Flow Regime Map Research Articles

Structure of air–water two-phase flow in helically coiled tubes

Air–water two-phase flow in helically coiled tubes is investigated experimentally to elucidate the effects of centrifugal acceleration on the flow regime map and the spatial and the temporal flow structure distribution. Three kinds of test tubes with 20 mm inner diameters including a straight tube are used to compare the turbulent flow structure. Superficial velocities up to 6 m/s are tested so that the centrifugal Froude number covers a range from 0 to 3. The interfacial structure is photographed from two directions by a high-speed video system with synchronized measurement of local pressure fluctuations. The results reveal that the flow transition line alters due to centrifugal force acting on the liquid phase in the tube. In particular, the bubbly flow regime is narrowed significantly. The pressure fluctuation amplitude gets large relatively to the average pressure loss as void fraction increases. The frequency spectra of the pressure fluctuation have plural peaks in the case of strong curvature, implying that the periodicity of slugging two-phase flow is collapsed by an internal secondary flow activated inside the liquid phase. Moreover, under large Froude number conditions, the substantial velocity of the gas phase that biases to the inner side of the helical coil is slower than the total superficial velocity because the liquid flow is allowed to pass through the outer side and so resembles a radial stratified flow.

Development of a Drift-flux Model for Heavy Liquid Metal/Gas Flow

The gas lift pump concept based on the bubbling of an inert gas into the primary reactor coolant to enhance natural circulation is currently considered in a number of PbBi-cooled reactor concepts. Thus, the analysis of available void fraction data and the development of two-phase heavy liquid metal/gas flow calculational models have become an important issue in the study of advanced nuclear reactor systems. In the absence of the detailed two-phase flow information needed to develop a flow regime map and the associated interfacial relations, drift-flux models have often been used in the thermal-hydraulic analysis of nuclear and other systems. Accordingly, we consider, in the current paper, the analysis of five sets of experimental data with different geometries, working fluids, flow rates and void fraction ranges, with a view to obtaining a best fit to the data in the form of a drift-flux model. The results of the analysis show that, for systems with flowing fluid, it is possible to represent the heavy liquid metal void fraction data in the form of a drift-flux correlation with a residual error of as low as 0.016, thus offering an improvement over existing void correlations.

Slug flow in a blast furnace taphole

Excessive splashiness of a blast furnace taphole stream, which is caused by entrained blast air, can result in premature trough refractory wear. This splashiness has been found to occur when the gas–liquid flow regime of slug flow is present in the taphole tube. The taphole stream trajectories reported by He et al. [Q. He, P. Zulli, F. Tanzil, B. Lee, J. Dunning, G.M. Evans, Flow characteristics of a blast furnace taphole stream and its effects on trough refractory wear, ISIJ Int. 42 (2002) 235–242] can be predicted by the theory proposed herein that recognises that the average velocity in the liquid slug is equal to the sum of gas and liquid superficial velocities. The splashy flow observed by He et al. [Q. He, P. Zulli, F. Tanzil, B. Lee, J. Dunning, G.M. Evans, Flow characteristics of a blast furnace taphole stream and its effects on trough refractory wear, ISIJ Int. 42 (2002) 235–242] may be simply predicted by reference to a two-phase flow regime map. It is suggested that a splashy taphole stream will be avoided if the taphole points downwards or if the cast rate is above around 5 m s −1.

