Interaction between Taylor bubbles rising in stagnant non-Newtonian fluids

The interaction between Taylor bubbles rising in stagnant non-Newtonian solutions was studied. Aqueous solutions of carboxymethylcellulose (CMC) and polyacrylamide (PAA) polymers were used to study the effect of different rheological properties: shear viscosity and viscoelasticity. The solutions studied covered a range of Reynolds numbers between 10 and 714, and Deborah numbers up to 14. The study was performed with pairs of Taylor bubbles rising in a vertical column (0.032 m internal diameter) filled with stagnant liquid. The velocities of the leading and trailing bubbles were measured by sets of laser diodes/photocells placed along the column. The velocity of the trailing bubble was analysed together with the liquid velocity profile in the wake of a single rising bubble (Particle Image Velocimetry data obtained from the literature). For the less concentrated CMC solutions, with moderate shear viscosity and low viscoelasticity, the interaction between Taylor bubbles was similar to that found in Newtonian fluids. For the most concentrated CMC solution, which has high shear viscosity and moderate viscoelasticity, a negative wake forms behind the Taylor bubbles, inhibiting coalescence since the bubbles maintain a minimum distance of about 1D between them. For the PAA solutions, with moderate shear viscosity but higher viscoelasticity than the CMC solutions, longer wake lengths are seen, which are responsible for trailing bubble acceleration at greater distances from the leading bubble. Also in the PAA solutions, the long time needed for the fluid to recover its initial shear viscosity after the passage of the first bubble makes the fluid less resistant to the trailing bubble flow. Hence, the trailing bubble can travel at a higher velocity than the leading bubble, even at distances above 90D.     

Liquid flow patterns about barbotage bubbles

This paper summarizes the results of a flow visualization study on the liquid motion around barbotage bubbles during growth and departure. Flow patterns, as well as for the first time, instantaneous velocities, are reported as a function of time and location about the bubbles. The experiments, employing the hydrogen-bubble technique and high-speed cine photography, were with: water as the liquid, air as the bubbled gas, orifice diameters of 0.116 and 0.252 cm, and different air flow rates; the two limiting cases of constant supply pressure and constant volumetric flow rate were covered. It was found that the liquid around a barbotage bubble assumes two velocity maxima, the first an outward maximum during bubble growth and the second in the opposite direction approximately at the time of bubble departure; further, liquid velocities were found to be higher close to the bubbling site. Certain differences in liquid velocities between the constant pressure and constant flow cases are explained in terms of available theoretical solutions to the bubble growth rate. Qualitative comparisons of the barbotage liquid flow patterns and those recently reported for boiling flow patterns are also presented.     

Measurement of pressure loss in the flow of polymer solutions through wavy channels

In order to examine the flow behavior of polymer solutions through porous media, the measurement of pressure loss and the experiment for flow visualization were carried out with wavy channels as one of the model channels of porous media. The test fluids used are aqueous solutions of polyacrylamide (PAA) with two different concentrations. The occurrence of the excess pressure loss, which was not due to the effect of the centrifugal force, was found for the PAA solutions. The relations between the friction factor ratio and the Deborah number were similar to that obtained for the flow through porous media. Furthermore, the results of the flow visualization suggest that the elongational property of the PAA solutions is connected with the occurrence of the excess pressure loss.     

Unobstructed particle image velocimetry measurements within an axial turbo-pump using liquid and blades with matched refractive indices

Performing PIV measurements within complex turbomachinery with multiple blade rows is difficult due to the optical obstruction to the illuminating sheet and to the camera caused by the blades. This paper introduces a refractive index matched facility that overcomes this problem. The rotor and stator blades are made of transparent acrylic, and the working fluid has the same optical refractive index as the blades. A 64% by weight solution of sodium iodide in water is used for this purpose. This liquid has a kinematic viscosity of about 1.1᎒^-6 m²/s, which is almost the same as that of water enabling operation at high Reynolds numbers. Issues related to operating with this fluid such as chemical stability, variations in transmittance and solutions to these problems are discussed. This setup allows full optical access to the entire rotor and stator passages both to the laser sheet and the camera. The experiments are conducted at different streamwise locations covering the entire flow fields around the rotor, the stator, the gap between them, and the wakes behind. Vector maps of the instantaneous and phase-averaged flow fields as well as the distribution of turbulent kinetic energy are obtained. Measurements at different magnifications enable us to obtain an overview of the flow structure, as well as detailed velocity distributions in the boundary layers and in the wakes.     

