Regime Transition Criteria Research Articles

Flow regime transition criteria for co-current downward two-phase flow

Downward two-phase flow is observed in light water reactor accident scenarios such as loss of coolant accident (LOCA) and loss of heat sink accident (LOHS) due to loss of feed water or a secondary pipe break, and so, it is vital to have a thorough understanding of the flow mechanisms and regimes. With this point of view, flow regime transition criteria for vertical downward flow for a range of pipe diameters of 24–101.6 mm has been developed. Several models looked at the radial distribution of the bubbles and the wake effect of leading bubbles while others looked into the Kelvin-Helmholtz instability seen at the gas-liquid interface. The newly developed criteria have been compared to flow regime maps obtained via subjective and objective means, consisting of air-water data at atmospheric conditions as well as at an elevated pressure of 0.2 MPa. The new model is also compared to flow regime maps developed with different inlet conditions. Overall, the present model showed good agreements with the available data, with the exception of several 50.8 mm ID flow regime maps of different inlet conditions as well as a self-organizing neural network. This study also highlights the need for a more objective and consistent flow regime map data for large diameter pipes, the identification of cap-bubbly and churn-turbulent flows in these maps, and the deviations observed between a supervised and self-organizing neural network (SONN).

Recognition and measurement gas-liquid two-phase flow in a vertical concentric annulus at high pressures

The air-water flow characteristics under pressure in the range of 1–6 MPa in a vertical annulus were evaluated in this report. Time-resolved bubble rising velocity and void fraction were also measured using an electrical void fraction meter. The results showed that the pressure has remarkable effect on the density, bubble size and rise velocity of the gas. Four flow patterns (bubble, cap-bubble, cap-slug, and churn) were also observed instead of Taylor bubble at high pressure. Additionally, the transition process from bubble to cap-bubble was investigated at atmospheric and high pressures, respectively. The results revealed that the flow regime transition criteria for atmospheric pressure do not work at high pressure, hence a new flow regime transition model for annular flow channel geometry was developed to predict the flow regime transition, which thereafter exhibited high accuracy at high pressure condition.

Flow regime transition criteria for upward two-phase flow in vertical rod bundles

The demand of accurate prediction for two-phase flow behavior in a nuclear reactor core composed of fuel rod bundles or a heat exchanger composed of heat transfer tube bundles requires comprehensive understanding of flow regime, void fraction, heat transfer and pressure drop. In comparison with the great success in developing the two-phase flow regime transition criteria for simple geometries such as pipes, annulus and rectangular channels, limited researches have been performed for developing the flow regime transition criteria of upward two-phase flow in vertical rod bundles. In the vertical rod bundles, slug bubbles spanning the bundle casing cannot exist due to their surface instability and the two-phase flow characteristics in the vertical rod bundles are different from those in pipes, annulus or rectangular channels whose channel size are smaller than the length scale of the surface instability. This study has proposed a new flow regime transition criteria model based on the analysis on the underlying physics of the upward two-phase flow behavior in the vertical rod bundles. A reliable drift-flux correlation to predict void fraction in the vertical rod bundle developed recently has been used in modeling the flow regime transition criteria. This study has classified the flow regime into 6 distinct flow regime such as bubbly, finely dispersed bubbly, cap-bubbly, cap-turbulent, churn and annular flows. The newly developed flow regime transition criteria have been compared with existing 4 flow regime maps obtained in vertical rod bundles. The fluid systems include air-water and steam-water. A fairly good agreement with some discrepancies has been obtained between the newly developed transition criteria and the measured flow regime maps.

