Flow Regime Map Research Articles

Flow transition in periodically fully developed wavy channels

In the present study, numerical investigations have been performed to study the flow transition mechanism in wavy channels using finite volume-based open source field operation and manipulation. Two different wavy channel configurations are chosen, which represent two different flow destabilization mechanisms, viz., Kelvin–Helmholtz and centrifugal instabilities. Sinusoidal walls with out-of-phase and in-phase channel configurations have been considered in the present study. Steady to chaotic flow transition in two different channel configurations are investigated by varying Reynolds number. A detailed flow regime map is presented for the two different wavy channel configurations. Unsteady flow features have been illustrated with the help of instantaneous streamlines, velocity contours, vorticity contours, and iso-Q surfaces. For the out-of-phase configuration, the flow changes from two-dimensional steady to two-dimensional unsteady in the Re range of 175–185, and then three-dimensional unsteady flow is observed for the Re varying from 250 to 260. On the contrary, for the in-phase configuration, the transition happens directly from steady two-dimensional flow (Re < 101) to unsteady three-dimensional (Re > 102) in a very narrow range of Re. Transitions in the two different wavy channels have been examined in detail using Hilbert–Huang transformation, phase-space reconstruction, Poincaré section, recurrence plot, and dynamic mode decomposition. Frequency, growth rate, and vortex structures of the dominant modes are illustrated corresponding to each value of Re for the considered channel configurations.

Mechanism of transition from elongated bubbles to wavy-annular regime in flow boiling through microchannels

Transition from elongated bubbles to annular flow regime is one of the most intricate phenomena in flow boiling in microchannels. To identify this transition, flow regime maps have been introduced primarily based on visualizations and characterized using average flow properties. However, understanding the mechanism responsible for this transition demands accurate determination of the hydrodynamic characteristics of liquid films around the vapor core. But, the challenges associated with measurements of the thickness and velocity of the liquid films have made it difficult to determine the flow hydrodynamics. Here, we utilize a measurement technique to determine the liquid film thickness along with velocities of the two phases. These measurements have enabled accurate calculation of the cross-sectional void fraction, vapor quality, interfacial shear stress and superficial velocities, which form the coordinates of the existing flow regime maps. The results show that upon transition to the annular flow regime, a remarkable increase in the liquid film thickness occurs due to a substantial increase in the liquid-vapor interfacial shear stress. This shear stress, which is a function of the liquid film thickness, vapor velocity, and channel aspect ratio, is introduced as a criterion for transition from elongated bubbles (EB) to wavy-annular (WA) flow regime.

Recommended set of interfacial drag correlations for the two-phase flow under thermal–hydraulic conditions of a horizontal steam generator

A set of interfacial drag correlations is proposed for describing two-phase flows under thermal–hydraulic conditions of a horizontal steam generator (SG). This set of correlations takes into account the peculiarities of the two-phase flow in different regions of the SG. Four characteristic regions are considered: tube bundles, tube-free zones in the SG lower part, zone between the outlet from the tube bundles and the submerged perforated sheet (SPS), and the SG upper part. The flow regime map contains bubbly flow, cap-bubbly flow, churn-turbulent flow and mist flow. A two-group bubbly flow model is used to describe bubbly and cap-bubbly flows in the SG corridors/gaps and in the bubbling layer above the SPS. Churn-turbulent flow occurs at the outlet from the tube bundles. In zones with a high void fraction under the SPS and above the SPS, mist flow occurs as a result of steam escaping from the interface with water droplets being captured and carried up. Some coefficients of the correlations are adjusted on experimental data. The detailed description of the set of interfacial drag correlations is presented in the paper. These correlations are implemented in the STEG-M code, which is validated on the experimental data of the void fraction obtained at two slice models of a horizontal SG (PGV and PGV-1500) and the full-scale WWER-1000 SG. Good agreement between the calculated and experimental data is obtained. The mean absolute deviation of the void fractions is <5.0%.

