Phase Flow Patterns Research Articles

Visualization and Image Measurement of Buoyant Bubbly Flow through Circular Cylinders Arranged in a Narrow Channel.

Bubble-driven flow around circular cylinders in a narrow channel is experimentally investigated by means of image processing. The cylinders are settled in the channel with three kinds of arrangement pattern to clarify the difference of the internal two-phase flow structure. Thinning algorithm is applied to the detection of all the bubbles in order to measure the bubble motion in locally high void fraction. Size and spatial distribution of the bubbles are also obtained with the image analysis to discuss the bubble's coalescence/fragmentation due to the collision with the cylinders. The results imply that the rising velocity of the bubble is governed not only by bouncing on the cylinders but also by the liquid phase flow pattern that is sensitively dependent on the gas flow rate and the cylinders' arrangement pattern.

Two-phase Couette–Taylor flow: Arrangement of the dispersed phase and effects on the flow structures

This study investigates the mutual interactions between a continuous and a dispersed phase (noncondensable or condensable) in the well-known Couette–Taylor flow between two concentric cylinders at low Reynolds numbers, where the outer cylinder is immobilized. In this experiment, the turbulent structures take place progressively. The noncondensable dispersed phase (air) is introduced either by ventilation, generated by agitation of a free surface situated at the top of the gap between the two cylinders. The condensable dispersed phase is generated by cavitation due to a drop in pressure. Comparisons are made between the single phase flow patterns and those observed in ventilated or cavitating flow. Two particular arrangements of the dispersed phase are experimentally evident, according to the Reynolds number of the flow. For low Reynolds numbers, bubbles are trapped in the core of the Taylor cells, whereas they migrate to the outflow regions near the inner cylinder for higher Reynolds numbers. Assessment of the forces applied to the bubbles and computation of their equilibrium position can act as a base in describing the bubble capture. When bubbles are located near the wall in the outflow region, it is found that the three first instabilities are strongly influenced by the dispersed phase. The cavitating flow is also characterized by an earlier appearance of the third instability.

Two- phase flow patterns and heat transfer in parallel microchannels

Microchannel heat sinks with two-phase flow can satisfy the increasing heat removal requirements of modern micro electronic devices. One of the important aspects associated with two- phase flows in microchannels is to study the bubble behavior. However, in the literature most of the reports present data of only a single channel. This does not account for flow mixing and hydrodynamic instability that occurs in parallel microchannels, connected by common inlet and outlet collectors. In the present study, experiments were performed for air- water and steam- water flow in parallel triangular microchannels with a base of 200–300 µ m. The experimental study is based on systematic measurements of temperature and flow pattern by infrared radiometry and high-speed digital video imaging. In air-water flow, different flow patterns were observed simultaneously in the various microchannels at a fixed values of water and gas flow rates. In steam-water flow, instability in uniformly heated microchannels was observed. This work develops a practical modeling approach for two-phase microchannel heat sinks and considers discrepancy between flow patterns of air- water and steam- water flow in microchannels.

Heat Transfer and Evaporation from a Plane Surface into a Half-Space upon a Sudden Increase in Body Temperature

The one-dimensional time-dependent problem of evaporation from a plane body surface into a half-space filled by a gas (condensed phase vapor) upon a sudden increase in the body surface temperature is studied. The evaporation coefficient is the problem parameter and may take arbitrary values within the limits from zero to unity. The problem is formulated for the kinetic equation and solved by the finite-difference method. It is shown that a deviation of the evaporation coefficient from unity considerably modifies the gas phase flow pattern. However, the evaporation rate divided by the rate of evaporation into a vacuum at the given surface temperature is only weakly dependent on the evaporation coefficient.

The use of ozone as a gas tracer for kinetic modeling of aqueous environmental ozonation processes

Ozone has been used as a tracer for establishing the flow pattern of the gas phase in a bubble column. Negative step tracer experiments have been conducted to determine the residence time distribution function of the gas phase through a bubble column when flowing counter‐currently to a water stream. The results have been interpreted following the tank in series model and the dispersion model for non‐ideal flow. At the experimental conditions investigated the gas phase flow pattern can be characterized as the equivalent to two well mixed tanks in series. Values of the dispersion number were also calculated.

