Phase Flow Regimes Research Articles

Influence of Flow Regime on Mass Transfer in Different Types of Microchannels

The performance of microstructured reactors (or microchannels) for mass-transfer-controlled liquid-liquid reactions depends on flow regimes that define the specific interfacial area for the mass transfer. In the present work, experiments were carried out to investigate the two phase-flow regimes and the mass transfer at relatively high throughput for a single microchannel (of 1-18 mL/min) in five generic microchannel designs (with and without structured internal surfaces), using a nonreacting water-acetone-toluene system. When the flow results were analyzed collectively in all microchannels, six different flow regimes such as slug, slug-drop, deformed interface, parallel/annular, slug-dispersed, and dispersed flow were observed. The mass-transfer comparison shows that the microchannel with structured internal surfaces shows better performance, because it creates a very fine dispersion, providing high interfacial area, compared to other microchannels. Finally, the mass-transfer data were correlated, which can be used for a priori predictions of mass-transfer rates in microchannels.

Second law analysis of condenser by using new heat transfer and pressure drop model based on flow regimes

A new heat transfer and pressure drop model is used for second law analysis of condenser by considering three mainly occurring two phase flow regimes such as annular, stratified wavy and fully stratified flows. A single pass double pipe two phase heat exchanger is considered for the analysis with refrigerant R134a as two phase fluid. For each type of flow patterns, the behaviour of entropy generation number due to heat transfer and pressure drop were studied by varying different input parameters of two phase fluid flow such as diameter of two phase tube, mass velocity, difference between saturation and tube wall temperature, quality and saturation temperature.

Development of Gas-Particle Euler-Euler LES Approach: A Priori Analysis of Particle Sub-Grid Models in Homogeneous Isotropic Turbulence

A new large eddy simulation (LES) approach for particle-laden turbulent flows in the framework of the Eulerian formalism for inertial particle statistical modelling is developed. Local instantaneous Eulerian equations for the particle cloud are first written using the mesoscopic Eulerian formalism (MEF) proposed by Fevrier et al. (J Fluid Mech 533:1–46, 2005), which accounts for the contribution of an uncorrelated velocity component for inertial particles with relaxation time larger than the Kolmogorov time scale. Second, particle LES equations are obtained by volume filtering the mesoscopic Eulerian ones. In such an approach, the particulate flow at larger scales than the filter width is recovered while sub-grid effects need to be modelled. Particle eddy-viscosity, scale similarity and mixed sub-grid stress (SGS) models derived from fluid compressible turbulence SGS models are presented. Evaluation of such models is performed using three sets of particle Lagrangian results computed from discrete particle simulation (DPS) coupled with fluid direct numerical simulation (DNS) of homogeneous isotropic decaying turbulence. The two phase flow regime corresponds to the dilute one where two-way coupling and inter-particle collisions are not considered. The different particle Stokes number (based on Kolmogorov time scale) are initially equal to 1, 2.2 and 5.1. The mesoscopic field properties are analysed in detail by considering the particle velocity probability function (PDF), correlated velocity power spectra and random uncorrelated velocity moments. The mesoscopic fields measured from DPS+DNS are then filtered to obtain large scale fields. A priori evaluation of particle sub-grid stress models gives comparable agreement than for fluid compressible turbulence models. It has been found that the standard Smagorinsky eddy-viscosity model exhibits the smaller correlation coefficients, the scale similarity model shows very good correlation coefficient but strongly underestimates the sub-grid dissipation and the mixed model is on the whole superior to pure eddy-viscosity model.

Pressure drop in two phase slug/bubble flows in mini scale capillaries

A segmented two phase slug/bubble flow occurs where a liquid and a gas are pumped into the same tube over a range of Reynolds numbers. This segmented two phase flow regime is accompanied by an increase in pressure drop relative to the single phase flow where only one fluid is flowing in a capillary. This work experimentally and theoretically examines the pressure drop encountered by the slug/bubble flow with varying slug lengths in mini channels. In the experimental work the dimensionless parameters of Reynolds number and Capillary number span over three orders of magnitude, and dimensionless slug length ranges over two orders of magnitude to represent flows typical of mini- and micro-scale systems. It is found, in agreement with previous work, that these dimensionless groups provide the correct scaling to represent the pressure drop in two phase slug/bubble flow, although the additional pressure drop caused by the interface regions was found to be ∼40% less than previously reported.

