Liquid Flow Structure Research Articles

Co-current stratified gas–liquid downflow—Influence of the liquid flow field on interfacial structure

The transition from smooth to wavy stratified flow is studied for various pipe inclination angles with the aim to contribute to the realistic modeling and simulation of long distance two-phase pipe flow. The influence of the liquid flow field on interfacial structure is studied through local axial velocity measurements in the liquid phase in conjunction with other liquid layer characterization experiments. Observations based on fast-video recordings, are used to identify the main patterns of wave evolution. Liquid layer thickness time records are acquired using a parallel wire conductance technique from which mean layer thickness, RMS and power spectra of the fluctuations, as well as wave celerities are calculated. Laser Doppler Anemometry ( LDA) is employed to investigate the flow structure in the thin liquid layer both with and without interfacial shear induced by a co-current gas flow. The results reveal the influence of the liquid flow field development on the interfacial structure. In particular, the new data of axial velocity profiles and liquid layer characteristics suggest that the onset of the interfacial waves is strongly affected by the liquid flow structure, possibly by the laminar to turbulent transition within the layer.

Behavior of a single coherent gas bubble chain and surrounding liquid jet flow structure

The motion of a single nitrogen gas bubble chain and the structure of water flow field surrounding the chain were experimentally studied. We developed a bubble generator that can control both the bubble diameter and the generation frequency independently. Experimental conditions of bubble Reynolds number and bubble distance divided by bubble diameter were from 300 to 650 and from 6.5 to 300, respectively. We discuss the interaction effects on the motion of each bubble rising in a chain, as compared to the effects of a single rising bubble. The bubble trajectories and the surrounding water flow fields in the state of bubbles rising in a chain were investigated using a high-speed digital video camera and an analog single-lens-reflex camera. We observed two important physical phenomena. First, bubbles passed through a nearly identical path in the case of low frequency of bubble production. On the contrary, at a height of approximately 50 mm from the nozzle, the bubbles in the case of high frequency deviated and scattered from this path due to bubble–bubble interaction. Second, with higher bubble production frequency, coherent bubble chain and the characteristic structure of the surrounding water flow called “liquid jet” were observed near the nozzle. The direction of liquid jet flow differed from the bubble trajectory. We theoretically investigated the relation of coherent bubble chain and liquid jet by applying the conservation of liquid momentum.

Study of bubble induced flow structure using PIV

An experimental investigation of the flow structure induced by a chain of gas bubbles was carried out in a rectangular bubble column using particle image velocimetry (PIV). It is observed that the bubble rising trajectory changes from one dimension to three dimension as liquid viscosity reduces. The variation of bubble rising trajectory associates with the alternation of bubble motions—with or without oscillatory and rotational motion depending the bubble rising trajectory is 3-D or 1-D. The different behaviors of gas bubbles introduce various instantaneous and averaged liquid flow structures. In general, complex fluid velocity fields present in liquid system of low viscosity where free vortex, cross flow, and irregular circular flow can be observed. The liquid pseudo-turbulence measured in terms of turbulence intensity and Reynolds stress is more intense in liquid of low viscosity. The turbulence is also enhanced by the frequency of bubble formation.

Observations of high temperature impinging-jet boiling phenomena

A high-speed video camera and microphone were used to capture the flow behavior and boiling sound of a free-surface water jet impinging on a high temperature surface during quench cooling. It was found that depending on the superheat of the surface considerably different flow patterns appeared. For cases where the initial surface temperature was above about 300 °C an almost explosive pattern appeared. This was in contrast to slightly lower temperatures where a liquid sheet flow structure was apparent. The change in phenomena was accompanied by a sudden change in the boiling sound and an increase in the heat transfer rate.

Gas–liquid interfacial structure and pressure drop characteristics of churn flow

Oscillatory characteristics of churn flow in the intermediate region between the annular and slug flows were experimentally investigated. To clarify the effects of the oscillatory motion of the liquid flow on the frictional pressure gradient, simultaneous measurements of the pressure drop and the spatio-temporal gas–liquid interface structure were conducted. From a comparison of time-averaged frictional pressure gradients with conventional correlations for steady two-phase flow, it was found that the measured frictional pressure gradient in the oscillatory flow was much greater than that predicted by correlations especially for churn flow under low liquid flow rate conditions. The liquid flow structure of churn flow was characterized by the flooding-type large wave formed on the thin liquid film. As the liquid flow rate decreased, the wave velocity became much higher than the average liquid velocity, which suggested that the frictional pressure gradient was dominantly caused not by the steady liquid film flow, but by the propagation of large wave. A large wave model was proposed, and was confirmed to be valid for predicting the frictional pressure gradient in both the churn and annular flow regions. For a practical calculation, a simple predictive method of the frictional pressure gradient for the churn and annular flows was also presented by means of the correlations of the two-phase friction multiplier and the liquid holdup.