Two-phase flow regime maps for air-lift pump vertical upward gas–liquid flow

The flow pattern maps available in literature were first developed for the petrochemical industry (Baker, 1954) for flow of oil and gas in large diameter pipes. Subsequently, the adiabatic flow pattern maps were developed as general flow pattern maps (for example, Hewitt and Roberts, 1969; Taitel et al., 1980). In recent years, a number of flow pattern maps have been developed for specific conditions such as small diameter tubes, evaporation or condensation, and compact heat exchanger geometries. The main task is to group together the basic flow structures and define a few basic patterns. This is by no means well defined and indeed many flow patterns exist as individual researchers group the flow patterns somewhat differently depending on their own interpretations. Basically the attitude of most practitioners is to minimize the number of flow pattern groups and to group together the flow structures that has basically the same character pertaining to the distribution of the interfaces. When designing devices in which a two-phase flow occurs, the determination of a flow regime is one of the most essential problems. Different flow regimes may arise, which are depending on the flux of both phases, properties of various factors as well as dimensions and location of a channel. The ranges of occurrence of particular two-phase flow regimes are generally presented in diagrams, called flow regime maps, in the form of areas divided by transition lines. In the literature there is a large number of such works which vary considerably and there are only quite a few

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.

Two-Phase Flow Regime Transitions in Microchannels: A Comparative Experimental Study

In recent years, micro-technologies have become very important to cutting edge industries such as aerospace and biotechnology. In the fluidic research area, two-phase flow study in microchannels has been an emerging topic in the past few years. Characteristics of two-phase flow in microchannels such as flow regimes, pressure drop, void fraction, and heat transfer are now being extensively studied by numerous groups. The ultimate goal of this research ranges from compact heat exchangers to small-sized refrigeration systems. One of the major drawbacks in this area to date is the lack of a universal flow regime map enabling the prediction of flow regimes in microchannels. In the present study, a new test rig was designed and constructed to extend the range of the existing data on flow regimes. Several flow regime maps, for a hydraulic diameter of less than 1.0 mm obtained from a comprehensive literature review, are tested and compared. A total of approximately 1475 experimental data points from present and previous studies performed in channels with Dh ≤ 1.0 mm were used for comparison. Two universal flow regime maps were created, one each for horizontal and vertical channels with a hydraulic diameter ranging between 0.1 mm and 1.0 mm. The resulting universal maps presented here are based on flow regimes observed in all the studies on two-phase flow in microchannels. We suggest that the use of these flow regimes could help diminish the amount of discord present among research groups pertaining to the definition of flow regimes. Based on the comparisons between the universal maps and different experimental flow regime maps, appropriate conclusions on the effects of channel orientation and geometry are suggested.

Three Phase Oil-Water-Gas Horizontal Co-Current Flow: I. Experimental and Regime Map

Flow regimes are reported for horizontal co-current three-phase, oil/water/air pipe flow for two different diameters. In the validation of experimental method the effect of time dependent characteristics and calming lengths are outlined. Initial two phase air/water flow studies led to the development of a universal flow regime map which gave flow pattern prediction of the various regimes by using a combination of dimensionless numbers for each phase as the mapping parameters. An adaptation of the flow regime map is presented here that permits major pattern types to be predicted for both two and three phase systems.

Flow Visualization and Flow Pattern Identification With Power Spectral Density Distributions of Pressure Traces During Refrigerant Condensation in Smooth and Microfin Tubes

This paper presents a flow pattern identifier of the prevailing flow regime during refrigerant condensation inside smooth- and microfin tubes. The power spectral density distribution of the fluctuating condensing pressure signal was used to identify the prevailing flow regime, as opposed to the traditional (and subjective) use of visual-only methods, and/or smooth-tube flow regime maps. The prevailing flow regime was observed by using digital cameras and was validated with the use of the conventional smooth-tube flow regime transition criteria, as well as a new flow regime map for microfin-tube condensation. Experimental work was conducted for condensing refrigerants R-22, R-407C, and R-134a at an average saturation temperature of 40°C with mass fluxes ranging from 300–800 kg/m2 s, and with vapor qualities ranging from 0.05–0.15 at condenser outlet to 0.85–0.95 at condenser inlet. Tests were conducted with one smooth-tube condenser and three microfin-tube condensers (with helix angles of 10°, 18°, and 37° respectively). The power spectral density distributions of the condensing pressure signals distinguish the annular and intermittent (slug and plug) flows. A very low resonant frequency (<40 Hz) and low power spectral density amplitude of the pressure oscillation denoted stratified and wavy flows. As the annular flow regime was approached, the oscillations became larger and their frequencies increased (typically 40–120 Hz). Intermittent flow showed the most distinct character of all flow regimes. Its trace consisted of large amplitude pressure pulses occurring at fairly constant frequencies (approximately 50, 60, 80, 100, and 120 Hz). As the transition from intermittent to annular flow began, the pressure fluctuations became less regular and the amplitude dropped sharply.

