Showing papers on "Slug flow published in 2004"

Heat Transfer Mechanisms During Flow Boiling in Microchannels

Abstract: The forces due to surface tension and momentum change during evaporation, in conjunction with the forces due to viscous shear and inertia, govern the two-phase flow patterns and the heat transfer characteristics during flow boiling in microchannels. These forces are analyzed in this paper, and two new nondimensional groups, K1 and K2 , relevant to flow boiling phenomenon are derived. These groups are able to represent some of the key flow boiling characteristics, including the CHF. In addition, a mechanistic description of the flow boiling phenomenon is presented. The small hydraulic dimensions of microchannel flow passages present a large frictional pressure drop in single-phase and two-phase flows. The small hydraulic diameter also leads to low Reynolds numbers, in the range 100‐1000, or even lower for smaller diameter channels. Such low Reynolds numbers are rarely employed during flow boiling in conventional channels. In these low Reynolds number flows, nucleate boiling systematically emerges as the dominant mode of heat transfer. The high degree of wall superheat required to initiate nucleation in microchannels leads to rapid evaporation and flow instabilities, often resulting in flow reversal in multiple parallel channel configuration. Aided by strong evaporation rates, the bubbles nucleating on the wall grow rapidly and fill the entire channel. The contact line between the bubble base and the channel wall surface now becomes the entire perimeter at both ends of the vapor slug. Evaporation occurs at the moving contact line of the expanding vapor slug as well as over the channel wall covered with a thin evaporating film surrounding the vapor core. The usual nucleate boiling heat transfer mechanisms, including liquid film evaporation and transient heat conduction in the liquid adjacent to the contact line region, play an important role. The liquid film under the large vapor slug evaporates completely at downstream locations thus presenting a dryout condition periodically with the passage of each large vapor slug. The experimental data and high speed visual observations confirm some of the key features presented in this paper. @DOI: 10.1115/1.1643090#

Transport of bubbles in square microchannels

Abstract: Liquid/gas flows are experimentally investigated in 200 and 525 μm square microchannels made of glass and silicon. Liquid and gas are mixed in a cross-shaped section in a way to produce steady and homogeneous flows of monodisperse bubbles. Two-phase flow map and transition lines between flow regimes are examined. Bubble velocity and slip ratio between liquid and gas are measured. Flow patterns and their characteristics are discussed. Local and global dry out of the channel walls by moving bubbles in square capillaries are investigated as a function of the flow characteristics for partially wetting channels. Two-phase flow pressure drop is measured and compared to single liquid flow pressure drop. Taking into account the homogeneous liquid fraction along the channel, an expression for the two-phase hydraulic resistance is experimentally developed over the range of liquid and gas flow rates investigated.

Transport Phenomena in Two-Phase Micro-Channel Heat Sinks

Abstract: The design and reliable operation of a two-phase micro-channel heat sink require a fundamental understanding of the complex transport phenomena associated with convective boiling in small, parallel coolant passages. This understanding is the primary goal of this paper. This goal is realized by exploring the following aspects of boiling in micro-channels: hydrodynamic instability, two-phase flow patterns, pressure drop, and convective boiling heat transfer. High-speed photographic methods were used to determine dominant flow patterns and explore as well as characterize hydrodynamic instabilities. Two types of dynamic instability were identified, a severe pressure drop oscillation and a mild parallel channel instability, and a simple method is recommended to completely suppress the former. Predictions of three popular two-phase pressure drop models and correlations were compared to micro-channel water data, and only a separated flow (Lockhart-Martinelli) correlation based on the assumption of laminar flow in both phases gave acceptable predictions. Several popular heat transfer correlations were also examined and deemed unsuitable for micro-channel heat sinks because all these correlations are based on turbulent flow assumptions, and do not capture the unique features of micro-channel flow such as abrupt transition to slug flow, hydrodynamic instability, and high droplet entrainment in the annular regime. These findings point to the need for further study of boiling behavior and new predictive tools specifically tailored to micro-channel heat sinks.Copyright © 2002 by ASME

Adiabatic gas–liquid flow in microchannels

Abstract: This article presents a review of adiabatic two-phase flow in minichannels and microchannels. Differences between them are identified and explained based on this review and our own research. Severa...