Drag reduction in the turbulent pipe flow of polymers

The paper concerns an experimental study of the fully developed turbulent pipe flow of several different aqueous polymer solutions: 0.25%, 0.3% and 0.4% carboxymethylcellulose (CMC), 0.2% xanthan gum (XG), a 0.09%/0.09% CMC/XG blend, 0.125% and 0.2% polyacrylamide (PAA). The flow data include friction factor vs. Reynolds number, mean velocity and near-wall shear rate distributions, and axial velocity fluctuation intensity u′ at a fixed radial location as a laminar/turbulent transition indicator. For each fluid we also include measurements of shear viscosity, first normal-stress difference and extensional viscosity. At high shear rates we find that the degree of viscoelasticity increases with concentration (0.3% CMC is an exception) for a given polymer, and in the sequence XG, CMC/XG, CMC, PAA, whilst at low shear rates the ranking changes to CMC, CMC/XG, XG, PAA. The extensional viscosity ranking is XG/CMC, XG, CMC, PAA at high strain rates and the same as that for the viscoelasticity at low shear rates. We find that the observed drag-reduction behaviour is consistent for most part with the viscoelastic and extensional-viscosity behaviour at the low shear and strain rates typical of those occurring in the outer zone of the buffer region.Although laminar/turbulent transition is practically indiscernible from the friction factor vs. Reynolds number plots, particularly for PAA and XG, the u′ level provides a very clear indicator and it is found that the transition delay follows much the same trend with elasticity/extensional viscosity as the drag reduction.     

Effect of bubbles on the turbulence near the exit of a liquid jet

The objective of this paper is to examine the effect of bubbles on the turbulence levels of a water jet. Simultaneous measurements of the axial and radial velocity components were taken in a bubbly jet with a Laser Doppler Velocimeter (LDV) and then compared to the velocities of a single phase jet at the same liquid flow rate. Mean bubble diameters ranged from 0.6 to 2 mm and the void fractions were up to about 20%. The liquid Reynolds numbers were from 5,000 to 10,000 approximately. The measurements extended to from an axial distance of 4–12 cm. It was observed that bubbles did not affect significantly the average velocity profiles in the jet. However bubbles increased the turbulence intensities in the core of the jet near the jet exit. The increase in turbulence intensities was more pronounced at lower Reynolds numbers and at higher void fractions.     

Bubble formation in co-fed gas–liquid flows in a rotor-stator spinning disc reactor

The gas–liquid flow in a rotor-stator spinning disc reactor, with co-feeding of gas and liquid, is studied for high gas volumetric throughflow rates and high gas/liquid volumetric flow ratios. High speed imaging and spectral analysis of pressure drop signals are employed to analyse the flow. Two mechanisms of bubble formation are observed, one due to gas overpressure leading to large irregular bubbles, and one due to liquid turbulent vortices leading to small, well-defined bubbles. The two mechanisms lead to three distinct gas dispersion regimes, distinguished by their characteristic oscillations in pressure drop. At low rotational Reynolds numbers (Re_ω < 0.4 · 10⁶), in the gas spillover regime, the gas is dispersed as large bubbles only. Above this critical Re_ω, small bubbles are sheared off as well, thus forming a heterogeneous dispersion. At sufficiently high Re_ω, depending on the gas flow rate, the gas is homogeneously dispersed as small bubbles. The maximum gas flow that can be dispersed as small bubbles is linearly proportional to the local energy dissipation rate. The understanding of the bubble formation mechanisms and pressure signature allows prediction and detection of the prevailing hydrodynamic regime in scaled up spinning disc reactors and for different reaction fluids.     

Surface tension measurement of aqueous polymer solutions

The surface tension of aqueous polymer solutions of polyacrylamide (PAM), polyacrylic acid (PAA), carboxymethyl cellulose (CMC), and hydroxyethyl cellulose (HEC) was studied over a range of polymer concentrations by using the maximum bubble pressure method at temperatures ranging from 20 to 65°C. The surface tension of water was also measured by the maximum bubble pressure method as well as by the DuNoüy ring method over the same temperature range. The experimental water data are in excellent agreement with the well-established tabulated data in the literature.

