Churn Flow Research Articles

Operational regimes in a confined pulsatory two-phase thermosyphon

The major loopholes in existing criteria used for the classification of various flow characteristics/patterns in the confined two-phase closed thermosyphons (TPCTs), and the additional accountable parameters have been recognized. The phenomenology of flow regimes in confined TPCTs, and the influence of confinement on their heat transfer characteristics need to be understood better. Also, the mitigative solutions to enhance the operational limits of a confined TPCT need to be developed. So, a visual and detailed study of the thermo-hydrodynamics of TPCTs is essential. The current research presents a confined pulsatory two-phase thermosyphon (PTPT), which is a hybrid cooling device derived by combining the ideologies of a pulsating heat pipe (PHP) and a TPCT. The functioning of PTPT involves thermosyphon phenomena in its evaporator coupled with oscillations of a liquid–vapor interface in its condenser. An experimental study of different flow regimes, viz. bubbly, bubbly/slug, geyser, slug, churn and annular flows, and associated heat transfer mechanisms observed in the PTPT at different applied thermal boundary conditions has been performed. The two-phase flow in PTPT has been rigorously mapped and an empirical flow regime map has been developed using the confinement and Jakob numbers for regime characterization. Contrary to the conventional notions, highly confined as well as unconfined flow patterns have been observed within a narrow range of the confinement number, with unconfined ones at the highest confinement number. The film-flow along with the corresponding instabilities, nucleation and growth of vapor bubbles, inter-regime transitional states, and associated pressure fluctuations have been studied in detail.

Application of Flow Pattern Map for Solving Liquid Loading Problems in Well AA

About two-thirds of fields that exist in the world right now are classified as mature fields. Mature fields usually have a lot of problems that occurred during production. Mature field is classified as field which has produced as much as fifty percent of their established proved plus probable resources estimation or has produced for more than twenty-five years. Empowerment of mature well can be increasing the efficiency of well productivity. Productivity decline can be caused by low maintenance of flow pattern in a well. The other problems which commonly happen is decreasing of reservoir pressure of the well, liquid loading, slugging, and high water cut percentage. Several methods can be used to solve a lot of problems in mature well. The methods can be applied to get an optimum well productivity result. This study shows that Well AA has a slug flow pattern using matching method on a flow pattern diagram. Gas injection and velocity string are applied to solve the problems for Well AA. Gas rate of about 12 MMSCFD is obtained with gas injection method to get annular flow pattern and three different sizes of velocity string are used which are 1.25 inch, 1.5 inch, and 2.0625 inch with churn flow pattern. It is identified that the use of velocity string of 1.25 inch is the optimum method for Well AA.

Experimental analysis and model evaluation of gas-liquid two phase flow through choke in a vertical tube

Downhole chokes have been extensively implemented in gas and oil fields to improve well performance and facilitate equipment/chemical cost savings. Existing models have been developed based on wellhead choke data. However, compared with a horizontal pipe, vertical two-phase flow presents different flow behavior and patterns. Therefore, the flow patterns should be considered to evaluate the performance of the models for predicting the downhole choke flow rate. In this study, 40 data points were tested on both vertical and horizontal tubes with 4-mm chokes. Moreover, 350 data points (Appendix) were tested on a vertical tube with four choke sizes (2, 4, 8, and 12 mm). Flow patterns, pressure, and mass flow rates were measured and analyzed. From the results of comparative analysis, significant discrepancies between vertical and horizontal evaluation results were reported. The results demonstrated that the downhole chokes changed the flow pattern downstream of chokes, improved the stability of p d , and prevented the liquid phase downstream of chokes from falling to the upstream. The slug–churn flow transition boundary was accurately predicted by Taitel correlation, and the churn–annular flow transition boundary moved leftward with an increase in pressure. Under the critical flow condition, the Ashford model (RMSE = 0.005 kg/s and R 2 = 0.92), Sachdeva model (RMSE = 0.007 kg/s and R 2 = 0.80), and Al-Safran model (RMSE = 0.004 kg/s and R 2 = 0.96) achieved the highest prediction accuracies for slug, churn, and annular flow, respectively. Based on 170 subcritical flow data points, the Perkins model (RMSE = 0.007 kg/s and R 2 = 0.99), Ashford model (RMSE = 0.009 kg/s and R 2 = 0.84), and Sachdeva model (RMSE = 0.009 kg/s and R 2 = 0.76) are recommended for slug, churn, and annular flow, respectively. • Comparing the mass flow rate evaluation results of two-phase flow through both vertical and horizontal choke. • Evaluating the flow pattern transitions upstream of the chokes under relative high-pressure condition. • Flow behavior of two-phase flow through downhole choke was investigated experimentally. • Evaluating the performance of flow rate prediction models under different flow patterns upstream of the downhole chokes.