Cap bubbly-to-slug flow regime transition in a vertical annulus

Abstract: The transition of the cap bubbly-to-slug flow regime was studied by using impedance void meters in a vertical annular flow channel. The characteristics of various flow regimes were demonstrated by the acquired impedance signals. The statistical parameters from the impedance signals were then fed into a self-organizing neural network for pattern categorization. Based on flow visualization, the classified patterns were translated into corresponding flow regimes. In addition, an analytical model was developed to predict the regime transition from cap bubbly to slug flows. Good agreement was obtained between the results by the neural network and the analytical model. Furthermore, comparisons of the model with other researchers’ experimental data also showed satisfactory results.

Effects of Liquid Physical Properties on the Forces Acting on a Pipe Bend in Gas–Liquid Slug Flow

Abstract: The effects of liquid physical properties on the hydrodynamic forces acting on pipe bends in two-phase slug flow were investigated. The two properties of interest were liquid viscosity and liquid surface tension although density was also changed. Discussion will be centred around the effects of the liquid viscosity and liquid surface tension on the maximum forces acting on a pipe bend, which is important for the design of the bend support system. Comparison between the experimental results and predictions from a one-dimensional model, the piston flow model, proposed by Tay and Thorpe will be presented.

Inertial and buoyancy driven water flows under gas bubbling and thermal stratification conditions in a tundish model

Abstract: Steel flow dominated by inertial and buoyancy flows under gas bubbling and thermal stratification conditions, in a one-strand tundish, was studied using a 2/5 scale water model. The use of a turbulence inhibitor yields plug flow volume fractions well above 40 pct for a casting rate of 3.12 tons/min under isothermal conditions. Small flow rates of gas injection (246 cm3/min), through a gas curtain, improved the fluid flow by enhancing the plug flow volume fraction. Higher flow rates originated an increase of back-mixing flow, thus forming recirculating flows in both sides of this curtain. Step inputs of hot water drove streams of this fluid toward the bath surface due to buoyancy forces. A rise in gas flow rate led to a thermal homogenization in two separated cells of flow located at each side of the gas curtain. Step inputs of cold water drove streams of fluid along the tundish bottom. Use of the gas curtain homogenized the lower part of the tundish as well as the upper part of the bath at the left side of the curtain. However, temperature at the top corner of the tundish, in the outlet box, remained very different than the rest of the temperatures inside this tundish. High gas flow rates (912 cm3/min) were required to homogenize the bath after times as long as twice the mean residence time of the fluid. Particle image velocimetry (PIV) measurements corroborated the formation of recirculating flows at both sides of the gas curtain.

Flow in the negative wake of a Taylor bubble rising in viscoelastic carboxymethylcellulose solutions: particle image velocimetry measurements

Abstract: A simultaneous technique employing particle image velocimetry (PIV) and shadowgraphy was used to study vertical slug flow in non-Newtonian fluids. Two aqueous solutions of 0.8 and 1.0 wt% carboxymethylcellulose (CMC) were studied and the flow field around individual Taylor bubbles fully characterized. The rheological fluid properties and pipe dimension yielded Reynolds numbers of 8 and 4 and Deborah numbers of 0.2 and 0.4. A negative wake was found downstream of the Taylor bubbles in both fluids. Below the bubble trailing edge, along the axis region, the fluid flows in the opposite direction to the bubble (negative wake), originating rotational liquid movements in adjacent regions. Even far downward from the bubble, rotational liquid movements are clearly seen and measured. In the 1.0 wt% CMC solution, the bubble trailing edge has the shape of a two-dimensional cusp. This two-dimensional cusp, of small dimensions, is seen in different orientations during the bubble rise-indicating a fast rotational movement. The asymmetrical shape of the trailing edge is responsible for small asymmetries in the flow in the wake region (three-dimensional flow). The asymmetrical shape associated with the rotational movement is responsible for an unsteady flow of small amplitude. In the 0.8 wt% CMC solution, the shape of the trailing edge changes during the bubble rise. An axisymmetric axial oscillation a continuous expansion and contraction of the trailing edge, is the origin of this behaviour. This oscillatory movement is responsible for an unsteady flow of small amplitude in the wake region.

