-averaged jet thickness, t/c, and the non-dimensional flow parameters such as, Reair, Fn,Cq and the non-dimensional streamwise distance, x/xref, based on a regression analysis, where xref is a reference distance. The results are relevant to engineering applications where the air–water bubbly flow in a free-stream is important.

Development of Bubble Size Correlation for Adiabatic Forced Convective Bubbly Flow in Low Pressure Condition Using CFD Code

In a multidimensional two-phase flow analysis, bubble size significantly affects interfacial transfer terms such as mass, momentum, and energy. With regard to bubbly flow, the application of a simple correlation-type bubble size model presents certain advantages, including short calculation times and ease of usage. In this study, we propose a semi-theoretical correlation developed from a steady state bubble number density transport equation for predicting the distribution of local bubble size using a computational fluid dynamics (CFD) code. The coefficients of the new correlation were determined using the local bubble parameters obtained on the basis of three existing vertical air-water experiments. Finally, these were implemented in commercial CFD code and evaluated against experimental data, which showed that the proposed correlation exhibits good prediction capability for forced convective air-water bubbly flows under low pressure conditions.

The Fluidization Backwash Method of Filter Beds by Air-water Bubbly Flow

The Fluidization Backwash Method of Filter Beds by Air-water Bubbly Flow

Experimental Investigation of a Submerged Air-Water Bubbly Flow Jet in a Water Tank as a Model of Steel Casting

Experimental Investigation of a Submerged Air-Water Bubbly Flow Jet in a Water Tank as a Model of Steel Casting

Development and evaluation of data-driven modeling for bubble size in turbulent air-water bubbly flows using artificial multi-layer neural networks

In the present study, we consider a new reliable model of the bubble size based on multi-layer artificial neural networks (ANN). A multi-layer ANN is used to establish a function for the bubble size without any assumption on the form. In the training procedure, the proposed ANN is trained using data sets collected from open literature and experiments performed in the present study. An excellent agreement was obtained between the trained ANN and experimental data in the bubble size. Also, sensitivity analyses along with principal component analysis and random forest method provide important physical parameters for the bubble size. Next, in order to rigorously evaluate the prediction capability of the present model, flow simulations were conducted for turbulent bubbly flows, for which experimental data are available. The present validation results show that a regime-adaptive data-driven model for the bubble size achieves successful estimation for both wall and core peaking regimes.

Image processing techniques for the measurement of two-phase bubbly pipe flows using particle image and tracking velocimetry (PIV/PTV)

Particle Image Velocimetry (PIV) measurement in gas-liquid bubbly flows is a challenging task, mainly due to the dispersion of the laser light caused by the gas-liquid interfaces. A common solution adopted is the use of fluorescent seeding particles associated with a bandpass filter for the laser light. Therefore, the camera captures only the light fluoresced by the seeding particles, filtering the laser light dispersed by the gas-liquid interfaces and measuring a velocity field which corresponds to the liquid phase, seeded with the particles. However, even for relatively low gas fractions, the fluoresced light reflected by the interfaces distorts the measurements, as it illuminates particles out from the laser plane. In addition, the fluoresced light also reflects at the interfaces, distorting the measurements and trending to overshoot the measured liquid velocities In this work, PIV measurements of air-water bubbly flows in a small diameter pipe (D = 26.2 mm) are performed with fluorescent tracer particles (PIV/LIF). A new method for the phase discrimination in PIV was developed to overcome the problems caused by the presence of bubbles in the flow, which uses the pixels intensity information of each interrogation window, to identify if that window corresponds to liquid or bubble region. To validate the PIV procedure, a Particle Tracking Velocimetry (PTV) algorithm was also developed to measure the bubbles velocities, based on similar implementations found in literature, but some corrections were also proposed to overcome the bubble overlap phenomena which arises in bubbly pipe flows, when backward illumination is used. The PIV and the PTV methods were tested and validated by a series of distinct gas void fraction and bubble diameters experimental cases, confirming the accuracy and reliability of both methods. The proposed method was used to analyze a set of upward laminar and turbulent bubbly flows, showing that the averaged axial velocity and the dispersed gas bubbles modify the turbulence intensity profiles.