High heat flux flow boiling of R1234yf, R1234ze(E) and R134a in high aspect ratio microchannels

Flow boiling of refrigerants in narrow channels has a great potential to cope with future challenges in the thermal management of high heat flux electronics. This study investigated the flow boiling heat transfer of R1234yf, R1234ze(E), and R134a in multi-microchannels as the applied heat flux was varied from boiling incipience to the critical heat flux. A parametric analysis on the two-phase local heat transfer coefficients was conducted for channel mass fluxes from 415 kg/m2s to 1153 kg/m2s and saturation temperatures of 30.5 ∘C and 40.5 ∘C. The test section comprised 17 channels with inlet restrictions. The channels were 298 µm wide, 1176 µm deep and 10 mm long. Three boiling regimes (I, II, III) were identified according to the observed effect of the heat and mass flux. In boiling regime I, the two-phase local heat transfer coefficient was independent of the mass flux and increased with the heat flux. In boiling regime II, the flow departed from this behavior, and a significant effect of the mass flux appeared. Finally, in boiling regime III, the two-phase local heat transfer coefficient decreased steeply with the heat flux. Flow visualization showed that a predominantly bubbly flow was present in the channels during boiling regime I. In boiling regime II, churn flow was the predominant flow regime, but appeared with more frequent vapor slugs as the heat flux was increased, eventually leading to alternating churn and wispy-annular flow. In boiling regime III, the flow regime alternated between churn and wispy-annular, with a dryout incipience leading to the progressive decrease of the two-phase local heat transfer coefficient until the achievement of the critical heat flux. The experimental data for boiling regimes I and II, amounting to 2504 local values, were compared against the prediction of selected literature correlations.

Gas-liquid vertical pipe flow patterns convolutional neural network classification using experimental advanced wire mesh sensor images

Flow pattern identification for simultaneous gas-liquid flow in pipes is a central problem in the petroleum industry. However, there are many conventional methods used for flow patterns identification that come with some form of restrictions limited to specific operating conditions. Although previous studies made efforts to make flow patterns identification free from biases (subjectivity), objective predictions are yet to be fully explored. For the first time, this study employs a 4-layer convolutional neural network (CNN) architecture and threshold segmentation algorithm to classify industrial working fluids of air/silicone oil flow patterns in a vertical pipe. Also, an advanced wire mesh sensor (WMS) instrumentation was used to obtain the cross-sectional frame images from a series of 52 experimental runs. This study made an attempt to use the sequential cross-sectional frames original data obtained from the experimental WMS because it is the crossing wires of the WMS that interact with the flow field and also capture the main flow pattern transitional zones. The obtained experimental testing results indicate that the supervised CNN model has an accuracy of 99.90% better than the seven-benchmarked supervised machine learning classification models used in recognizing bubbly, slug and churn flow patterns. The CNN model experimental test accuracy also outwits other related flow patterns identified in the literature when compared, thereby affirming the model's robustness. Since the WMS provide high spatial and temporal resolutions data, the average pixel intensity values from the segmented images were used as the flow indicator in identifying the transition zones within the main flow patterns. Furthermore, the Locally Interpretable Model-agnostic Explanation (LIME) algorithm for the first time was used to explain and interpret features in the WMS flow images that were contributing to the overall CNN classification scores. Finally, the CNN trained model was able to classify the main flow patterns and segmented transitional zones with certainty and confidence without subjectivity which is inherent in the direct conventional methods.

