Bubble Flow Patterns Research Articles

CFD simulation of hydrodynamic behaviors and aerobic sludge granulation in a stirred tank with lower ratio of height to diameter

Flow patterns in stirred and aerated bioreactors are greatly influenced by the air flow rate and the stirrer’s rotation speed, which in turns, affects the granulation of aerobic sludge. A three-dimensional computational fluid dynamics (CFD) model was used to simulate the flow pattern under different combination of rotation speeds and superficial gas velocities (SGVs) in a lower ratio of height to diameter (H/D ratio) bioreactor with a plastic cuboid baffle agitator. Simulations showed that flow pattern transited from a single circulation cell to multiple vortical cells with increase of the rotation speed and SGV. Increase in rotation speed had a more significant contribution to gas holdup, liquid velocity, bubble size distribution, flow pattern and shear rate than that of SGV. A stirred and aerated SBR was established to perform aerobic granulation with rotation speed at 300 rpm and SGV at 1.05 cm/s, and the average shear rate of the bioreactor was investigated. The experiment results showed that aerobic granules were successfully formed with filamentous bacteria entangling on the surface of granules. The filamentous granular sludge had good settleability with a settling velocity at 19.6 ± 5.1 m/h, and its SVI value was between 30 and 90 mL/g. The diameter of the granules was up to 1.42 mm. The average shear rate of the bioreactor (12.62–27.52 1/s.) was mostly influenced by the rotation speed rather than the SGV.

Application of Electromagnetic Induction Technique to Measure the Void Fraction in Oil/Gas Two Phase Flow

In this work, electromagnetic induction technique of measuring void fraction in liquid/gas fuel flow was utilized. In order to improve the electric properties of liquid fuel, an iron oxide Fe3O4 nanoparticles at 3% was blended to enhance the liquid fuel magnetization. Experiments have been conducted for a wide range of liquid and gas superficial velocities. From the experimental results, it was realized that there is an existing linear relationship between the void fraction and the measured electromotive force, when induction coils were connected in series for excitation coils, regardless of increase or decrease CNG bubbles distribution in liquid fuel flow. Therefore, it was revealed that the utilized method yielded quite reasonable account for measuring the void fraction, showing good agreement with the other available measurement techniques in the two-phase flow, and also with the published literature of the bubbly flow pattern. From the results of the present investigation, it has been proven that the electromagnetic induction is a feasible technique for the actual measurement of void fraction in a Diesel/CNG fuel flow.

A study on the influence of the tube layout on sub-channel hydrodynamics in a bubble column with internals

In this work, the hydrodynamics of a bubble column with vertical heat exchanger internals in a narrow bubble column of Di=0.1m inner diameter with a clear liquid height of Lc=1.1m was comprehensively studied. We applied ultrafast X-ray tomography to obtain hydrodynamic parameters, such as gas holdup, bubble size distribution, bubble number flux and flow patterns at hitherto inaccessible positions within the sub-channels of the tube bundles. To investigate the influence of the tube bundle pattern, square and triangular pitches were considered. Tubes of do=8 and 13 mm outer diameter were installed to study the effect of tube size, while maintaining approx. Ac=25% coverage of the cross-sectional area, which is typical for, e.g., Fischer-Tropsch processes operated in bubble column reactors. The superficial gas velocity was varied from ug=2 to 20 cm s−1 to cover homogeneous and heterogeneous flow regimes. Internals’ type and tube diameter were found to crucially influence the gas holdup distribution across the column diameter, which is known to generate liquid circulation, to shape the gas velocity profile and to cause intensive bubble interactions. The higher flow resistance induced by triangular tube configurations and dense tube patterns, which is attributed to the smaller hydraulic diameter of the respective configurations, forces bubbles to preferably rise near the column wall. Within the tube bundle, the radial holdup profiles show a pronounced non-parabolic trend, indicating zones of reverse liquid flow directions between the internal tubes. Furthermore, the gas-liquid flow morphology within various sub-channels was analyzed revealing a slug-like flow formation at superficial gas velocities larger than 10 cm s−1.