Interfacial area transport models for horizontal air-water bubbly flow in different pipe sizes

The current work seeks to develop the interfacial area transport models for horizontal bubbly flow applicable to various pipe sizes. A steady-state one-dimensional, one-group interfacial area transport equation (IATE) for horizontal adiabatic air-water bubbly flow is presented. The models discussed include: (a) frictional pressure drop prediction based on Lockhart–Martinelli approach; (b) drift-flux based closure relations for void-weighted bubble velocity and void fraction; (c) bubble interaction mechanisms; and (d) local void fraction reconstruction to calculate covariance parameters associated with asymmetric bubble distribution in horizontal bubbly flow. To develop these models, experiments are performed to establish local databases in bubbly flow using the four-sensor conductivity probe in horizontal test facilities with three different inner diameters (38.1 mm, 50.8 mm, and 101.6 mm). In total, 23 test conditions with superficial liquid and gas velocities in the ranges of 3.00–6.00 m/s and 0.08–1.00 m/s, respectively, are employed in the current work. The range of void fraction for the established database is 0.017–0.193. It is demonstrated that, the void weighted bubbly velocity (<<vg>>), void fraction (<α>), and frictional pressure loss can be predicted very well with an averaged percent difference of ±4.8%, ±4.7%, and ±2.1%, respectively. The covariance parameters can be predicted generally within ±30% using the current approach for void fraction reconstruction. As a result, the IATE can predict the measured interfacial area concentration with an averaged percent difference of ±5.9%, except for the test conditions with large bubbles generated as flow develops. Meanwhile, it is found that decreasing liquid velocity decreases the contributions to the total change of interfacial area concentration by random collision induced bubble coalescence, and turbulent impact induced bubble breakup.

On the Partition Method of Frictional Pressure Drop for Dispersed Two-Phase Flows in the RELAP5/MOD3, TRACE V5, and SPACE Codes

Frictional pressure drop (also called wall drag) for a two-phase flow has been investigated for several decades. However, the two-phase frictional pressure drop models in the state-of-the-art thermal-hydraulic system codes are significantly different from each other, especially in the way to partition the wall friction force of liquid and vapor phases in the two-fluid momentum equations. This may lead to unphysical results in some flow conditions.In this technical note, the two-phase wall frictional pressure drop models in the RELAP5/MOD3, TRACE V5, and SPACE codes are discussed in terms of the wall friction partition into the liquid and vapor momentum equations. To show the effect of different partition methods in the three codes, we simulated air-water bubbly flows in a horizontal pipe. The results of the calculations show that the partition method has a direct effect on the relative velocity of the two phases, and it may lead to unphysical behaviors of dispersed bubbles and droplets. It is strongly recommended to revisit the two-fluid formulation and the partition method of two-phase wall drag in the state-of-the-art system codes.

Experimental study of bubbly flow using image processing techniques

This paper presents an experimental study of bubbly flows at relatively high void fractions using an advanced image processing method. Bubble overlapping is a common problem in such flows and the past studies often treat the overlapping bubbles as a whole, which introduces considerable measurement uncertainties. In this study, a hybrid method combining intersection point detection and watershed segmentation is used to separate the overlapping bubbles. In order to reconstruct bubbles from separated segments, a systematic procedure is developed which can preserve more features captured in the raw image compared to the simple ellipse fitting method. The distributions of void fraction, interfacial area concentration, number density and velocity are obtained from the extracted bubble information. High-speed images of air-water bubbly flows are acquired and processed for eight test runs conducted in a 30mm×10mm rectangular channel. The developed image processing scheme can effectively separate overlapping bubbles and the results compare well with the measurements by the gas flow meter and double-sensor conductivity probe. The development of flows in transverse and mainstream directions are analyzed and compared with the prediction made by the one-dimensional interfacial area transport equation (IATE) and the bubble number density transport equation.