Single Bubble Chain Behavior and Liquid Flow Structures in the Vicinity of a Nozzle (Liquid Jet Developments by Single Bubble Chains)

The behavior of single nitrogen gas bubble chains and the structures of flow field in the vicinity of a nozzle were experimentally studied. Both bubble diameters and bubble generation frequencies were precisely controlled independently to be 2.2 mm, and from 1 Hz to 10 Hz (corresponding bubble distance ; 300∼30 mm), respectively, by using innovative acoustic device. Careful investigations of flow field in the vicinity of a nozzle by using Particle Image Velocimetry enabled us to investigate the following two significant physical phenomena. First, The lift forces acting on bubbles in the bubble chains were observed to increase as bubble generation frequency increases, and were largely affected by the preceding bubbles. Second, developments of characteristic liquid jet were identified to play a dominant role in generation of lift forces on each bubble in single bubble chains, and were theoretically investigated by applying the conservation of liquid momentum.

209 単一気泡列中の気泡挙動および周囲液体流れ場構造 : 気泡列による液ジェットの生成

209 単一気泡列中の気泡挙動および周囲液体流れ場構造 : 気泡列による液ジェットの生成

Prediction of Two-Phase Frictional Pressure Loss by Hydrodynamic Flow Model.

Simultaneous measurements of pressure gradients and spatio-temporal characteristics of gasliquid interface are conducted in vertical upward two-phase flow. To clarify the mechanism of frictional pressure losses for slug, churn and annular flow regimes, a hydrodynamic flow model is presented which permits predictions of the frictional pressure gradient for a wide range of flow regimes. The liquid flow structure is assumed to consist of waves propagating upward and base film flow. The contributions of waves and base film flow to the frictional pressure gradients are presented by considering P. D. F. for liquid holdup. At low liquid flow rate, the wave velocity is much higher than the absolute velocity of the base film, and the frictional pressure gradient is dominantly caused by the wave propagation. At high liquid flow rate, the contribution of the base film flow becomes comparable to that of waves. The agreement between the predicted frictional pressure gradients by the present model and experimental data is fairly good.

Residence time, mass transfer and back-mixing of the liquid in trickle flow reactors containing porous particles

A residence time distribution (RTD) model to describe the liquid trickle flow in a trickle-bed reactor packed with porous particles and operated both under partially and fully wetted conditions was proposed based on a simple representation of the liquid flow structure. The model views the external liquid stream as divided into a dynamic zone where an axially dispersed plug flow pattern prevails, and an external stagnant (or static) zone contiguous to both the dynamic zone and the partially wetted porous particles. The model incorporates mass transfer between (i) external dynamic and stagnant zones, (ii) dynamic zone and nearby partially wetted porous particles, (iii) stagnant zone and adjacent partially wetted particles, and (iv) finally intraparticle diffusion. The model parameters were derived from liquid residence time distribution tests with various air/Newtonian and air/non-Newtonian systems. Analysis of the dynamic tracer impulse–response data of the liquid revealed the significance of the mass transfer resistance between static liquid and adjacent wetted particles, intraparticle diffusion resistance, as well as partial wetting. By properly accounting for intraparticle diffusion, peculiarly high liquid axial dispersion coefficients were obtained for low liquid velocities and high carboxymethyl-cellulose (CMC) concentrations. Finally, the deficiency of lumping static liquid–solid mass transfer, internal diffusion, and partial wetting in the Péclet number and the number of transfer units was discussed.

Structure and regimes of liquid film flow in spinning cone columns

Analyses of the spatial structure and hydrodynamic regimes of the liquid-film flow over a rotating cone surface and application of this analysis to the liquid-phase flow in spinning-cone-distillation columns (SCC) are presented. Based on the results obtained in dimensionless form, a diagram of the flow zones (the inner inlet-dependent, the intermediate Coriolis, and the outer centrifugal zones) and regimes (the laminar, the wavy-laminar, and the fully turbulent regimes) in terms of the SCC operating conditions is developed, and a relationship between the changes in flow pattern and the scale of SCCs is established. Specifically, we find that the radial flow structure is similar at all scales of the presently available range of spinning cone columns, and the flow regime is turbulent for all operating conditions in commercial scale, but not necessarily in laboratory-scale columns. Implications of these findings to the possible nature of flooding in SCCs are discussed.