Investigation of Holdup and Pressure Drop Behavior for Oil-Water Flow in Vertical and Deviated Wells

Two-phase flow of oil and water is commonly observed in wellbores, and its behavior under a wide range of flow conditions and inclination angles constitutes a relevant unresolved issue for the petroleum industry. Among the most significant applications of oil-water flow in wellbores are production optimization, production string selection, production logging interpretation, down-hole metering, and artificial lift design and modeling. In this study, oil-water flow in vertical and inclined pipes has been investigated theoretically and experimentally. The data are acquired in a transparent test section (0.0508 m i.d., 15.3 m long) using a mineral oil and water (ρo/ρw = 0.85, μo/μw = 20.0 & σo−w = 33.5 dyne/cm at 32.22°C). The tests covered inclination angles of 90, 75, 60, and 45 deg from horizontal. The holdup and pressure drop behaviors are strongly affected by oil-water flow patterns and inclination angle. Oil-water flows have been grouped into two major categories based on the status of the continuous phase, including water-dominated and oil-dominated flow patterns. Water-dominated flow patterns generally showed significant slippage, but relatively low frictional pressure gradients. In contrast, oil-dominated flow patterns showed negligible slippage, but significantly large frictional pressure gradients. A new mechanistic model is proposed to predict the water holdup in vertical wellbores based on a drift-flux approach. The drift flux model was found to be adequate to calculate the holdup for high slippage flow patterns. New closure relationships for the two-phase friction factor for oil-dominated and water-dominated flow patterns are also proposed.

97/05038 Two phase flow patterns and boiling heat transfer in tubes containing helical wire inserts. Part II—critical heat flux studies: Lan, J. et al. Enhanced Heat Transfer, 1997, 4, (4), 283–296

Particle size effects on boiling performances of micro-porous enhanced surfaces are studied using five different sizes of diamond particles. By comparing the coating thicknesses with the superheated liquid layer thickness, the coatings are classified into two groups: ‘micro-porous’ and ‘porous’ coatings. The superheated liquid layer thickness is calculated using one-dimensional transient thermal conduction. Micro-porous coating shows different characteristics of boiling performances compared to porous coating in incipient superheat, nucleate boiling and CHF.

97/05037 Two phase flow patterns and boiling heat transfer in tubes containing helical wire inserts part I—flow patterns and boiling heat transfer coefficients: Lan, J. et al. Enhanced Heat Transfer, 1997, 4, (4), 269–281

97/05037 Two phase flow patterns and boiling heat transfer in tubes containing helical wire inserts part I—flow patterns and boiling heat transfer coefficients: Lan, J. et al. Enhanced Heat Transfer, 1997, 4, (4), 269–281

Uniqueness And Simultaneous Predictability of Relative Permeability And Capillary Pressure By Discrete And Continuous Means

Abstract Requirements for unique estimation of two-phase flow functions are investigated. This is achieved by increasing the number of observable parameters by using both external and internal data, by representing the flow functions via global empirical functions, discrete values, and piece-wise local functions and by implementing the imulated annealing method for non-linear global optimization. Applications involving unsteady-state drainage and imbibition unsteady-state core flood tests are presented. It is shown that global functional representations of the relative permeability and capillary pressure data are not sufficient and a discrete representation of flow functions leads to non-smooth functions. However, a piece-wise functional representation of relative permeability and capillary pressure data can be determined uniquely when the transient-state internal saturation profiles and the overall pressure differentials are used together for history matching. Introduction Petrophysical properties of multiphase flow systems in porous rock are complex functions of the morphology and topology of the porous medium, interactions between rock and fluids, phase distribution and flow patterns and regimes. It is impractical to deconvolute the effect of the individual factors and forces from the macroscopic petrophysical properties. Therefore, the effect of these properties on the flow behaviour is lumped in the form of empirically determined relative permeability and capillary pressure functions which are used as the primary flow parameters for the macroscopic description of multiphase flow in porous media. However, development of reliable methods for the simultaneous determination of relative permeability and capillary pressure data from laboratory core flood tests is a challenging task and is of continuing interest to the oil and gas industry. This paper addresses the issue of uniqueness in the determination of relative permeability and capillary pressure functions by means of the history matching of unsteady-state displacement data obtained from laboratory core flow tests. History matching (the inverse problem) requires a reliable porous media averaged, macroscopic flow description model (the forward problem) to predict the values of the observable parameters such as cumulative production, pressure drop histories and saturation history profiles so that the best estimates of the relative permeabilities and capillary pressure functions can be determined. However, some model parameters may be overdetermined while leaving the others underdetermined, unless the interpretation method is designed to assimilate a proper set of experimental data. Lack of intrinsic data, experimental uncertainties, and an accurate physical representation of the complex flow affect the reliability of the predictions. When a problem is ill-posed, its solution may not necessarily be unique and, therefore, perturbation of any model parameters may lead to arbitrary variations of the solutions. As demonstrated in the present study, observed values are quite sensitive to the flow functions. In reservoir simulation, the flow functions are usually chosen as the first model parameters to be tuned for history matching. Presently, there are no satisfactory general theoretical flow functions available. For the prediction of two phase relative permeabilities alone, over thirty different empirical models have been proposed (Honarpoor et al.(1), and Siddiqui et al.(2)). Although these empirical models are applicable only to specific conditions, in many applications, they have been selected arbitrarily without any particular basis.