Pressure drop and heat transfer characteristics of boiling water in sub-hundred micron channel

The current work focuses on the pressure drop, heat transfer and stability in two phase flow in microchannels with hydraulic diameter of less than one hundred microns. Experiments were conducted in smooth microchannels of hydraulic diameter of 45, 65 μm, and a rough microchannel of hydraulic diameter of 70 μm, with deionised water as the working fluid. The local saturation pressure and temperature vary substantially over the length of the channel. In order to correctly predict the local saturation temperature and subsequently the heat transfer characteristics, numerical techniques have been used in conjunction with the conventional two phase pressure drop models. The Lockhart–Martinelli (liquid–laminar, vapour–laminar) model is found to predict the two phase pressure drop data within 20%. The instability in two phase flow is quantified; it is found that microchannels of smaller hydraulic diameter have lesser instabilities as compared to their larger counterparts. The experiments also suggest that surface characteristics strongly affect flow stability in the two phase flow regime. The effect of hydraulic diameter and surface characteristics on the flow characteristics and stability in two phase flow is seldom reported, and is of considerable practical relevance.

Diameter effect on critical heat flux

The critical heat flux look-up table (CHF LUT) is widely used to predict CHF for various applications, including design and safety analysis of nuclear reactors. Using the CHF LUT for round tubes having inside diameters different from the reference 8 mm involves conversion of CHF to 8 mm. Different authors [Becker, K.M., 1965. An Analytical and Experimental Study of Burnout Conditions in Vertical Round Ducts, Aktiebolaget Atomenergie Report AE 177, Sweden; Boltenko, E.A., et al., 1989. Effect of tube diameter on CHF at various two phase flow regimes, Report IPE-1989; Biasi, L., Clerici, G.C., Garriba, S., Sala, R., Tozzi, A., 1967. Studies on Burnout, Part 3, Energia Nucleare, vol. 14, pp. 530–536; Groeneveld, D.C., Cheng, S.C., Doan, T., 1986. AECL-UO critical heat flux look-up table. Heat Transfer Eng., 7, 46–62; Groeneveld et al., 1996; Hall, D.D., Mudawar, I., 2000. Critical heat flux for water flow in tubes – II subcooled CHF correlations. Int. J. Heat Mass Transfer, 43, 2605–2640; Wong, W.C., 1996. Effect of tube diameter on critical heat flux, MaSC dissertation, Ottawa Carleton Institute for Mechanical and Aeronautical Engineering, University of Ottawa] have proposed several types of correlations or factors to describe the diameter effect on CHF. The present work describes the derivation of new diameter correction factor and compares it with several existing prediction methods.

Influence of stirrer type on mixture homogeneity in continuous powder mixing: A model case and a pharmaceutical case

Continuous powder mixers offer a viable alternative to batch processes, but have received very little attention in scientific literature and in the industrial world. Mixer design is still very empirical and is not based on assessed methodologies. In this paper, we report experiments that aimed to compare two very different types of stirrers for a pilot-scale continuous powder mixer, and for two types of mixtures: a model mixture and a real pharmaceutical mixture. The first stirrer A is of the frame type with inclined paddles and internal transporting screw, the other stirrer B is of the shaft type with paddles mounted on it. Results are first presented from the viewpoint of bulk powder flow by hold-up determination and correlation with operating conditions. General relationships are derived which show that the mobile B leads to higher hold-ups, which may be an important drawback. The study of mixture homogeneity globally confirms these findings, especially in a dense phase flow regime. In the fluidised regime, where the stirrer B can be used, attention is drawn to the negative effect of excessive rotational speeds on the quality of the mixtures.

Hydrodynamics of a high pressure three‐phase fluidized bed subject to foaming

AbstractThe effects of pressure and surfactants on the phase holdups and flow regime transition velocities of gas–liquid–solid fluidized beds were investigated. The effect of pressure on the bed phase holdups is significant and more pronounced at larger gas flow rates where pressure has a greater effect on the equilibrium bubble size. The addition of a surfactant leads to an increase in the gas holdup and a lowering of the solids and liquid holdups. The presence of a surfactant with a liquid flow results in shearing of the bubbles across the gas–liquid distributor, limiting the effect of pressure. Finally, for all conditions, gas holdups in the freeboard region were greater than in the bed.

Textural and fractal properties of CuO/Al2O3 catalyst supports

Abstract Reactive amorphous aluminas with a high surface area, suitable as a precursor for catalyst supports, are obtained by flash calcination of gibbsite in a reactor for pneumatic transport in the dilute two phase flow regime. In the present paper we study the effects of the gibbsite dehydration temperature on the textural and fractal properties of activated aluminas. The parameters of the pore structure for all samples were evaluated from nitrogen adsorption–desorption isotherms. On the basis of the sorption-structure analysis, the fractal dimension of the aluminas surface were determined by three methods, according to Pfeifer and Avnir, Neimark et al. and Mahnke and Mogel, with a goal to compare obtained values. As expected, a good agreement between the results of these methods was achieve. The values of the fractal dimension of activated aluminas increase with the increase of the temperature of thermal treatment, indicating that the irregularities of their surfaces are greater.