For a fixed concentration, all of the polymer solutions exhibited a decrease in surface tension with increasing temperature level. When compared with water at a fixed temperature level, the PAM and CMC solutions showed slightly higher surface tension values, whereas the PAA solutions yielded values equal to those found for water. In the case of the HEC solutions, the measured surface tensions decreased with concentration at a fixed temperature level and were lower than the values found for water. For a concentration of 2000 wppm the surface tension values for the hydroxyethyl cellulose were of the order of 10% lower than those for water at a fixed temperature level.

A comparison of the new measurements with the relatively limited previously published studies showed good agreement.     

Effects of sound on flow separation from blunt flat plates

The effect of sound on the flow around plates with semicircular or square leading edges and square trailing edges located in a low turbulence open jet has been studied. In all circumstances the length of the leading edge separation bubbles associated with square leading edge plates was found to oscillate. When sound was applied to the flow around these plates, the leading edge shear layers reattached closer to the leading edge and the oscillations in bubble length occurred at the applied sound frequency, generating patches of concentrated vorticity in the boundary layers. These vorticity patches moved downstream near the plate surface and then beyond the trailing edge to form vortex cores in a street with a Strouhal number equal to the applied sound value. Sometimes these vortex streets are unstable and break down into streets with Strouhal numbers approaching those observed without sound. These effects of sound were not observed in the flow around plates with semicircular leading edges. Without sound, square leading edge plates of intermediate length did not shed regular vortex streets.     

Flow of wormlike micelle solutions past a confined circular cylinder

In this paper, we present the results of an investigation into the flow of a series of viscoelastic wormlike micelle solutions past a confined circular cylinder. Although this benchmark flow has been studied in great detail for polymer solutions, this paper reports the first experiments to use a viscoelastic wormlike micelle solution as the test fluid. The flow kinematics, stability and pressure drop were examined for two different wormlike micelle solutions over a wide range of Deborah numbers and cylinder to channel aspect ratios. A combination of particle image velocimetry and pressure drop measurements were used to characterize the flow kinematics, while flow-induced birefringence measurements were used to measure the micelle deformation and alignment in the flow. The pressure drop was found to decrease initially due to the shear thinning of the test fluid before increasing at higher flow rates as elastic effects begin to dominate the flow. Above a critical Deborah number, an elastic instability was observed for just one of the test fluids studied, the other remained stable for all Deborah number tested. Flow-induced birefringence and velocimetry measurements showed that observed instability originates in the extensional flow in the wake of the cylinder and appears not as periodic counter-rotating vortices as has been observed in the flow of polymer solutions past circular cylinders, but as a chaotic rupture event in the wake of the cylinder that propagates axially along the cylinder. Reducing the cylinder to channel aspect ratio and the degree of shearing introduced by the channel walls had a weak impact on the stability of the flow. These measurements, when taken in conjunction with previous work on flow of wormlike micelle solutions through a periodic array of cylinders, definitively show that the instability can be attributed to a breakdown of the wormlike micelle solutions in the extensional flow in the wake of the cylinder.     

Opposed bubbly jets at different impact angles: Jet structure and bubble properties

The structure of two colliding water jets containing small gas bubbles is studied experimentally. The effects of the separation distance between jets, as well as the orientation angle, on the spatial distribution of bubbles have been considered. Results on the global structure of the final jet and bubble properties have been obtained using a high-speed video camera, and measurements of the positions of coalescence events are presented. Jets are introduced through inclined pipes (with a diameter of 0.7 mm) into a large water tank to avoid wall effects. Inclination angle has been changed from 0° to 45° with respect to the horizontal, resulting in a 0° up to 90° impact angle between jets. Generation of bubbles is controlled by a T-junction device where a regular slug-flow is created prior to injection. Bubble sizes have been measured, and a mean diameter of around 1 mm has been obtained using high values of the liquid flow rate. In the studied range of separation distances between the bubbly jets, a more homogeneous dispersion of bubbles is created as the distance between jets is decreased and the momentum flux of each jet is increased. Higher numbers of coalescences are observed when using smaller distance between jets, and the obtained measurements revealed that the number of bubble coalescence events is reduced significantly using high values of liquid flow rates.     