Flow regime transition criteria for upward two-phase cross-flow in horizontal tube bundles

In comparison with great success in the two-phase flow research for internal flows, relatively limited two-phase researches have been performed for external flows such as two-phase cross-flow in horizontal tube bundles. The recent demand of compact and efficient design of shell-tube type heat exchangers requires comprehensive understanding of void fraction, heat transfer, pressure drop, flow-induced vibration and flow regime in the shell side. The flow regimes are strongly related to the two-phase external-flow structure and affect the heat transfer performance in the heat exchangers. To this end, extensive literature survey has been performed to understand the flow regime appearing in the upward two-phase cross-flow in horizontal tube bundles. This study has classified the flow regime into 5 distinct flow regimes such as bubbly, cap bubbly, churn, finely dispersed bubbly and annular flows. A new model of the flow regime transition criteria has been proposed based on the analysis on the underlying physics of the upward two-phase cross-flow behavior in horizontal tube bundles. The newly developed flow regime transition criteria have been compared with 12 flow regime maps measured in horizontal tube bundles with normal square pitch array, normal triangular pitch array and parallel triangular pitch array. The test conditions of the 12 flow regime maps include the pitch/diameter ratio ranging from 1.17 to 1.5 and the tube diameter ranging from 0.00979m to 0.020m. The fluid systems are air-water, boiling iso-butane, boiling propane, boiling n-pentane, boiling R134a and R235fa. A fairly good agreement with some discrepancy has been obtained between the newly developed transition criteria and the measured flow regime maps.

Flow regime identification for upward two-phase flow in helically coiled tubes

Abstract Helically coiled tubes have excellent compactness, heat transfer efficiency, thermal stress adaptability and two-phase flow stability. The increasing application of two-phase flow in helically coiled tube devices necessitates a deep knowledge of phase distribution features for different flow regimes as well as flow regime transitions characteristics. This paper identified the flow regimes of air–water adiabatic two-phase flow in three helically coiled tubes using a double-sensor conductivity probe and high speed camera. Similar to the inclined straight tube, a total of six flow regimes were found in the helically coiled tubes. The influence of centrifugal force on the phase distribution was analyzed for each regime. A set of straight-forward flow regime transition criteria were proposed and then validated by benchmark experiments conducted in an inclined straight tube. These criteria were then used to generate flow regime maps based on the experiments with liquid superficial velocities ranging from 0.03 to 4.12 m/s and gas superficial velocities ranging from 0.03 to 16 m/s. Compared to the horizontal and inclined straight tubes, the helically coiled tube experienced shrinkage in the bubbly flow region; also, wavy and annular flows appeared at slower gas superficial velocities.

Process intensification characteristics of a microreactor absorber for enhanced CO2 capture

Gas separation processes, including post-combustion carbon capture (PCCC) by chemical absorption using liquid solvents can be substantially enhanced using high performance micro-structured surfaces to enhance the surface area available for reaction. The present paper studies the hydrodynamics and mass transfer performance of gas–liquid absorption of CO2 into aqueous diethanolamine in a micro-structured reactor. The system was designed to comprise 15 straight parallel channels in a cross flow inlet configuration. The hydraulic diameter of each channel was 456μm. The performance of the reactor was studied with respect to the absorption efficiency, mass transfer coefficient, acid gas loading ratio, and pressure drop. A flow pattern map was developed using available regime transition criteria. Parametric studies varying the gas and liquid flow rates, as well as their respective concentrations at the reactor inlet, were conducted. The two-phase pressure drop was compared against the predictions of a piecewise model and a reasonably good agreement was obtained. Absorption efficiencies close to 100% were observed under certain operating conditions. The presently achieved values of liquid-side volumetric mass transfer coefficients were between 1–3 orders of magnitude higher than those reported for most conventional gas–liquid absorption systems, which can be attributed to the inherent high specific interfacial area provided through micro-structured surfaces. The results reported here indicate the substantial levels of process intensification that can be achieved using microreactors.

Flow regimes and void fractions during condensation of hydrocarbons in horizontal smooth tubes

Phase change in hydrocarbons is of interest to the petroleum, refining, and chemical processing industry, as well as in HVAC and in organic Rankine power cycles. An experimental investigation of flow regimes and void fractions during condensation of propane flowing through smooth horizontal tubes with internal diameters of 7.0mm and 15.0mm was conducted. Measurements were made over mass fluxes ranging from 75 to 450kgm−2s−1, operating pressures ranging from 952 to 1218kPa, and nominal vapor qualities ranging from 0.05 to 0.95. Results from this experimental study are compared with predictions from correlations in the literature, and agreement and differences are interpreted and discussed. Appropriate flow regime transition criteria between the wavy and annular flow regimes are highlighted. Detailed analyses of the video frames are also used to develop a new multi-regime void fraction model that uses the drift flux model framework. The model provides improved agreement with the experimental results when compared to correlations from the literature, and predicts the trends with mass flux, tube diameter and pressure well.

