Annular Regime Research Articles

Flow regime identification and volume fraction prediction in multiphase flows by means of gamma-ray attenuation and artificial neural networks

Abstract This work presents a new methodology for flow regimes identification and volume fraction predictions in water–gas–oil multiphase systems. The approach is based on gamma-ray pulse height distributions (PHDs) pattern recognition by means the artificial neural networks (ANNs). The detection system uses appropriate fan beam geometry, comprised of a dual-energy gamma-ray source and two NaI(Tl) detectors adequately positioned in order measure transmitted and scattered beams, which makes it less dependent on the regime flow. The PHDs are directly used by the ANNs without any parameterization of the measured signal. The system comprises four ANNs. The first identifies the flow regime and the other three ANNs are specialized in volume fraction predictions for each specific regime. The ideal and static theoretical models for annular, stratified and homogeneous regimes have been developed using MCNP-X mathematical code, which was used to provide training, test and validation data for the ANNs. The energy resolution of NaI(Tl) detectors is also considered on the mathematical model. The proposed ANNs could correctly identify all three different regimes with satisfactory prediction of the volume fraction in water–gas–oil multiphase system, demonstrating to be a promising approach for this purpose.

A singular integro-differential equation model for dryout in LMFBR boiler tubes

A 2D steady model for the annular two-phase flow of water and steam in the steam-generating boiler pipes of a liquid metal fast breeder reactor is proposed. The model is based on thin-layer lubrication theory and thin aerofoil theory. The exchange of mass between the vapour core and the liquid film due to evaporation of the liquid film is accounted for using some simple thermodynamics models, and the resultant change of phase is modelled by proposing a suitable Stefan problem. Appropriate boundary conditions for the flow are discussed. The resulting non-linear singular integro-differential equation for the shape of the liquid film free surface is solved both asymptotically and numerically (using some regularization techniques). Predictions for the length to the dryout point from the entry of the annular regime are made. The influence of both the traction τ provided by the fast-flowing vapour core on the liquid layer and the mass transfer parameter η on the dryout length is investigated.

Two-Phase Flow Pressure Drop Characteristics in Trapezoidal Silicon Microchannels

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper focuses on experimentally studying the pressure drop characteristics for two-phase flow in microchannels of hydraulic diameter 109 <formula formulatype="inline"><tex Notation="TeX">$\mu {\rm m}$</tex> </formula>, over a relatively large range of heat flux of (0–30 <formula formulatype="inline"><tex Notation="TeX">${\rm W}/{\rm cm}^{2}$</tex></formula>) and mass flow rate values (44–1114 <formula formulatype="inline"><tex Notation="TeX">${\rm kg}/{\rm m}^{2}\hbox{-}{\rm s}$</tex></formula>). Three fluid flow regimes (single-phase, two-phase, and dryout) have been covered in this paper, with deionized water as the working fluid. For a given heat flux, the variation of average pressure drop with flow rate can be classified into three distinct regimes. In the first regime (higher flow rate), the pressure drop decreases linearly with decrease in flow rate. In the second regime (lower flow rate), pressure drop increases with decreasing flow rate and reaches a maximum (with a minimum on either side). Finally, in the very low flow rate regime, pressure drop increases rapidly with decreasing flow rate. The average pressure drop in the two-phase regime is predicted well by the annular flow model. In addition to absolute pressure drop values, we also report pressure fluctuations. The magnitude of pressure fluctuations appears to be correlated to the underlying flow regime, such as bubbly, slug, and annular regimes, which have been identified through the flow visualization. An important outcome of this study is the identification of as many as four operating points with similar pressure drop penalty. This may help to choose the right operating conditions for a microchannel-based heat sink for use in cooling electronics. These detailed experimental results are also expected to be useful for modeling two-phase flow in microchannels. </para>

A heat transfer model for evaporation of coalescing bubbles in micro-channel flow