Void fraction measurements of bubbly flow in a horizontal 90° bend using wire mesh sensors

Wire mesh sensors were used to investigate the void fraction distribution of two-phase gas-liquid bubbly flow in a 50.8 mm diameter horizontal test section containing a 90° bend. The 90° bend had a radius of curvature of 76.2 mm, and its inlet was installed at an axial distance of one hundred pipe diameters from the test section's inlet. Wire mesh sensors measurements were recorded at axial locations of 97.5, 112.8, and 130.8 pipe diameters from the air-water inlet, which included axial locations both upstream and downstream of the 90° bend. Deionised water and compressed air were used as the working fluids, with the bubbly flow regime achieved at superficial liquid velocities ranging from 3.50 m/s to 5.42 m/s and superficial gas velocities, referenced at atmospheric conditions, ranging from 0.10 to 1.25 m/s. The effects of the 90° bend, as well as superficial gas and liquid velocities, on the void fraction distribution were parametrically investigated. Extensive measurements of the area-averaged, time-averaged, and bulk-averaged void fraction are reported for a total of 27 individual test cases. Radial profiles of the time-averaged void fraction are presented for all test cases along both vertical and horizontal axes. Results demonstrated the effects of superficial liquid and gas velocities on phase distribution before and after the bend. Statistical techniques were applied to discretize a transitional bubbly-slug flow pattern into constituent small bubble and large bubble flow patterns.

Numerical study on bubble behavior in magnetic nanofluid used for waste heat recovery power generation concept

Electrical energy can be generated by the bubble motion inside the magnetic nanofluid under the influence of an external magnetic field. The relative movement of the magnetic particles dispersed in the magnetic fluid is induced through the movement of the bubbles rising by buoyancy force. This disturbs the external magnetic field associated with the generator coil, and electrical energy can be generated. The bubble movement in this complex physical environment was studied through 2D numerical analysis. Commercial magnetic fluids EFH1 and EFH3, manufactured by Ferrotec, were selected as the working fluid for the investigation. A level set method was used to analyze the 2-phase flow of bubbles motion in the magnetic fluid. The effect of magnetic particle concentration on the behavior of bubbles and the change of bubble flow patterns through interaction between bubbles were observed by analysis. In addition, the influence of the magnetic force caused by the external magnetic field on the behavior of the bubble was also investigated. The following results can be obtained through the analysis of this study. The high concentration of magnetic particles increases the viscosity and attenuates the rising velocity and the lateral oscillation of the bubbles. The interaction of the 2 bubbles depends on the initial relative distance. Merging occurs only between 2 bubbles within a certain initial distance, which maximizes disturbance of the surrounding magnetic fluid. The magnetic force exerted by the permanent magnets externally applied is relatively small in comparison with gravity. Therefore, the effect on the rise behavior of the bubble is not significant. In consideration of the overall external force and flow conditions, the pattern of the bubble flow that maximizes the efficiency in the present electric energy generation concept was found.

A combined CFD/visualized investigation of two-phase heat and mass transfer inside a horizontal loop thermosiphon

A horizontal two-phase loop thermosiphon (HLTS) which incorporates a U-turn section to prevent bidirectional flow was investigated in this study via flow visualization and three-dimensional computational fluid dynamics (CFD). The experimental device consisted of a glass-water HLTS which was tested under a range of filling ratios and operating conditions to visualize the two-phase flow in the loop. At the same time, a three-dimensional, volume of fluid (VOF) CFD model was developed to further elucidate the underlying heat and mass transfer regimes for this HLTS. It was found that by increases the filling ratio, the flow regime changes from smooth unidirectional flow to geyser oscillation flow. At a filling ratio of <70%, nucleate boiling and drop-wise condensation occur in the presence of clear unidirectional flow. At >83% filling ratios, geyser oscillation occurs, in which ascending/descending flow of the working fluid was repeatedly observed. During ascending flow, a plug flow pattern was observed in the riser and a bubble flow pattern was observed in the downcomer section. During descending flow, an annular flow pattern was observed in the riser, while plug-to-annular flow was observed in the downcomer section. The simulation results revealed these same two-phase flow regimes and predicted and measured temperatures and thermal resistances deviated by less than 9%. Overall, this valid CFD model can easily be extended to evaluate the heat and mass transfer mechanisms and performance of any geometrically similar HLTS systems across the range of filling ratios and flow regimes studied in this paper.