Air-water two-phase flow regime and transition criteria in vertical upward narrow rectangular channels

Upward two-phase flow in two different vertical narrow gaps was investigated experimentally. For better understanding the characteristics of air-water flow in rectangular channels, the visual images were captured through high-speed cameras and the void fraction results were obtained through three impedance void meters. The flow pattern was identified through ReliefF-FCM clustering algorithm to avoid the subjectivity of human observation. The flow regime maps were plotted in two different forms, superficial velocities and void fraction and compared with the previous model in the open literature. Through the analysis of the experimental results, the air-water flow in narrow rectangular channels is characterized by the limitation of bubble distribution, coalescence and movement. The probability of bubble coalescence in narrow rectangular channels is increased compared with those in conventional channels due to the restriction of the narrower side walls. New transition criteria was obtained according to the drift-flux model and the force balance analysis which shows satisfactory agreements with existing experimental data for gaps of 1–6 mm.

Experimental Investigation and Visualization Study of the Condensation Characteristics in a Propylene Loop Heat Pipe

At present, the structural design of a loop heat pipe (LHP) condenser mostly relies on experience. To analyze the condensation flow pattern in an LHP and to find a more accurate empirical correlation that can better guide condenser design in existing empirical correlations, a test system was designed and fabricated to observe the condensation phenomenon in the condenser and to evaluate the LHP’s heat transfer performance. Since propylene has a wide temperature range to different temperature control requirements, it was chosen as the working fluid. In this paper, the condensation process in the condenser was observed, and the operating parameters of a propylene LHP were measured at 283 K. The condensation flow patterns in the 2.5 mm ×2.5 mm channel mainly included stratified flow, wavy flow and intermittent flow. By comparing the experimental results with four typical condensation flow regime maps, it was found that the Breber and Cavallini flow regime maps can successfully predict most of the condensation flow patterns of the LHP even with ultra-small mass flux. In addition, four condensation heat transfer coefficient correlations were selected to compare the predicted values of heat transfer coefficient with the experimental results. The study showed that the predicted values from the correlation by Cavallini et al. matched 90% of the experimental values well with an error of less than 20%, and this correlation is recommended for highly accurate LHP design.

Development of a Real-Time Objective Gas-Liquid Flow Regime Identifier Using Kernel Methods.

Currently, flow regime identification for closed channels has mainly consisted of direct subjective methods. This presents a challenge when dealing with opaque test sections of the pipe or at gas-liquid flow rates where unclear regime transitions occur. In this paper, we develop a novel real-time objective flow regime identification tool using conductance data and kernel methods. Our experiments involve a flush-mounted conductance probe that collects voltage signals across a closed channel. The channel geometry is a horizontal annulus, which is commonly found in many industries. Eight distinct flow regimes were observed at selected gas-liquid flow rate settings. An objective flow regime identifier was then trained by learning a mapping between the probability density function (PDF) of the voltage signals and the observed flow regimes via kernel principal components analysis and multiclass support vector machine. The objective identifier was then applied in realtime by processing a moving time-window of voltage signals. Our approach has: 1) achieved more than 90% accuracy against visual observations by an expert for static test data; 2) successfully visualized conductance data in 2-D space using virtual flow regime maps, which are useful for tracking flow regime transitions; and 3) introduced an efficient real-time automatic flow regime identifier, with only conductance data as inputs.

Effects of density, viscosity and surface tension on flow regimes and pressure drop of two-phase flow in horizontal pipes

