Flow Regime Map Research Articles

Experimental investigation of condensation flows in circular cross-section channel

In this work, condensation flows of pure ethanol in a horizontal water-cooled circular cross-section heat exchanger were investigated for a mass velocity ranging between 30 and 100 kg m−2 s−1. The inner channel of condensation has inner and outer diameters of 4 mm and 10 mm, respectively, with a heat exchange length of 1020 mm. An adiabatic section consisted of a transparent circular cross-section channel, of 4 mm inner diameter and of 130 mm visible length, was placed downstream of the condenser in order to visualise the condensation flows by shadowgraph technique. Two main flow regimes were identified: the wavy-stratified regime and the wavy-annular regime. A flow regime map was developed and compared with flow regime maps available in the literature. Besides, the local vapour quality and internal heat transfer coefficient were determined and compared with prediction models of the literature. Finally, the total pressure drop in the condenser was determined.

Experimental study of two-phase oil – Polymer flow in horizontal flow path

Two-phase flow inside the oil and gas wells and flow path lines is a usual occurrence in petroleum and petrochemical studies. Due to its complexity, flow transfer parameters such as hold up and pressure drop and their dependency on the flow regime maps are not well understood yet. This study is aimed at analyzing holdup for oil- water system and evaluation of this parameter by means of Image processing under different available flow regime conditions. In addition, different oil-polymer systems have been studied to evaluate the impact of polymer concentration on transport criteria. The experimental setup used for this study is consisted of a 12 m long Plexiglas pipe with an inner diameter of 30 mm. The results revealed that the only polymer solutions with polymer concentration below than 333 ppm behave as Newtonian fluids. The results showed that low concentration of polymer solution acts as drag reducing agent and reduces the amount of pressure gradient in comparison with oil – water system. Furthermore, the change in flow pattern map between oil-low concentration polymer solution and oil-high concentration polymer solution systems have been analyzed. The results showed that high concentration polymer solution not only leads to higher degrees of pressure drop but also impacts the flow pattern map and more specifically the Three-Layer flow regime cannot be observed in such flow systems. Therefore, a generalized model has been developed to define the flow regime map and the transition criteria between different flow regimes as well as pressure drop and hold up for oil – polymer systems.

A unique methodology of objective regime classification for two phase flow based on the intensity of digital images

It is proposed that digital images can uniquely be used to identify the regimes during the flow of mixed phases using the temporal and spatial features of the flow. To demonstrate the potential of the postulation, the digital videographs of gas-liquid two-phase flow through a 50 mm diameter vertical tube are considered. The intensity of each pixel is estimated through image processing. The time series of area-averaged intensity contains unique features to identify the flow regimes like that obtained from any conventional sensor. Further, the intensity values along lines in the direction of flow and perpendicular to it can be utilised to produce the spatio-temporal plots and the time history of the interfacial evolution which give signatures of the flow phenomenon hitherto unavailable from the use of conventional sensors. The methodology, compared successfully with the well-established flow regime map has enough potential to be applied for multiphase flow of various types.

Examining the Effect of Flow Rate Ratio on Droplet Generation and Regime Transition in a Microfluidic T-Junction at Constant Capillary Numbers

The focus of this work is to examine the effect of flow rate ratio (quotient of the dispersed phase flow rate over the continuous phase flow rate) on a regime transition from squeezing to dripping at constant capillary numbers. The effect of the flow rate ratio on the volume of droplets generated in a microfluidic T-junction is discussed, and a new scaling law to estimate their volume is proposed. Existing work on a regime transition reported by several researchers focuses on the effect of the capillary number on regime transition, and the results that are presented in this paper advance the current understanding by indicating that the flow rate ratio is another parameter that dictates regime transition. In this paper, the transition between squeezing and dripping regimes is reported at constant capillary numbers, with a transition region identified between squeezing and dripping regimes. Dripping is observed at lower flow rate ratios and squeezing at higher flow rate ratios, with a transition region between the two regimes at flow rate ratios between 1 and 2. This is presented in a flow regime map that is constructed based on the observed mechanism. A scaling model is proposed to characterise droplet volume in terms of flow rate ratio and capillary number. The effect of flow rate ratio on the non-dimensional droplet volume is presented, and lastly, the droplet volume is expressed in terms of a range of parameters, such as the viscosity ratio between the dispersed and the continuous phase, capillary number, and the geometrical characteristics of the channels.