NUMERICAL STUDY OF FLOW AND HEAT TRANSFER CHARACTERISTICS IN SHELL-AND-PLATE HEAT EXCHANGERS

The shell-and-plate heat exchanger is a completely welded heat exchanger that combines the best features of a shell-and-tube heat exchanger with a plate heat exchanger. The flow between the circular corrugated plates behaves as a first expanding then converging flow, leading to the intricate nature of flow structures, flow resistance, and phase-change heat transfer. The present paper numerically investigates the flow behaviors and heat transfer characteristics in the channel between the circular corrugated plates of 0.2 m in diameter, 0.0022 in corrugation depth, and 75° in corrugation angle. In the simulation, the gas velocity ranges from 0.25-6.00 m/s while the liquid velocity from 0.01-0.20 m/s. The wall is set as adiabatic and constant temperature condition of 350 K, respectively. For the single-phase flow, a longitudinal spiral flow along the main flow direction is formed, and the local velocity at the edge of the circular plate is generally higher and more turbulent than that in the central area. Additionally, the increase of Reynolds number leads to a more significant pressure drop but an apparent heat transfer enhancement. Based on the analysis of the friction factor and Nusselt number with Reynolds number, the empirical correlations for predicting the flow resistance and heat transfer characteristics in the channel are proposed. For the two-phase flow, we employed the VOF model to investigate the distribution of the phases in the downward flows. Compared with the experimental data, the numerical results are demonstrated to provide an accurate analysis of the phases distribution, particularly in the cross section of the channel. Accordingly, the flow patterns in the downward two-phase flows are identified as bubbly flow, slug flow, film flow, and churn flow. Based on the variation of the averaged void fraction, the film flow can be identified as α > 0.8 while bubbly flow as α < 0.5. Additionally, the void fraction in the cross section of the flow channel along the flow direction is analyzed in detail. Since the effect of the expansion and contraction of structures will go away with an increase in the gas mass flow rate, it can be anticipated that the effect of uniform distribution is more prominent for slug flow and is negligible for film flow.

Flow pattern, heat transfer and frictional pressure drop investigation of R365mfc condensation in a micro-structured corrugated gap with mixed angles

• Experimental visualization results for condensation of R365mfc. • Mixed 27°/63° plate gap with microstructure are used. • Analysis of flow path, flow patterns, heat transfer and frictional pressure drop. • Comparison between empirical correlations in literature and experimental data. • A new heat transfer model based on condensation mechanism is developed. This paper experimentally investigates the condensation of R365mfc in a micro-structured corrugated gap with mixed 27°/63° chevron angle. The condensation mechanisms are analyzed by visualizing the flow patterns through a transparent polyurethane plate, and the measurements are carried out at saturation pressure 1.12 bar ( p red = 0.034), mass flux between 6.14 and 44.56 kg/m 2 s and a mean vapor quality from 0.58 to 0.92. Six flow patterns are identified as the mass flux is decreasing: annular flow, wispy-annular flow, partial film flow, smooth liquid film flow, churn flow and slug flow. A flow pattern map is drawn as a function of mass flux and vapor quality, G ≈ 20 kg/m 2 s is considered as a transition, after which no partial film flow is observed. The combination of G ≈ 20 kg/m 2 s and x ≈ 0.83 is the transition of the condensation mechanism between gravity-controlled condensation and a combination of gravity-controlled and convection condensation. The frictional pressure drop increases with the increase of mass flux and vapor quality and is not influenced by flow patterns. The experimental heat transfer coefficients and frictional pressure drop are compared with the prediction results calculated by the correlations in the literature and a new heat transfer model is developed, which includes the individual heat transfer models of a gravity-controlled regime, a transition regime and a shear force-controlled regime as well as the transition criterion between these regimes. The new model predicts 97% of experimental data within ±30%, the root mean square error and the mean absolute error is 12.7% and 9% respectively.

