Two-phase Flow Pattern Transitions Research Articles

Saturated/subcooled flow boiling heat transfer characteristics for R134a, HFE-7100 and Deionized water inside micro/mini-channels: Part I - Visualization of flow patterns and new diabatic two-phase flow pattern transition regimes

An experimental investigation on flow saturated/subcooled flow boiling heat transfer for R134a, HFE-7100 and Deionized water inside micro/mini-channels (L/dh = 40/53/70) was presented. Flow patterns and their transitions of three working fluids at various liquid subcoolings (0 ∼ 70 K) were visualized and analyzed. Experiments were carried out at a saturated pressure of 770 kPa for R134a and at atmospheric pressure for HFE-7100 and Deionized water. The mass flux varied from 400 to 800 kg/m2s, and heat fluxes up to 270.1 W/cm2 over the entire range of vapor quality (0 ∼ 1). The effects of mass flux, heat flux, liquid subcoolings and geometry parameters on flow pattern evaluation and transition were discussed. Bubble (isolated and confined bubbles), slug, churn, annular and dry-out flows were observed for the micro/mini-channels. The results indicated that the flow patterns have a significant difference on flow boiling under different liquid subcoolings. There is obvious flow pattern transition from slug flow to annular flow for nearly saturated boiling (ΔTsub ≤ 10 K). However, with the increase of inlet liquid subcooling, only obvious bubble flow and slug flow were observed during the entire boiling process, and the annular flow was not obvious before triggering the CHF. Besides, the flow pattern transition criterion is modified and the new flow pattern transition regimes including saturated/subcooled flow boiling for different working fluids are developed based on present experimental results.

Applying a non-invasive multi-spectral sensing technique to two-phase flow measurements for pipeline monitoring

"Smart" sensors, with a fusion-based approach, pave the way for potentially changing how pipeline network systems are controlled, maintained, and monitored. This is typically done through structural modeling or spot monitoring using micro-electromechanical systems, strain gauges or through general conventional sensing. One of the challenges facing new developments in structural health monitoring (SHM) of a deployed asset is application to inaccessible locations. This work presents the use of a non-intrusive integrity monitoring sensor system utilising a multi-spectral approach to give more trusted insights to structural integrity as well as flow characteristics. The paper addresses the fundamental problems in identifying and characterising two-phase (air–water and oil–water) flow patterns and flow pattern transitions using numerous sensing techniques: strain gauges, optical Fibre Bragg Gratings, thermocouples (K-type), accelerometers, acoustic emission analysis and gyroscopes. By providing low noise, un-biased strain measurements, the non-intrusive sensor system can verify and improve integrity monitoring, providing a reliable alternative to conventional sensing. In addition, the non-intrusive sensor system enables pressure monitoring, providing flow assurance data and, provides good data integrity, making it an ideal tool as an enhanced surveillance strategy in real time for SHM of pipeline networks.

Flow pattern observations and flow pattern map for adiabatic two-phase flow of carbon dioxide in vertical upward and downward direction

A test facility to investigate flow pattern transitions of vertical two-phase flow of CO2 has been built within the scope of the high-luminosity detector upgrades at the European Organization for Nuclear Research (CERN). Adiabatic flow pattern observations for both vertical up- and downflow are recorded with high-speed imaging in tubes of 8 mm inner diameter, with saturation temperatures in the range of −25°C to +5°C and mass velocities ranging from 100kgm−2s−1 to 450kgm−2s−1. A database of 431 flow pattern observations in upward and 123 in downward direction has been compiled. The recorded data have been analysed with machine learning techniques and a previously trained Frame- and Flow-Regime-Classifier is used for the flow regime classification. The observed two-phase flow pattern transitions did not match the transition lines of existing flow pattern maps. As a consequence, new transition lines for the bubbly-to-slug, slug-to-churn and churn-to-annular transitions have been developed for both vertical upflow and downflow respectively and condensed into new flow pattern maps. It is concluded, that the flow regime transitions are strongly depended on vapour quality, mass velocity, the flow direction and the fluid properties. Compared to horizontal flow, a dryout region is not observed and the liquid film of the annular flow regime dries out symmetrically at vapour qualities close to x=1.