Experimental study of vertical co-current slug flow in terms of flow regime transition in relatively small diameter tubes

In order to investigate the evolution characteristics of structure parameters during flow regime transition process in gas-liquid slug flow, the vertical upward slug flow in a wide range of liquid superficial velocity (0.03< jl<1.6m/s) are studied in a tubular test section with the inside diameter of 25.4mm. Impedance void meters are employed to measure the time series of cross-sectionally averaged void fraction of separated two parts corresponding to Taylor bubble and liquid slug. With increasing flow gas flow rates at constant liquid flow rate, the present research studies the evolution of length and void fraction of each part in slug unit. New structure of turbulent slug unit in strong relation with transition process is observed. It is found that such special structure plays an important role in transition from slug flow to churn-turbulent flow. It is also found that the flooding determines whether the flow regime evolves into churn-turbulent flow or directly into annular flow. By studying the evolution of flow regimes, two structure parameters, void fraction of liquid slug and length ratio of Taylor bubble to liquid slug, can be used as the transition criterion. New criterion for the transition from slug flow to churn-turbulent flow was also proposed based on these two parameters. It is found that the current results agree well with the existing flow regime map for relatively small diameter tubes in literature.

Adaptive mesh method applied to the thermal hydraulic program of system code with a judging criterion based on the matrix error

In this paper, an adaptive mesh method is described and applied to the one-dimension thermal hydraulic program in a system code for simulating the two-phase flow problems. The adaptive mesh method has been widely used to solve various engineering problems, but it is still rarely applied in existing thermal hydraulic system codes which are of great significance for reactor design and analysis. Compared with the uniform mesh, the mesh size could be adjusted automatically. It is helpful to locate the discontinuity points or deal with the large gradient solutions with this method, especially for some local phenomena that occur in phase transfer or two phase flow problems. Based on it, the computational accuracy and efficiency are improved, when solving the partial difference equations. The applicability of the adaptive mesh method in a system code is a major issue, and the first step is studying the feasibility of the adaptive mesh method in thermal hydraulic programs. In this paper, the finite volume method and the Newton-Raphson algorithm are applied in the code as the numerical method of two-fluid model. Furthermore, the code also includes heat transfer models, friction models and a flow regime map, which are important in simulating hydraulic phenomena. Depending on the algorithm, a judging criterion for determining the location where mesh refinement is needed is proposed. Three typical numerical examples are given to verify the feasibility and effectiveness of the adaptive mesh algorithm in solving the one-dimension two-phase flow problems of local phenomena and show great agreement with theoretical or experiment results.

Local flow regime and bubble size distribution in the slender particle-containing scrubbing-cooling chamber of an entrained-flow gasifier

A dual-tip conductivity probe was used to measure local radial gas holdups and bubble chord lengths to study the local flow regime and bubble size distribution in a scrubbing-cooling chamber cold model apparatus containing slender particles. The results showed that the cross-section averaging gas holdups with different fiber volume fractions and aspect ratios could be estimated within errors of ±10% using the modified Kataoka-Ishii bubbly flow semi-empirical correlation at superficial gas velocities ranging from 0.074 m/s to 0.37 m/s. The local flow regime map showed that an obvious bubbly flow formed near the inner wall of the liquid bath due to the effect of wall shear stress, while a cap-bubbly flow formed in other annulus regions due to the emergence of cap bubbles. Chord length distributions were transformed into bubble size distributions by decomposing the measured chord length distributions and estimating the bubble shape factors. Bubble size was categorized into two types—small spherical and large non-spherical bubbles—depending on if the equivalent diameter of the bubbles was smaller or larger than 2 mm. Liquid turbulence, shear stress between the fluid and the wall and liquid backmixing were enhanced by increasing the superficial gas velocity, which affected the size distributions of rising and descending bubbles as well as that of bubble populations. The extent of turbulence suppression and “fiber separation walls” varied with the fiber volume fractions, which affected the bubble size distribution in different regions. The fiber number density characterizing the variation of the fiber aspect ratio with different fiber diameter and length was used to study the bubble size distribution. The bubble size distribution at different aspect ratios was affected by interactions among fibers, bubbles, the fluid and the wall.