Bubble size and flow characteristics of bubbly flow downstream of a ventilated cylinder

Bubbly flows downstream of a ventilated cylinder in a water tunnel are experimentally studied. Emphasis is placed upon the relationship between bubble property and carrier flow parameters. Under no-ventilation condition, the pure-water wake flow is measured with particle image velocimetry technique. Bubbles are generated with ventilation and the bubbly flow is visualized using shadow image velocimetry. The separation and statistical treatment of bubbles in the captured images are accomplished with an in-house code. The influence of upstream flow velocity and air flow rate is examined. Sauter mean diameter of the bubbles and bubble velocity distribution are obtained. Instantaneous bubbly flow pattern is in accordance with the carrier flow characteristics. Across the high-vorticity region, bubbles experience a remarkable bubble size variation, large bubbles are annihilated. As for cross-sectional bubble size distribution, the tendency obtained with image processing agrees with the result obtained with the formula associating turbulent kinetic energy dissipation with bubble size. As upstream velocity increases, the percentage of small bubbles increases. Both bubble volume fraction and the most predominant bubble size increase with air flow rate. The percentage of large bubbles varies slightly with air flow rate.

Two-phase flow patterns for condensation of ethanol-water mixtures in triangular microchannels

An array of isosceles triangular microchannels with sidewalls inclined at 54.7° were fabricated by the wet etching method. The condensation of water and ethanol binary mixtures was visualized with additional measurements to investigate the condensation flow patterns in the silicon microchannels with hydraulic diameters of approximately 155μm. The visualization study indicated that the flow patterns were influenced by the inlet ethanol concentration and the mass flux. Annular, injection, droplet-injection and bubble flow patterns were observed along the channels. The condensation instability effects were stronger with increased equivalent surface free energy differences. The increasing equivalent surface free energy differences caused droplets to form in the microchannel that moved from the condensing surface to the top corner due to the Marangoni effect, which led to fluctuating flow patterns. The frequency of the injection flow increased with increasing Rev and input ethanol concentration. The mass flux and the steam quality during condensation were measured at various locations in the experiments. Two phase flow pattern maps were developed for various inlet ethanol concentrations.

Stream-wise distribution of skin-friction drag reduction on a flat plate with bubble injection

To investigate the stream-wise distribution of skin-friction drag reduction on a flat plate with bubble injection, both experiments and simulations of bubble drag reduction (BDR) have been conducted in this paper. Drag reductions at various flow speeds and air injection rates have been tested in cavitation tunnel experiments. Visualization of bubble flow pattern is implemented synchronously. The computational fluid dynamics (CFD) method, in the framework of Eulerian-Eulerian two fluid modeling, coupled with population balance model (PBM) is used to simulate the bubbly flow along the flat plate. A wide range of bubble sizes considering bubble breakup and coalescence is modeled based on experimental bubble distribution images. Drag and lift forces are fully modeled based on applicable closure models. Both predicted drag reductions and bubble distributions are in reasonable concordance with experimental results. Stream-wise distribution of BDR is revealed based on CFD-PBM numerical results. In particular, four distinct regions with different BDR characteristics are first identified and discussed in this study. Thresholds between regions are extracted and discussed. And it is highly necessary to fully understand the stream-wise distribution of BDR in order to establish a universal scaling law. Moreover, mechanism of stream-wise distribution of BDR is analysed based on the near-wall flow parameters. The local drag reduction is a direct result of near-wall max void fraction. And the near-wall velocity gradient modified by the presence of bubbles is considered as another important factor for bubble drag reduction.

Time-dependent limited penetrable visibility graph analysis of nonstationary time series

Recent years have witnessed the development of visibility graph theory, which allows us to analyze a time series from the perspective of complex network. We in this paper develop a novel time-dependent limited penetrable visibility graph (TDLPVG). Two examples using nonstationary time series from RR intervals and gas–liquid flows are provided to demonstrate the effectiveness of our approach. The results of the first example suggest that our TDLPVG method allows characterizing the time-varying behaviors and classifying heart states of healthy, congestive heart failure and atrial fibrillation from RR interval time series. For the second example, we infer TDLPVGs from gas–liquid flow signals and interestingly find that the deviation of node degree of TDLPVGs enables to effectively uncover the time-varying dynamical flow behaviors of gas–liquid slug and bubble flow patterns. All these results render our TDLPVG method particularly powerful for characterizing the time-varying features underlying realistic complex systems from time series.