Flow-Accelerated Corrosion in Pipe Wall Downstream of Orifice for Water and Air-Water Bubble Flows

An orifice is used widely as a flow meter or a contraction device in pipeline systems in hydro-power plants, thermal power plants, and chemical plants because of its simple construction, high reliability, and low cost. However, it is well known that flow-accelerated corrosion (FAC) occurs on the pipe wall downstream of the orifice. Some of the authors have examined FAC through experimental and numerical analyses and have reported that one of the major governing parameters of FAC for single-phase water flow is the pressure fluctuation p’ on the pipe wall, and also that pipe wall thinning rate TR can be estimated by p’. In addition, they have presented the effects of the ori-fice geometry on p’ or TR, and have described a method for suppressing p’ or TR. In the present study, FAC for a two-phase air-water bubble flow is examined and compared with the single-phase water flow experimentally. Further, it is shown that because p’ is also considered a governing parameter of FAC for a two-phase air-water bubble flow, TR can be estimated using p’. It is also indicated that, by using a downstream pipe with a smaller diameter than that of the upstream pipe, p’ or TR can be suppressed.

Flow and heat transfer of petal-shaped double tube (Water and air-water bubbly flow)

Flow and heat transfer of petal-shaped double tube (Water and air-water bubbly flow)

Interphase force analysis for air-water bubbly flow in a multiphase rotodynamic pump

Purpose – Interphase forces between the gas and liquid phases determine many phenomena in bubbly flow. For the interphase forces in a multiphase rotodynamic pump, the magnitude analysis was carried out within the framework of two-fluid model. The purpose of this paper is to clarify the relative importance of various interphase forces on the mixed transport process, and the findings herein will be a base for the future study on the mechanism of the gas blockage phenomenon, which is the most challenging issue for such pumps. Design/methodology/approach – Four types of interphase forces, i.e. drag force, lift force, virtual mass force and turbulent dispersion force (TDF) were taken into account. By comparing with the experiment in the respect of the head performance, the effectiveness of the numerical model was validated. In conditions of different inlet gas void fractions, bubble diameters and rotational speeds, the magnitude analyses were made for the interphase forces. Findings – The results demonstrate that the TDF can be neglected in the running of the multiphase rotodynamic pump; the drag force is dominant in the impeller region and the outlet extended region. The sensitivity analyses of the bubble diameter and the rotational speed were also performed. It is found that larger bubble size is accompanied by smaller predicted drag but larger predicted lift and virtual mass, while the increase of the rotational speed can raise all the interphase forces mentioned above. Originality/value – This paper has revealed the magnitude information and the relative importance of the interphase forces in a multiphase rotodynamic pump.

Buoyancy effects in steeply inclined air-water bubbly shear flow in a rectangular channel

We report measurements of two-dimensional (B/D = 5) fully turbulent and developed duct flows (overall length/depth, L/D = 60; D-based Reynolds number Re > 104) for inclinations to 30° from vertical at low voidages (< 5 % sectional average) representative of disperse regime using tap water bubbles (4–6 mm) and smaller bubbles (2 mm) stabilised in ionic solution. Pitot and static probe instrumentation, primitive but validated, provided adequate (10 % local value) discrimination of main aspects of the mean velocity and voidage profiles at representative streamwise station i.e L/D = 40. Our results can be divided into three categories of behaviour. For vertical flow (0°) the evidence is inconclusive as to whether bubbles are preferentially trapped within the wall-layer as found in some, may be most earlier experimental works. Thus, the 4-mm bubbles showed indication of voidage retention but the 2-mm bubbles did not. For nearly vertical flow (5°) there was pronounced profiling of voidage especially with 4-mm bubbles but the transverse transport was not suppressed sufficiently to induce any obvious layering. In this context, we also refer to similarities with previous work on one-phase vertical and nearly vertical mixed convection flows displaying buoyancy inhibited mean shear turbulence. However, with inclined flow (10+ degrees) a distinctively layered pattern was invariably manifested in which voidage confinement increased with increasing inclination. In this paper we address flow behavior at near vertical conditions. Eulerian, mixed and VOF models were used to compute voidage and mean velocity profiles.