Flow Regimes of Upward Gas-liquid Two-Phase Flow in a Vertical Pipe with an Axisymmetic Sudden Expansion

Experimental study was made on the multi-dimensional behavior of upward air-water two-phase flow through a vertical pipe with an axisymmetric sudden expansion, which is one of the typical multi-dimensional channel geometry. The aims of this study are to clarify the multidimensional behavior of bubbly flow or slug flow affected by sudden expansion channel geometry, and to accumulate the experimental data for two-phase flow analysis, which is applicable to predict the multi-dimensional behavior with appropriate accuracy. The observation using a high-speed video camera was performed to clarify the multi-dimensional dynamic flow behavior with the sudden expansion. It revealed the characteristic phenomena such as gas-slug breakup and bubble deformation and bubble size below and above the sudden expansion point. Furthermore the effect of the sudden expansion channel on the flow regime map can be also described by using these experimental result.

IMPROVED INFLOW PERFORMANCE RELATIONSHIPS AND IMPROVED WELL BORE PERFORMANCE

The inflow performance relationship for oil proposed by Vogel (1968) and extended by Brown (1981) has been modified empirically to allow for variations in water cut and gas liquid ratio.The well bore performance of wells flowing or gas lifting continuously is known to be affected adversely by the separation of fluids at the eductor (tubing or annulus) intake. Performance is affected mostly in wells with large well bore storage.The design of the tubing intake also affects separation. A tubing intake that will reduce separation in flowing wells and increase separation in pumped wells has been designed and tested in a transparent model.A qualitative estimate of the gas separation efficiency of this device has been made. The estimate is based on the Ros (1961) flow regime map and on observations by Beggs and Brill (1973) that liquid hold-up increases with well deviation up to 40º. Further tests are warranted.Rod pump performance is analysed in detail. A spreadsheet allows comparison of pump capacity with well inflowperformance.Even at very low gas/liquid ratios, gas separation at the tubing intake affects significantly both single and two stage pump performance.

Transition velocities in the riser of a circulating fluidized bed

Abstract We attempted to study the hydrodynamics and flow regime transitions in a circulating fluidized bed (CFB). In conventional gas–solids fluidization, the flow regimes include the fixed bed, bubbling fluidization and turbulent fluidization, in which the regimes mainly depend on the gas velocity. However, in a CFB, the flow regime depends on both the solids circulation rate and gas velocity. The flow regimes in a CFB include the high-velocity fluidized bed and pneumatic conveying. Experiments were conducted in a column of 0.052 m internal diameter and 1.2 m long covering a wide range of operating conditions. The transport velocity was estimated by emptying time and extrapolation techniques. It was noticed from the present experimental observations that Geldart B-type particles move more or less as individual particles, while Geldart A-type particles have a tendency to form clusters. Empirical correlations have been proposed for the transport velocity Utr and the chocking velocity U ch. A qualitative map of different flow regimes of a gas-solid fluidized bed has been proposed based transition velocities.

Development of a Drift-flux Model for Heavy Liquid Metal/Gas Flow

The gas lift pump concept based on the bubbling of an inert gas into the primary reactor coolant to enhance natural circulation is currently considered in a number of PbBi-cooled reactor concepts. Thus, the analysis of available void fraction data and the development of two-phase heavy liquid metal/gas flow calculational models have become an important issue in the study of advanced nuclear reactor systems. In the absence of the detailed two-phase flow information needed to develop a flow regime map and the associated interfacial relations, drift-flux models have often been used in the thermal-hydraulic analysis of nuclear and other systems. Accordingly, we consider, in the current paper, the analysis of five sets of experimental data with different geometries, working fluids, flow rates and void fraction ranges, with a view to obtaining a best fit to the data in the form of a drift-flux model. The results of the analysis show that, for systems with flowing fluid, it is possible to represent the heavy liquid metal void fraction data in the form of a drift-flux correlation with a residual error of as low as 0.016, thus offering an improvement over existing void correlations.