Flow Regime Study of a Light Material in an Industrial Scale Cold Flow Circulating Fluidized Bed

A series of experiments was conducted in the 0.3meter diameter circulating fluidized bed test facility at the National Energy Technology Laboratory (NETL) of the U. S. Department of Energy. The particle used in this study was a coarse, light material, cork, which has a particle density of 189kg∕m3 and a mean diameter of 812μm. Fluidizing this material in ambient air approximates the same gas-solids density ratio as coal and coal char in a pressurized gasifier. The purpose of this study is twofold. First, this study is to provide a better understanding on the fundamentals of flow regimes and their transitions. The second purpose of this study is to generate reliable data to validate the mathematical models, which are currently under development at NETL. Utilization of such coarse, light material can greatly facilitate the computation of these mathematical models. Furthermore, the ratio of density of cork to air under ambient conditions is similar to the density ratio of coal to gas at the gasification and pressurized fluidized bed combustion environment. This paper presents and discusses the data, which covered operating flow regime from dilute phase, fast fluidization, and to dense phase transport by varying the solid flux, Gs at a constant gas velocity, Ug. Data are presented by mapping the flow regime for coarse cork particles in a ΔP∕ΔL‐Gs‐Ug plot. The coarse cork particles exhibited different behavior than the published literature measurements on heavier materials such as alumina, sand, FCC, silica gel, etc. A stable operation can be obtained at a fixed riser gas velocity higher than the transport velocity, e.g., at Ug=3.2m∕s, even though the riser is operated within the fast fluidization flow regime. Depending upon the solids influx, the riser can also be operated at dilute phase or dense phase flow regimes. Experimental data were compared to empirical correlations in published literature for flow regime boundaries as well as solids fractions in the upper dilute and the lower dense regions for fast fluidization flow regime. Comparisons of measured data with these empirical correlations show rather poor agreements. These discrepancies, however, are not surprising since the correlations for these transitions were derived from experimental data of comparative heavier materials such as sands, FCC, iron ore, alumina, etc.

Identification of two phase flow regimes by void fraction measurements

This paper will present a method to identify flow patterns (bubble & slug flow) in vertical air-water two-phase flow by void fraction measurement (using an impedance probe) at Inst. of Mechanics Hanoi.

Improving the power consumption in pneumatic conveying systems by a daptive control of the flow regime

The pneumatic conveying of solids in a gas stream is a recurrent process in petrochemical industries. However, due to practical limitations the majority of existing systems have capacities ranging from 1 to 400 tones per hour over distances less than 1000 m, mainly because of a high power consumption per transported unit mass. More specifically, to avoid the formation of dense structures such as dunes and plugs, which, depending on the characteristics of the material and on the availability of a pressure head from the carrier phase may cause a violent pressure surge or a possible line blockage, the system is preferably operated at homogeneous dispersed flow. To sustain such a flow regime high velocities are needed and, accounting for the resulting higher pressure drops, higher power consumption is demanded. An optimized pneumatic conveying system can be conceived with the help of adaptive control techniques. In the context described above, lower transport velocities are allowed if the formation of aggregates that precedes the transition to dense phase flow regimes are automatically detected and destroyed, thus, artificially stabilizing the light phase homogeneous flow regime. This work assesses the reduction in the necessary power that the application of such adaptive control technique could produce. Experimental results are presented for a 45 mm i.d. pneumatic conveying system used to transport Setaria Italica seeds. The instrumentation used to identify the flow regime is constituted of several pressure sensors installed along the transport line. The proposed control strategy is based on processing these signals through a neural network model to assess the flow condition and to mimic an optimized gain scheduled PID algorithm. Preliminary results show that reductions in power consumption can reach 50% when compared with classical non controlled transport.

Theoretical analysis of the flow regimes and their characteristics in vertically flowing gas–solids suspensions

This paper analyses the detailed flow structure characteristics of a vertical, fully developed gas–solids suspension. It is based on an asymptotic approach to general multiphase flow equations, in the range of low or moderate solids concentrations. Three distinct flow structures are identified, defining the Similar Profiles, Transition and Dense Phase Flow Regimes, as well as the local mechanism generating the transitions. The predicted properties of the trends of variation of local flow variables, with overall solids loading are, then, compared with existing experimental data. The observed wide agreement encourages the use of the theory for the interpretation and the prediction of the behavior of existing industrial units.