Two-phase gas/liquid flow in pipes is a common occurrence in the petroleum, chemical, nuclear and geothermal industry. In the petroleum industry, it is encountered in the production, transportation, and processing of hydrocarbons from the oil and gas fields. In designing these systems, accurate prediction of pressure drop is imperative, which is determined from the flow pattern and flow regime map. Unfortunately, most of the flow regime maps were developed for the air-water system and widely used for the oil/gas system. Despite the practical importance, the general applicability of these maps is not addressed. In order to improve the generality of flow regime maps, it is necessary to evaluate the effect of density, viscosity, and surface tension, which differ by great magnitude on flow regime maps. Thus, this study evaluates the effect of density, viscosity, and surface tension on the flow regime map. To evaluate the effect of fluid properties, experiments were conducted using a horizontal flow loop of 9.15 m (30 ft) pipe length and 0.0254 m (1-inch) pipe diameter with a two-phase air/liquid system. The surface tension was varied using the surfactant solution, viscosity was varied with the aid of glycerin, and density was varied with the aid of calcium bromide. The superficial velocity of the liquid ranges from 0 to 3.048 m/s (0–10 ft/s) and superficial gas velocity ranges from 0 to 18.288 m/s (0–60 ft/s) respectively. The experimental data were used to generate a flow pattern map and to predict the effect of these properties on the variation in the boundaries of different flow patterns and pressure drop. The results show that the flow patterns and transition boundaries were affected by fluid properties. The boundary between the annular and slug-annular flow at high velocity, the slug flow and pseudo-slug flow at medium velocity, and inertial wave and ripple wave flow at low velocities are affected by the variation in surface tension, viscosity, and density. The decrease in surface tension shifted the boundary between annular and slug flow toward the left-bottom corner of the flow regime map, while, similar effects were observed due to an increase in viscosity, which is opposite to the effect of surface tension. The increase in density shifts the boundary toward the top-right corner of the flow regime map. The effect of density on the flow pattern transition is significant compared to the viscosity, while the viscosity effect is higher than the surfactant. The effect of fluid properties on pressure drop is noticeable at high velocities, however, the pressure drop is not evident to reveal the flow pattern and boundary transition quantitatively.

Flow Regime Visualization and Identification of Air–Water Two-Phase Flow in a Horizontal Helically Coiled Rectangular Channel

Experiments of flow regime visualization and identification of air–water two-phase flow were conducted in a horizontal helically coiled rectangular channel. The test superficial liquid and gas velocities are 0.09–2 m/s and 0.18–16 m/s, respectively. Flow regimes were observed with a high-speed video camera and the corresponding local and average void fractions were measured with an electric conductivity probe method and with a quick-close valve method, respectively. Four main flow regimes including unsteady pulsating flow, bubbly flow, intermittent flow, and annular flow were observed. The bubbly flow identification criteria depend on more than 90% bubbles whose chord length was smaller than the channel equivalent diameter. The annular flow identification criteria is the local void fraction in the gas core larger than 0.97. Then the flow regimes and their transition mechanisms are analyzed. Furthermore, new transition criteria among these flow regimes have been proposed. The results show that the critical transition average void fraction from bubbly to intermittent flow is 0.23. A complete air–water flow regime map has been developed for the horizontal helically coiled rectangular channel and the map predicts the observed flow regimes well.

Passage of a rising bubble through a liquid‐liquid interface: A flow map for different regimes

AbstractThe passage of a rising air bubble through a stratified horizontal interface between two Newtonian liquids is studied numerically. A ternary phase‐field model has been utilized for capturing the interface between three immiscible fluids. According to the previous studies, the density, viscosity, and surface tension of the two liquids, in addition to the bubble diameter, are the effective parameters of bubble interaction with the interface. By changing these variables, three main flow patterns are identified in numerical simulations. Penetration flow regime is observed when the bubble enters the upper liquid alone and does not raise the lower liquid. Entrainment flow regime occurs when the bubble lifts some of the heavier liquid to the lighter liquid but still rises alone. If the bubble holds a film of the denser liquid when it rises in the upper liquid, envelopment flow regime takes place. A flow regime map is represented to distinguish the aforementioned flow patterns using Weber and Morton numbers and determine regime transition criteria based on these two dimensionless parameters. For Weber numbers lower than 30, or Morton numbers higher than 1.7 × 10−2, the penetration regime is observed. On the contrary, when the Weber number is higher than 65 and the Morton number is lower than 7 × 10−5, the envelopment regime occurs. The entrainment pattern happens between these ranges and two additional limiting relations of 320Mo0.18 &lt; We &lt; 1500Mo0.23.