Measurement of Oil–Gas–Water Flows in Vertical Pipes Using Electromagnetic Flowmeter and Dual-Conductance Sensors

Three-phase flows are significantly complicated in comparison to single-phase flows and gas&#x2013;liquid two-phase flows. So far, faced with the presence of two nonconductive dispersed phases in oil&#x2013;gas&#x2013;water three-phase flows, the measurement characteristics of electromagnetic flowmeter (EMF) in three-phase flows are an unresolved problem. The flowrate of three-phase flows is difficult to measure due to the nonuniformity arising from interactions between dispersed phases. Aiming at the difficult problem of production logging in oil wells of three-phase flows, this article reports an experimental study on measuring three-phase flows via the EMF and the dual-conductance sensors. Among them, the phase holdups of three-phase flow in vertical pipe are extracted through newly designed distributed coaxial double-ring conductance sensor (DRCS) and rotating electric field conductance sensor (REFCS). A three-phase flow loop test in a pipe of 20 mm inner diameter is carried out. The results show that the EMF has the ability to substantively measure the flowrate of water phase under three flow patterns (bubble, slug, and churn flows) when the water cut is above 60&#x0025;, and the average absolute percentage deviation (AAPD) for prediction of superficial water velocity is 3.55&#x0025;. The measurement of superficial gas and oil velocities is implemented by solving the drift-flux models of three-phase flow based on the phase holdups and superficial water velocity, and the AAPDs for predictions of superficial gas and oil velocities are 4.33&#x0025; and 5.95&#x0025;, respectively. This research provides a new measurement strategy for production logging of oil, gas, and water three-phase flow production profile by combining the EMF with dual-conductance sensors.

МОДЕЛИРОВАНИЕ ВЛИЯНИЯ ПОЛОЖЕНИЯ ТРУБОПРОВОДА НА РЕЖИМ МНОГОФАЗНОГО ПОТОКА

The paper investigates the effect of an inclination angle of a straight pipeline on the gas-liquid two-phase flow regime at the same inlet conditions. The homogeneous Volume of Fluid (VOF) model in Ansys Fluent is used in this study for simulating the spread of the gas phase, changes in the entrance region and establishing of the multiphase flow regime. The authors consider a horizontal, vertical and inclined (60 degrees) pipelines where the stratified, bubbly and churn flow regimes are formed respectively, and the results are presented in terms of the gas phase distribution in the volume of the carrying liquid and the velocity field.

Experimental Examination of Gas-liquid Two-phase Flow Patterns in an Inclined Rectangular Channel with 90° Bend for Various Vertical Lengths

In order to realize the behavior of two-phase flow in an inclined rectangular channel with 90̊ bend for various vertical lengths, an experimental investigation has performed. The test section (a chanel with 30 m length) contains two horizontal parts and a middle vertical section. Three vertical lengths of 50, 100 and 150 cm were utilized along the test section. The cross section of the rectangular channel is 10 cm × 15 cm. Air and water were the fluids used in this work. Various flow patterns are created by varying the volumetric flow-rates of gas and liquid; the air and water flow rates were in the range of 2 to 10 and 34 to 235 , respectively. The high speed video system was utilized to achieve image subsequence of the flow under various conditions. The effects of vertical length on flow regimes and pattern transition borders are examined. According to the flow visualization, no vortex was observed in the vertical section. The significant flow regime in the vertical section is churn flow regime. It is concluded that the flow pattern structures are not greatly affected by changing the vertical length.

Three-Phase Flow Tomography System in Upward-Vertical High-Viscous-Oil/Water/Gas Flow