Acoustic emission-based flow noise detection and mechanism analysis for gas-liquid two-phase flow

The interphase forces of gas-liquid two-phase flow produce flow noise, which contains abundant two-phase flow information, such as two-phase flow rate, flow pattern, void fraction, etc. In this study, the acoustic emission technique is utilized to measure the flow noise of gas-liquid two-phase flow quantitatively. The signal is separated and identified by the Hilbert Huang transform, the R/S analysis, and the flow noise mathematical model. The Autoregressive Integrated Moving Average (ARIMA) time series model is used to predict the energy change of bubble flow. The results show that the Hilbert Huang transform can separate the noise signal. According to the R/S analysis, IMF1-IMF3 is the interaction noise of the gas and liquid. The results show that the prediction results all fall within the 95% confidence interval. It provides new approaches for the quantitative analysis of gas-liquid two-phase flow and flow pattern transitions.

Surface hydrophobicity change on adiabatic two-phase flow pattern transitions in horizontal tubes

The effect of surface hydrophobicity on the air-water two-phase flow pattern transition was experimentally investigated. Experiments and the two-phase flow visualization were carried out in a hydrophobic polymer tube (4.76 mm ID). The tube material is fluorinated ethylene propylene (FEP) with a hydrophobic surface, and the static contact angle of a water droplet on a smooth surface is 97.28°. The channel surface was roughened to increase the hydrophobicity. The increased surface roughness results in a microstructured surface texture that forms a fine air trapped layer below the liquid droplet on the polymer surface. The channel surface was rotary ground by sandpaper number grit 400, 320, 100, and 80 to develop microstructure textured surface. The measured static contact angle of the roughened surface was 110.7°, 113.26°, 121.03°, and, 126.28° in order of increasing surface roughness. The two-phase flow of the channels with different contact angles was visualized by a high-speed camera and the flow pattern maps were constructed. Even under the same phase superficial velocity conditions, differences in the degree of hydrophobicity lead to transitions to entirely different patterns. The two-phase flow pattern map with the change in hydrophobicity is partitioned by the change in the modified Weber number as a factor indicative of the degree of surface hydrophobicity. The modified Weber number is defined as the ratio of flow inertia to adhesion force, and the new pattern map of the two-phase flow is redefined to take into account the wall hydrophobicity change.

Classification of natural circulation two-phase flow image patterns based on self-organizing maps of full frame DCT coefficients

Many of the recent nuclear power plant projects use natural circulation as heat removal mechanism. The accuracy of heat transfer parameters estimation has been improved through models that require precise prediction of two-phase flow pattern transitions. Image patterns of natural circulation instabilities were used to construct an automated classification system based on Self-Organizing Maps (SOMs). The system is used to investigate the more appropriate image features to obtain classification success. An efficient automated classification system based on image features can enable better and faster experimental procedures on two-phase flow phenomena studies. A comparison with a previous fuzzy inference study was foreseen to obtain classification power improvements.In the present work, frequency domain image features were used to characterize three different natural circulation two-phase flow instability stages to serve as input to a SOM clustering algorithm. Full-Frame Discrete Cosine Transform (FFDCT) coefficients were obtained for 32 image samples for each instability stage and were organized as input database for SOM training. A systematic training/test methodology was used to verify the classification method. Image database was obtained from two-phase flow experiments performed on the Natural Circulation Facility (NCF) at Instituto de Pesquisas Energéticas e Nucleares (IPEN/CNEN), Brazil.A mean right classification rate of 88.75% was obtained for SOMs trained with 50% of database. A mean right classificationrate of 93.98% was obtained for SOMs trained with 75% of data. These mean rates were obtained through 1000 different randomly sampled training data. FFDCT proved to be a very efficient and compact image feature to improve image-based classification systems. Fuzzy inference showed to be more flexible and able to adapt to simpler statistical features from only one image profile. FFDCT features resulted in more precise results when applied to a SOM neural network, though had to be applied to the full original grayscale matrix for all flow images to be classified.