Hydraulic model of steady state multiphase flow in wellbore annuli

Abstract Based on the classification and flow behaviors of two phase flow in wellbore annuli, the hydraulic models for slug flow and annular flow in annuli for vertical or inclined wells were established, and the flow regime transition criteria were also obtained. Based on the flow behavior research of multiphase flow in wellbore annuli, the liquid film zone was used as the control volume, and the effect of the tubing liquid film, casing liquid film and the droplets in gas core area on the mass and momentum transfers were considered. The mass and momentum conservation equations of slug and annular flows were obtained. Then the evaluation criterion of flow pattern transitions were established, including dispersed flow to slug flow, bubble flow to slug flow and slug flow to annular flow. The model prediction results were compared under the experimental conditions from the previous literatures. The predictions of flow pattern, liquid holdup and pressure gradient were compared between the new model and the pipe flow model modified by using the hydraulic diameter. The results show that the flow pattern, liquid holdup and pressure gradient can be predicted by the new model more accurately, and the prediction of liquid holdup and pressure gradient are better.

Flow regime transitions during condensation in microchannels

Heat transfer and pressure drop for two-phase flow inside tubes are closely related to the corresponding flow mechanisms. The flow patterns formed in microchannels during condensation differ from those observed in conventional tubes. Using an extensive R134a condensation flow-regime database (1 < Dh < 4.91 mm, 150 < G < 750 kg m−2 s−1), new flow regime transition criteria are proposed. The data are used to understand the physical mechanisms and the governing influences for each of the identified flow regimes and develop dimensionless transition criteria. These criteria can be utilized to identify the flow regimes and transitions for various fluids, operating conditions and channel sizes, thereby generalizing their applicability. This mechanistic determination of condensation flow regimes in different operating conditions and geometries will assist in the development of models for predicting condensation heat transfer and pressure drop, enabling the development of optimized microchannel heat exchangers.

Effect of bubble interactions on the prediction of interfacial area in TRACE

The conventional thermal-hydraulic nuclear reactor system analysis codes utilize a two-field, two-fluid formulation for two-phase flows. To provide closure, static flow regime transition criteria and algebraic relations are employed to estimate the interfacial area concentration (ai). To better reflect the dynamic evolution of two-phase flow, an experimental version of TRACE is developed to estimate the ai using the one-group interfacial area transport equation (IATE), which includes mechanistic models for bubble coalescence and disintegration. These models account for: (1) bubble break-up due to impact of a turbulent eddy, (2) bubble coalescence due to random collision driven by turbulent eddies, and (3) bubble coalescence due to the acceleration of a bubble in the wake region of a preceding bubble. To assess the impact of including bubble interaction models in TRACE, code predictions of experimental data measured by a multi-sensor conductivity probe are compared to both the IATE and flow regime based predictions. In total, 50 air–water vertical co-current upward and downward bubbly flow conditions in pipes with diameters ranging from 2.54 to 20.32cm are evaluated. It is found that TRACE, using the conventional flow regime relation, always underestimates ai by predicting a larger bubble size than observed in the experimental data. Additionally, the axial trend of the ai prediction is always linear because ai in the conventional code is predominantly determined by the pressure. However, TRACE with the one-group IATE significantly improves the prediction results, yielding a ±13.0% difference with the data. It is found that the non-linear developments observed in the experimental data, which reflect bubble interactions, are predicted well using the IATE. Moreover, in several conditions dominated by bubble interactions, the ai trend displayed is opposite to the effect of pressure. In these cases, the conventional TRACE relation predicts an incorrect trend in ai, while the IATE predicts the experimental data well.