A new three-phase flow tomography system is presented in this work. The system was developed to be applied in high-viscous-oil/water/gas flow arranged in different flow patterns, with the three fluids present simultaneously in a crossing point. The primary measurement is based on a simpler and reliable digital in-phase/quadrature (IQ) demodulator and a wire-mesh sensor (WMS), but the system is capable of including different types of primary measurements. Dynamic three-phase flow tests were carried out to verify the system performance and compare its results with the ones obtained by quick-closing valves (QCVs) and a high-speed video camera. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In situ</i> volumetric fractions and flow patterns were compared. The experiments were conducted in a vertical glass pipe of 50-mm i.d. and 12-m height. The observed three-phase flow patterns were water-continuous churn flow with dispersed bubbles and oil drops (WChBoDo), water-continuous slug flow with Taylor bubbles, spherical bubbles, and oil drops (WPiBoDo), and oil-continuous slug flow with Taylor bubbles, spherical bubbles, and water drops (OPiBoDi). Compared with the measurements using QCVs, the average relative errors of the measured volumetric phase fractions were 12% for water, 13% for air, and 22% for oil. The experimental results, including the details of the flow inside the pipe, the phase distributions, and the objective characterization of flow patterns, suggest that the proposed system is a promising option for three-phase flow-related studies and developments. The proposed technique can reach 8930 frames/s with a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$16\times16$ </tex-math></inline-formula> WMS.

Two-Group Bubble Interfacial Area Concentration Correlation for Upward Bubbly to Churn Flows in Vertical Rod Bundle Flow Channels

Two-Group Bubble Interfacial Area Concentration Correlation for Upward Bubbly to Churn Flows in Vertical Rod Bundle Flow Channels

A novel comprehensive model for predicting production of downhole choke wells

The mass-flow rate that flows through wellhead/downhole chokes is typically used to measure oil/gas production. Published mass-flow rate prediction models are proposed based on wellhead choke data; however, because of the different flow behavior between vertical and horizontal tubes, unacceptable errors may occur when used on downhole choke wells. These models can be improved by three additional factors; flow patterns, gravity, and gas expansion coefficient. In this study, 180 pressure, mass-flow rate, and flow pattern data points were measured with four different size chokes on a vertical tube. According to the flow behavior analysis, a new comprehensive model was proposed to improve the accuracy of multiphase mass-flow rate prediction for downhole chokes. The experimental results show that the downhole choke redistributes the gas and liquid phase, causing the flow patterns downstream of the choke to change. Additionally, pressure data analysis results show that the pressure difference between the Pu (pressure upstream of the chokes) and Pd (pressure downstream of the chokes) can be used to identify the flow pattern transitions upstream of the chokes. The statistical error results of the new model coupled with various slip models reveal that the Simpson (RMSE = 0.0018 Kg/s, R2 = 0.99) and Schuller (RMSE = 0.002 Kg/s, R2 = 0.97) models achieve the highest accuracy prediction for the intermittent and continuous flow, respectively. Furthermore, the critical pressure ratio measured on a vertical tube can be accurately predicted by the new comprehensive model. Compared to other models, the new comprehensive model is the most accurate, yielding the best mass-flow rate predictions for the dataset; slug (RMSE = 0.0018 Kg/s and R2 = 0.98), churn (RMSE = 0.0018 Kg/s and R2 = 0.99), and annular flow conditions (RMSE = 0.002 Kg/s and R2 = 0.97).

An experimental study of interfacial dynamics during stratified oil-water draining from storage tank

Oil-water stratification is an inevitable phenomenon in the petroleum industry. The present study describes stratified oil-water draining from a square tank through a 9 mm diameter drain pipe. The purpose of this study is to understand the flow physics of oil as well as air entrainment in drain pipe during drainage of water. In order to observe the influence of the viscosity of oil on the entrainment and flow structure, two different oil-water stratifications are selected. In these two cases, the viscosity ratios of oil to water are 1.64 and 55, respectively. While draining, the flow of single-phase liquid in the drainpipe is transformed to two-phase liquid-liquid flow and then three-phase flow. A pointed tip of oil is formed as the oil-water interface approaches the drainpipe inlet. Different flow patterns, namely bubbly flow, slug flow, churn flow, elongated slug flow, jet flow, and annular flow, are observed as air and oil enter the drain pipe. Reverse flow is observed in the case of a higher viscosity ratio. An empirical correlation based on non-dimensional numbers has been proposed for describing the condition of no air entrainment in the drain pipe. Optical probes are used successfully to detect the presence of several regimes of air-oil-water and oil-water in the drainpipe.

