Showing papers on "Slug flow published in 2005"

Inertial and Interfacial Effects on Pressure Drop of Taylor Flow in Capillaries

Abstract: In a capillary, the two-phase pressure drop in Taylor slug flow was measured. A carefully designed inlet section for the capillary allowed the independent variation of gas bubble and liquid slug length. Gas and liquid superficial velocities were varied from 0.04 m/s to 0.3 m/s. If the slug length was smaller than 10 times the capillary diameter, the length-averaged friction factor for the liquid slug increased drastically from the single phase value (f = 16/Re) due to differences in curvature at the front and the back of the bubble. The use of different liquids allowed the independent variation of Re and Ca. The flow of elongated bubbles in capillaries was simulated using the CFD code FIDAP. It was found both numerically and experimentally that for Re ≫ 1, the extra pressure terms may be taken account using (Ca/Re) as a parameter. The numerical results agreed with the experimental data, provided that Marangoni effects of impurities are taken into account. The results allow the determination of slug length from pressure drop measurements in closed equipment where the slug length cannot otherwise be measured easily, such as monoliths and microreactors. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Internal Circulation within the Liquid Slugs of a Liquid-Liquid Slug-Flow Capillary Microreactor

Abstract: A so-called “slug-flow” capillary microreactor has been proposed for the investigation of mass-transfer-limited liquid−liquid reactions. Internal circulation within the slug leads to an intensified and tunable mass transfer. Understanding the development of the circulatory flows and the influence of operating parameters upon them is thus crucial. In this study, experiments were carried out to visualize the internal circulations using particle image velocimetry (PIV) technique. State-of-the-art computational fluid dynamics (CFD) simulations were used to predict the internal circulation within the liquid slugs and a CFD particle tracing algorithm employed to visualize them. Each slug was modeled as a distinct single-phase flow domain. The effect of the flow velocity and slug length on the velocity profile and stagnant zones of the internal circulations for a slug with and without a wall film is discussed. The internal circulations could be qualitatively and quantitatively characterized with the help of the ...

Hydrodynamics of Taylor Flow in Vertical Capillaries: Flow Regimes, Bubble Rise Velocity, Liquid Slug Length, and Pressure Drop

Abstract: Two-phase flow hydrodynamics in vertical capillaries of circular and square cross sections were experimentally studied, using air as the gas phase and water, ethanol, or an oil mixture as the liquid phase. The capillary hydraulic diameters ranged from 0.9 mm to 3 mm, with the superficial gas and liquid velocities covering a span of 0.008−1 m/s, which is typical of that obtained in monolith reactors. Using a high-speed video camera, four distinct flow regimes were observed within the range at which experiments were conducted: bubbly, slug-bubbly, Taylor, and churn flows. Annular flow was observed at excessively high gas and low liquid flow rates, well beyond those of interest to this study. Based on the definition of a two-class flow regime, the combination of two parametersthe slip ratio (S) and the ratio of the superficial gas velocity to two-phase superficial velocity (UG/UTP)was observed to be suitable for determining the transition from homogeneous flow to nonhomogeneous flow. The influence of capill...

Design for mixing using bubbles in branched microfluidic channels

Abstract: This letter describes a method for producing chaotic transport trajectories in planar, microfluidic networks prepared by standard, single-step lithography and operated with a steady-state inflow of the fluids into the device. Gaseous slugs flowing through the network produce temporal variation of pressure distribution and lead to stretching and folding of the continuous fluid. Stabilization of the bubbles by surface-active agents is not necessary, and the method is compatible with the wide range of reactions performed in on-chip bioassays.

Inhomogeneous MUSIG Model - a Population Balance Approach for Polydispersed Bubbly Flows

Abstract: Many flow regimes in Nuclear Reactor Safety (NRS) Research are characterized by multiphase flows, with one phase being a continuous liquid and the other phase consisting of gas or vapour of the liquid phase. In the range of low to intermediate volume fraction of the gaseous phase the multiphase flow under consideration is a bubbly or slug flow, where the disperse phase is characterized by an evolving bubble size distribution due to bubble breakup and coalescence processes. The paper presents a generalized inhomogeneous Multiple Size Group (MUSIG) Model. Within this model the disperse gaseous phase is divided into N inhomogeneous velocity groups (phases) and each of these groups is subdivided into M bubble size classes. Bubble breakup and coalescence processes between all bubble size classes are taken into account by appropriate models. The derived inhomogeneous MUSIG model has been validated against experimental data from the TOPFLOW test facility at the Research Center Rossendorf (FZR). Comparisons of gas volume fraction and velocity profiles with TOPFLOW-074 test case data are provided, showing the applicability and accuracy of the model for polydispersed bubbly flow in large diameter vertical pipe flow.

