Two-phase flow patterns across triangular tube bundles for air–water upward flow

This paper presents flow pattern experimental results obtained during two-phase upward flow across a horizontal tube bundle. Experiments were performed for flows across a normal triangular tube bundle with 19 mm OD tubes and transversal pitch of 24 mm. Results were obtained for gas and liquid superficial velocities ranging from 0.13 to 10.00 m/s and 0.02 to 1.50 m/s, respectively. Flow patterns were identified subjectively based on visual observations through side windows, and objectively using the k-means clustering method based on signals of a differential pressure transducer and a capacitive sensor. Bubbles, large bubbles, dispersed bubbles, churn, intermittent and annular flow patterns were identified subjectively. The clustering method satisfactorily identified groups of data corresponding to the distinct flow patterns, which were compared with predictive methods available in the open literature. New predictive methods for transitions between flow patterns are proposed based on the flow patterns identified objectively. The proposed methods predicted accurately the data obtained in the present study as well as experimental results and flow pattern maps available in the open literature for distinct geometries.     

线性稳定性理论在圆管充分发展两相流型研究中的应用

圆截面光滑直管内充分发展的两流体同心环状流的线性稳定性研究不仅具有重要的学术意义,而且在预测两相流型转换方面也有着重要应用.本文评述了该流动构型的线性稳定性研究进展,着重分析了该流动构型的失稳机制及其与两相流型转换间的关系,并针对微重力气-液两相流地面模拟实验问题,探讨了今后需要着重研究的若干方面.      

Some features of spray breakup in effervescent atomizers

The near orifice spray breakup at low GLR (gas to liquid ratio by mass) values in an effervescent atomizer is studied experimentally using water as a simulant and air as atomizing gas. From the visualizations, the near orifice spray structures are classified into three modes: discrete bubble explosions, continuous bubble explosions and annular conical spray. The breakup of the spray is quantified in terms of the mean bubble bursting distance from the orifice. The parametric study indicates that the mean bubble bursting distance mainly depends on airflow rate, jet diameter and mixture velocity. It is also observed that the jet diameter has a dominant effect on the bubble bursting distance when compared to mixture velocity at a given airflow rate. The mean bubble bursting distance is shown to be governed by a nondimensional two-phase flow number consisting of all the aforementioned parameters. The location of bubble bursting is found to be highly unsteady spatially, which is influenced by flow dynamics inside the injector. It is proposed that this unsteadiness in jet breakup length is a consequence of varying degree of bubble expansion caused due to the intermittent occurrence of single phase and two-phase flow inside the orifice.     

Two-phase flow patterns in a 90° bend at microgravity

Bends are widely used in pipelines carrying single-and two-phase fluids in both ground and space applications. In particular, they play more important role in space applications due to the extreme spatial constraints. In the present study, a set of experimental data of two-phase flow patterns and their transitions in a 90° bend with inner diameter of 12.7 mm and curvature radius of 76.5 mm at microgravity conditions are reported. Gas and liquid superficial velocities are found to range from (1.0≈23.6) m/s for gas and (0.09≈0.5) m/s for liquid, respectively. Three major flow patterns, namely slug, slug-annular transitional, and annular flows, are observed in this study. Focusing on the differences between flow patterns in bends and their counterparts in straight pipes, detailed analyses of their characteristics are made. The transitions between adjoining flow patterns are found to be more or less the same as those in straight pipes, and can be predicted using Weber number models satisfactorily. The reasons for such agreement are carefully examined. The project supported by the Canadian Space Agency (CSA) and the visiting scholar program of the Chinese Academy of Sciences (CAS)     

Vortices and wall shear stresses in asymmetric and symmetric channels with sinusoidal wavy walls for pulsatile flow at low Reynolds numbers