Regimes and critical conditions of detonation propagation in expanding channels in gas suspensions of ultrafine aluminum particles

Under study are the regimes of detonation propagation in channels with linear expansion filled with monodisperse mixtures of oxygen and ultrafine aluminum particles of various loading; the methods of numerical simulations are used. The detonation combustion of submicron aluminum particles is described within the semi-empirical model of reduced kinetics with due regard to the transition from the diffusion-limited regime of combustion to the kinetic one. Waves of both planar and developed cellular detonation are considered as initial conditions. The characteristics of the main flow regimes are obtained and described: the subcritical (detonation failure), critical (detonation failure in some part of the channel) and supercritical (continuous detonation propagation). The maps of flow regimes in suspensions of 200-nm – 400-nm particles are presented in the plane of parameters: the channel width, expansion angle. The obtained critical conditions are similar to those observed in the gas detonation. The critical channel width linearly depends on the expansion angle up to a first critical value (35°–38°). Behind the second critical value (50°), the channel width is independent on the expansion angle. Between these values, there is an interval of nonmonotonicity similar to the detonation of micro-sized suspensions of aluminum particles. The effect of particle loading on the critical conditions in poor mixtures appears in the form of a sharp increase in the critical channel width, if the mass concentration falls below 0.25.

Data-Driven-Based Forecasting of Two-Phase Flow Parameters in Rectangular Channel

In the current nuclear reactor system analysis codes, the interfacial area concentration and void fraction are mainly obtained through empirical relations based on different flow regime maps. In the present research, the data-driven method has been proposed, using four machine learning algorithms (lasso regression, support vector regression, random forest regression and back propagation neural network) in the field of artificial intelligence to predict some important two-phase flow parameters in rectangular channels, and evaluate the performance of different models through multiple metrics. The random forest regression algorithm was found to have the strongest ability to learn from the experimental data in this study. Test results show that the prediction errors of the random forest regression model for interfacial area concentrations and void fractions are all less than 20%, which means the target parameters have been forecasted with good accuracy.

Internal two-phase flow and spray characteristics of outside-in-liquid twin-fluid atomizers

Despite its wide industrial applications, the internal-mixing twin-fluid atomization suffers a major problem of spray unsteadiness. This work links the flow characteristics inside a newly designed outside-in-liquid (OIL) atomizer with the spray stability and quality by flow visualization and digital image processing. The results show that the inhomogeneous bubbly and slug flow regimes cause an unwanted intermittent flow inside the exit orifice and an unstable coarse spray, while the stable-annular or the wavy-annular chamber flows provide a favorable continuous annular flow inside the exit orifice and a stable fine spray. By proposing a new parameter considering liquid/gas momentum ratio per liquid injection hole, a flow regime map is constructed, and the transition criterion between the favorable annular flow and the unfavorable intermittent flow regimes inside the exit orifice is obtained. A direct relation between the Sauter mean diameter of the spray droplets and the internal flow transition parameter is found. This study indicates that, under specific internal geometries, the optimized OIL atomizer can overcome the spray unsteadiness problem and hence is suitable for applications requiring high spray stability and quality.

Downward Annular Flow of Air–Oil–Water Mixture in a Vertical Pipe

The paper presents the results of a study concerned with the hydrodynamics of an annular downward multiphase flow of gas and two mutually non-mixing liquids through a vertical pipe with a diameter of 12.5 mm. The air, oil and water were used as working media in this study with changes in superficial velocities in the ranges of jg = 0.34–52.5 m/s for air, jo = 0.000165–0.75 m/s for oil, and jw = 0.02–2.5 m/s for water, respectively. The oil density and viscosity were varied within the ranges of ρo = 859–881 kg/m3 and ηo = 29–2190 mPas, respectively. The research involved the identification of multiphase flow patterns and determination of the void fraction of the individual phases. New flow patterns have been identified and described for the gravitational flow conditions of a two-phase water–oil liquid and a three-phase air–water–oil flow. New flow regime maps and equations for the calculation of air, oil and water void fractions have been developed. A good conformity between the calculated and measured values of void fraction were obtained. The map for the oil–water–air three-phase flow is valid for the following conditions: j3P = 0.35–53.4 m/s (velocity of three-phase mixture) and oil in liquid concentration βo* = 0.48–94% (oil in liquid concentration). In the case of a downward annular oil–water two-phase flow, this map is valid for liquid mixture velocity jl = 0.052–2.14 m/s and βo* = 0.48–94%.

