Void Fraction Fluctuations Research Articles

Vertically downward gas-liquid flow: Void fraction and pressure drop

Using an air-water mixture and 34 mm ID pipe, a series of pressure drop and void fraction measurements were collected. The observed flow regimes were falling film, annular, bubbly, cap bubble, slug, and churn flows. The temporal fluctuations of void fraction and pressure drop measurements were firstly studied using signals visualization and PDF. It was revealed that both signals can be used to identify the flow regimes. A new flow pattern map is proposed based on the Froude number, calculated using the mixture velocity of the two phases, and the ratio of gas-to-liquid superficial velocities. The behaviors of the total and frictional pressure drops were studied and the influence of the superficial velocities as well as the flow regimes were identified. The experimental results were compared to four existing vertically downward frictional pressure drop models, none of which provided accurate predictions.

Pulsatile gas-liquid flow resembling Decompression Sickness: Computational Fluid Dynamics simulation and experimental validation.

This work performs two-dimensional Computational Fluid Dynamics (CFD) simulations of pulsatile bubbly flow in a column resembling the flow inside human vena cava during Decompression Sickness (DCS), aiming to illustrate the effect of certain parameters in bubbly blood flow and so facilitate the design of the: a) corresponding in-vitro bubbly flow experiments under pulsatile flow conditions inside a flow loop and b) in-vivo trials on swines for assessing a novel electrical impedance spectroscopy technique on the detection of bubbles (as those found during DCS) in their bloodstream. The commercially available ANSYS 2019-R3 CFD code was employed to simulate the pulsatile bubbly flow that resembled DCS. Simulations were validated against experiments conducted in a vertical co-current upward pulsatile bubbly flow provided by a flow loop equipped with electrical, optical and pressure diagnostics. CFD simulations under pulsatile conditions were initially validated by oscillatory in-vitro bubbly flow experiments. Then, the influence of pulsation parameters on void fraction, α, and flow velocity, U, profiles was computationally investigated. Intense periodic fluctuations of void fraction were observed along the column and their intensity increases with pulsation amplitude. Moreover, U and α radial profiles were uniform for bubbles 30 μm but showed a core-peaking profile for bubbles 300 μm. CFD simulations of pulsatile bubbly flow resembling DCS provided unconventional information about the influence of different-sized sub-millimetre bubbles on the flow velocity and void fraction profiles, which are expected to improve the design of in-vitro and in-vivo trials for the detection of bubbles such as those found in DCS.

Experimental investigation on the void fraction characteristics of a sudden expansion tube using wire-mesh sensors

Void fraction is an essential parameter of gas-liquid two-phase flow and experiments were executed to investigate the void fraction fluctuation characteristics of gas-liquid two phase flow through a sudden expansion tube. Two 16 × 16 wires mesh sensors were applied to measure the phase distribution of upstream pipe(pipe-32) and downstream pipe(pipe-50). The superficial gas velocity is in the range of 3.46 m/s - 22.46 m/s and the superficial liquid velocity ranges from 0.034 m/s to 0.414 m/s. Flow pattern evolution of upstream and downstream pipes was reconstructed and compared. The experiment results show that, in contrast to pipe-32, the void fraction of pipe-50 shows different trends with the increase of liquid and gas velocity. Liquid-carrying capacity is essential in the relationship between the void fraction of pipe-32 and pipe-50. The critical superficial liquid and gas velocities are proposed to characterize the liquid-carrying capacity. The maximum critical superficial gas and liquid velocity is 15.56 m/s and 0.207 m/s, respectively. Besides, a model is proposed to describe the relationship of void fraction between pipe-32 and pipe-50. It is found that the prediction error is less than ±10% in the case of annular flow.

Computational fluid dynamics modelling of multiphase flows in double elbow geometries

This study investigates the effects of elbow on the transition and development of multiphase flow using computational fluid dynamics modelling techniques. The Eulerian - Multifluid VOF model coupled with an Interfacial Area Transport Equation has been employed to simulate air-water two-phase flow in a pipe with two standard 90 degree elbows mounted in series. Turbulence effects were accounted for by the RNG k-ε model. The effects of separation distance on two-phase flow development have been studied for initial slug and churn flow regimes. Computational fluid dynamics simulation results of phase distribution and time series of void fraction fluctuations were obtained and they showed good agreement with available experimental data. The results show that for initial slug flow regime, there is no flow regime transformation upstream and downstream of the two elbows. While at initial churn flow regime, flow regime transformation occurs at different sections of the flow domain before and after the two elbows. It was noticed that irrespective of the flow regime, the amplitudes and frequencies of void fraction fluctuation become smaller as the fluid flows along the pipe. Changes in the separation distance between the two elbows have larger effects on the flow at churn flow regime.

