Wall Shear Stress Induced by Taylor Bubbles in Inclined Flow Channels (original) (raw)
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Wall shear stress induced by a large bubble rising in an inclined rectangular channel
International Journal of Multiphase Flow, 2014
The rise of single air bubbles in inclined rectangular channels was experimentally investigated. Two-segment electrodiffusion probes were used to measure wall shear rate profiles along the passing bubbles. They provided information on reverse flow in a liquid film separating the bubble from the wall, capillary waves appearing at the bubble tail, and near-wall flow fluctuations in the bubble wake. The corresponding bubble shapes and rise velocities were obtained from simultaneous visual observations done by a high-speed camera. The experiments were carried out for three channel depths (1.5, 4, and 8 mm), various channel inclinations (from 5 to 90°), bubble volumes (from 1 to 80 ml), and liquid up-flow velocities (from 0 to 0.2 m/s).
Inertial and buoyancy effects on the flow of elongated bubbles in horizontal channels
International Journal of Multiphase Flow, 2021
When a long gas bubble travels in a horizontal liquid-filled channel of circular crosssection, a liquid film is formed between the bubble and the channel wall. At low Reynolds and Bond numbers, inertial and buoyancy effects are negligible, and the liquid film thickness is a function of the capillary number only. However, as the tube diameter is increased to the millimetre scale, both buoyancy and inertial forces may become significant. We present the results of a systematic analysis of the bubble shape, inclination, and liquid film thickness for a wide range of capillary, Bond, and Reynolds numbers, namely 0.024 ≤ Ca l ≤ 0.051, 0.11 ≤ Bo ≤ 3.5, and 1 ≤ Re l ≤ 750. Three-dimensional numerical simulations of the flow are performed by employing the Volume-Of-Fluid method implemented in OpenFOAM. In agreement with previous studies, we observe that buoyancy lifts the bubble above the channel axis, making the top liquid film thinner, and thickening the bottom film. As the Bond number approaches unity, the cross-sectional shape of the bubble deviates significantly from a circular shape, due to flattening of the bottom meniscus. The simulations demonstrate the existence of a cross-stream film flow that drains liquid out of the top film and drives it towards the bottom film region. This drainage flow causes inclination of the bubble, with a larger inclination angle along the bottom plane of the bubble than the top. As buoyancy becomes even more significant, draining flows become less effective and the bubble inclination reduces. A theoretical model for the liquid film thickness and bubble speed is proposed embedding dependencies on both capillary and Bond numbers, which shows good agreement with the reported numerical results. Inertial forces tend to shrink the bubble cross-section and further lift the bubble above the channel centreline, so that the bottom film thickness increases significantly with the Reynolds number, whereas the top film thickness is less sensitive to it.
Dynamic film thickness between bubbles and wall in a narrow channel
Experiments in Fluids, 2011
The present paper describes a novel technique to characterize the behavior of the liquid film between gas bubbles and the wall in a narrow channel. The method is based on the electrical conductance. Two liquid film sensors are installed on both opposite walls in a narrow rectangular channel. The liquid film thickness underneath the gas bubbles is recorded by the first sensor, while the void fraction information is obtained by measuring the conductance between the pair of opposite sensors. Both measurements are taken on a large two-dimensional domain and with a high speed. This makes it possible to obtain the two-dimensional distribution of the dynamic liquid film between the bubbles and the wall. In this study, this method was applied to an air-water flow ranging from bubbly to churn regimes in the narrow channel with a gap width of 1.5 mm.
Study of Stream Flow Effects on Bubble Motion
Nuclear Technology, 1983
The formation of air bubbles at constant-pressure, submerged orifices was investigated in both quiescent and moving streams inside a vertical tube. Parameters affecting the bubble rise velocity, such as bubble generating frequency and diameter, were studied and analyzed for bubbles rising in a chain and homogeneous mixture. A special technique for measuring bubble motion parameters has been developed, tested, and employed throughout the experimental investigation. The method is based on a water-air impedance variation. Results obtained in stagnant liquid show that increasing the bubble diameter serves to increase bubble rise velocity, while an opposite trend has been observed for stream liquid where the bubble diameter increase reduces the bubble rise velocity. The increase of bubble generation frequency generally increases the bubble rise velocity. Experimental data covered with bubble radial distribution showed symmetrical profiles of bubble velocity and frequency, and the radial distribution of the velocity profiles sometimes has two maxima and one minimum depending on the liquid velocity. Finally, in stagnant liquid, a normalized correlation has been developed to predict the terminal rise velocity in terms of bubble generating frequency, bubble diameter, single bubble rise velocity, and conduit dimensions. Another correlation is presented for forced bubbly flow, where the bubble rise velocity is expressed as a function of bubble generating frequency, bubble diameter, and water superficial velocity.