Experimental investigation of horizontal air–water bubbly-to-plug and bubbly-to-slug transition flows in a 3.81 cm ID pipe

The present study seeks to investigate horizontal bubbly-to-plug and bubbly-to-slug transition flows. The two-phase flow structures and transition mechanisms in these transition flows are studied based on experimental database established using the local four-sensor conductivity probe in a 3.81 cm inner diameter pipe. While slug flow needs to be distinguished from plug flow due to the presence of large number of small bubbles (and thus, large interfacial area concentration), both differences and similarities are observed in the evolution of interfacial structures in bubbly-to-plug and bubbly-to-slug transitions. The bubbly-to-plug transition is studied by decreasing the liquid flow rate at a fixed gas flow rate. It is found that as the liquid flow rate is lowered, bubbles pack near the top wall of the pipe due to the diminished role of turbulent mixing. As the flow rate is lowered further, bubbles begin to coalesce and form the large bubbles characteristic of plug flow. Bubble size increases while bubble velocity decreases as liquid flow rate decreases, and the profile of the bubble velocity changes its shape due to the changing interfacial structure. The bubbly-to-slug transition is investigated by increasing the gas flow rate at a fixed liquid flow rate. In this transition, gas phase becomes more uniformly distributed throughout the cross-section due to the formation of large bubbles and the increasing bubble-induced turbulence. The size of small bubbles decreases while bubble velocity increases as gas flow rate increases. The distributions of bubble size and bubble velocity become more symmetric in this transition. While differences are observed in these two transitions, similarities are also noticed. As bubbly-to-plug or bubbly-to-slug transition occurs, the formation of large elongated bubbles is observed not in the uppermost region of bubble layer, but in a lower region. At the beginning of transitions, relative differences in phase velocities near the top of the pipe cross-section to those near the pipe center become larger for both gas and liquid phases, because more densely packed bubbles introduce more resistance to both phases.     

Wake properties of a single gas bubble in three-dimensional liquid-solid fluidized bed

Experiments were performed to investigate the wake properties of a single gas bubble in a three-dimensional liquid-solid fluidized bed via a video camera moving at the same speed as the bubble. The solids holdup in the fluidized bed varied up to around 10%. The bubble size varied from 5 to 20 mm with corresponding bubble Reynolds numbers ranging from 1000 to 6500. The bubble was observed to have two types of wake configurations depending on the bubble size: the asymmetric/helical vortex wake for small bubbles and the symmetric wake for large bubbles. The bubble shape and relative rise velocity in the fluidized bed can be well-represented by correlations developed for single bubbles in liquid media, although the bubble shape in liquid-solid media is slightly more flattened compared to that in liquid media. The bubble rocking frequency was found to be independent of particle properties and to correspond in magnitude to the vortex shedding frequency in a two-dimensional liquid-solid fluidized bed. The average primary wake size in three dimensions is comparable to that in two dimensions.     

Stereo-PIV measurements of vapor-induced flow modifications in confined jet impingement boiling

A single subcooled jet of water which undergoes boiling upon impingement on a discrete heat source is studied experimentally using time-resolved stereo particle image velocimetry (PIV). The impinging jet issues from a 3.75 mm diameter sharp-edged orifice in a confining orifice plate positioned 4 orifice diameters from the target surface. The behavior at jet Reynolds numbers of 5,000 and 15,000 is compared for a constant jet inlet subcooling of 10 °C. Fluorescent illumination allows for simultaneous imaging of both the flow tracers and the vapor bubbles in the flow. Flow structure, time-averaged velocities, and turbulence statistics are reported for the liquid regions within the confinement gap for a range of heat inputs at both Reynolds numbers, and the effect of the vapor generation on the flow is discussed. Vapor generation from boiling is found to modify the liquid velocities and turbulence fluctuations in the confinement gap. Flow in the confinement gap is dominated by vapor flow, and the vapor bubbles disrupt both the vertical impinging jet and horizontal wall jet flow. Moreover, vapor bubbles are a significant source of turbulence kinetic energy and dissipation, with the bubbly regions above the heated surface experiencing the most intense turbulence modification. Spectral analysis indicates that a Strouhal number of 0.023 is characteristic of the interaction between bubbles and turbulent liquid jets.     

Robust numerical analysis of the dynamic bubble formation process in a viscous liquid

The dynamics of bubble formation from a submerged nozzle in a highly viscous liquid with relatively fast inflow gas velocity is studied numerically. The numerical simulations are carried out using a sharp interface coupled level set/volume-of-fluid (CLSVOF) method and the governing equations are solved through a hydrodynamic scheme with formal second-order accuracy. Numerical results agree well with experimental results and it is shown that the sharp interface CLSVOF method enables one to reproduce the bubble formation process for a wide range of inflow gas velocities. From numerical results, one can improve their understanding of the mechanisms regarding the dynamics of bubble formation. For example, it is found that for some sets of parameters that the bubble formation process reaches steady state after several bubbles are released from the nozzle. At steady state, bubbles uniformly rise freely in the viscous liquid. It is observed that the fluid flow around a formed bubble has a significant role in determining the overall dynamic process of bubble formation; e.g. the effect of the fluid flow from the preceding bubble can be seen on newly formed bubbles.     