A pressure drop model for condensation accounting for non-equilibrium effects

A mechanistic pressure drop model is proposed in this paper for condensation in horizontal smooth round tubes in order to account for the non-equilibrium effects. The model makes use of a flow regime map and void fraction correlation as well as a mechanistic heat transfer model that are all developed for condensation of superheated vapor in a vapor-compression system. The model provide seamless transition between single-phase and two-phase regions including the superheated, condensing-superheated, two-phase, condensing-subcooled and subcooled regions. Diabatic flow visualizations are used to analyze the effects on pressure drop from the formation of waves. An enhancement factor to represent the frequency and magnitude of the waves is established using Kelvin-Helmholtz and Rayleigh-Taylor instability. The two-phase pressure drop is modeled based on the single-phase pressure drop correlations, the flow regimes, void fractions as well as the enhancement factor. Data obtained from R134a, R32, R1234ze(E), R1233zd(E) and R245fa with mass fluxes from 100 kg m−2s−1 to 400 kg m−2s−1and heat fluxes from 5 kW m−2 to 15 kW m−2 inside two different tubes of 4 and 6 mm are used to validate the model.

Downward two phase flow experiment and general flow regime transition criteria for various pipe sizes

Flow regime map is often used in choosing constitutive correlations for the two-phase flow model. The related research mainly concentrates on the vertical upward and horizontal flow, while it is not sufficient in the vertical downward flow. Downward flow is very important as it is frequently encountered in the industrial applications. To enrich the downward flow research, an experiment is performed on a piping system with an inner diameter of 0.1524 m. Four different flow patterns (bubbly flow, cap bubbly flow, churn turbulent flow, and annular flow) are classified with the artificial neural network method. The probability density function (PDF) profile of each flow pattern is discussed. The proposed flow regime map is compared with the other experiments and the effect of the pipe size is discussed. The existing downward flow regime boundary criteria are assessed with the experiment results. It is found that these criteria cannot fit the experiment results well. A set of general boundary criteria are still needed. In this paper, the criteria for the boundary of the bubbly flow, the boundary between the cap bubbly flow and the slug flow, and the boundary of the falling film regime are proposed. They are verified with the experiments on different size pipes. A significant inlet effect on the flow regime boundary is found. The falling film boundary criterion proposed cannot be applied when a sparger is used to inject gas into the downward test section.

Flow regimes in gas-liquid-solid mini-fluidized beds with single gas orifice

Flow regimes and their transitions in the gas-liquid-solid mini-fluidized beds (MFBs) with single gas orifice were studied experimentally in this paper. The diameter of the fluidized beds varied from 3 mm to 5 mm and the vertical column height was 50 mm. The ranges of superficial gas velocity and liquid velocity were 1.96 × 10−4–4.73 × 10−3 m/s and 5 × 10−6–4.2 × 10−2 m/s, respectively. The solid particles ranged in size from 50 μm to 300 μm. Half-fluidization, slug, dispersed bubble and transport flow regimes in the three-phase MFBs were identified by analyzing minimum fluidization velocity, pressure drop, and entrainment velocity of solid particles based on experimental data of pressure drop and fluidization velocity as well as flow observations. The effects of solid particle, liquid properties, bed, gas orifice sizes, and static bed height on the flow regimes and transitions were investigated. Results showed that the behavior of gas bubbles and the wall effect affected the flow regimes and transitions. Obvious wall effect increased minimum fluidization liquid velocity in the liquid-solid mini-fluidized beds, which increased the transition liquid velocity from the half-fluidization to slug flow regimes. Moreover, wall effect made Taylor bubbles bigger at lower superficial liquid velocities due to the bubble coalescence and the transition boundaries expansion. Solid particles aggregation, wall effect and bubble wake behavior were responsible for increment of the minimum entrainment velocity of solid particles. Flow regime maps of the gas-liquid-solid MFBs were presented and correlations were suggested for the flow regime boundaries.