Lagrangian slug flow modeling and sensitivity on hydrodynamic slug initiation methods in a severe slugging case

Severe slugging is a dynamic two-phase flow phenomenon with regular liquid accumulation and blow-out in flow-line riser geometries. This paper discusses the applicability of a slug tracking model on a case where hydrodynamic slug initiation in a horizontal part of the pipeline upstream the riser base affects the severe slugging cycle period. The given experimental case is from the Shell laboratories in Amsterdam: air–water flow in a 100m long pipe (65m horizontal and 35m −2.54° downwards) followed by a 15m long vertical riser.A Lagrangian slug and bubble tracking model is described. A two-fluid model is applied in the bubble region and the slug region is treated as incompressible flow, with an integral momentum equation. Slug initiation from unstable stratified flow can be captured directly by solving the two-fluid model on a fine grid (a hybrid capturing and tracking scheme). Alternatively, slug initiation can be made from sub grid models, allowing for larger grid sizes. The sub grid models are based on the two established flow regime transition criteria derived from the stability of stratified flow and from the limiting solution of the unit cell slug flow model.Sensitivity studies on hydrodynamic slug initiation models on the severe slugging characteristics are presented. No hydrodynamic slug initiation (e.g. large grid size in the capturing scheme) overestimates the severe slug period compared with the experiments. Slug capturing and sub grid initiation models both give good predictions for small grid sizes (provided the detailed inlet configuration is included in the capturing case). Good predictions are also shown for larger grid sizes (factor of 50) and sub grid initiation models.The numerical tests show that correct prediction of the severe slugging cycle is sensitive to the initiation of upstream hydrodynamic slugs, but less sensitive to the local structure of the slug flow (frequencies and lengths) in the upstream region.

Visualized investigation on flow regimes for vertical upward steam–water flow in a heated narrow rectangular channel

Flow regimes are very important in understanding two-phase flow resistance and heat transfer characteristics. In present work, two-phase flow regimes for steam–water flows in a single-side heated narrow rectangular channel, having a width of 40mm and a gap of 3mm, were visually studied at relatively low pressure and low mass flux condition. The flow regimes observed in this experiment could be classified into bubbly, churn and annular flow. Slug flow was never observed at any of the conditions in our experiment. Flow regime maps at the pressure of 0.7MPa and 1.0MPa were developed, and then the pressure effect on flow regime transition was analyzed. Based on the experimental results, the comparisons with some existing flow regime maps and transition criteria were conducted. The comparison results show that the bubbly transition boundary and annular formation boundary of heated steam–water flow were consistent with that of adiabatic air–water flow. However, the intermediate flow pattern between bubbly and annular flow was different. Hibiki and Mishima criteria could predict the bubbly transition boundary and annular formation boundary satisfactorily, but it poorly predicted churn zone in present experimental data.

Two-phase flow structure in large diameter pipes

Flow in large pipes is important in a wide variety of applications. In the nuclear industry in particular, understanding of flow in large diameter pipes is essential in predicting the behavior of reactor systems. This is especially true of natural circulation Boiling Water Reactor (BWR) designs, where a large-diameter chimney above the core provides the gravity head to drive circulation of the coolant through the reactor. The behavior of such reactors during transients and during normal operation will be predicted using advanced thermal–hydraulics analysis codes utilizing the two-fluid model. Essential to accurate two-fluid model calculations is reliable and accurate computation of the interfacial transfer terms. These interfacial transfer terms can be expressed as the product of one term describing the potential driving the transfer and a second term describing the available surface area for transfer, or interfacial area concentration. Currently, the interfacial area is predicted using flow regime dependent empirical correlations; however the interfacial area concentration is best computed through the use of the one-dimensional interfacial area transport equation (IATE). To facilitate the development of IATE source and sink term models in large-diameter pipes a fundamental understanding of the structure of the two-phase flow is essential. This understanding is improved through measurement of the local void fraction, interfacial area concentration and gas velocity profiles in pipes with diameters of 0.102m and 0.152m under a wide variety of flow conditions. Additionally, flow regime identification has been performed to evaluate the existing flow regime transition criteria for large pipes. This has provided a more extensive database for the development and evaluation of IATE source and sink models. The data shows the expected trends with some distortion in the transition region between cap-bubbly and churn-turbulent flow. The flow regime map for the 0.102m and 0.152m diameter test sections agree with the existing flow regime transition criteria. It may be necessary to perform further experiments in larger pipes and at higher gas flow rates to expand the range of conditions for which models can be developed and tested.