Flow regimes for horizontal two-phase flow of CO 2 in a heated narrow rectangular channel

Two-phase flow patterns of CO 2 in a narrow rectangular channel or a small diameter tube are very important in understanding heat transfer characteristics and in developing an appropriate heat transfer correlation. Geometric effects on two-phase flow patterns of CO 2 are very significant in many respects. In the present study, the flow boiling process of CO 2 in a horizontal narrow rectangular channel, having a width of 16 mm and a height of 2 mm, is visualized at various test conditions, and then the effects of mass flux are analyzed. Based on the test results, a flow regime map is developed and then compared with the existing maps and flow regime transition criteria that are selected by considering physical phenomena occurred during the flow regime transition processes of CO 2. In addition, the flow transition criteria for bubbly–slug, slug–annular, and bubbly–annular flow in a horizontal narrow channel are proposed by comparing the test data with existing transition criteria.

Flow behavior and regime transition in a high-density circulating fluidized bed riser

Flow behavior and flow regime transitions were determined in a circulating fluidized bed riser (0.203 m i.d. × 5.9 m high) of FCC particles ( d p = 70 μ m , ρ p = 1700 kg / m 3 ). A momentum probe was used to measure radial profiles of solids momentum flux at several heights and to distinguish between local net upward and downward flow. In the experimental range covered ( U g = 5 – 8 m / s ; G s = 22 – 345 kg / m 2 s ), the fast fluidization flow regime was observed to coexist with dense suspension upflow (DSU). At a constant gas velocity, net downflow of solids near the wall disappeared towards the bottom of the riser with increasing solids mass flux, with dense suspension upflow achieved where there was no refluxing of solids near the riser wall on a time-average basis. The transition to DSU conditions could be distinguished by means of variations of net solids flow direction at the wall, annulus thickness approaching zero and flattening of the solids holdup versus G s trend. A new flow regime map is proposed distinguishing the fast fluidization, DSU and dilute pneumatic transport flow regimes.

Characterization and Selection of Materials for Air Biofiltration in Fluidized Beds

A methodology is described for the selection of the best material to employ in a fluidized biofilter applied to volatil organic compounds (VOC) treatment. Two different supports were considered: one natural, scrap wood, and one synthetic, polyurethane foam. In a first part, the main objective was to establish the fluidizability of the selected solids. Particles of increasing moisture content were tested in rigs equipped with different air distribution technologies. Maps of flow regimes were drawn by varying air velocity and bed height. In the second part, the sorption capacities of the selected materials were evaluated at different humidity levels for a soluble organic compound (ethanol) and an insoluble one (toluene). Their ability for supporting viable microorganisms was then measured. Fine particles with 45% moisture content fluidized with the tuyere distributor in channeling and slugging regimes, whereas the conical distributor generated a spouted bed regime. Coarse wood particles with 45% moisture commonly used in fixed bed biofilters were considered as less suitable due to limitations borne by their fluidization regimes. Finally polyurethane foam filled with Agar gel could not endure the mechanical stress to which particles are subjected in fluidized beds. It was shown that a 45% moisture content was optimum to ensure transfer for both soluble and hydrophobic compounds. Considering the biological criterion, it appeared that a 45% humidity level was sufficient to ensure biomass growth and biodegradation of pollutants on fine scrap wood particles.