High-Pressure Condensing Refrigerant Flows through Microchannels, Part I: Pressure Drop Models

ABSTRACTCondensation of high-pressure refrigerants in small-diameter channels over a wide range of reduced pressures approaching the critical point is investigated in this two-part study. In this paper, Part I of the study, a multi-regime pressure drop model for condensing fluids in small-diameter channels is presented. Pressure drop measurements were conducted on condensing R404A in circular channels (inner diameter = 0.508, 1.00, 3.05 mm) over the entire quality range. Saturation temperatures were varied from 30 to 60°C, and mass fluxes from 200 to 800 kg m−2 s−1, to evaluate their effect on pressure drop. The saturation temperatures investigated here correspond to reduced pressures between 0.38 and 0.77. The pressure drop models are developed using a microchannel flow regime map and the void fraction models presented by the authors in previous work. The resulting model predicts 85.5% of the data within ±25%. Part II of the study presents a corresponding heat transfer model.

High-pressure Condensing Refrigerant Flows Through Microchannels, Part II: Heat Transfer Models

ABSTRACTCondensation of high-pressure refrigerants in small-diameter channels over a wide range of reduced pressures approaching the critical point is investigated in this two-part study. Part I presented pressure drop measurements and a two-phase pressure drop model. In this paper, Part II of the study, a condensation heat transfer model is presented. Heat transfer coefficients were measured during condensation of R404A in circular channels (inner diameter = 0.86, 1.55, 3.05 mm) over the entire quality range. The saturation temperature was varied from 30 to 60°C, and mass flux from 200 to 800 kg m-2 s-1, to evaluate their effects on condensation heat transfer coefficient. The heat transfer model is developed using a microchannel flow regime map and the void fraction model previously developed by the authors. The resulting model predicts 93.6% of the data within ±25%. The model exhibited good agreement with data from condensing ammonia and carbon dioxide, predicting 84.8% and 97% of their data within ±25%, respectively.

Investigation of subcooled and saturated boiling heat transfer mechanisms, instabilities, and transient flow regime maps for large length-to-diameter ratio micro-channel heat sinks

This study investigates the interfacial behavior and heat transfer mechanisms associated with flow boiling of R-134a in a micro-channel test module. The test module features 100 of 1 × 1 mm2 square micro-channels. Large length of micro-channels used (609.6 mm) is especially important to capturing broad axial variations of both flow and heat transfer behavior. The fluid is supplied to the test module in subcooled state to enable assessment of both the subcooled boiling and saturated boiling regions. The study employs a combination of temperature measurements along the test module and high-speed video to explore crucial details of the flow, including dominant flow regimes, flow instabilities, and downstream dryout effects. It is shown that, unlike macro-channel flows, where flow regimes can be clearly demarcated, flow regimes in micro-channels are associated with transient fluctuations that are induced by flow instabilities. The dominant flow behavior and associated dryout effects are characterized with the aid of a new transient flow regime map and a dryout map, respectively. Two sub-regions of the subcooled boiling region, partially developed boiling (PDB) and fully developed boiling (FDB), are examined relative to dominant interfacial and heat transfer mechanisms, and a previous correlation is identified for accurate prediction of the heat transfer coefficient for both PDB and FDB. The saturated boiling region is shown to consist of three separate sub-regions: nucleate boiling dominated for qualities below 0.3, combined nucleate and convective boiling for qualities between 0.3 and 0.5, and convective boiling dominated for qualities above 0.5. Above 0.5, dryout effects begin to take effect, causing a gradual decline in the heat transfer coefficient followed downstream by a more severe decline. A previous correlation is identified for prediction of the heat transfer coefficient in the saturated boiling region.