Particle-scale study of gas-solid flows in a bubbling fluidised bed: Effect of drag force and collision models

Inter-particle and inter-phase interactions of gas-solid flows dominate macro-scale bed hydrodynamics in fluidised bed systems. In this work, typical fluidisation behaviours are presented. The effects of different drag models (i.e., Di Felice drag model, Gidaspow drag model) and collision models (i.e., hard-sphere model, soft-sphere model) on gas-solid behaviours in a bubbling fluidised bed are comprehensively compared. Besides, the influence of critical operating conditions (i.e., superficial gas velocity) is discussed. The simulation results show that Di Felice drag model shows better agreement with the experimental data than the Gidaspow drag model. Both the hard-sphere model and soft-sphere model can well predict particle velocity profiles. Larger superficial gas velocity and hard-sphere model respectively cause more vigorous inter-phase momentum exchange and more frequent collision events, indicating the more intense bubble evolution and the resulting fluctuations of void fraction. The rotational velocity obtained from the soft-sphere simulation is larger than that from the hard-sphere one, but the rotational velocity fluctuation is much weaker.

Spontaneous and artificial void wave propagation beneath a flat-bottom model ship

Although the practical use of bubbly drag reduction for marine vessels has begun to spread, it is unclear how bubbles migrate along spatially developing turbulent boundary layers. We measured the spatiotemporal distribution of the local void fraction beneath a 4-m-long fully transparent flat-bottom model ship towed in a 100-m-long water tank with a ship speed of up to 3.00 m/s. Bubbles were injected with both constant and periodically fluctuated air flow rates into the turbulent boundary layers. With constant flow rates, 2–5-mm bubbles caused spontaneous void waves, i.e., quasiperiodic fluctuation of the local void fraction, standing at 4–8 Hz. When repetitive bubble injection, which formed artificial void waves downstream, was applied, void waves propagated at 47%–53% of the ship speed. During the propagation, we found the apparent diffusion coefficient of the void wave taking values down to 10−4 m2/s and even negative values when the repetition frequency was set lower than the spontaneous frequency of void waves. This indicates that the void waves persist for several tens of meter in application to ship hulls.

Nature and magnitude of operating forces in a horizontal bend conveying gas-liquid slug flows

Operating forces and magnitude of loads from gas-liquid slug flows exerted on a horizontally orientated 90o bend are investigated. The distributed forces are either Newtonian, associated with the fluids motion or Configurational, inherent to the internal distributions of the phases. The forces are derived through the conventional balances of mass and linear momentum arising from the volume of fluid (VOF) description of gas-liquid flows. The study uses the integral form of the momentum balance to estimate the operating forces budget. Invoking dynamical time scales separation discloses the connection of the Lamb vector (vortex-force) to the local time rate of momentum. An interesting outcome being an explicit expression for Favre-Reynolds stress that reveals the contribution of void fraction fluctuations in the redistribution of the stress across the interface. Numerical simulations are performed to determine the magnitude of Newtonian loads on bend using a segmented domain technique to represent the fully established slug flow regime. The time-dependent traces of the relevant flow variables such as liquid hold-up, flow rates and resultant forces on the bend are recorded and analysed. Compared to the isotropic component, the deviatoric stresses are shown to have a marginal contribution to the total forces. It is also shown that loading cycles on bends are much higher than slugging cycles; this is an important feature for the structural integrity assessment of pipelines with bends.