The current study presents a one-dimensional model of confined coalescing bubble flow for the prediction of micro-channel convective boiling heat transfer. Coalescing bubble flow has recently been identified as one of the characteristic flow patterns to be found in micro-scale systems, occurring at intermediate vapor qualities between the isolated bubble and the fully annular regimes. As two or more bubbles bond under the action of inertia and surface tension, the passage frequency of the bubble-liquid slug pair declines, with a redistribution of liquid among the remaining flow structures. Assuming heat transfer to occur only by conduction through the thin evaporating liquid film trapped between the bubbles and the channel wall, the present model includes a simplified description of the dynamics of the formation and flow of the liquid film and the thin film evaporation process, taking into account the added mass transfer by breakup of the bridging liquid slugs. The new model has been confronted against experimental data taken within the coalescing bubble flow mode that have been identified by a diabatic microscale flow pattern map. The comparisons for three different fluids (R-134a, R-236fa and R-245fa) gave encouraging results with 83% of the database predicted within a +/-30% error band. Furthermore, the new model is able to predict a boiling curve with an exponent of 0.74 typical of numerous micro-channel flow boiling studies, thus suggesting film evaporation as the controlling heat transfer mechanism rather than nucleate boiling. (C) 2009 Elsevier Inc. All rights reserved.

Flow map and measurement of void fraction and heat transfer coefficient using an image analysis technique for flow boiling of water in a silicon microchannel

The present work focuses on the generation of the flow regime map for two-phase water flow in microchannels of a hydraulic diameter of 140 µm. An image analysis algorithm has been developed and utilized to obtain the local void fraction. The image processing technique is also employed to identify and estimate the percentage of different flow regimes and heat transfer coefficient, as a function of position, heat flux and mass flow rate. Both void fraction and heat transfer coefficient are found to increase monotonically along the length of the microchannel. At low heat flux and low flow rates, bubbly, slug and annular flow regimes are apparent. However, the flow is predominately annular at high heat flux and high flow rate. A breakup of the flow frequency suggests that the flow is bistable in the annular regime, in that at a fixed location, the flow periodically switches from single-phase liquid to annular and vice versa. Otherwise, the occurrence of three regimes—single-phase liquid, bubbly and slug are observed. These results provide several useful insights about two-phase flow in microchannels besides being of fundamental interest.

Flow boiling heat transfer of R134a, R236fa and R245fa in a horizontal 1.030 mm circular channel

This research focuses on acquiring accurate flow boiling heat transfer data and flow pattern visualization for three refrigerants, R134a, R236fa and R245fa in a 1.030 mm channel. We investigate trends in the data, and their possible mechanisms, for mass fluxes from 200 to 1600 kg/m 2s, heat fluxes from 2.3 kW/m 2 to 250 kW/m 2 at T sat = 31 °C and Δ T sub from 2 to 9 K. The local saturated flow boiling heat transfer coefficients display a heat flux and a mass flux dependency but no residual subcooling influence. The changes in heat transfer trends correspond well with flow regime transitions. These were segregated into the isolated bubble (IB) regime, the coalescing bubble (CB) regime, and the annular (A) regime for the three fluids. The importance of nucleate boiling and forced convection in these small channels is still relatively unclear and requires further research.

BOILING HEAT TRANSFER IN A VERTICAL MICROCHANNEL: LOCAL ESTIMATION DURING FLOW BOILING WITH A NON INTRUSIVE METHOD

This paper summarizes the results of experimental and numerical studies concerning boiling heat transfer inside vertical minichannels. The objective here is to provide basic knowledge on the systems of biphasic cooling in minichannels for several gravity levels (μg, 1g, 2g). To fully understand the high heat transfer potential of boiling flows in microscale’s geometry, it is vital to quantify these transfers. To achieve this goal, an experimental device has been set up and in order to study the influence of gravity, the device has been embarked on board A300 Zero-G to make Parabolic Flights experiments. Analysis is made up by using an inverse method in order to estimate the local heat coefficient while boiling occurs inside a minichannel. The results concern two-phase flow during convective boiling and this investigation leads to solve an inverse heat conduction problem (IHCP). The estimation consists in inversing experimental data measurements located in the heating rod of our experiment to obtain the wall temperature and the heat flux on the minichannel surface. Images and video sequences have been performed with a high speed camera. The experiments are conducted with HFE-7100 as this fluid has a low boiling temperature at the cabin pressure of the A300. This choice was governed by the capillary length which is 4.3 mm in microgravity (0.05 m.s). The experimental loop allows the control of the heat flux and the liquid flow rate for three hydraulic diameters (DH): 0.49 mm; 0.84 mm and 1.18 mm. The influence of three different parameters on the heat transfer coefficient are carried out: the gravity level, the Reynolds number and the vapor quality. First results show that whatever the gravity level, the local heat transfer decreases sharply from the inlet to the outlet channel. In the annular slug regime, the average heat transfer is found around 6000 W.m.K and we observe that the heat transfer coefficient is higher in microgravity (30 %) compared to normal and hypergravity conditions.