Effects of pipe size on horizontal two-phase flow: Flow regimes, pressure drop, two-phase flow parameters, and drift-flux analysis

This paper performs experimental studies to investigate the effects of pipe size on horizontal air–water two-phase flow in a wide range of flow configurations. Some of the major characteristic two-phase flow phenomena of interest include flow regime transitions, two-phase frictional pressure drop, local interfacial structures, and drift-flux analysis. Based on the experimental results obtained in two pipe sizes, the flow regime maps and the correlations for two-phase frictional pressure drop employed in conventional nuclear reactor system analysis codes (TRACE and RELAP) are evaluated. It is found that the flow regime maps in the codes incorrectly predict plug and slug flows as bubbly flow. Effects of pipe size on flow regime transitions are observed, while these effects are not reflected on the maps employed in the codes. The frictional pressure drop analysis suggests that the closure relations used in both TRACE and RELAP can be improved for horizontal two-phase flow. The local time-averaged two-phase flow parameters acquired by a four-sensor conductivity probe including void fraction, interfacial area concentration, bubble velocity, and bubble Sauter-mean diameter provide quantitative descriptions for the interfacial structures in different pipe sizes. It is found that increasing the pipe size tends to cause bubbles to be more concentrated near the top wall in bubbly flow. As a result, the bubble layer occupies a smaller portion of the flow area as pipe diameter increases. Based on the experimental database, drift-flux analysis using both the 〈〈vg〉〉 vs. 〈 j〉 and 〈α〉 vs. 〈β〉 methods is performed. The effect of pipe size on drift-flux analysis is found to be negligible. The global slip is found to be less than one, indicating that the gas phase moves slower than the liquid phase in horizontal bubbly flow. This analysis also provides a predictive method to estimate the void-weighted bubble velocity and void fraction, which can be predicted with an average percent difference of ±3.3% and ±3.1%, respectively.

Effect of Surface Tension Variation of the Working Fluid on the Performance of a Closed Loop Pulsating Heat Pipe

ABSTRACTIn this article, the effect of surface tension variation of the working fluid on the thermal performance of a pulsating heat pipe (PHP) is presented. A two-turn closed loop PHP is fabricated with a combination of copper and quartz tubes having 2 mm inner diameter. Three filling ratios are considered in the present study. A common surfactant namely sodium dodecyl sulfate is used to vary the surface tension of the distilled water which is the base working fluid tested in the PHP. Visualization as well as heat transfer studies is performed in the PHP. Thermal resistances of working fluids with different surface tension are estimated for each filling ratio. Further, the effect of surfactant concentration on the hydrodynamics of the PHP is also discussed. Addition of surfactant induces formation of foam after the filling process and in some phase of PHP operation. It changes the flow regime boundaries and lowers the evaporator temperature under specific operating conditions. A flow regime map is also constructed for the present PHP. The working fluid with a lower surface tension gives a minimum evaporator temperature in the vertical orientation and thus, lowest thermal resistance has been obtained. However, surface tension does not influence the performance in the horizontal position appreciably.