Heat Transfer Characteristics of an Impinging Air Jet Laden with Solid Particles Recirculating in an Enclosure. 1st Report. Flow Characteristics and Volume Fraction Distribution of Gas-Solid Suspension.

To avoid the complicated system and additional pump power required for a gas-solid suspension flow, the authors proposed a new concept of an impinging air jet laden with solid particles recirculating in an enclosure, and the heat transfer characteristics were experimentally studied. As the first step of this study, this paper deals with the flow characteristics of gas-solid suspension in the enclosure. Since a preliminary experiment showed that the configuration of the cell and the position of the gas exhaust hole strongly affect the flow pattern and the impinging particle volume fraction, gas-solid two-phase flow within two types of cells, namely parabolic plane cell and inclined plane cell, was examined. In addition to the observation of flow pattern and measurement of pressure drop, the volume fraction distribution of particles was obtained through an optical experiment. By comparing the results of the two cells, it was clearly shown that the flow pattern within the parabolic plane cell is a smooth two phase flow pattern and thus the volume fraction of impinging particles is higher than that of the inclined plane cell. The results enable us to predict that the parabolic plane cell has more favorable characteristics for heat transfer enhancement.

Delay of Nonsteady Cavitation in Restrictors for Fluid Power Systems.

In fluid power systems, cavitation under nonsteady flow conditions is a major factor limiting the response and reliability. As the most fundamental phenomenon, the time lag of nonsteady cavitation in restrictors is discussed. First, the times required to establish the single phase flow pattern and for bubbles to grow from cavitation nuclei are estimated and found to be much shorter than that required to generate the nuclei. Then, by applying both the hole theory and theory of rate processes, a predication formula for the time lag is derived. The calculated results coincide with the experimental ones.

Transport phenomena in bubble colunm reactor II: Pressure drop

The two-phase frictional pressure drop was measured in a 0.15 m internal diameter bubble column operated in a continuous co-current manner. The fractional gas hold-up was measured using the gamma-ray absorption technique and the pressure drop using modified differential manometry. The superficial gas and liquid velocities were varied in the ranges 0.127–0.451 m s −1 and 0.047–0.236 m s −1 respectively. The effects of liquid viscosity, non-Newtonian behaviour, and the presence of electrolyte were investigated. A coherent model was developed to predict the pressure drop in homogeneous (bubbly flow) and heterogeneous (churn-turbulent) regimes. For this purpose, knowledge of the liquid phase flow pattern was used. A comparison between model predictions and experimental observations is presented. On the basis of the pressure drop, a criterion was developed to predict the transition from the homogeneous to the heterogeneous regime. A favourable comparison is shown between model predictions and experimental observations.

Transport phenomena in bubble column reactors I: Flow pattern

A new model has been developed for the liquid phase flow pattern in bubble columns. This model satisfies the mass, momentum and energy balances for the gas and liquid phases. Further, it takes into account the variations in the flow pattern due to changes in the physical properties of the liquid phase. The model can be used for batch as well as continuous (co-current and counter-current) modes of operation. A satisfactory agreement is shown between model predictions and experimental observations. A simplified procedure is suggested for quick estimation of the liquid phase velocity profile. The proposed model is expected to be useful for the rational prediction of transfer coefficients for momentum, mass and heat. The prediction and measurement of the pressure drop is discussed in the companion paper.

Design of pressurized coil of the small medical Dewar

Two phase flow pattern in pressurized coil has been predicted. The formulae, procedure, general computer program and flow sheet for calculating pressure drop and heat transfer in the coil are presented. The work attempts to provide optimum design method for small medical Dewar and provide a clear understanding of two phase flow heat transfer in the pressurized coil.