Flow regime transition criteria for two-phase flow in a vertical annulus

In this work, a new flow regime transition model is proposed for two-phase flows in a vertical annulus. Following previous works, the flow regimes considered are bubbly (B), slug (S) or cap-slug (CS), churn (C) and annular (A). The B to CS transition is modeled using the maximum bubble package criteria of small bubbles. The S to C transition takes place for small annulus perimeter flow channels and it is assumed to occur when the mean void fraction over the entire region exceeds that over the slug–bubble section. If the annulus perimeter is larger that the distorted bubble limit the cap-slug flow regime will be considered since in these conditions it is not possible to distinguish between cap and partial-slug bubbles. The CS to C transition is modeled using the maximum bubble package criteria. However, this transition considers the coalescence of cap and spherical bubbles in order to take into account the flow channel geometry. Finally, the C to A transition is modeled assuming two different mechanisms, (a) flow reversal in the liquid film section along large bubbles; (b) destruction on liquid slugs or large waves by entrainment or deformation. In the S to C and C to A flow regime transitions the annulus flow channel is considered as a rectangular flow channel with no side walls. In all the modeled transitions the drift-flux model is used to obtain the final correlations. The final equations for every flow regime transition are easy to be implemented in computational codes and not experimental input is needed. The prediction accuracy of the newly developed model has been checked against air–water as well as boiling flow regime maps. In all the cases, the new developed model shows better predicting capabilities than the existing correlations most used in literature.

Flow regime transition criteria for two-phase flow at reduced gravity conditions

Flow regime transition criteria are of practical importance for two-phase flow analyses at reduced gravity conditions. Here, flow regime transition criteria which take the frictional pressure loss effect into account were studied in detail. Criteria at reduced gravity conditions were developed by extending an existing model from normal gravity to reduced gravity conditions. A comparison of the newly developed flow regime transition criteria model with various experimental datasets taken at microgravity conditions showed satisfactory agreement. Sample computations of the model were performed at various gravity conditions, such as 0.196, 1.62, 3.71 and 9.81 m/s 2 corresponding to micro-gravity and lunar, Martian and Earth surface gravity, respectively. It was found that the effect of gravity on bubbly–slug and slug–annular (churn) transitions in a two-phase flow system was more pronounced at low liquid flow conditions, whereas the gravity effect could be ignored at high mixture volumetric flux conditions. While for the annular flow transitions due to flow reversal and onset of droplet entrainment, higher superficial gas velocity was obtained at higher gravity level.

Implementation and evaluation of one-group interfacial area transport equation in TRACE

The present study implements the one-dimensional interfacial area transport equation into the TRACE code, being developed by the U.S. Nuclear Regulatory Commission. The interfacial area transport equation replaces the conventional flow regime dependent correlations and the regime transition criteria for furnishing the interfacial area concentration in the two-fluid model. This approach allows dynamic tracking of the interfacial area concentration by mechanistically modeling bubble coalescence and disintegration mechanisms. Thus, it eliminates potential artificial bifurcations or numerical oscillations stemming from the use of conventional static correlations. To implement the interfacial area transport equation, a three-field version of TRACE is utilized, which is capable of tracking both the continuous liquid and gas fields as well as a dispersed gas field. To demonstrate the feasibility of the present approach, the steady-state one-group interfacial area transport equation applicable to adiabatic air–water bubbly two-phase flow is first tested in the present study. Data obtained in 18 different flow conditions from two vertical co-current upward air–water bubbly two-phase flow experiments performed in round pipes (25.4mm and 48.3mm) are used to help evaluate the implementation. Results obtained from TRACE with the interfacial area transport equation (TRACE-T) and those from TRACE without the transport equation (TRACE-NT) are compared to demonstrate the enhancement in prediction accuracy. The predictions made by TRACE-T agree well with the data with an average percent difference of approximately ±8%. It is also evident from the results that while TRACE-T accounts for dynamic interaction of bubbles along the flow field, the predictions made by TRACE-NT are attributed primarily to the pressure change.