Quadrant analysis for fluctuations of turbulent shear stress and void fraction in a horizontal bubbly channel flow

Quadrant analysis for fluctuations of turbulent shear stress and void fraction in a horizontal bubbly channel flow

Detached eddy simulations of rising Taylor bubbles

A numerical study of turbulent flow induced by Taylor bubbles rising in a stagnant liquid column is presented in this paper. Predictions of the void fraction, mean velocity and turbulent fluctuations are compared with available experimental results. In addition, we analysed the wall pressure and differential wall pressure fields as well as the temperature field produced by the interaction of a Taylor bubble with a heated tube wall. Implications for pressure measurements and thermal fluctuations in two-phase systems are discussed. A second simulation was performed to shed light on Taylor bubble coalescence, specifically highlighting the distinct mechanisms that operate in different parts of the wake region.

Bubble convection and bubbly flow turbulent time and length scales in two-dimensional plunging jets

Air entrainment and air-water mixing by a flow impingement are enhanced by the turbulent shear layer and associated instabilities in the receiving waterbody. This paper presents an experimental study aiming at a quantitative description of bubble-turbulence interplay in two-dimensional supported plunging water jets. In addition to the basic air-water flow properties, the turbulence intensity in the highly-aerated plunging pool was estimated based on void fraction and total pressure fluctuation measurements, while the turbulent time and length scales were recorded systematically. The coupling of bubble convection and formation of macroscopic turbulent structures was characterised in terms of bubble clustering behaviours and turbulent length and time scales of the bubbly flow eddy structures. The effects of jet impact velocity were investigated for a fixed jet length. These advanced data analyses were applied to plunging jet two-phase flow for the first time. The results would provide new benchmark data for numerical modelling of intense air-water flow at a higher level than the basic two-phase flow dynamic properties. A discussion was developed at the end on the turbulent length scales and the Schmidt number in the bubbly flow regions of horizontal hydraulic jump and vertical supported plunging jet.

Nanoparticle coating of a microchannel surface is an effective method for increasing the critical heat flux

Results are presented of an investigation into water boiling in a single microchannel 0.2 mm high, 3 mm wide, and 13.7 mm long with a smooth heating surface or with a coating from aluminum oxide nanoparticles. The experimental procedure and the test setup are described. The top wall of the microchannel is made of glass so that video recording in the reflected light of the process can be made. A coating of Al2O3 particles is applied onto the heating surface before the experiments using a method developed by the authors of the paper. The experiments yielded data on heat transfer and void fraction and its fluctuations for the bubble and transient boiling in the microchannel. The dependence was established of the heat flux on the temperature of the microchannel wall with a smooth surface or a surface with Al2O3 nanoparticle coating for various mass flows in the microchannel. The boiling crisis has been found to occur in the microchannel with a nanoparticle coating at a considerably higher heat flux than that in the channel without coating. The experimental data also suggest that the nanoparticle coating improves heat transfer in the transition boiling region. Processing of the data obtained using a high-speed video revealed void fraction fluctuations enabling us to describe two-phase flow regimes with the flow boiling in a microchannel. It has been found that a return flow occurs in the microchannel under certain conditions. A hypothesis for its causes is proposed. The dependence of the void fraction on the steam quality in the microchannel with or without a nanoparticle coating was determined from the video records. The experimental data on void fraction for boiling in the microchannel without coating are approximated by an empirical correlation. The experiments demonstrate that the void fraction during boiling in the microchannel with a nanoparticle coating is higher than during boiling in the channel without coating (where φ and х are the void fraction and the steam quality, respectively) in the region of a sharp increase in the φ(х) curve.

Two-phase flow pattern recognition in a varying section based on void fraction and pressure measurements

In a hydroelectric turbine, the air injected during operation has an impact on the yield of the machine leading to important losses of energy. To understand those losses and be able to reduce them, a first step is to understand the pattern of the two-phase flows and describe their characteristics in the turbine. Those two-phase flows can be bubbly, intermittent, or annular, with different types of intermittent flow possible. Two-phase flow patterns are well defined in classical geometries such as cylinders with reliable descriptions available [5]. But, there is a critical lack of knowledge for flow patterns in other geometries. In our present work we take interest into a geometry that is a pipe with periodical changes of the section and realize a flow pattern map. To realize this map, we measure the pressure variations and void fraction fluctuations while changing the flow rates of water and air in our test section. We then use our physical understanding of the phenomena to analyze data and identify different flow patterns, characterize them, and build a new flow pattern map.