Two-phase wavy-annular flow in small tubes

Base liquid film thickness distribution, wave behavior, and pressure loss measurements have been obtained for 237 horizontal two-phase (air–water) flow conditions in 8.8 and 15.1 mm I.D. tubes in the wavy and annular regimes. The behaviors measured indicate the presence of a transitional wavy-annular regime at flow rates traditionally labeled as annular in these small diameter tubes. Data for a 26.3 mm I.D. tube do not show these same trends.

Analysis of the Acceleration Region in a Circulating Fluidized Bed Riser Operating above Fast Fluidization Velocities

In commercial circulating fluidized bed (CFB) processes the acceleration zone greatly contributes to solids mixing, gas and solids dispersion, and particle residence times. A new analysis was developed to describe the relative gas-solids concentration in the acceleration region of a transport system with air as the fluidizing agent for Geldart-type B particles. A theoretical expression was derived from a drag relationship and momentum and continuity equations to describe the evolution of the gas-solids profile along the axial direction. The acceleration zone was characterized using nondimensional analysis of the continuum equations (balances of masses and momenta) that described multiphase flows. In addition to acceleration length, the boundary condition for the solids fraction at the bottom of the riser and the fully developed regions were measured using an industrial scale CFB of 0.3 m diameter and 15 m tall. The operating factors affecting the flow development in the acceleration region were determined for three materials of various sizes and densities in core annular and dilute regimes of the riser. Performance data were taken from statistically designed experiments over a wide range of Fr (0.5-39), Re (8-600), Ar (29-3600), load ratio (0.2-28), riser to particle diameter ratio (375-5000), and gas to solids densitymore » ratio (138-1381). In this one-dimensional system of equations, velocities and solid fractions were assumed to be constant over any cross section. The model and engineering correlations were compared with literature expressions to assess their validity and range of applicability. These expressions can be used as tools for simulation and design of a CFB riser and can also be easily coupled to a kinetics model for process simulation.« less

Comparison of hydrodynamic and mass transfer performances of an emulsion loop-venturi reactor in cocurrent downflow and upflow configurations

Hydrodynamic parameters (gas-induced flow rate and gas hold-up) and mass transfer characteristics (kLa, kL and a) have been investigated in a gas–liquid reactor denoted “Emulsair” in which the distributor is an emulsion-venturi and the gas phase is self-aspired by action of the kinetic energy of the liquid phase at the venturi throat. Two configurations, respectively cocurrent downflow and cocurrent upflow were compared. A chemical method involving the dispersion of a CO2–air mixture in a monoethanolamine (MEA) aqueous solution was used to measure mass transfer parameters. Experimental results showed that only the homogeneous bubbling regime prevailed in the upward configuration, while an annular regime could also be observed for cocurrent downflow at low liquid flow rate. Gas-induced flow rate and gas hold-up were usually smaller for cocurrent upflow, both at constant liquid flow rate and specific power input. The same stood for mass transfer properties. Conversely, specific power requirements were lower at constant liquid flow rate and mass transfer characteristics were enhanced at constant gas-induced flow rate for cocurrent upflow. A comparison with other gas–liquid contacting devices showed that the Emulsair reactor is a versatile tool avoiding the presence of mechanically moving parts when high and quickly adaptable dissolved gas supply is required. The cocurrent upflow configuration can be preferred when high gas flow rates are desired because the evolutions of gas-induced flow rate and mass transfer characteristics exhibit a stronger dependence on specific power input in the homogeneous bubbling regime for this configuration.