Stabilizing control and controllability. Control solutions to avoid slug flow in pipeline-riser systems

Abstract: Riser slugging is a flow regime that can occur in multiphase pipeline-riser systems, and is characterized by severe flow and pressure oscillations. The irregular flow caused by riser slugging can c ...

Development of an X-ray computed tomography (CT) system with sparse sources: application to three-phase pipe flow visualization

Abstract: This paper describes the application of a two-beam X-ray computed tomography (CT) system to multiphase (gas–oil–water) flow measurement. Two high-voltage (160 keV) X-ray sources are used to penetrate a 4-in. (101.6 mm ID) pipeline. A rotating filter wheel mechanism is employed to alternately “harden” and “soften” the X-ray spectra to provide discrimination between the three phases. Because this system offers only two projections, conventional back-projection algorithms are ineffective and thus a new reconstruction technique has been developed. A matrix equation is formed, to which additional “smoothing equations” are added to compensate for the lack of projection data. The tomographic result is obtained by computing an inverse matrix. This is a one-off computation and the inverse is stored for repeated use; reconstructed images from synthesized data demonstrate the effectiveness of this technique. Three-phase tomographic images of a horizontal slug flow are presented, which clearly show the mixing of oil and water layers within the slug body. The relevance of this work to the offshore oil and gas industry is summarized.

A non-intrusive probe for bubble profile and velocity measurement in horizontal slug flows

Abstract: A new technique was developed for measuring the profile and mean velocity of elongated bubbles in horizontal air–water slug flows. It is based on the capacitance between two thin electrodes mounted on the external surface of a dielectric pipe, and has advantages in relation to the traditional parallel wire technique, since it is not intrusive, the presence of impurities in the liquid phase has no influence on the probe response, and it is applicable to very low electrical conductivity liquids, such as oils and deionized water. Tests were performed in an experimental facility with a 5 m long, 34 mm internal diameter Plexiglas pipeline. The elongated bubble mean velocity was determined by using a cross correlation technique applied to the signals coming from two identical capacitance probes, mounted 50 mm distant from each other. The results were compared with an empirical correlation from the literature. Discordance was observed only for flows near the flow pattern transition regions in the flow pattern map.

Solids deposition in low-velocity slug flow pneumatic conveying

Abstract: Solids deposition in the horizontal pipeline of a pneumatic conveying system was studied both mathematically and experimentally. Mathematically modelled results using the coupled discrete element method (DEM) and computational fluid dynamics (CFD) approach have demonstrated an intensive exchange of particles between the stationary layer (deposited particles) and the moving slug and a variation of solids concentration and pressure and velocity distributions across the slug. Slug flows were also visualised experimentally through a glass section and analysed by a high-speed video camera. The amount of particle deposition in the pipeline after a conveying was calculated by controlling the solids feeding rate using a rotary valve and by monitoring the solids flow out of the system using dynamic load cells. Experimentally generated data have quantitatively shown a tendency of more solids deposition with lower gas mass flow rate in slug flows except that, below a certain amount of solids mass flow rate, the deposition becomes independent of gas flow rate.

Experimental two-phase fluid flow in microchannels

Abstract: Micro or mini heat spreaders are used in the interest of providing higher cooling capability for microtechnologies. Heat spreaders using micro or mini channels are not yet well studied, for this the fundamentals of two-phase heat transfer in microchannels are being studied. Here, a comprehensive experimental two-phase flow study has been carried out on two single round tubes (D = 0.509 and 0.790 mm) and for two different fluids: R-134a and R-245fa. An optical measurement method for two-phase flow characterization in microtubes has been applied to determine the frequency of bubbles exiting a microevaporator, the coalescence rates of these bubbles and their lengths as well as their mean two-phase vapor velocity. Four principal flow patterns (bubbly flow, slug flow, semi-annular flow and annular flow) with their transitions (bubbly/slug flow and slug/semi-annular flow) were observed. A new type of flow pattern map for evaporating flow in microchannel has been developed. The first zone corresponds to the isolated bubble regime. It includes both bubbly flow or/and slug flow and is present up to the onset of coalescence. The second zone is the coalescing bubble regime. It is present up to the end of coalescence process. The third zone is the annular zone and is limited by the fourth zone of this diabatic map, the onset of critical heat flux. This flow pattern map can be used for heat transfer model and design of micro evaporator. The vapor velocity or cross sectional void fraction have been measured. For R-134a, the flow can be considered to be homogeneous (or near homogeneous). For R-245fa, more tests exhibit instabilities and surprisingly show vapor velocities below those of homogeneous flow. Frictional two-phase pressure drops have been measured over a wide range of conditions for the two microchannels and two fluids. Three regimes are distinguishable when regarding to the variation of the adiabatic frictional pressure drop with the vapor quality or the two-phase friction factor with the two-phase Reynolds number: a laminar regime for ReTP < 2000, a transition regime for 2000 ≤ ReTP ≤ 8000 and a turbulent regime for ReTP ≥ 8000. The turbulent two-phase flows are best predicted by the Muller-Steinhagen correlation. New accurate CHF data have been measured with the test facility. A new microchannel version of the Katto-Ohno correlation has been developed to predict the CHF in circular, uniformly heated microchannels. Moreover, a new transition curve from annular flow to dryout has been proposed.