Gas–liquid–hydrate flow characteristics in vertical pipe considering bubble and particle coalescence and breakage

The gas–liquid flow behavior laden by hydrate particles is significant for the flow assurance of submarine natural gas hydrate production. An integrated multiphase flow model considering the coalescence and breakage of bubbles and hydrate particles was developed to investigate the gas–liquid–hydrate flow problem in a vertical pipe. The population balance theory was adopted to describe the evolution of the bubble and hydrate particle sizes in a flow field. The effects of particle behavior on flow pattern transition were analyzed. The results indicated that the phase distribution became inhomogeneous during the flow process, and bubbles tended to aggregate at the central region of the pipe. Most hydrate particles agglomerated in the liquid film region, and the particle size increased. The coalescence and breakage of bubbles induced a flow pattern transition from a bubbly to slug and churn flows. However, the agglomeration of hydrate particles increased the viscosity of the liquid phase, further reduced the turbulent intensity, and enhanced the stability of the bubbles. Thus, a higher gas superficial velocity was required for the formation of slug and churn flows. An annular flow was easier to form because the hydrate particles entrained in the liquid phase stabilized the liquid film. This study contributes to the flow assurance of submarine natural gas hydrate production and relevant multiphase flow modeling studies with hydrate particles.

Investigation of Huge Wave in Gas-Liquid Two-Phase Churn Flow

Churn flow frequently occurs in power plants, chemical engineering, petroleum, and other industrial applications. Due to its chaotic nature, churn flow has a significant influence on safety and management control. As one of the essential characteristics of churn flow, depth knowledge of the huge wave is crucial for a better understanding churn flow. However, relevant studies on these issues are still in shortage because it is difficult to capture its behaviours experimentally. In this study, we employed the high-speed camera to capture the evolution and properties of huge waves under churn flow conditions in a vertical pipe. The inner diameter of the pipe is 19 mm. Based on the observation, the flooding of the falling film in churn flow is demonstrated to be the slug/churn transition mechanism. Additionally, the liquid distribution in the cross-section of the tube is provided and discussed in detail. Compared with the existing experiment data, we carefully analyze the properties of huge waves, such as frequency and amplitude.

Gas-liquid flow in small channels: Artificial neural network classifiers for flow regime prediction

To design and operate multiphase apparatus with mini- and microchannels, it is important to know how the fluids stream inside. Most literature reports the occurrence of flow regimes in dependence on gas and liquid superficial velocities in flow maps that are valid for fluids with similar properties and channels with similar geometry. Attempts to develop universally applicable flow maps show limitations in the number and variation of considered model parameters or in the number of considered flow regimes. This paper presents artificial neural network classifiers able to predict all relevant flow regimes: (a) Taylor flow, (b) bubbly flow, (c) Taylor-annular flow, (d) churn flow, (e) dispersed flow, (f) annular flow, (g) rivulet flow, and h) parallel flow in dependence on geometric and operational parameters as well as fluid properties with a high precision (R=0.92...0.95 and classification rates were generally above 80%). The classifiers were developed and validated by using more than 13,000 experimental data on gas-liquid flows extracted from 97 flow maps and are based on 7 significant dimensionless groups, namely, ReG, ReL, WeG, WeL,CaL, Θ*, and the channel form factor FC.

The Effect of Hydraulic Diameter on Flow Boiling within Single Rectangular Microchannels and Comparison of Heat Sink Configuration of a Single and Multiple Microchannels