Numerical simulation and flow visualization were performed to study the dynamical behavior of vortices generated in channels with two different geometries, i.e., a periodically converging–diverging channel and serpentine channel, both having sinusoidal wavy walls. This system for pulsatile flow is used to enhance heat and mass transfer in very viscous liquids. The numerical results predict well the dynamical behavior of vortices and agree with the flow visualizations. For both channels, the vortex expands in each furrow of the channel walls during the deceleration phase and shrinks during the acceleration phase, which leads to fluid exchange between the vortex and the mainstream. The time-averaged vortex strength and wall shear stresses increase, as the frequency of fluid oscillation increases under a fixed oscillatory fraction of the flow rate. However, above a certain value of the frequency, they reversely decrease due to viscous effects. This frequency for the serpentine channel is smaller than that for the converging–diverging channel. The channel geometries are found to have an important effect of the flow characteristics.     

Two-phase flow patterns in short horizontal rectangular microchannels

The two-phase flow in a short horizontal channel of a rectangular cross-section with the height of 100–500 µm and width of 9–40 mm was studied experimentally. The use of the Schliren and fluorescent methods made it possible to reveal the flow of liquid in the channel and to determine its characteristics quantitatively. The features of the churn, jet and drop flow patterns were studied in details. Two particular regimes that can be distinguished represent formation of immobile drops on the channel walls because of the liquid film or liquid bridges breakage and appearance of mobile drops due to the two-phase flow instabilities. It is found out that formation of various two-phase flow patterns and transitions between them are determined by instabilities of the liquid–gas flow in the side parts of a channel. Frontal instability has been observed during the liquid–gas interaction in the region of liquid output from the nozzle. It is shown that a change in the height and width of the horizontal channels has a substantial effect on the boundaries between the flow regimes. One of the results is that the region of the churn regime increases significantly with decreasing thickness of the channel.     

微喷混合中的颗粒破碎效应

采用TAB模型,基于气-粒两相流程序对液态金属铅颗粒在氦气中的破碎过程进行了分析,获得了破碎特征时间、破碎后颗粒尺度等关键力学量;并对充气微喷射实验进行了数值模拟,考虑颗粒破碎效应后的模拟结果与实验结果较好符合。结果证实了充气条件下液态金属颗粒的再次破碎现象,可加深认识微喷混合的物理规律。     

Effect of pipe rotation on downward co-current air–water flow in a vertical pipe

The present research experimentally studied the effect of pipe rotation on the flow patterns of downward gasliquid two-phase flow. Two-phase flow patterns and their transition boundaries were observed and analyzed at different pipe revolutions. The experimental setup was fabricated to show flow patterns in a downward direction. The setup includes a transparent vertical pipe with a diameter of 50 mm and an aspect ratio (L/d) of 80 that can rotate at different speeds. Eight flow maps were obtained at revolutions of 0, 60, 120, 180, 240, 300, 400 and 500 rpm by changing the air and water velocities at any revolution (a total of 2205 points). The gasliquid downward two-phase flow regimes were analyzed using image processing. The experimental results were compared with published flow maps for vertical flow. It was found that pipe rotation has major effect on flow patterns map and their transitions boundaries. Increasing pipe rotation cause slug and annular flow start at lower V_SG.     

One-group interfacial area transport equation and its sink and source terms in narrow rectangular channel

The characteristics of two-phase flow in a narrow rectangular channel are expected to be different from those in other channel geometries, because of the significant restriction of the bubble shape which, consequently, may affect the heat removal by boiling under various operating conditions. The objective of this study is to develop an interfacial area transport equation with the sink and source terms being properly modeled for the gas–liquid two-phase flow in a narrow rectangular channel. By taking into account the crushed characteristics of the bubbles a new one-group interfacial area transport equation was derived for the two-phase flow in a narrow rectangular channel. The random collisions between bubbles and the impacts of turbulent eddies with bubbles were modeled for the bubble coalescence and breakup respectively in the two-phase flow in a narrow rectangular channel. The newly-developed one-group interfacial area transport equation with the derived sink and source terms was evaluated by using the area-averaged flow parameters of vertical upwardly-moving adiabatic air–water two-phase flows measured in a narrow rectangular channel with the gap of 0.993 mm and the width of 40.0 mm. The flow conditions of the data set covered spherical bubbly, crushed pancake bubbly, crushed cap-bubbly and crushed slug flow regimes and their superficial liquid velocity and the void fraction ranged from 0.214 m/s to 2.08 m/s and from 3.92% to 42.6%, respectively. Good agreement with the average relative deviation of 9.98% was obtained between the predicted and measured interfacial area concentrations in this study.     