Phase Distribution Characteristics of Bubbly Flow in Mini Pipes Under Normal and Microgravity Conditions

The axial development of the void fraction, interfacial area concentration and Sauter mean bubble diameter profiles of adiabatic air-water bubbly flows in 5.0 and 3.0 mm-diameter pipes were measured using a stereo image processing method under two gravity conditions, vertical upward (normal gravity) and microgravity. The flow measurements were performed at four axial locations. The axial distances from the pipe inlet (z) normalized by the pipe diameter (D) were z/D = 5.5, 34, 72 and 110 for 5.0 mm-diameter pipe and z/D = 15, 62, 120 and 188 for 3.0 mm-diameter pipe. Data were collected for superficial gas and liquid velocities respectively in the ranges of 0.00434–0.0500 m/s and 0.205–0.754 m/s. The effect of gravity on the radial distribution of bubbles and the axial development of two-phase flow parameters is discussed in detail, based on the obtained database. The phase distributions in pipe cross-sections were classified into 3 basic patterns: core peak, intermediate peak and wall peak distributions, based on two normalized parameters: a normalized void peak position and a normalized void peak intensity. Phase distribution pattern maps under normal and microgravity conditions were generated for bubbly flows in 5.0 and 3.0 mm-diameter pipes. The data obtained in the current experiment are expected to contribute to the benchmarking of CFD simulation of void fraction and interfacial area concentration distribution patterns in forced convective pipe flow under microgravity conditions.

Effects of 90-deg Vertical Elbows on the Distribution of Local Two-Phase Flow Parameters

The present study focuses on developing a database to investigate the effects of 90-deg vertical elbows on the transport and distribution of local two-phase flow parameters in air-water bubbly flows. The experimental facility consists of both vertical and horizontal sections made out of 50.8-mm inner diameter pipes and interconnected via 90-deg glass elbows. Six different flow conditions within or near the bubbly flow regime at the inlet are investigated in the current study. A multisensor conductivity probe is employed to measure detailed local two-phase flow parameters at ten axial locations along the test section, within which 90-deg elbows are installed at L/D = 63 and 244.7 from the inlet. The data show that the elbow makes a significant impact on the two-phase pressure drop, bubble distribution, and bubble velocity. The bubbles moving across the vertical-upward elbow are entrained along the secondary flow streamlines leading to a bimodal distribution. For the test conditions investigated in the present study, this bimodal distribution is independent of the bubble distribution upstream of the vertical-upward elbow. In the case of the vertical-downward elbow, on the other hand, the large inertia of the axial liquid flow results in the bubbles migrating toward the inside of the elbow curvature.

Liquid Turbulence Kinetic Energy Budget of Co-Current Bubbly Flow in a Large Diameter Vertical Pipe

The liquid turbulence kinetic energy transfer between the liquid and gas phases was investigated for upward air-water bubbly flow in a 200 mm diameter pipe. The liquid and gas axial momentum equations were analyzed to estimate the interfacial drag from experimental measurements, and hence the liquid turbulence production due to the relative velocity of the bubbles. The liquid turbulence production due to the bubbles was significantly higher than that due to the liquid shear. The liquid turbulence kinetic energy budget indicates that the turbulence production due to the bubbles is approximately balanced by the viscous dissipation, estimated assuming an isotropic turbulence structure, with negligible dissipation due to the bubbles. The liquid turbulence kinetic energy spectra showed an addition of energy at length scales in the range corresponding to the bubble diameter. A model for the turbulence energy production spectra due to the bubbles is proposed and used to investigate the spectral turbulence energy budget. The model indicates that when there is a liquid turbulence augmentation, most of the production occurs in the low wave number range with only a small overlap with the viscous dissipation region. In the case of a turbulence suppression, most of the bubble production occurs in the same wave number range as the viscous dissipation.