Magnetic resonance imaging of liquid flow and pore structure within packed beds

Magnetic resonance imaging (MRI) volume- and velocity-measurement techniques are used to probe structure-flow correlations within the interparticle space of a packed bed of ballotini. Images of the three mutually orthogonal components of the velocity field are obtained in two perpendicular slices within a bed of 5 mm diameter ballotini packed within a glass column of internal diameter 4.6 cm. Comparison of flow images obtained for two beds of identical column-to-particle diameter ratio but of differing length show that velocity enhancements at the walls of the bed are greater in the shorter, more ordered, bed. A three-dimensional volume image of each bed is also obtained and analysed to partition the interparticle space into individual pores and determine the location of pore necks. Correlations between volume flow rate and the surface area of the constrictions (pore necks) within the interparticle space lie between two limiting behaviours. For pores associated with low local Reynolds number, the volume flow rate through the constrictions scales as the square of the cross-sectional area of the constriction, whereas at the extreme of high local Reynolds number pores show volume flow rates scaling with cross-sectional area.

Impact on the surface of a compressible liquid

The propagation of a weak shock wave produced by an abrupt impact on a rigid boundary of a slightly compressible liquid is considered. Initially the liquid is at rest and occupies the region enclosed by a circular cylindrical surface. At some instant of time the points of this surface obtain velocities directed radially inwards. The velocity value depends on both the time and the angular coordinate and is small compared with the sound velocity. The liquid flow is assumed to be plane. A weak shock wave is formed under the impact and propagates to the centre of the liquid region. Initially the shock front is circular and bends later on due to non-linear effects. The form of the front is very complicated near the centre and depends on the distribution of the initial impact velocity. The evolution of the shock front at this stage is described by the geometrical acoustic theory. As a result a local focusing of separate parts of the shock wave is observed. The position of the shock wave and the liquid flow behind it are described within the framework of the transonic approximation within those zones. The structure of the liquid flow close to the focusing points is shown to be unaffected by any details of the external action and to be described by the solution of the non-linear boundary problem free from parameters.

Mass transfer in packed columns: The cylinder model

AbstractIn order to predict mass transfer in packed columns, it is necessary to know the interfacial area. The well‐known and often cited equation (Onda, Kolev, Zech, etc.) do not yield precise values for the mass transfer coefficient βL ae or the interfacial area ae, especially for modern packing elements. A new model will be presented which takes into account the structure of packing (cylinder model) as well as the structure of the liquid hold‐up. This model allows the separation of liquid flow through packings into its main constituents, i.e. rivulets and drops (freely falling). The determination of the structure of liquid flow allows the prediction of effective interfacial areas as well as of volumetric mass transfer coefficients for irrigated large packing elements manufactured from plastics, metals or ceramic. The applicability to novel future packings is one of the advantages of the new model.

Identification and modelling of liquid flow structures in bubble column reactors

The liquid circulation patterns were studied in the bubble columns using visualisation of liquid flow, joint analysis of the wall pressure fluctuations and measurements of residence time distribution of both phases. The results of our experiments proved that none of the existing hydrodynamic models can give us a complete picture of macro-scale flow patterns occuring in bubble columns. The analysis of macrocirculation flow models clearly favorized the concept of spiral upward flow of central macrovortex.

Hydrodynamics and heat transfer in a vertical cylinder exposed to periodically varying centrifugal forces (accelerated crucible-rotation technique)

Investigations are made of the thermal and hydrodynamic structure of liquid flow in a vertical cylinder exposed to the simultaneous effect of thermal gravitational and periodically varying centrifugal forces. The latter were produced by periodic variation of the rotational speed of the cylinder, the axis of which coincided with that of rotation. A vortex flow is found at the bottom of the cylinder, the direction of which depends on the rotational speed of the latter. The hydrodynamics and heat transfer of the liquid stably stratified over the height and exposed to periodically varying centrifugal forces are investigated. It is shown that there are periodic changes of temperature throughout the entire liquid layer; the values of temperature and velocity and their variations at different liquid layer heights are found. The effect of periodically varying forces on the quality of a growing crystal is analysed. The paper might be of interest for thermal physicists and specialists dealing with the growth of monocrystals.