Experiment investigation of two-phase flow pattern transition and frictional pressure drop of R600a in a horizontal tube

With the growing concern of environment problems, there are urgent demands for environment-friendly refrigerants in the refrigeration industry. As a natural refrigerant, R600a not only has zero ozone depletion potential and quite low global warming potential, but also has better cooling performance than other refrigerants. Due to its good cooling performance and environment-friendly characteristics, R600a has been used as an alternative refrigerant in heat transfer applications such as refrigerators, freezers, and heat pumps. It is known that accurate prediction of two phase flow pattern transition and frictional pressure drop of R600a in a horizontal tube play an important role in design and optimization of refrigerators, freezers, and heat pumps. According to this issue, a detailed experimental study was carried out to investigate the two-phase flow pattern transition and frictional pressure drop characteristics of R600a in a smooth horizontal tube with an inner diameter of 6 mm. The experiments were performed at conditions covering saturation temperature from 282.4 to 304 K, mass fluxes from 67 to 194 kg/(m2 s). Based on a high speed camera, four main flow regimes can be observed: plug flow, stratified-wavy flow, slug flow and annular flow. The flow map of R600a has been drawn and comparisons between the experimental data of flow patterns and transition lines in the literature have been made. The results showed that with the increase of mass flux, the inception of the annular flow regime occurs earlier. The intermittent flow regime takes place over a narrow range of vapor quality and annular flow regime occupies a wide range of vapor quality. In addition, the trend of Barbieri and Ong and Thome I / A transitions lines are consistent with the experimental data. However, both of them overestimate the inception of the annular flow regime. After taking the influence of vapor inertia, liquid viscous and surface tension forces into account, a modified Weber number We * has been introduced to the new flow pattern transition model. Based on the relationship of X tt and We *, a new correlation has been proposed and it can well fit with the experimental data of flow pattern. Furthermore, a detailed discuss of frictional pressure drop of annular flow and non-annular flow has been made. The results indicated that frictional pressure drop increases with the increases of vapor quality. The tendency of frictional pressure drop presents different the changing laws in different flow regime. The frictional pressure drop increases slowly in the non-annular flow regime while it increases rapidly when the flow pattern turns into the annular flow regime. The reason is that the vapor-phase pressure drop often dominates the total two-phase frictional pressure drop. The annular flow regime owns higher void fraction than the non-annular flow regime. Nine classic prediction correlations of pressure drop were evaluated and the results showed that the correlation of Gronnerud and Muller-Steinhagen and Heck give the best fit to the experimental frictional pressure drop of annular flow and non-annular flow with a mean absolute relative deviation of 38.5% and 19.7% respectively.

Flow pattern transition during condensation of R134a and R1234ze(E) in microchannel arrays

Two-phase flow pattern transition during the condensation of R134a and R1234ze(E) was experimentally investigated in a microchannel array made of borosilicate glass cooled symmetrically on the top and bottom by forced convection of water. The microchannel array consisted of 50 channels with the aspect ratio of 2.46, the hydraulic diameter of 301.6μm and the length of 50mm. Saturation temperatures are 20–25°C with mass fluxes varying from 6 to 50kg/m2/s. Observed patterns are annular flow, steady injection flow, quasi-symmetric wave flow and slug/bubbly flow. In addition, three flow pattern transition modes including steady injection, wave-coalescence injection, and liquid bridging were observed and mapped in terms of liquid Webb number versus condensation number. The steady injection transition was more studied in detail by measuring the bubble length variation and the moving velocity of bubbles and slugs with different mass fluxes of R134a and R1234ze(E). The transition mechanism of injection flow was identified by analyzing the force balance based on the experimental results. The flow pattern transition locations and frequencies were measured and compared with the prediction correlations in the previous investigations.

Two-phase flow pattern map for R170 in a horizontal smooth tube

An experimental investigation of two-phase flow patterns and their transitions for R170 in a horizontal smooth tube was presented. The tests were conducted at various saturation pressures from 1.5MPa to 2.5MPa for different mass fluxes from 100kgm−2s−1 to 250kgm−2s−1 over the entire vapor quality range. The effects of mass flux and saturation pressure on two-phase flow pattern transitions were investigated and analyzed. The physical properties of velocity, viscosity and surface tension are important to the flow pattern transition criteria under our experimental conditions. Several two-phase flow pattern maps in horizontal tubes were compared with the experimental data. Finally, a new flow pattern map for R170 in a horizontal tube based on the Kim et al. (2012) map was carried out.