Numerical and experimental study of the heat transfer and fluid flow by thermocapillary convection around gas bubbles

 At liquid–gas or liquid–liquid interfaces thermocapillary or Marangoni convection develops in the presence of a temperature or concentration gradient along the interface. This convection was not paid much attention up to now, because under terrestrial conditions it is superimposed by the strong buoyancy convection. In a microgravity environment, however, it is the remaining mode of natural convection. During boiling in microgravity it was observed at subcooled conditions. Therefore the question arises about its contribution to the heat transfer. Thus the thermocapillary convection was intensively studied at single gas bubbles in various liquids both experimentally and numerically. Inside a temperature gradient chamber, the overall heat transfer around single bubbles of different volume was measured with calorimetry and the liquid flow with PIV and LDV. In parallel to the experiment, a 2-dimensional mathematical model was worked out and the coupled heat transfer and fluid flow was simulated with a CV-FEM method both under earth gravity level and under microgravity. The results are described in terms of the dimensionless Nusselt-, Peclet-, Marangoni-, Bond- and Prandtl-number. Received on 23 August 1999     

An experimental investigation of the motion of long bubbles in inclined tubes

The relative motion of single long air bubbles suspended in a constant liquid flow in inclined tubes has been studied experimentally. Specially designed instrumentation, based on the difference in refractive properties of air and liquid with respect to infrared light, has been constructed and applied to measure bubble propagation rates.A series of experiments were performed to determine the effect of tube inclination on bubble motion with liquid Reynolds and Froude numbers, and tube diameter as the most important parameters.Particular aspects of the flow are described theoretically, and model predictions were found to compare well with observations. A correlation of bubble and average liquid velocities, based on a least squares fit to the data is suggested. Comparisons with other relevant models and data are also presented.     

绕水翼超空化流动形态与速度分布

为揭示超空化流场结构特性,利用高速全流场显示技术,观察了绕hydronautics水翼的超空化流动形态,并利用数字粒子图像测速仪(DPIV)测量了其速度分布. 在测量空穴内部流速分布时,采用空化流场中的空化泡作为示踪粒子来显示流动结构. 结果表明:随着空化数的降低,超空化流动显现出了明显的阶段特征,其中水汽混合相和汽相的分布决定了空化区域的形态与流速分布;空化区和主流区的汽液交界面处存在着较大的速度梯度;低速分布区域随着空化数的降低由水翼吸力面中后部向水翼下游移动;在空化区域内部,水汽混合区的速度相对较低,而汽相区则与主流区有着相近的速度分布.关键词超空化水翼、DPIV、高速摄像、空化形态、流速分布      

The minimum in-line coalescence height of bubbles in non-Newtonian fluid

The minimum in-line coalescence height of bubbles generated from a submerged nozzle was investigated experimentally in shear thinning non-Newtonian fluid at lower Reynolds number (2∼60). Carboxymethyl cellulose sodium (CMC) aqueous solution and carbon dioxide were used as the liquid phase and the gas phase, respectively. The process of the formation, movement and in-line coalescence of bubbles was visualized and recorded by a high-speed digital camera. The influences of bubble size, bubble generation frequency and liquid property on the minimum in-line coalescence height of bubbles were investigated by changing nozzle diameter, gas flow rate and the mass concentration of CMC aqueous solutions. For a given liquid, the generating frequency and size of bubbles increased but the minimum coalescence height of in-line bubbles decreased when the nozzle diameter and gas flow rate were increased. When the nozzle diameter and gas flow rate were fixed, the shear-thinning effect of CMC aqueous solution became stronger with increasing CMC mass concentration, which led to the increase in both the terminal rise velocity and average acceleration of the trailing bubble, consequently, the minimum in-line coalescence height of bubbles decreased. An empirical correlation for estimating the minimum in-line bubble coalescence height was proposed, the calculating values accords well with experimental data with a mean relative deviation only 7.6%.