Sub-channel flow regime maps in vertical rod bundles with spacer grids

The accurate prediction of thermal-hydraulic parameters is based on the flow regime maps for rod bundle, which is important for the safety of nuclear reactor. An air-water two phase flow experiment has been performed to study the sub-channel flow regime (SCFR) maps in 5 × 5 rod bundles section with two distinct spacer grids, including simplified spacer grid (SSG) and mixing vane spacer grid (MVSG). To obtain the objective SCFR maps in rod bundles, a sub-channel impedance void meter is newly developed to measure the time series void fraction in sub-channels. Besides, the random forest clustering algorithm has been adopted to identify the sub-channel flow regimes objectively based on a training sample and 13 selected feature values of time series void fraction. The feature values include mean value, standard deviation value, sample entropy value and 10 proportion values. In this way, the objective sub-channel flow regime maps are obtained at four different locations with different spacer grids. Distinct features have been observed for different SCFRs. As for the SCFR transitions in different sub-channels over the same cross-section, almost all of them arise in the inner sub-channel firstly for the effect of casing tube. Moreover, the dissipation length of spacer grid is larger than 19.5 L/D. In the influencing region of spacer grid, the transition from cap bubbly to cap turbulent flow occurs in the corner sub-channel at 19.5 L/D downstream of spacer grid firstly and then at 6.8 L/D for low liquid velocity, while firstly occurs at 6.8 L/D for high liquid velocity. The magnitude of the spacer grid effect on SCFR transition depends on the superficial liquid and gas velocity, as well as the structure of spacer grid for current flow conditions. Only Liu and Hibiki’s model is applicable for the transition from bubbly to cap bubbly flow in sub-channel. Therefore, new transition models or correlations should be developed for other sub-channel flow regime transitions.

Investigations on the penetration length of steam-air mixture jets injected horizontally and vertically in quiescent water

Gas submerged jet penetration length is a key parameter for many industrial applications. Many experimental and theoretical works have been performed on pure steam and non-condensable gas submerged jets. However, the steam-air mixture gas submerged jets are not yet fully understood. So, a series of visualization experiments are performed to investigate the effect of air mass fraction, jetting direction, water temperature and inlet pressure on mixture gas submerged jet penetration length. Two flow regimes are distinguished based on the evolution of the interface between water and mixture gas, which are referred to as stable regime and oscillation regime. A flow regime map is developed based on inlet pressure and air mass fraction. A momentum balance model for predicting jet penetration length is proposed. Results show that the mixture gas jet penetration length increases with the increasing inlet pressure and decreases with the increasing air mass fraction. The water temperature has little effect on the jet penetration length. A new correlation for dimensionless vertical jet penetration length is developed based on the form of model analysis. The discrepancy between predicted results and present experimental data is within ±5%. A new correlation for dimensionless horizontal jet penetration length is developed based on experimental results. The discrepancy between predicted dimensionless horizontal jet penetration length and experimental value is within ±18%.

Visual observations of flow patterns in downward air-water two-phase flows in a vertical narrow rectangular channel

Flow patterns in downward air–water two-phase flows in a narrow rectangular channel were visually observed. A test section was constructed using transparent acrylic plate with a test channel 2.35 mm thick, 66.6 mm wide, and 780 mm long. The experiments determined that at high liquid velocity the flow patterns for downward flow are similar to those observed in upward flow. However, at low liquid velocity some significant differences were observed between downward and upward flows. The new flow patterns, falling film flow, large-bubbly flow, and intermediate flow, were identified in the low liquid velocity region, where buoyancy force is influential. A flow regime map for downward flow in a narrow rectangular channel was constructed using the experimental data and analyses.

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).

Confinement and vapour production rate influences in closed two-phase reflux thermosyphons Part A: Flow regimes

This study investigates the boiling regimes in a small diameter (D = 8 mm) transparent thermosyphon. The influence of confinement on the boiling regimes was studied using a range of fluids. The confinement of the vapour phase leads to boiling regimes that differ from those traditionally described for thermosyphon evaporator boiling physics, widely considered as a combination of pool boiling and film evaporation. The boiling behaviour of small dimension thermosyphons was investigated by designing and constructing a fully transparent thermosyphon, enabling simultaneous thermal and visual analysis. Three working fluids, water, ethanol and HFE-7000, were used to characterise the thermosyphon behaviour with varied characteristic bubble length scales. The observed flow regimes could be characterised in terms of the degree of confinement and rate of vapour production. A flow regime map was developed based on these observations to predict thermosyphon flow in terms of both confinement and the rate of vapour production. It was determined that for low confinement and high rates of vapour production, the boiling regimes resemble those of pool boiling. In contrast, at high levels of confinement and high heat flux, an unsteady regime exists where relatively large bubbles and vapour generation rates result in a pulsatile geyser-type flow regime.