Backlight imaging tomography for gas?liquid two-phase flow in a helically coiled tube

Abstract: Air–water two-phase flow in a helically coiled tube is investigated using backlight imaging tomography to elucidate the effect of centrifugal acceleration on phase distribution and interfacial structure. Superficial velocities up to 6 m s−1 in 20 mm diameter tube are tested. We focused on a slug flow regime in which centrifugal acceleration dominates the flow. The interfacial structure is visualized in six directions using a set of originally designed mirror-mounted water jackets. A temporal expansion image is made from line-sampled images and is used to reconstruct phase distribution through a linear backward projection algorithm. The present topography measurement showed various new features of gas–liquid two-phase flow in a helically coiled tube, such as a wall-covering effect in the case of high superficial velocity.

Method and apparatus for preventing slug flow in pipelines

Abstract: Disclosed is a method and system for controlling the formation of liquid or gas slugs along a pipeline (120). In embodiments, an injection unit (104) injects a liquid surface tension reducing agent, such as a foamant, into the pipeline upstream of the high point. A control unit (208) can be used to control the injection unit. In certain arrangements, the control unit adjusts the injection of the agent based on measured parameters of interest. In embodiments where the control unit utilizes temperature measurements, one or more temperature sensors (132) are positioned along the pipeline. The control unit utilizes the temperature measurements to determine whether a predetermined condition exists or a liquid or gas slug is present.

Experimental Study on the Transition in the Velocity of Individual Taylor Bubbles in Vertical Upward Co-Current Liquid Flow

Abstract: An experimental study is presented concerning the transition in the velocity of individual Taylor bubbles in vertical co-current liquid flow. Velocities of individual Taylor bubbles rising in co-current liquids (kinematic viscosities from 10 −6 to 5.7 × 10 −6 m 2 s −1 ) in acrylic columns of 22 mm, 32 mm and 52 mm internal diameter were measured for a wide range of Reynolds number of the flowing liquid using two non-intrusive experimental techniques. The measuring section was located at 6.0 m from the gas injection. The operating conditions used correspond to inertial controlled regime. The data showed an unexpected feature of the bubble motion: the velocity coefficient C changes even when the flow regime in the liquid ahead the bubble is still laminar, i.e., the transition in the bubble velocity starts at liquid Reynolds numbers much lower than 2100. Additional experiments, employing PIV measurements, showed a developed laminar liquid flow ahead the bubble nose. Based on a dimensional analysis, the most important dimensionless numbers for the phenomena were identified and, after processing all data, an empirical correlation was established to predict the velocity coefficient C for a large range of operation conditions. This information is very important for vertical two-phase slug flow modelling.

Experimental characterization of flow boiling heat dissipation in a microchannel heat sink with different orientations

Abstract: Experimental characterization of FC-72 flow boiling in a microchannel heat sink is presented for three different orientations: vertical upflow (VU), vertical downflow (VD) and horizontal flow with horizontal facing (HH). The heat sink consisted of 21 microchannels in parallel, with the channel dimensions of 0.2mm (W) times 2mm (H) times 15mm (L). The heat sink was attached onto a flip chip BGA package with a thermal grease at the interface to minimize the contact resistance. The experiments were conducted based on a subcooling around 31 degC and inlet thermodynamic equilibrium quality x=-0.4 with varying flow rates and power inputs. Boiling with bubbles was observed at the heat sink outlet even if the exit quality x<0. It was found that the flow boiling regime can be represented by isolated bubble flow, slug flow in the present microchannels. Both the junction to inlet thermal resistances and pressure drops are presented as functions of the power input. The effect of the flow orientations on both thermal and hydraulic performances was examined and discussed

Homotopy analysis of Couette and Poiseuille flows for fourth grade fluids

Abstract: The steady flow of a fluid, called a fourth grade fluid, between two parallel plates is considered. Depending upon the relative motion of the plates we analyze four types of flows: Couette flow, plug flow, Poiseuille flow and generalized Couette flow. In each case, the nonlinear differential equation describing the velocity field is solved using perturbation technique and homotopy analysis method. The pressure distribution is also found. It is observed that the homotopy analysis method is more efficient and flexible than the perturbation technique.