The effect of channel diameter on adiabatic two-phase flow characteristics in microchannels

The effect of channel diameter on two-phase flow was investigated to identify the phenomena which distinguish microchannels from minichannels. Experiments were conducted with a mixture of nitrogen gas and water in circular channels of 530, 250, 100, and 50 μm diameter. The temperature, pressure and flow rates of the liquid and gas were measured and images of the flow patterns recorded. The two-phase flow was characterized by the flow regime map, void fraction, and frictional pressure drop. In the 530 and 250 μm channels, the two-phase flow characteristics were similar to those typically observed in minichannels of ∼ 1 mm diameter. In the 100 and 50 μm channels, the two-phase flow behaviour departed from that observed in minichannels––the occurrence of slug flow dominated, the void fraction–volumetric quality relationship departed from a linear Armand-type correlation, and mass flux no longer influenced the two-phase frictional multiplier. Clearly, the channel diameter has an effect on two-phase flow in the range of channel diameters investigated. A new slug flow model is also proposed to gain physical insight into the observed flow characteristics in microchannels. The model can predict the two-phase frictional pressure gradient for the 100 and 50 μm channels, if the actual void fraction data are used, substantiating the assumed flow mechanism.

Interfacial structures and interfacial area transport in downward two-phase bubbly flow

An adiabatic, air–water, co-current, vertically downward bubbly flow was studied to gain a better understanding of interfacial structures and flow characteristics. The experimental test sections were round pipes with internal diameters of 25.4 and 50.8 mm. Flow regime map was obtained using characteristic signals obtained from an impedance void meter, and a neural network-based identification methodology to minimize the subjective judgment in determining the flow regimes. A four-sensor conductivity probe was used to measure the local two-phase flow parameters that characterize the interfacial structures. The parameters measured were: void fraction, interfacial area concentration, bubble velocity, and bubble Sauter mean diameter. Furthermore, a laser Doppler anemometer (LDA) system was used to measure local axial liquid velocity and turbulence. The local profiles of these parameters as well as their axial development revealed the nature of the interfacial structures and the bubble interaction mechanisms occurring in the flow. Based on previous study of interfacial area transport for upward flows, the interfacial area transport equation applicable to downward flow was developed with certain modifications in bubble interaction terms.

Measurement of Void Fraction and Pressure Drop of Air-Oil Two-Phase Flow in Horizontal Pipes

The void fractions, flow regimes, and pressure drop of air-oil two-phase flow in a half-inch diameter pipe over a wide range of test conditions have been investigated. The flow regimes were identified with the aid of a 1000 frames per second high-speed camera. A capacitance sensor for instantaneous void fraction measurements was developed. The mean and probability density function of the instantaneous void fraction signal can be used to effectively identify the different flow regimes. The current flow regime data show significant differences in the transitional boundaries of the existing flow regime maps. Property correction factors for the flow regime maps are recommended. The pressure drop measurements were compared to the predictions from four existing two-phase flow pressure drop models. Though some of the models performed better for certain flow regimes, none of the models were found to give accurate results over the entire range of flow regimes.

Evaluation of Influence of Pipe Width on Interfacial Growth of Horizontal Stratified Two-Phase Flow in Rectangular Pipe by Lattice Boltzmann Method

The horizontal stratified two-phase flow in the rectangular pipe whose width is smaller than the height is simulated by the one-component two-phase lattice Boltzmann method. The interfacial growth between two phases is measured for three cases with the different pipe width and the measured dimensionless number chasacterizing the two-phase flow is compared with the flow regime map proposed by Taitel and Dukler. It is found that the boundary separating the interfacial growth from the non-growth which is obtained by the simulations is larger than that in the flow regime map when the pipe width is narrower.

ADIABATIC GAS–LIQUID FLOW IN MICROCHANNELS

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. Several channels of decreasing diameter were used in our experiments to determine the effect of the channel size on the two-phase flow of nitrogen gas and water. The effect of channel geometry was examined by characterizing the two-phase flow in a circular and square microchannel of similar size. Only slug flow was observed in the microchannels. Four new sub-classes of slug flow were subsequently defined. A new correlation was developed for the time-averaged void fraction data in the microchannels. The two-phase pressure drop in microchannels was predicted by treating the two phases as being separate with a large velocity difference. Regarding the effect of microchannel geometry, the transition boundaries on the two-phase flow regime maps were shifted for the slug flow subcategories.