Experimental investigation of gas-brine liquid flow in horizontal pipeline

Pipelines that are transporting fluids of different salinity in different pressurized conditions are met with different behaviors of flow. Flow behavior of two-phase gas–liquid system in simulated horizontal pipeline conditions are investigated experimentally with three different brine salinity concentrations comparable to the reservoir fluid properties within the Malay Basin. A test facility in total length of 80-feet was designed and configured for data acquiring of flow regimes which were observed via a 1.5-in diameter and a 7.5-feet long transparent acrylic pipe. A flow regime map where the transition of flow pattern from stratified to intermittent to mixed is proposed as the superficial gas velocity and superficial liquid velocity increase at increasing brine concentration. There is a combination flow pattern presumed to be the combination of plug flow, slug flow and thin annular. When the brine salinity increases, the pressure drop of the system increases. The lowest pressure drop in the form of percentage for brine concentration of 1000, 15,000, and 30,000 ppm at brine superficial velocity of 3.0 m/s were 57.1, 50, and 42.9% respectively. Pressure drop experimental results from this study were compared with previous mathematical correlations. Results also indicate that water holdup increases when water input fraction increases.

Gas-Liquid Flow and Interphase Mass Transfer in LL Microreactors

This work investigates the impact of fluid (CO2(g), water) flow rates, channel geometry, and the presence of a surfactant (ethanol) on the resulting gas–liquid flow regime (bubble, slug, annular), pressure drop, and interphase mass transfer coefficient (kla) in the FlowPlateTM LL (liquid-liquid) microreactor, which was originally designed for immiscible liquid systems. The flow regime map generated by the complex mixer geometry is compared to that obtained in straight channels of a similar characteristic length, while the pressure drop is fitted to the separated flows model of Lockhart–Martinelli, and the kla in the bubble flow regime is fitted to a power dissipation model based on isotropic turbulent bubble breakup. The LL-Rhombus configuration yielded higher kla values for an equivalent pressure drop when compared to the LL-Triangle geometry. The Lockhart–Martinelli model provided good pressure drop predictions for the entire range of experimental data (AARE &lt; 8.1%), but the fitting parameters are dependent on the mixing unit geometry and fluid phase properties. The correlation of kla with the energy dissipation rate provided a good fit for the experimental data in the bubble flow regime (AARE &lt; 13.9%). The presented experimental data and correlations further characterize LL microreactors, which are part of a toolbox for fine chemical synthesis involving immiscible fluids for applications involving reactive gas–liquid flows.

Two-Phase Heat Transfer and Flow Regimes in Pin Fin-Enhanced Microgaps—Effect of Pin Spacing

Abstract An experimental investigation of the flow boiling of dielectric refrigerant R245fa is conducted in microgaps with enhancement features. A silicon microgap of height 200 μm populated with pin fin arrays of diameter 150 μm with spacing 200 μm in both horizontal and vertical directions is examined. For five different test conditions and in a wide range of mass flux from 781 to 5210 kg/m2s, and inlet temperatures in the range of 13–18 °C, average single-phase and two-phase heat transfer coefficients, pressure drop, and exit vapor quality are reported. Three major flow patterns are observed in the pin finned area using high-speed flow visualization at frame rate of 2229 fps: foggy, bubbly, and slug flow. Based on the experimental data, a flow regime map is constructed.