Experimental and numerical analysis of transient liquid slug motion in a voided line

Abstract: In this paper, the hydrodynamics of a single transient slug in a voided line was investigated numerically and experimentally. In the experiments, the liquid slugs of various lengths were propelled into an inclined smooth steel pipe under several different driving air pressures. The pipe segment terminates in an open ended elbow; the pressure time histories are recorded at this elbow. The recorded peak pressures are correlated to the tank pressures and pipe and slug geometry. Dimensionless parameters are developed to present the experimental data. Moreover, the flow is investigated numerically using several Godunov type schemes, namely, basic Godunov Scheme, Total Variation Diminishing (TVD), based Weighted Average Flux (WAF) method and two different Monotone Upstream Schemes for Conservation Laws (MUSCL) methods. These schemes employ Godunov 's approach facilitating exact and Harten, Lax and van Leer's modified approximate solver (HLLC) type solution methods solving the Riemann problem of gas dynamics equations. Furthermore, the recorded peak pressures at the elbow are compared with the results of the numerical analysis and results of two earlier studies.

Slug flow hydrodynamics in the presence of catalyst rods

Abstract: A design for a slug flow reactor, referred to as the confined slug flow (CSF) reactor, is presented and some hydrodynamic issues for this reactor are considered. Particular attention is given to the prediction of gas bubble and liquid slug velocities and lengths. These parameters are found to be described adequately by modified forms of relationships for conventional slug flow operation. In combination with the continuity equations for slug flow, the modified velocity relationships enable the evaluation of liquid film properties (and subsequently mass transfer coefficients), as well as liquid and gas residence times. The studies demonstrate the possibilities of using catalyst inserts within monolith blocks under slug flow operation, and thus improved flexibility in the design of mass-transfer-enhanced, three-phase, structured reactor systems. © 2004 Institution of Chemical Engineers.

Local Liquid Velocity in Vertical Air-Water Downward Flow

Abstract: We present an experimental study of local liquid velocity measurement in downward air-water bubbly and slug flows in a 50.8 mm inner-diameter round pipe. The axial liquid velocity and its fluctuations were measured by a laser Doppler anemometry (LDA) system. The maximum liquid velocity in a downward two-phase flow could occur off the pipe centerline at relatively low liquid flow rates and this observation is consistent with other researchers' results

Bubble size effect on the gas-lift technique

Abstract: The gas-lift technique uses gas injection in vertical oil wells to decrease the gravitational pressure drop. The decreased pressure in the production pipe results in an increase of the pressure drop from the reservoir to the oil well and enhances the oil production. In this thesis we investigated the influence of the bubble size and initial concentration distribution on the gas-lift technique efficiency. A vertical upward bubbly pipe flow of air and water is used, with a height of 18m and a diameter of 72mm. The flow velocity conditions investigated in the experiments were representative of practical gas-lift circumstances. Different bubble injectors were tested in order to vary the size and initial concentration distribution of the bubbles. The influence of the initial concentration distribution was limited. The effect of bubble size, on the contrary, was significant. It was due to three contributions: the effect of the bubble size on the flow pattern changes, on the radial distribution (of void fraction and of gas and liquid velocity) and on the relative velocity between the gas and the liquid. To study separately these different contributions associated with bubble size changes we developed and used local measurement techniques. We investigated the possibility of Laser-Doppler Anemometry (LDA) measurements in bubbly flows. These tests were conducted in two different experimental configurations, corresponding to a stirred vessel and a pipe flow. To determine the size and the velocity of the bubbles a four-point optical fibre probe was validated and used. We also developed a method for estimating the bubble shape and orientation based on the time series provided by the four-point optical fibre probe. This method was first validated and then applied to our multiple bubbles pipe flow conditions. We then studied the effects of bubble size and concentration distribution on the gas-lift technique. In general it can be concluded that a decreasing bubble size increases the efficiency of the gas-lift technique. This is, among others, caused by the fact that the initial bubble size significantly affected the flow pattern transition from bubbly flow to slug flow. This effect could be described by using a bubble size dependent critical void fraction relation for the transition from bubbly flow to slug flow. With decreasing bubble size the transition shifts to larger values of the void fraction. Measurements were conducted to measure the influence of the bubble size on the velocity profile of gas and liquid and on the bubble concentration profile. For this purpose the four-point probe optical fibre probe and the LDA measurement technique were used. The velocity and concentration profiles were strongly influenced by the bubble size. Also the parameters C0 and Udrift of the drift-flux model are therefore bubble-size dependent. Based on the measurements, mathematical relations have been developed to describe these bubble size effects on the drift-flux parameters. We also developed a numerical model based on the Euler-Euler modelling approach for predicting the radial profiles of the void fraction and velocity as a function of the relevant parameters, such as bubble size. The predictions were in reasonable agreement with experimental data.