Phase change heat transfer within microchannels is considered one of the most promising cooling methods for the efficient cooling of high-performance electronic devices. However, there are still fundamental parameters, such as the effect of channel hydraulic diameter Dh whose effects on fluid flow and heat transfer characteristics are not clearly defined yet. The objective of the present work is to numerically investigate the first transient flow boiling characteristics from the bubble inception up to the first stages of the flow boiling regime development, in rectangular microchannels of varying hydraulic diameters, utilising an enhanced custom VOF-based solver. The solver accounts for conjugate heat transfer effects, implemented in OpenFOAM and validated in the literature through experimental results and analytical solutions. The numerical study was conducted through two different sets of simulations. In the first set, flow boiling characteristics in four single microchannels of Dh = 50, 100, 150, and 200 μm with constant channel aspect ratio of 0.5 and length of 2.4 mm were examined. Due to the different Dh, the applied heat and mass flux values varied between 20 to 200 kW/m2 and 150 to 2400 kg/m2s, respectively. The results of the two-phase simulations were compared with the corresponding initial single-phase stage of the simulations, and an increase of up to 37.4% on the global Nu number Nuglob was revealed. In the second set of simulations, the effectiveness of having microchannel evaporators of single versus multiple parallel microchannels was investigated by performing and comparing simulations of a single rectangular microchannel with Dh of 200 μm and four-parallel rectangular microchannels, each having a hydraulic diameter Dh of 50 μm. By comparing the local time-averaged thermal resistance along the channels, it is found that the parallel microchannels configuration resulted in a 23.3% decrease in the average thermal resistance R¯l compared to the corresponding single-phase simulation stage, while the flow boiling process reduced the R¯l by only 5.4% for the single microchannel case. As for the developed flow regimes, churn and slug flow dominated, whereas liquid film evaporation and, for some cases, contact line evaporation were the main contributing flow boiling mechanisms.

Elongated bubble velocity estimation in vertical liquid-gas flows using flow-induced vibration

The multiphase flow occurs in various major industrial fields and nature. Furthermore, due to the single and multiphase flow characteristics, they generate vibration on the structure conveying or subjected to the flow. This paper investigates in an experimental apparatus, the possibility to use the flow-induced vibration from a vertical liquid-gas two-phase flow conveyed by a pipe to obtain the elongated bubble velocity. The vibration signal analysis in time domain showed that the structural excitation due to the elongated bubble rising in the stagnant liquid was significant to be distinguished from other excitations. The time-frequency analysis of slug and churn flow showed a significant amplitude variation in a specific frequency band. Finally, it was possible to obtain the elongated bubble velocity with reasonable accuracy by cross-correlating the envelope of a vibration signal filtered in a particular frequency band of two accelerometers. Thus, this paper presents a non-invasive and simple to mount technique to estimate the elongated bubble velocity in stagnant and moving liquid conditions.

Thermo-fluidic characteristics and performance in a distribute heating bubble pump generator

In some thermally driven two-phase natural circulation systems, bubble pumps serve as the key driving powers for the cycles. Recently a type of distribute heating bubble pump generator (BPG) is gradually receiving attention due to its compact structure and great potentials to utilize solar energy and low-grade waste heat recovery. The BPG provides a variety of promising features (e.g., passive heat transfer, enhanced reliability), which can benefit the advancing of heat transfer technology. For the primary study, we performed an experiment in a distributed heating BPG. Through utilizing multiple lift tubes and partial visualization configurations, it provides accesses to observe the flow pattern transition and monitor the flow instability, and thus to explore some of the underlying mechanisms affecting BPG performance.Results showed that heat input and immersion height were crucial parameters to enable the operation of distribute heating BPG. With low heat input or high inlet water subcooling level, the flow within the pump was unstable with intermittent flow interruptions. As the heat input increased, the fluid flow became more stable, the vapor generation increased linearly, while the lifted liquid flow rate initially increased then decreased. Correspondingly, the flow pattern at the outlet section of lift tubes gradually changed from slug flow to churn flow, and then to annular flow. The higher of the immersion was, the higher heat input was needed for the flow pattern transition. It was in the churn flow regime at the outlet of lift tubes for the BPG to lift a maximum liquid. At lower immersion level, liquid reflux in the lift tubes was obvious and affected the flow stability as well as the lifting performance. At higher immersion level, the fluid flow was more stable and faster, which lifted more liquid while generated less vapor depending on the inlet subcooling. In general, the BPG showed better performance (both the lifted liquid and vapor generation increased) at smaller inlet subcooling level or lower system pressure. This study highlights the flow pattern evolution and flow stability, which is helpful to the reliable design and effective operation of the distributed heating BPG.