Gas–water two-phase flow characterization with Electrical Resistance Tomography and Multivariate Multiscale Entropy analysis

Flow behavior characterization is important to understand gas-liquid two-phase flow mechanics and further establish its description model. An Electrical Resistance Tomography (ERT) provides information regarding flow conditions at different directions where the sensing electrodes implemented. We extracted the multivariate sample entropy (MSampEn) by treating ERT data as a multivariate time series. The dynamic experimental results indicate that the MSampEn is sensitive to complexity change of flow patterns including bubbly flow, stratified flow, plug flow and slug flow. MSampEn can characterize the flow behavior at different direction of two-phase flow, and reveal the transition between flow patterns when flow velocity changes. The proposed method is effective to analyze two-phase flow pattern transition by incorporating information of different scales and different spatial directions.

An experimental investigation on the effect of middle length and inclination angle of S-shaped channel on two-phase flow patterns

An experimental investigation is conducted to study the effect of middle length and inclination angle of an S-shaped channel on adiabatic two-phase flow patterns. To study this effect, three channel lengths of 0.25, 0.50, and 1.00m, and four bends at the angles of 22.5°, 45°, 67.5° and 90° were used along the test section which is 35.83m long. The channel cross section dimensions were 0.05×0.10m and the test fluids were air and water. The observed flow patterns in 12 cases were studied. Each flow pattern was identified by visual observation and recorded with video camera for further analysis of the collected data points. The momentum transfer phenomenon between the phases for passing the upward middle section along the S-shaped channel was closely investigated. Five flow patterns of vortex plug, vortex slug, vortex wavy-annular, churn and mist packet annular were observed. The vortex type flow patterns were recorded with high speed video camera. The physical mechanisms and momentum transfer related to formation of these flow patterns were explained in detail. Flow pattern maps were obtained and then compared for different middle lengths and inclination angles to perform a parametric study on the S-shaped channel. Moreover, effect of upstream/downstream of the S-shaped channel on the maps and transition lines were discussed. It was revealed that upstream effects reflect the interactions and momentum transfer between the phases. Therefore, flow pattern types mainly depend on upstream behavior. On the other hand, the amount of backward flow resulting in the establishment of countercurrent flow and creation of vortexes along middle section is determined by downstream effects. Increasing the middle length or inclination angle causes the transition to vortex plug flow pattern to occur at higher liquid velocities. It also causes the area with churn flow pattern on the map to expand ending in smaller vortex slug flow pattern area. However, increasing middle length or decreasing inclination angle, ends in smaller area of mist packet annular area. It is also observed that for higher middle lengths, increasing the inclination angle shifts the transition lines more remarkably.A dimensionless analysis of obtained flow pattern maps is also carried out. It is shown that effect of geometry changes along S-shaped pipeline on two-phase flow pattern transitions is determined by either inclination angle or elevation difference between two horizontal lines. The explained flow mechanisms, obtained flow pattern maps and the transition lines can be used to predict the particular flow pattern that will be established for a given mass flux, inclined length and angle along pipelines.

Enhancement of Interphase Transport in Mini-/Microscale Applications Using Passive Mixing

Structured mini-/microscale reactors continue to receive attention from both industry and academia due to their low pressure drop, high heat and mass transfer rates, and ease of scale-up relative to conventional reactor technology. Commonly considered for reactions such as hydrogenations, hydrodesulfurization, oxidations, and Fischer–Tropsch synthesis, the performance of these systems is highly dependent on mixing and the interfacial area between phases. While existing literature describes the initial flow patterns generated by a broad range of two-phase contactors, few studies explore the dynamic impacts of downstream passive mixing elements. Experimental and computational methodologies for characterizing two-phase flow pattern transitions, pressure drop, and heat and mass transfer are discussed, with relevant examples for serpentine and Venturi-based passive mixing designs. The efficacies of these two configurations are explored in the context of pressure drop, conditions leading to significant interface renewal, and design considerations for optimizing mass transfer. Challenges associate with the characterization of multiphase flow through these systems are highlighted, and strategies suggested for both experimental and computational analysis of dynamic flow patterns and fluid–fluid interactions.