High-Speed Visualization of Two-Phase Flow in a Micro-Scale Pin-Fin Heat Exchanger

Flow regimes and bubble growth are observed in a pin-fin micro-scale heat exchanger with R-11 as the working fluid. The heat exchanger is machined in silicon and derived from a DNA micro-array consisting of 150 μm-square fins separated by 50 μm-square passages. The fins are staggered and oriented 45 degrees to the flow direction such that approximately 750 channel intersections occur within the volume of the exchanger. The purpose of the study is to determine if this multiply-connected geometry produces the flow blockage, reversal, and other instabilities observed in single and parallel micro-channel configurations. The upper surface of the exchanger is a glass plate that provides optical access. High-speed digital photography and microscope optics are used to obtain real-time images of the flow at a framing rate of 5 kHz. The lower surface is electrically heated and instrumented with a heat flux gage. Inlet and outlet temperatures and pressures, heater and wall temperatures, and volumetric flow rate are monitored. Nucleation is observed near the entrance of the heat exchanger. In the central section, developed vapor regions are composed of broad slug-like vapor fronts immediately followed by a slowly growing bubbly flow. An annular regime dominates the downstream section of the exchanger with drop-like liquid structures appearing at the downstream edge of fins. The heat transfer coefficient decreases with exit quality as in other micro-scale exchangers; however, the flow instability present in parallel channel exchangers is not observed in this configuration.

Measurement of the Deposition Rate of Droplets in a Vertical Tube Containing a Flow Obstacle

The critical heat flux in an annular two-phase flow regime is influenced significantly by an obstacle placed in a flow channel. Since the transition to critical heat flux condition in this flow regime is caused by the depletion of liquid film, it is probable that the flow obstacle has a notable influence on the rate of droplet deposition and, consequently, the film flow rate in the annular regime. Also, the obstacle’s effect on the deposition rate would be important in predicting the critical heat flux in a boiling water reactor core because the grid spacer can be regarded as a flow obstacle placed in the subchannel. The obstacle effect was studied experimentally for vertical upward air-water annular flow; placing 12 small tubes of different cross sections concentrically in the test section tube one by one, the influence of obstacle geometry on the deposition rate was investigated. The rate of droplet deposition markedly increased if the present tubular obstacle was placed; the rate of increase was between ~30 and 200% and depended primarily on the obstacle shape. Using the experimental data, an empirical correlation to account for the obstacle’s effect was proposed.

A new type of diabatic flow pattern map for boiling heat transfer in microchannels

Flow pattern data and bubble measurements for two small diameter sight glass tubes are used to propose a new type of flow pattern map for evaporating flows in microchannels. Rather than segregating the observations into the traditional flow regimes and an adiabatic map, the new diabatic map classifies flows into three types: (i) the isolated bubble regime, where the bubble generation rate is much larger than the bubble coalescence rate and includes both bubbly and slug flows, (ii) the coalescing bubble regime, where the bubble coalescence rate is much larger than the bubble generation rate and exists up to the end of the coalescence process and (iii) the annular regime, whose extent is limited by the vapor quality at the onset of critical heat flux. This formulation is thought to be more useful for phenomenological modeling of the processes controlling boiling heat transfer and two-phase pressure drops in microchannels, and it also visually defines the feasible operating limit of microchannel heat spreaders at the critical vapor quality corresponding to critical heat flux (CHF). The database covers two refrigerants (R-134a and R-245fa) and two channel diameters (0.509 and 0.790 mm). The micro-evaporator length was varied from 20 to 70 mm, the inlet subcooling from 2 to 15 °C, the mass flux from 200 to 2000 kg m−2 s−1 and heat fluxes up to 597 kW m−2. Three different saturation temperatures were tested: 26, 30 and 35 °C.

Experimental study of R134A condensation heat transfer inside the horizontal micro-fin tubes