On the prediction of the phase distribution of bubbly flow in a horizontal pipe

Horizontal bubbly flow is widely encountered in various industrial systems because of its ability to provide large interfacial areas for heat and mass transfer. Nonetheless, this particular flow orientation has received less attention when compared to vertical bubbly flow. Owing to the strong influence due to buoyancy, the migration of dispersed bubbles towards the top wall of the horizontal pipe generally causes a highly asymmetrical internal phase distributions, which are not experienced in vertical bubbly flow. In this study, the internal phase distribution of air-water bubbly flow in a long horizontal pipe with an inner diameter of 50.3 mm has been predicted using the population balance model based on direct quadrature method of moments (DQMOM) and multiple-size group (MUSIG) model. The predicted local radial distributions of gas void fraction, liquid velocity and interfacial area concentration have been validated against the experimental data of Kocamustafaogullari and Huang (1994). In general, satisfactory agreements between predicted and measured results were achieved. The numerical results indicated that the gas void fraction and interfacial area concentration have a unique internal structure with a prevailing maximum peak near the top wall of the pipe due to buoyancy effect.

Comparison of Local Interfacial Structures Around 90 and 45-Degree Elbows in Horizontal Bubbly Flows

This study presents a comparison of the geometric effects of 90 deg and 45 deg elbows in horizontal two-phase air-water bubbly flow. Two separate experiments were performed in the horizontal test section made out of 50.3 mm inner diameter glass tubes. The first set of data was collected with a 90 deg elbow installed, and then a 45 deg elbow was added to the existing facility to acquire the second set of data. A total of 15 different flow conditions, all within the bubbly flow regime, were identified for the 90 deg experiment, and very similar flow conditions were extended to the 45 deg experiment. A double-sensor conductivity probe was employed to acquire the local data at seven different axial positions along the test section, out of which four measurement locations are associated with the 90 deg experiment and three with the 45 deg experiment. The data show that the elbows have a significant effect on the development of interfacial structures as well as the bubble interaction mechanisms. Furthermore, there are characteristic similarities and differences between the effects of the two elbows. While the effect of the 45 deg elbow is evident immediately after the elbow, the 90 deg elbow effect tends to propagate further downstream of the elbow rather than immediately after the elbow. Moreover, it is shown that both elbows induce spatial oscillations in the interfacial structures and two-phase flow parameters, but the degree and the nature of oscillations differ. The effects of the elbows are also compared for the axial transport of the two-phase flow parameters.

Interfacial Area Transport in Horizontal Bubbly Flow with 90-deg Elbow

The present study develops an interfacial area transport equation applicable to an air-water horizontal bubbly flow with a flow restriction. The experiments are performed in a round glass pipe of 50.3-mm inner diameter, along which a 90-deg elbow is installed at L/D = 206.6 from the two-phase mixture inlet. In total, 15 different flow conditions in the bubbly flow regime are studied. The detailed local two-phase flow parameters are acquired by a double-sensor conductivity probe at four different axial locations. The effect of the elbow is evident in the distribution of local parameters as well as in the development of interfacial structures. The elbow clearly promotes bubble interactions resulting in significant changes in both the void fraction and interfacial area concentration. In the present study, the elbow is found to promote the coalescence mechanism while reducing the disintegration mechanism. These geometric effects are also reflected in the axial development of one-dimensional two-phase flow parameters. In the present analysis, the interfacial area transport equation is developed in one-dimensional form via area-averaging based on the existing model for vertical flow. In the averaging process, characteristic nonuniform distributions of the two-phase flow parameters in horizontal two-phase flow are treated mathematically by covariance calculations. Furthermore, the change in pressure due to the minor loss of the elbow is taken into consideration by using a newly developed correlation analogous to Lockhart and Martinelli’s. In total, 60 area-averaged data points are employed to benchmark the present model. The present model predicts the data well with an average percent difference of approximately ±10%.