Curvature induced flow pattern transitions in serpentine mini-channels

Experimental investigation of the phase interactions in two-phase mini-channel serpentine systems is performed with a focus on determining the effect of radius of curvature of the serpentine on two-phase flow pattern transitions. The initial formation of two-phase flow patterns in T-junction contactors and the resulting effect on the flow through serpentine geometries are studied to predict the initiation of bubble breakup and/or coalescence in planar serpentine arrangements. Bubble breakup maps are developed for each of the serpentine geometries, identifying curvature-induced shifts in the transitions between flow patterns. Single-phase dimensionless analysis for curved geometries is extended to two-phase flow to identify the geometric dependence of critical bubble breakup. Further analysis performed shows that the characteristic length for curved geometries encountered in the Dean number for single-phase flow is suitable for capturing the effects of curvature on the initiation of bubble breakup. The dependence of Weber number on the characteristic length is reported and a critical We CDL C = 10 is identified for predicting bubble breakup inception in the serpentine system.

Two-phase flow pattern transitions of short airlift pumps

The flow behaviour and flow pattern transitions of two-phase flow of short airlift pumps were investigated in a wide range of submergence ratios and air discharges. This was obtained using conductivity signals from point-conductivity probes along with visual observations. It was found that the transitions between the various flow patterns depend on the submergence ratio and that the two-phase flow structure can change characteristics along the riser tube. A flow regime map is presented which takes into account the operational parameters, namely superficial gas velocity and submergence ratio, of the airlift pump.

Macro-to-microchannel transition in two-phase flow: Part 1 – Two-phase flow patterns and film thickness measurements

The classification of macroscale, mesoscale and microscale channels with respect to two-phase processes is still an open question. The main objective of this study focuses on investigating the macro-to-microscale transition during flow boiling in small scale channels of three different sizes with three different refrigerants over a range of saturation conditions to investigate the effects of channel confinement on two-phase flow patterns and liquid film stratification in a single circular horizontal channel (Part 2 covers the flow boiling heat transfer and critical heat flux). This paper presents the experimental two-phase flow pattern transition data together with a top/bottom liquid film thickness comparison for refrigerants R134a, R236fa and R245fa during flow boiling in small channels of 1.03, 2.20 and 3.04 mm diameter. Based on this work, an improved flow pattern map has been proposed by determining the flow patterns transitions existing under different conditions including the transition to macroscale slug/plug flow at a confinement number of Co ≈ 0.3–0.4. From the top/bottom liquid film thickness comparison results, it was observed that the gravity forces are fully suppressed and overcome by the surface tension and shear forces when the confinement number approaches 1, Co ≈ 1. Thus, as a new approximate rule, the lower threshold of macroscale flow is Co = 0.3–0.4 while the upper threshold of symmetric microscale flow is Co ≈ 1 with a transition (or mesoscale) region in-between.

Flow Boiling Visualization of R-134a in a Vertical Channel of Small Diameter

The present work reports on flow boiling visualization of refrigerant R-134a in a vertical circular channel with an internal diameter of 1.33 mm and 235 mm in heated length. A quartz tube with a homogeneous Indium Tin Oxide coating is used to allow heating and simultaneous visualization. Flow patterns have been observed along the heated length with the aid of high-speed complementary metal oxide semiconductor (CMOS) digital camera. From the flow boiling visualization, seven distinct two-phase flow patterns have been observed: isolated bubbly flow, confined bubbly flow, slug flow, churn flow, slug-annular flow, annular flow, and mist flow. Two-phase flow pattern observations are presented in the form of flow pattern maps. The effects of the saturation temperature and the inlet subcooling degree on the two-phase flow pattern transitions are elucidated. Finally, the experimental flow pattern map is compared with models developed for conventional sizes as well as to a microscale map for air-water mixtures available in literature, showing a large discrepancy.