Visualization and pressure distribution of gas-liquid flow in ¾ inch plexyglass pipe diameter with the slope orientation upward, horizontal and downward

The flowing phase in the pipe is affecting on the continuity of fluid transport process, this is due to the interaction when the fluid mixture flowing at a certain superficial velocity. Therefore it will be studied the distribution of pressure and flow pattern of a transparent pipe (plexiglass) ¾ inches, and its influence on the orientation of the pipe 10°, 5°, 0°, -5o and -10° by the experimental equipment. The research was carried out experimentally with the water flow 10 - 20 lpm and air flow 5 - 15 lpm. Results showed the flow pattern was produced on every slope of the pipe is a bubble, elongated bubble, plug, plug-slug transition, plug bubble, bubble slug, slug, stratified wavy and stratified with mixture interfaces. On the upward slope of the bubble flow patterns likely to appear more and result the highest pressure of 1690 N/m2 on Jmix = 9.3 m/s, on the horizontal position bubble slug flow pattern that often appears with the highest pressure drop of 593 N/m 2 on Jmix = 8.23 m/s, while the downward slope of the wavy stratified flow pattern likely to appear more with the highest pressure drop of 685 N/m 2 on Jmix = 5.88 m/s.

Research on the Conductivity-Based Detection Principles of Bubbles in Two-Phase Flows and the Design of a Bubble Sensor for CBM Wells.

The parameters of gas-liquid two-phase flow bubbles in field coalbed methane (CBM) wells are of great significance for analyzing coalbed methane output, judging faults in CBM wells, and developing gas drainage and extraction processes, which stimulates an urgent need for detecting bubble parameters for CBM wells in the field. However, existing bubble detectors cannot meet the requirements of the working environments of CBM wells. Therefore, this paper reports findings on the principles of measuring the flow pattern, velocity, and volume of two-phase flow bubbles based on conductivity, from which a new bubble sensor was designed. The structural parameters and other parameters of the sensor were then computed, the “water film phenomenon” produced by the sensor was analyzed, and the appropriate materials for making the sensor were tested and selected. After the sensor was successfully devised, laboratory tests and field tests were performed, and the test results indicated that the sensor was highly reliable and could detect the flow patterns of two-phase flows, as well as the quantities, velocities, and volumes of bubbles. With a velocity measurement error of ±5% and a volume measurement error of ±7%, the sensor can meet the requirements of field use. Finally, the characteristics and deficiencies of the bubble sensor are summarized based on an analysis of the measurement errors and a comparison of existing bubble-measuring devices and the designed sensor.

Experimentally‐based constitutive relations for co‐current gas‐liquid flow in randomly packed beds

Experimental Observations on average pulse velocity and frequency in concurrent gas‐liquid (down) flow through randomly packed beds are used to extract constitutive relations for the gas‐liquid interaction and mean curvature terms that appear in a recently proposed volume‐averaged two‐fluid model for bubbly flow. The proposed closures lead to a reasonably quantitative prediction of the average pressure drop and liquid saturation under bubbly flow conditions and in the near pulse regime. In addition, the proposed closures provide realistic estimates for the location of the bubble‐to‐pulse transition in microgravity and in 1g down‐flow and predict the disappearance of the bubbly flow pattern at low liquid fluxes in 1g down‐flow. © 2016 American Institute of Chemical Engineers AIChE J, 63: 812–822, 2017

Experimental investigation on submerged gas-liquid mixture injection into water through a micro-channel