The kinetics of the partial dehydration of gibbsite to activated alumina in a reactor for pneumatic transport

The dehidration kinetics of gibbsite to activated alumina was investigated at four different temperatures between 883 K and 943 K in a reactor for pneumatic transport in the dilute two phase flow regime. The first order kinetic behavior of this reactionwith respect to the water content of the solid material was proved and an activation energy of 66.5 kJ/mol was calculated. The effect of residence time on the water content is given and compared with theoretical calculations. The water content and other characteristics of the products depend on two main parameters, one is the short residence time and the other is the temperature of the dehydration of gibbsite. The short residence time of the gibbsite particles in a reactor for pneumatic transport prevents crystallization into new phases, as established from XRD analysis data. Reactive amorphous alumina powder, with a specific surface area of 250 m2/g, suitable as a precursor for catalyst supports is obtained.

Stochastic Analysis of Gas-Liquid-Solid Flow in Three-Phase Circulating Fluidized Beds.

Characteristics of gas-liquid-solid flow behavior in a riser are investigated in a three-phase circulating fluidized bed (0.102 m I.D.×3.5 m in height). Local gas holdup, solid holdup distribution, and pressure fluctuations in the riser have been measured and utilized to describe the gas-liquid-solid flow behavior more conveniently. The resultant pressure fluctuations have been analyzed by adopting the chaos method: The time series of pressure fluctuations have been interpreted by means of phase space portraits and Kolmogorov entropy. The effects of gas and liquid velocities and solid circulation rate on the local gas holdup, solid axial distribution, phase space portrait, and Kolmogorov entropy of pressure fluctuations, as well as on the flow behavior of gas-liqid-solid mixture in the riser are determined. It is found that pressure fluctuations can be a quantitative tool to characterize the flow behavior and flow regime transition of multiphases in the riser. The relations between the pressure fluctuations and the distribution of phase holdup and flow regime in the riser are also discussed.

Hydrodynamics of annular circulating fluidised bed drier

Hydrodynamics of the annular circulating fluidised bed drier (ACFBD) with variable gap opening was studied with ragi particles of average diameter 1530 μm and density 1272 kg/m 3. For openings greater than 40 mm, the mass flux of solids was constant for all air rates studied. A correlation relating mass flux of solids with air velocity and opening is proposed. Models are proposed for calculating pressure drop separately for the dense slugging flow and dilute phase flow regimes. A correction factor, called erratic factor, has been incorporated for the dense slugging flow regime. The average deviation in the calculated values of pressure drop using the proposed model was 10%.

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.

Identification of two phase flow regimes via diffusional analysis of experimental time series

The problem of identifying different two phase flow regimes from experimental time series by employing the method of diffusional analysis is addressed. This technique, recently applied to the multiphase flow field, is described and compared with other techniques used to characterize multiphase flow regimes. Diffusional analysis is applied to experimental time-series obtained from both a γ-densitometer and capacitance probes. The choice of the appropriate experimental signal to be processed is also discussed. The experimental time series were obtained from a rig with air and light oil. The results obtained confirm the advantages of the method proposed in identifying the features of different flow regimes. The advantages are particularly evident when comparing diffusional analysis with the widely applied Rescaled Range technique.

Phase Portraits and Flow Regimes in a Circular Couette System with Counter-Rotating Cylinders

Experiments have been conducted on the circular Couette system, established recently in our laboratory. By using an optical-electronic modulation feedback Laser Doppler Velocimetry technique, the velocity time series data with higher signal-noise ratio, power spectra, phase portraits and Poincare sections were obtained. Two scenarios to chaos were observed. One is the Ruelle-Takens scenario, while the other (when both cylinders of the Couette system are counter-rotating) is quite different from it. A tremendous variety of flow regimes and a new state were observed. A tentative explanation is made for the new state.

Axial Dispersion and Phase Holdup Characteristics in Reciprocating Plate Extraction Columns

Abstract Axial dispersion and phase holdup characteristics have been determined in a 0.102-m i.d. × 3.5 m high QVF glass column. The axial dispersion coefficient decreases with increasing reciprocating frequency (f) and amplitude (A) in the inhomogeneous dispersed phase flow regime, whereas it increases in the emulsion flow regime. The axial dispersion coefficient with a perforated plate increases with continuous and dispersed phase velocities. However, the effect of phase velocities on axial dispersion is less pronounced with the fan plate. The axial dispersion coefficient can be correlated with A 2 f, fluid velocities, and the free fractional opening area of the plates. The dispersed phase holdup increases with an increase in agitation intensity. Af, and decreases with the free opening area of the plate.