Granular material flow regime map for planar silos and hoppers

This study presents a new theoretical model for granular materials flow regimes during discharge from wedge-shape (planar) silos. The new model takes into account the influence of granular material properties, material height, bin and hopper geometrical dimensions and consider the possible difference between the bin and the hopper wall friction coefficients. In addition, the model allows to found an appropriate transition between mass flow and two kinds of funnel flow regimes. By comparing the new model assessments to experimental results, which were found in the literature, good agreement was achieved. Based on the new model equations a generalized flow regime map was presented. The proposed generalized flow regime map should help designers to increase accuracy and flexibility in the granules flow regime assessment and therefore might improve significantly the process efficiency.

Two-phase flow regime identification based on the liquid-phase velocity information and machine learning

Two-phase flow regime identification in a horizontal pipe was realized based on the liquid phase velocity information and the machine learning method. Ultrasound Doppler velocimetry was employed to measure the liquid velocity. Statistical features extracted from the velocity time series data, such as mean, root mean square, and power spectral density, were used to realize real-time flow regime identification. In addition, two novel parameters—maximum velocity ratio and maximum velocity difference ratio—were proposed to identify plug and decaying slug flow. Different classification algorithms were employed to achieve a high identification accuracy. Moreover, transient flow regime identification with a fast response was realized based on two classification algorithms—long–short term memory and convolutional neural network. The results show that the accuracy of real-time flow regime identification based on a flow regime map can reach up to 93.1% using support vector machine, the maximum velocity ratio and maximum velocity difference ratio are effective in identifying plug and decaying slug flow, and transient flow regime identification under slug flow condition can be realized with an accuracy of 94% based on a convolutional neural network (CNN). Decaying slugs with long lengths confuse the CNN and are responsible for the error in identification. The results presented herein are expected to expand the available knowledge on two-phase flow regime identification.

Hydrodynamics of gas/shear-thinning liquid two-phase flow in a co-flow mini-channel: Flow pattern and bubble length

The flow patterns and bubble characteristics formed during gas–liquid flows in a circular co-flow mini-channel with carboxymethyl cellulose (CMC) aqueous solutions are investigated experimentally. The pattern transition and bubble length are elucidated by systematically analyzing the influences of the various factors of the ratio of gas–liquid flow rates, CMC solution mass fraction, and surfactant [sodium dodecyl sulfate (SDS)] mass fraction. Five kinds of flow regimes, namely, bubbly flow, Taylor flow, Taylor-annular flow, annular flow, and churn flow, are identified visually in the fully developed region of the inlet side of the channel, and a universal flow-regime map in terms of the gas and liquid inlet flow rates is constructed using water, CMC solution, and polyacrylamide solution to cover a broad range of material properties. It is found that the ratio of gas–liquid flow rates has a remarkable influence on the flow pattern transitions. The CMC solution mass fraction and SDS mass fraction can also affect the flow-regime map by varying the flow drag force and surface tension acting on the bubble in the mini-channel. The bubble length increases with the ratio of gas–liquid flow rates but decreases with the increase in the CMC fraction and SDS fraction. Based on consideration of the rheological properties of the liquid, a scaling law of bubble length in a co-flow mini-channel with shear-thinning liquids is developed, and the results predicted by it can agree with the measurement data very well under present conditions.

Prediction of two-phase flow patterns in upward inclined pipes via deep learning

The industrial process involving gas liquid flows is one of the most frequently encountered phenomena in the energy sectors. However, traditional methods are practically unable to reliably identify flow patterns if additional independent variables/parameters are to be considered rather than gas and liquid superficial velocities. In this paper, we reported an approach to predict flow pattern along upward inclined pipes (0–90°) via deep learning neural networks, using accessible parameters as inputs, namely, superficial velocities of individual phase and inclination angles. The developed approach is equipped with deep learning neural network for flow pattern identification by experimental datasets that were reported in the literature. The predictive model was further validated by comparing its performance with well-established flow regime forecasting methods based on conventional flow regime maps. Besides, the intensity of key features in flow pattern prediction was identified by the deep learning algorithm, which is difficult to be captured by commonly used correlation approaches.