The effect of bubbles on the wall drag in a turbulent channel flow
Physics of Fluids, 2005
The effect of a few relatively large bubbles injected near the walls on the wall drag in the "minimum turbulent channel" is examined by direct numerical simulations. A front-tracking/finite-volume method is used to fully resolve all flow scales including the bubbles and the flow around them. The Reynolds number, using the friction velocity and the channel half-height, is 135 and the bubbles are 54 wall units in diameter. The results show that deformable bubbles can lead to significant reduction of the wall drag by suppression of streamwise vorticity. Less deformable bubbles, on the other hand, are slowed down by the viscous sublayer and lead to a large increase in drag.
Shear flow of a suspension of bubbles rising in an inclined channel
Journal of Fluid Mechanics, 2004
A weak, laminar shear flow of a monodisperse suspension of high-Reynolds-number, low-Weber-number bubbles is studied in a novel experimental configuration. Nitrogen bubbles are formed through an array of small capillaries at the base of a tall channel with a small inclination from the vertical. The bubbles generate a unidirectional shear flow, in which the denser suspension near the bottom wall falls and the lighter suspension near the top wall rises. Profiles of the bubble and liquid velocities and the bubble volume fraction are obtained using hot-film and dual impedance probes. To our knowledge, measurements of the laminar shear properties of a nearly homogeneous bubble suspension have not previously been reported.
Chemical Engineering Research and Design, 2005
A n experimental study is presented concerning the transition in the velocity of individual Taylor bubbles in vertical co-current liquid flow. Velocities of individual Taylor bubbles rising in co-current liquids (kinematic viscosities from 10 26 to 5.7 Â 10 26 m 2 s 21 ) in acrylic columns of 22 mm, 32 mm and 52 mm internal diameter were measured for a wide range of Reynolds number of the flowing liquid using two nonintrusive experimental techniques. The measuring section was located at 6.0 m from the gas injection. The operating conditions used correspond to inertial controlled regime. The data showed an unexpected feature of the bubble motion: the velocity coefficient C changes even when the flow regime in the liquid ahead the bubble is still laminar, i.e., the transition in the bubble velocity starts at liquid Reynolds numbers much lower than 2100. Additional experiments, employing PIV measurements, showed a developed laminar liquid flow ahead the bubble nose. Based on a dimensional analysis, the most important dimensionless numbers for the phenomena were identified and, after processing all data, an empirical correlation was established to predict the velocity coefficient C for a large range of operation conditions. This information is very important for vertical two-phase slug flow modelling.
Application of the electrodiffusion method for near-wall flow diagnostics
EPJ Web of Conferences, 2014
The electrodiffusion method is presented as a measuring technique suitable for the flow measurement done in close proximity of the wall, thus in the region difficultly accessible by standard anemometric techniques (e.g. PIV, LDA). The experimental results obtained in different flow configurations (backward-facing step flow, wavy film flow, turbulent channel flow, rising Taylor bubbles) document application capabilities of this technique, e.g. for mapping of wall shear stresses, delimitation of stable flowrecirculation zones, detection of short-time reversal of the near-wall flow, investigation of the near-wall turbulence, or detection of moving fluid particles. A new technology of sensor fabrication based on photolithography is introduced and possible applications of microsensors in microfluidic devices discussed. This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Chemical Engineering Science, 1991
The numerical method presented in Part I is used to simulate the liquid flow around a single Taylor bubble in a vertical tube. A modified low Reynolds number k--E model is incorporated in the simulation for accurate prediction of the wall shear stress when liquid flow is turbulent. A model for free surface damping of turbulence is also included in the numerical process. The predicted rise velocily and the shape of the bubble as well as the film thickness and the wall shear stress are in good agreement with new experiments as well as earlier data.
Experimental Analysis of Multiple Air-Bubbles Rise in Water Channel Using A Submerged Needle
2019
Article history: Received 25 September 2019 Received in revised form 19 October 2019 Accepted 2 November 2019 Available online 10 November 2019 2019 This paper studies the ascent of multiple air bubbles experimentally utilizing a highframerate camera (HSC) (400 fps) which has the ability to capture fast moving objects which can then be played back in slow motion. Experiments conducted previously suggested that small air bubble pursues a rectilinear rise motion. Bubbles that have radius less than 0.81 mm approximately follows rectilinear path as concluded by [1] [2]. Notwithstanding, in the present study it has been seen that by utilizing a needle inside the water channel, multiple bubbles pursue a spiral movement in a path rather instead of a rising in a straight line. Motion is studied in both of these planes (X-Y and Y-Z). Image processing techniques have been utilized thus to upgrade the quality of the images and along these lines to examine the flow features of the multiple air ...