Submerged gas-liquid mixture injection into water through a micro-channel shows unique fluid characteristics by various flow regimes in the micro-channel generating special jetting phenomena which have never been systematically studied. We injected air-water mixtures into water through submerged 1.0 and 1.5mm diameter micro-channels. Bubbly, Taylor, bubble-train Taylor, churn and Taylor-annular flows were observed, and generated respective jetting flow regimes. Overall, two flow regimes namely “unstretched bubbles” and “stretched bubbles” were identified in the jet region. The former consists of three flow patterns, namely discrete bubbles, gathered bubbles and grown irregular bubbles. The latter occured when liquid slugs having strong enough momentum to stretch the bubble interface, from which single-bubble stretched, twin-bubbles stretched and even bubble rupture were further identified. We compared flow regime maps in micro-channels with those of submerged jets, and proposed the correspondence relationship, whose universality was confirmed by the consistency for different sized micro-channels. Transitions between two overall flow regimes of submerged jets, as well as flow patterns of unstretched bubbles were found determined by flow regime transitions in the micro-channel. Besides, transition laws between flow patterns of stretched bubbles were affected by the uniformity of slug length throughout the micro-channel, which were quantitatively analyzed.

Analytical Model With Nonadiabatic Effects for Pressure Drop and Heat Transfer During Boiling and Condensation Flows in Conventional Channels and Minichannels

In this paper a method developed earlier by the authors is applied to calculations of pressure drop and heat transfer coefficient for boiling flow and condensation flow with account of nonadiabatic effects for some recent data collected from the literature. The first effect, the modification of interface shear stresses in an annular flow pattern, is considered through incorporation of the so-called “blowing parameter.” The mechanism of modification of shear stresses at the vapor–liquid interface for such case is presented in detail in the paper. In the case of annular flow it contributes to thickening and thinning of the liquid film, which correspond to condensation and boiling, respectively. There is also another influence of the wall heat flux, where it is influencing the bubble nucleation in the case of the bubbly flow pattern. As a result, a modified general form of the two-phase flow multiplier, applicable both to boiling flow and condensation flow, is obtained, in which the nonadiabatic effects are clearly pronounced. The obtained model of the two-phase multiplier, incorporating the nonadiabatic effects, is additionally used in predictions of the heat transfer coefficient.

Confined bubble growth and heat transfer characteristics during flow boiling in microchannel

Bubble behaviors are closely related to the heat transfer performance during flow boiling in microchannel, however, the effect of channel cross-section decreasing on the bubble growth is still not fully understood at present. In this work, an experimental investigation is conducted to investigate the bubble growth characteristics during flow boiling in a single microchannel with 0.5mm×1mm rectangular cross-section, and the heat transfer performance of flow boiling and its influencing factors are studied. Experiments are conducted with subcooled deionized water and the bubble behaviors are visualized by a high speed CCD camera installed upon the test section. Depending on the heat flux, different growth features are observed in the bubble growth process. Two kinds of bubble growth model are identified: the power law model in initial growth period and the linear law model in later period. The confinement effect of the microchannel is deemed as the mechanism causing the alteration of bubble growth models during its growth process. The deformation features of confined bubble are discussed to illustrate the intensification of evaporation on the liquid–vapor (LV) interface at bubble root, which increases the growth rate of bubble in its confined growth period as well as the heat transfer capability of bubble. Therefore, the maximum local heat transfer coefficient along the channel is found in the region where confined bubble and/or short elongated bubble flow pattern are dominant. Moreover, the heat flux is found to have great influence on the overall heat transfer performance of flow boiling in microchannel, but the effect of mass flux is much less.

New dimensionless number for gas–liquid flow in pipes

Two-phase flow modeling is a general problem in science and engineering. Two-phase flow phenomenon is inherently complicated and characterized by a large number of flow variables. It is historically known that the lack of proper dimensionless numbers in two-phase flow is one of the major shortcomings as compared to single-phase flow. A new dimensionless number (Slippage Number) for gas–liquid flow in pipes is proposed in this paper. The number is defined as the ratio of the difference in the gravitational forces between slip and no-slip conditions to the inertial force of the gas. It is found to be a function of Froude number based on the mixture velocity especially in the elongated bubble, slug, churn, bubble, and high film thickness wavy annular flow patterns. The liquid holdup data for a wide range of fluid and flow conditions (different viscosities, densities, pipe diameters, inclination angles, gas and liquid flow rates) can be correlated with a single curve using the Slippage Number. The value of the number varies from highest to lowest for bubble, elongated bubble, slug, churn, stratified and annular flow patterns, respectively. It is close to zero for homogeneous flow patterns like mist and dispersed bubble flows. We also show that this number may be used as a flow pattern identifier.