Local characteristic of horizontal air–water two-phase flow by wire-mesh sensor

Two-phase flow widely exists in many industries. Understanding local characteristics of two-phase flow under different flow conditions in piping systems is important to design and optimize the industrial process for higher productivity and lower cost. Air–water two-phase flow experiments were conducted with a 16×16 conductivity wire-mesh sensor (WMS) in a horizontal pipe of a multiphase flow facility. The cross-sectional void fraction time series was analysed by the probability density function (PDF), which described the void fraction fluctuation at different flow conditions. The changes and causes of PDFs during a flow regime transition were analysed. The local structure and flow behaviour were characterized by the local flow spectrum energy analysis and the local void fraction distribution (horizontal, vertical and radial direction) analysis. Finally, three-dimensional transient flow fluctuation energy evolution and characteristic scale distribution based on wavelet analysis of air–water two-phase flow were presented, which revealed the structural features of each phase in two-phase flow.

Effect of bubble size on void fraction fluctuations in dispersed bubble flows

It is known that bubble size affects seriously the average void fraction in bubbly flows where buoyant velocities vary considerably with bubble size. On the contrary, there is no systematic literature report about bubble size effects on the intensity and frequency of void fraction fluctuations around the average void fraction. This work aims to provide such information. An electrical impedance technique is employed along with non-intrusive ring electrodes to register void fraction fluctuations down to 10−5. Bubble size fluctuations are estimated from high resolution optical images. Experiments are conducted in co-current upward dispersed bubble flow inside a 21mm tube with average bubble size between ∼50 and ∼700μm. Water and blood simulant are used as test liquids with velocity from ∼3 to ∼30cms−1. The above resemble conditions of Decompression Sickness (DCS) in the bloodstream of human vena cava. It is found that the intensity and frequency of void fraction fluctuations vary appreciably with bubble size at constant gas and liquid flow rates. Moreover, these variations are not random but scale with bubble size. As a first step to quantify this effect, an empirical expression is derived that relates average bubble size to the ratio standard deviation/average value of void fraction.

Air entrainment and turbulent fluctuations in hydraulic jumps

A hydraulic jump is the sudden transition from a high-velocity impinging flow into a turbulent roller in an open channel. Substantial amounts of air are entrapped at the impingement point, and significant free-surface fluctuations take place above the roller. In the present study, some physical modelling was conducted in a relatively large sized facility. The flow conditions included a wide ranges of inflow Froude numbers and Reynolds numbers (3.8 < Fr1 < 10.0, 2.1 × 104 < Re < 1.6 × 105). The fluctuating features of free-surface and roller position were investigated non-intrusively with a series of acoustic displacement meters. The characteristic frequencies of the fluctuating motions were documented, and some major roller surface deformation patterns were revealed. The air-water flow properties were investigated with an intrusive phase-detection probe. The void fraction and bubble count rate data were documented in the jump roller, together with the interfacial velocity distributions. The rate of air entrainment was estimated based upon the void fraction and interfacial velocity distribution data. Some simultaneous measurements of instantaneous void fraction and free-surface fluctuations as well as longitudinal jump front oscillations were conducted. The relationship between the rate of air entrainment and turbulent fluctuations is discussed. Both the turbulent fluctuation and aeration properties are basic design parameters in urban water systems in which a hydraulic jump may take place. The present work provides relevant information for water systems including covered channels and partially-filled pipes.

Pebble bed packing in prismatic containers

New analyses of previous tomography investigations show in detail void fraction fluctuations close to walls generated by regular pebble arrangements. Local packing factors within the pebble bed were determined for characteristic zones. These results are very helpful for the interpretation of the packing experiments performed with spherical pebbles in different kinds of Plexiglas containers dominated by flat walls.The packing factors for single-size pebbles in the containers with a piston on top are fairly independent of bed height unless the height to diameter ratio becomes less than 10. For the closed rectangular containers, the development of structured packings is rendered more difficult. However, for blanket relevant bed heights, both for orthosilicate and beryllium pebbles, the packing factors obtained which agree well with previously obtained reference values.

0312 周期的気泡注入により持続されるボイド率変動(OS3-3 流れの抵抗低減,OS3 流れの抵抗低減)

In the frictional drag reduction using bubbles, we proposed a periodic bubble injection technique to improve the performance and the new technique improved the performance 66% in Re 2200 at a horizontal channel flow. In this paper, the void fraction fluctuation in higher Reynolds number condition is investigated by visualization. The uncontrolled fluctuation of the void fraction fluctuation doesn't have any specific frequency, however the artificial fluctuation of void fraction maintain its frequency in streamwise. We indicate that it is possible to control density and shape of a swarm of bubbles by controlling the injection frequency and gas volume.