Multiphase Reactors: Models and Experimental Verification

Identification and quantification of flow regimes, phase holdup distributions, flow patterns and backmixing is essential for proper design and scale-up of multiphase reactors. Existing models often suffer from inadequate experimental confirmation. Here, we describe recent progress made in our laboratory in characterizing liquid circulation and backmixing in bubble columns and in quantifying, via a phenomenological model, the behavior of trickle-beds in the low interaction regime. The need for imaging techniques such as fan-beam tomography and PET is illustrated. Liquid mixing in bubble columns caused by global liquid convection and by turbulent eddies is usually described by the axial dispersion model (ADM) which, at least in the churn turbulent regime, lacks a theoretical basis. In the ADM the two mixing mechanisms are lumped into a single effective dispersion coefficient. An alternative backmixing description assumes multiple liquid circulation cells, with height equal to column diameter, to occur in the column. We have successfully demonstrated that instantaneous and time averaged liquid velocity profiles in the column can be determined by monitoring the motion of a neutrally bouyant tracer particle via a Computer Aided Radioactive Particle Tracking (CARPT) technique. It can now be shown that a single strong liquid circulation cell exists in columns of a variety of diameters and at different operating conditions but that smaller circulation cells can form in the distributor region. Model predicted one dimensional axial time averaged liquid velocity profile agrees well with the data and applies to the middle section of the column. The CARPT technique allows the computation of Lagrangian autocorrelation coefficients, the rms fluctuating velocities, Lagrangian integral time scales and the turbulent dispersion coefficients. Thus CARPT provides the necessary information i. e. velocity profiles and turbulent dispersion coefficients, for implementation of a convection dispersion model. The planned implementation of fan-beam tomography would quantify the two-dimensional holdup distribution and together with CARPT provide all the data necessary for verification of fundamental two phase flow models. Pressure drop, holdup, flow regime transition and phase distribution in trickle-beds has received considerable attention but no generally accepted theory for prediction of these quantities exists. Here we describe a phenomenological model for the low interaction regime which views the bed as an array of slits with liquid film flow. Universal velocity profile is used to describe the flow in both liquid films and the gas core and a large data bank for pressure drop and holdup was used to confirm that in the low interaction regime each of the two phases ignores the presence of the other one. The final model contains only two parameters, which are determined from single phase flow experiments, and contains no constants fitted to two phase flow data. The model predicts pressure drop and holdup in the uniform low interaction regime better than any of the existing models. Furthermore, introducing the Kapitza's criterion for laminar liquid film instability into the model the flow regime transition to pulsing is predicted well for all the data that satisfy the conditions for the theory to apply. Other pore level transition mechanisms are suspected for other data. The above phenomenological model is also used as a basis for predicting liquid distribution in a cell model of a trickle-bed. Experimental confirmation awaits the application of PET technology.

Fluidisation of fine and very dense hardmetal powders

The fluidisation characteristics of a hardmetal powder, of extremely high density and small primary particle size such that it belongs to Group C in Geldart's classification, has been investigated. Surprisingly, it was found that in this case the powder could be fluidised via self-agglomeration, though such behaviour is not predicted from the primary particle diameter and/or its density, or the Geldart classification. When the gas is flowing for the first time through a bed of this powder, three regimes of two phase flow patterns were distinguished with increasing gas velocity: gas—‘solid powder’; gas—‘solid powder’ and agglomerates; gas—agglomerates. Subsequently, only the third condition is found with the agglomerate size dependent on gas velocity. A theoretical analysis of the first pattern is also presented, as well as a general qualitative description of the whole process.

Transition of two phase flow patterns. Transition of bubble flow to other flow patterns in air and water at atmospheric pressure and temperature.

Progressive change of flow pattern of two phase flow was studied experimentally in the transition region from bubble flow to slug flow or annular flow. In the experiment, three transparent straight circular tubes which had different diameters were employed for the test section. The water flew upward in the test section and the air was issued from a circular nozzle located at the bottom of the test section. The experiment was performed at atmospheric pressure and temperature. The transition from bubble flow to slug flow is classified into three parts for the region of flow state in the result. The first part is a lump of air contacting the nozzle, the second part is a developing region from bubble flow to bubble cap and the third part is a region between the bubble cap and Taylor bubble. The entrance region from the air nozzle to the Taylor bubble is independent of Reynolds number of the water flow, but depends considerably on the ratio of air flow rate to the flow rates of air and water.

Design of semi-batch gas-liquid-liquid polymerization reactors with application to interfacial condensation

A physical and chemical model for use in the analysis of gas-liquid-liquid stepwise condensation polymerizations carried out in a continuous flow stirred tank reactor and other related modes of operation is presented. The model accounts for finite mass transfer resistances between the gas-liquid and partially miscible liquid-liquid phases and includes either plug-flow or complete backmixing as choices for the gas phase flow pattern. Polymerization kinetics involving multiple reactions with a chain growth stopper are described, various limiting cases of the model are given, and methods for solving the model equations are summarized. The model equations are applied to the gas-liquid-liquid interfacial polymerization of bisphenol-A to polycarbonate as an example of a complex system having industrial importance. The effects of various model parameters on the polymer chain length distribution are examined for a particular limiting case.