Effect of soluble and insoluble gas bubbling methods on ultrafiltration fouling control

Fouling phenomenon is well known as a main obstacle in membrane separation technology. In this study, the effects of bubbling with various hydrodynamic factors and gas–liquid solubility were evaluated on fouling control and permeation flux in ultrafiltration of skimmed milk solution. Direct gas injection and carbonated feed, as a new bubbling method, were used for bubbling. In the direct gas injection technique, various two-phase flow patterns (slug and bubble), gas flow rates, and bubbling modes (continuous and intermittent) were investigated during a cross-flow ultrafiltration. The results showed that the both of gas bubbling methods improved the permeation flux during 30-min filtration. The permeation flux was enhanced up to 72 and 40% by direct injecting of N2 and CO2, respectively, while it was only enhanced up to 58% with carbonated feed. The evaluation of hydrodynamic resistance of membranes indicated that the gas bubbling by carbonated feed was more effective on the fouling resistance, while the cake resistance was affected by the gas injection during infiltration. In addition, the slug flow pattern was more effective than the bubble flow pattern on decline of membrane fouling. In the slug flow pattern, the permeate flux increased when medium flow rate was applied. Furthermore, ultrafiltration performance was improved using the intermittent gas bubbling mode. This study indicated that slug flow pattern with insoluble gas has a higher performance than the other ones in preventing fouling and flux enhancement during ultrafiltration processing.

Visualization study of gas–liquid two-phase flow patterns inside a three-stage rotodynamic multiphase pump

Visualization experiments were conducted to investigate (under different operating conditions) the gas–liquid two-phase flow patterns at the entry section and the impellers inside a three-stage rotodynamic multiphase pump designed by the authors. A mixture of water and air was selected as the medium. The high-speed photography technology was used to investigate the flow field. The flow pattern presented at the entry section was observed to be bubbly flow. The average diameter of the bubbles increased with the increasing inlet gas volume fraction (IGVF) but decreased with the increasing rotational speed. The flow patterns in the impellers could be classified into four categories with the increasing IGVF. For the tiny IGVF, isolated bubbles flow emerged in all the three impellers. For small values of the IGVF, no obvious gas pocket was observed and the bubbly flow was the main flow pattern in three stages. As the IGVF increased, the gas pocket first appeared in the first stage, while the bubbly flow pattern remained as the main flow pattern in the second and third stages. With further increasing IGVF, the three stages all exhibited as gas pocket flow. The gas pocket originally formed at the suction surface of the blades and then moved to the pressure surface as the gas pocket flowed downstream with the fluid. In addition, some bubbles moved upstream leave the diffuser entrance along the axial direction. Segregated gas flow was observed as the final catagory with the increasing IGVF. The flow pattern maps of the gas–liquid two-phase flow were drawn under different conditions, the ranges of flow patterns extended in the direction of high IGVF with the increasing rotational speed and shrank in the direction of low IGVF with the increasing liquid volume flow rate. With continued increases in the IGVF, the differential pressure of the pump decreased sharply in the range of the gas pocket flow and then decreased smoothly in the range of segregated gas flow.

Capacitance wire mesh imaging of bubbly flows for offshore treatment applications

The impact of ship motion on bubbly flow was emulated using a swell simulator to expose flow structure changes emerging in bubble columns relevant to offshore floating applications. Roll, roll+pitch, yaw, heave and sway were implemented at various frequencies and changes in bubbly flow resulting from the imposed motions were monitored for the first time by means of a dual capacitance wire mesh sensor to measure local gas holdup and velocity. Visualizations of the two-phase flow revealed that roll, roll+pitch, and high-frequency sway were the most impactful in terms of bubble zigzag and swirl, and bubble-clustering and segregation due to vessel dynamic inclinations. As a consequence of these motions, lateral migration of bubbles and their clustering enhanced liquid recirculation and local streamwise gas velocity. Compared to static vertical bubble column, bubbly flow pattern was barely altered by yaw and low-frequency sway except the heave displacements which tended to slowdown the bubble rise.