Finite volume-based volume-of-fluid method for the simulation of two-phase flows in small rectangular channels

Abstract: The topic of the present thesis is the direct numerical simulation of gas-liquid two-phase flow in rectangular channels with hydraulic diameter of the order of 1 mm with heat transfer. A new volume-averaged equation for enthalpy is derived and implemented in the finite volume code TURBIT-VoF for the case when both fluids are considered as incompressible. The numerical approximation of this equation reduces the oscillations associated with the discontinuities at the interface using an accurate reconstruction of the convective and conductive heat fluxes. To model convective heat transfer for a spatially periodic two-phase flow in a channel with large length-to-hydraulic diameter ratio, a new concept, called periodic fully developed flow and heat transfer, is proposed. After a few hydraulic diameter away from the channel inlet the flow characteristics are free from entrance effects. For this region, the identification of the periodicity characteristics of the flow enables to restrict the analysis of the flow field and temperature distribution to a single isolated module. As typical example of periodic gas-liquid two-phase flow, the slug flow in small channels is considered. The flow of a train of large bubbles uniformly distributed along a channel with square cross-section is simulated. The bubble shape, the flow structure inside the bubble and in the liquid slug are analyzed. The bubble diameter, bubble velocity and relative bubble velocity for two different Capillary numbers are computed and compared with the experimental data from the literature showing good agreement. The convection and conduction of heat inside the channel due to a uniform, both axially and perimetrically, heat flux is also considered. The modification of the temperature field due to the presence of the bubble is analyzed.

Stress-Field Modeling and Pressure Drop Prediction for Slug-Flow Pneumatic Conveying in an Aerated Radial Stress Chamber

Abstract: Slug-flow pneumatic conveying is a full-bore mode of flow within the dense-phase flow regime where bulk materials are transported in the form of slugs at conveying speeds below saltation velocity. The mechanism of slug-flow pneumatic conveying consists of the particles being picked up from the stationary bed in front of a moving slug while the same amount of material is deposited behind the slug. Stress field modeling of the material slug is the first step in developing a prediction model for the pressure drop along a pneumatic conveying line. However, a reliable prediction strongly relies on an accurate assessment of several factors, including the particle properties, pipeline dimensions, and operating conditions. So far, the particle diameter has always been one of the crucial parameters, which is not desirable in regards to the limitations it imposes on the choice of bulk materials. This article focuses on one parameter, the stress transmission coefficient kw, which relates the lateral wall stress with...

Development of a Slug-Flow PCR Chip with Minimum Heating Cycle Times

Abstract: A microfluidic PCR device is presented capable of rapid temperature ramping and handling of sample volumes in the microliter and submicroliter range. The PCR chip comprises a straight micro channel in which a sample slug is periodically moved over three temperature zones. As part of the sample preparation a method for metering of the sample volume was developed. The PCR chip and the chips for sample preparation and fluidic actuation were fabricated by ultra-precision milling in polymer substrates. Computer simulations suggest that the sample slugs are heated or cooled on a time scale of some ten milliseconds when transported to a different temperature zone. The fluidic actuation based on a ferrofluid transducer is capable of positioning the sample volumes with a high accuracy after a large number of cycles. The design developed should be ideally suited for fast PCR of small sample amounts in a highly parallel manner.

Void Fraction Measurement in Oil-Gas Transportation Pipeline Using an Improved Electrical Capacitance Tomography System

Abstract: To measure the void fraction online in oil-gas pipeline, an improved electrical capacitance tomography (ECT) system has been designed. The capacitance sensor with new structure has twelve internal electrodes and overcomes the influence of the pipe wall. The data collection system is improved by using high performance IC (integrated circuit). Static tests of bubble flow, stratified flow and annular flow regime are carried out. Measurements are taken on bubble flow, stratified flow and slug flow. Results show that the new ECT system performs well on void fraction measurement of bubble flow and stratified flow, but the error of measurement for slug flow is more than 10%.

Monitoring system of gas-liquid two-phase flow

Abstract: A new monitoring system of the flow states of gas-liquid two-phase flow has been developed by using a differential pressure transducer combined with wavelet analysis and fuzzy pattern recognition technique. The differential pressure signal of gas-liquid two-phase flow was obtained under the states of the different flow patterns, including bubble flow, slug flow, plug flow and annular flow. The acquired differential pressure signals were analyzed by the wavelet transform and the eigenvalues were extracted. The flow state of the system was monitored by fuzzy pattern recognition technique. The experimental results show that the developed system can monitor flow states of horizontal gas-liquid two-phase flow effectively.