The Investigation of the two-phase flow patterns and their transitions during the condensation has gained increasing interest and importance from the well-known phenomenon that the heat transfer characteristics are strongly dependent on the flow patterns. Therefore, it is very important to study on which heat transfer enhancement approach is suitable for an individual flow pattern inside a condenser, so that an accurate heat transfer mechanism can be understood that is consistent with the flow patterns. The condensation heat transfer for R134a in the two kinds of in-tube three-dimensional (3-D) micro-fin tubes with different geometries is experimentally investigated. Based on the flow pattern observations, the flow patterns in the Soliman flow regime map are divided into two-flow regimes; one with the vapor-shear-dominant annular regime and the other with the gravitational-force-dominant stratified-wavy regime. The flow regime transition criterion between the annular regime and the stratified-wavy regime is at Fr equal to 2. In the annular regime, the heat transfer coefficients h of the two kinds of in-tube 3-D micro-fin tubes decreases as the vapor quality x decreases. The regressed condensation heat transfer correlation from the experimental data of the annular flow region is obtained. The dispersibility of the experimental data is inside the limits of ±25%. In the stratified-wavy regime, the average heat transfer coefficient h of the two kinds of in-tube 3-D micro-fin tubes increases as the mass flux increases and the number of micro fins in the 3-D micro-fin tube is not the controlling factor for the performance of a condensation heat transfer. The regressed condensation heat transfer correlation of the stratified-wavy flow regime is experimentally obtained. The dispersibility of the experimental data is inside the limits of ±22%. Combined with the criteria of flow pattern transitions, the correlations can be used for the design of a condenser with 3-D micro-fin tubes.

Experiments for liquid phase mass transfer rate in annular regime for a small vertical tube

The double film extraction technique was used to measure the deposition rate and the entrainment rate of droplets for vertical upward annular two-phase flow in a small diameter tube. The test section was a round tube of 5 mm in inside diameter, air and water were used as test fluids and the system pressure was varied within 0.14–0.76 MPa. It was shown in the present experimental conditions that the deposition rate was primarily influenced by the droplet concentration in the gas core and that the entrainment rate was correlated well with the dimensionless number denoting the ratio of interfacial shear force to surface tension force acting on the surface of liquid film. These results were consistent with available empirical correlations that were developed using the experimental data for larger diameter tubes.

TRIALS OF MOMENTUM BALANCE FOR TWO PHASE HORIZONTAL PIPE FLOW

Momentum balance models were tested against reliable data for both hold up and pressure drop. Best prediction performance was achieved using models that considered the actual shape of the liquid phase in the pipe. In such circumstances the momentum balance calculation tended to under predict both the hold up and pressure drop for some of the annular and stratified regimes. Suggestions are made for improvements in the momentum balance approach

Transport Phenomena in Two-Phase Micro-Channel Heat Sinks

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.

Prediction of the critical heat flux in annular regime in various vertical channels

The axial variation of film flow rate in annular regime was analyzed to predict the critical heat flux in water saturated flow boiling in various vertical channels. In the analyses, several quantities including the deposition and entrainment rates of droplets were evaluated with the correlations that were based on the separate experimental data of film flow rate and droplet flow rate in annular flow; model closure was achieved without introducing arbitrary constants. The predicted results were compared with extensive data of critical heat flux in flow boiling. Fairly good agreements were observed for the critical heat fluxes in round tubes with uniform and axially non-uniform heating and in non-circular shaped channels without sharp corners. It is hence expected that the important processes for the onset of critical heat flux condition in annular regime were satisfactorily described in the model. However, the critical heat fluxes were generally overestimated in thin rectangular channels with sharp corners. The non-uniform distribution of liquid film in channel cross-section would be the primary reason of the disagreement.

Momentum Balance for Two‐Phase Horizontal Pipe Flow Part 2: Testing of Models

AbstractMomentum balance models were tested against reliable data for both holdup and pressure drop. The best prediction performance was achieved using a model that considered the actual shape of the liquid phase in the pipe. In such circumstances the momentum balance calculation tended to under predict both the holdup and pressure drop for some of the annular and stratified regimes. Suggestions are made for improvements in the momentum balance approach.

2133 レーザーフォーカス変位計による FEP マイクロチャンネル内液膜界面の計測

This paper presents a new method for measuring the interface of a liquid film in microchannels using a laser focus displacement meter (LFD). To prevent the tube wall signal from disturbing that of the gas-liquid interface, a fluorocarbon tube with water box was used; the refraction index of this device was the same as that for water. Simultaneous measurements of the interface in fluorocarbon tubes of 0.5 and 1mm I.D. using the LFD and a high-speed video camera with a microscope showed that the LFD could measure the interface of a liquid film with high spatial and temporal resolution during annular, slug, and piston flow. The data also clarified the existence of a thin liquid film less than 1μm in thickness in slug and annular regimes.