0214 超音波流速分布計を用いた水平チャネル内気液二相乱流のモニタリング(OS2-3 混相流の多次元可視化計測,OS2 混相流の多次元可視化計測)

In the frictional drag reduction by using bubbles, fluctuations of void fraction take an important role to enhance the effect. We have investigated the mechanisms on formation of bubble clusters that provide the large fluctuation. The objective of this study is establishing a new measurement system of passing interval of bubbles with ultrasonic wave in order to find out the property of void fluctuation. We conducted synchronous measurement of ultrasonic pulse emission and video camera recording of bubble images in a horizontal bubbly channel flow. The present technique provided reasonable results on the passing interval in comparison with the estimates from the images. Probability distributions of the passing bubbles obtained at three different points in the downstream represent influence of bubble coalescence.

A newly developed photoelectric optical fiber probe for simultaneous measurements of a CO2 bubble chord length, velocity, and void fraction and the local CO2 concentration in the surrounding liquid

In a bubbly flow, the mass transfer from bubble swarms to the surrounding liquid and the concentration of dissolved gas transported in the liquid are deeply related to time-spatial structures of the gas and liquid phases. Various techniques to measure bubble characteristics or solution concentration in gas-liquid two-phase flows have been developed. However, detailed discussions about the gas absorption process in consideration of the above relations are not found, because there was no instrument to enable simultaneous measurement of bubble characteristics and concentrations of the surrounding liquid. The purpose of the present study was to develop a new type of probe that enables the simultaneous measurement of these features. We have developed a platinum (Pt)-plated optical fiber probe (POFP: Photoelectric Optical Fiber Probe), which can perform the functions of an optical fiber probe and an electrical probe. The POFP is able to simultaneously measure a CO2-included bubble chord length, velocity, and time-series void fraction as well as the liquid-phase local CO2 concentration. The bubble velocities and chord lengths measured via the POFP method satisfactorily agreed with those obtained with high-speed visualization. Based on the theoretical analysis and primary experimental results, the POFP was considered to possess satisfactory accuracy and response speed for the simultaneous measurement of a bubble velocity, a bubble chord length, a void fraction and a CO2 concentration in the surrounding water. We demonstrated the performance of the POFP by examining it in a bubble column. The CO2 concentration fluctuation at the bottom zone of the bubble column correlated rationally with the void fraction fluctuation. The correlation faded out toward the upper zone. The POFP is considered a useful tool for revealing the relations between flow structures of a bubbly flow and concentration transportation.

Influence of gravity on gas–liquid two-phase flow in horizontal pipes

Based on volume of fluid (VOF) method, a three-dimensional unsteady mathematical model for gas–liquid two-phase flow in horizontal pipe under various gravities is developed. The flow patterns, void fraction distribution and fluctuation in circular pipes with diameters of 7mm and 10mm under various gravities of 10−4g0, 0.17g0, 0.38g0, and g0 (g0=9.8m/s2) are all presented. The quantitatively characterization for intermittent features of gas–liquid two-phase flow under various gravities is realized by using power spectral density (PSD) analysis to extract the dominant frequencies from massive void fraction fluctuation signals. The results indicate that the gravity plays a significant role in the flow pattern, void fraction distribution and void fraction fluctuation. The flow pattern transition appears as a result of the coalescence of bubbles/plugs induced by bubbles/plugs accumulation and gravity driven drainage of liquid film between coalescing bubbles/plugs with increasing gravity. Moreover, the thicker liquid film at the bottom of bubble/plug unit occurs with increasing gravity, resulting in a larger void fraction at the top layer of pipes and a longer region with zero void fraction at the bottom layer. And also with the increasing gravity, both the average peak value and dominant frequencies of void fraction fluctuation decrease. For the slip ratio, it not only increases with increasing gravity but also with increasing two-phase superficial velocity. However, the influence of gravity on the two-phase flow is weakened by the enhanced role of surface tension due to the reduction of diameter as well as the enhancement of inertial force due to the increase of two-phase velocity.