Interfacial Area Transport Equations for Gas-Liquid Flow

In relation to the modeling of the interfacial transfer terms in the two-fluid model, the concept of the interfacial area transport equation has been proposed to develop a constitutive relation for the interfacial area concentration. The interfacial area transport equation can replace the traditional flow regime maps and regime transition criteria. The changes in the two-phase flow structure can be predicted mechanistically and the effects of the boundary conditions and flow development are efficiently modeled by this transport equation. A successful development of the interfacial area transport equation can make a significant improvement in the two-fluid model formulation and the prediction accuracy of three-dimensional two-phase flow simulations. In this paper, the basic concept of the interfacial area transport equation and its formulation have been briefly described. Available models of the interfacial area sink and source terms and existing databases have been reviewed and the interfacial area transport equation has been benchmarked using adiabatic bubbly flow and condensation bubbly flow data. Future direction for this research has been suggested.

Study of two-phase flows in reduced gravity using ground based experiments

Experimental studies have been carried out to support the development of a framework of the two-fluid model along with an interfacial area transport equation applicable to reduced gravity two-phase flows. The experimental study simulates the reduced gravity condition in ground based facilities by using two immiscible liquids of similar density namely, water as the continuous phase and Therminol 59® as the dispersed phase. We have acquired a total of eleven data sets in the bubbly flow and bubbly to slug flow transition regimes. These flow conditions have area-averaged void (volume) fractions ranging from 3 to 30% and channel Reynolds number for the continuous phase between 2,900 and 8,800. Flow visualization has been performed and a flow regime map developed which is compared with relevant bubbly to slug flow regime transition criteria. The comparison shows that the transition boundary is well predicted by the criterion based on critical void fraction. The value of the critical void fraction at transition was experimentally determined to be approximately 25%. In addition, important two-phase flow local parameters, including the void fraction, interfacial area concentration, droplet number frequency and droplet velocity, have been acquired at two axial locations using state-of-the-art multi-sensor conductivity probe. The radial profiles and axial development of the two-phase flow parameters show that the coalescence mechanism is enhanced by either increasing the continuous or dispersed phase Reynolds number. Evidence of turbulence induced particle interaction mechanism is highlighted. The data presented in this paper clearly show the marked differences in terms of bubble (droplet) size, phase distribution and phase interaction in two-phase flow between normal and reduced gravity conditions.

Local Flow Regime Transition Criteria of Gas-Liquid Two-phase Flow in Vertical Upward Tube with a Horizontal Rod

The upward multiphase cross flow and heat transfer in the vertical tube may occur in oil production and chemical facilities. In this study, the local flow patterns of an upward gas-water two phase cross flow in a vertical tube with a horizontal rod have been investigated with an optical probe and the digital high speed video system. The local flow patterns are defined as the bubble, slug, churn and annular flow patterns. Optical probe signals are analyzed in terms of probability density function, and it is proved that the local flow patterns can be recognized by this method. The transition mechanisms between the different flow patterns have been analyzed and the corresponding transitional models are proposed. Finally, local flow pattern maps of the upward gas-water two-phase flow in the vertical tube with a horizontal rod are constructed.

The characterization of flow regimes with power spectral density distributions of pressure fluctuations during condensation in smooth and micro-fin tubes

This paper presents an objective predictor of the prevailing flow regime during refrigerant condensation inside smooth-, micro-fin and herringbone tubes. The power spectral density (PSD) distribution of the fluctuating condensing pressure signal was used to predict 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, Froude rate criteria, as well as a new flow regime map that was developed for micro-fin tube condensation. Experimental work was conducted for condensing R-22, R-407C, and R-134a at an average saturation temperature of 40 °C with mass fluxes ranging from 300 to 800 kg/m 2 s, and with vapour qualities ranging from 0.85–0.95 at condenser inlet to 0.05–0.15 at condenser outlet. Tests were conducted with one smooth-tube condenser and three micro-fin tube condensers (with helix angles of 10°, 18°, and 37°, respectively). It is shown that the micro-fin tubes cause a delay in the transition from annular to intermittent flow by at least 19% (compared to the smooth tube), thus significantly contributing to the enhancement of heat transfer.