Characteristics of two-phase flow pattern transitions and pressure drop of five refrigerants in horizontal circular small tubes

An experimental investigation on the characteristics of two-phase flow pattern transitions and pressure drop of R-22, R-134a, R-410A, R-290 and R-744 in horizontal small stainless steel tubes of 0.5, 1.5 and 3.0 mm inner diameters is presented. Experimental data were obtained over a heat flux range of 5–40 kW/m 2, mass flux range of 50–600 kg/(m 2 s), saturation temperature range of 0–15 °C, and quality up to 1.0. Experimental data were evaluated with Wang et al. and Wojtan et al. [Wang, C.C., Chiang, C.S., Lu, D.C., 1997. Visual observation of two-phase flow pattern of R-22, R-134a, and R-407C in a 6.5-mm smooth tube. Exp. Therm. Fluid Sci. 15, 395–405; Wojtan, L., Ursenbacher, T., Thome, J.R., 2005. Investigation of flow boiling in horizontal tubes: part I – a new diabatic two-phase flow pattern map. Int. J. Heat Mass Transfer 48, 2955–2969.] flow pattern maps. The effects of mass flux, heat flux, saturation temperature and inner tube diameter on the pressure drop of the working refrigerants are reported. The experimental pressure drop was compared with the predictions from some existing correlations. A new two-phase pressure drop model that is based on a superposition model for two-phase flow boiling of refrigerants in small tubes is presented.

Two-phase flow pattern transition across micropillars: Size scale effect at the microscale

Size scale effect of adiabatic two-phase cross-flow over micropillars was experimentally investigated, and the mechanisms controlling the formation and transition of the recently discovered bridge flow was examined. A transition criterion, based on the Weber and Euler numbers, for the breakage of the bridges, is provided. The bridges break from the surface when the product of the Weber and Euler numbers is larger than ∼1; otherwise, the bridges cease to exist by coalescing with liquid slugs.

Experimental investigation of R-134a and R-245fa two-phase flow in microchannels for different flow conditions

An optical measurement method for two-phase flow characterization in microtubes has been applied to determine the frequency of bubbles exiting a microevaporator, the coalescence rates of these bubbles and their lengths as well as their mean two-phase vapor velocity. The tests were run in 0.5 mm and 0.8 mm diameter glass channels using R-134a and R-245fa at 26, 30 and 35 °C saturation temperatures. Four flow patterns (bubbly flow, slug flow, semi-annular flow and annular flow) with their transitions (bubbly/slug flow and slug/semi-annular flow) were detected. The two-phase flow pattern transitions observed with R-134a did not compare well to a leading macroscale flow map for refrigerants nor to a microscale map for air–water flows. No significant influence of the inlet subcooling (2, 3, 5 °C) nor the saturation pressure has been observed on the flow pattern transitions. Furthermore, changing the heated length from 70 to 30 mm before the glass tube did not influence the location of the transition lines in the flow pattern map. The 0.8 mm diameter also did not show any significant difference to the 0.5 mm channel, although bubbly/slug flow was present over a wider range of mass flux. The transitions between flow regimes are less influenced by the mass flux for R-245fa than for R-134a; thus for low mass fluxes the difference is significant and the flow regimes appear earlier in terms of vapor quality.

Characterization of diabatic two-phase flows in microchannels: Flow parameter results for R-134a in a 0.5 mm channel

An optical measurement method for two-phase flow pattern characterization in microtubes has been utilized to determine the frequency of bubbles generated in a microevaporator, the coalescence rates of these bubbles and their length distribution as well as their mean velocity. The tests were run in a 0.5 mm glass channel using saturated R-134a at 30 °C (7.7 bar). The optical technique uses two laser diodes and photodiodes to measure these parameters and to also identify the flow regimes and their transitions. Four flow patterns (bubbly flow, slug flow, semi-annular flow and annular flow) with their transitions were detected and observed also by high speed video. It was also possible to characterize bubble coalescence rates, which were observed here to be an important phenomena controlling the flow pattern transition in microchannels. Two types of coalescence occurred depending on the presence of small bubbles or not. The two-phase flow pattern transitions observed did not compare well to a leading macroscale flow map for refrigerants nor to a microscale map for air–water flows. Time averaged cross-sectional void fractions were also calculated indirectly from the mean two-phase vapor velocities and compared reasonably well to homogeneous values.