Nitrogen/non-Newtonian fluid two-phase upward flow in non-circular microchannels

This paper presents experimental investigations on nitrogen/non-Newtonian fluid two-phase flow in vertical noncircular microchannels, which have square or triangular cross-section with the hydraulic diameters being D_h = 2.5, 2.886 and 0.866 mm, respectively, by visualization method. Three non-Newtonian aqueous solutions with typical rheological properties, i.e., 0.4% carboxymethyl cellulose (CMC), 0.2% polyacrylamide (PAM) and 0.2% xanthan gum (XG) are chosen as the working fluids. The common flow patterns are identified as slug flow, churn flow and annular flow. The dispersed bubble flow is only found in the case with nitrogen/CMC solution two-phase flow in the largest channel. A new flow pattern of nitrogen/PAM solution two-phase flow, named chained bubble/slug flow, is observed in all the test channels. The flow regime maps are also developed and the results show that the rheological properties of the non-Newtonian fluid have remarkable influence on the flow pattern transitions. The geometrical factors of the microchannel such as the cross-section shape and hydraulic diameter of the channel can also affect the flow regime map. Finally, the results obtained in this work are compared with the available flow pattern transitions.     

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.     

Bifurcation phenomena in two-phase natural circulation

Multiple steady-state solutions have been found theoretically for a two-phase natural circulation loop. This bifurcation phenomenon is attributed to the nonmonotonic behavior of the buoyancy and friction forces, and the discontinuities due to transitions between two-phase flow patterns. The paper outlines a general and consistent modeling method to describe two-phase natural circulation at steady state. The solution of the one-dimensional governing conservation equations is performed by a combination of analytical and numerical stages. The method is implicit, to treat the highly nonlinear problem by multinested iteration loops. Results are presented and discussed for the multiple solutions of the flow rate, temperature, quality and void distributions and two-phase elevations. Pressure effects are also studied. A comparison with available data shows that the theoretical model simulates the important characteristics of two-phase thermosyphons.     

Experimental study of R134A condensation heat transfer inside the horizontal micro-fin tubes

The Investigation of the two-phase flow patterns and their transitions during the condensation has gained increasing interest and importance from the well-known phenomenon that the heat transfer characteristics are strongly dependent on the flow patterns. Therefore, it is very important to study on which heat transfer enhancement approach is suitable for an individual flow pattern inside a condenser, so that an accurate heat transfer mechanism can be understood that is consistent with the flow patterns. The condensation heat transfer for R134a in the two kinds of in-tube three-dimensional (3-D) micro-fin tubes with different geometries is experimentally investigated. Based on the flow pattern observations, the flow patterns in the Soliman flow regime map are divided into two-flow regimes; one with the vapor-shear-dominant annular regime and the other with the gravitational-force-dominant stratified-wavy regime. The flow regime transition criterion between the annular regime and the stratified-wavy regime is at Fr equal to 2. In the annular regime, the heat transfer coefficients h of the two kinds of in-tube 3-D micro-fin tubes decreases as the vapor quality x decreases. The regressed condensation heat transfer correlation from the experimental data of the annular flow region is obtained. The dispersibility of the experimental data is inside the limits of ±25%. In the stratified-wavy regime, the average heat transfer coefficient h of the two kinds of in-tube 3-D micro-fin tubes increases as the mass flux increases and the number of micro fins in the 3-D micro-fin tube is not the controlling factor for the performance of a condensation heat transfer. The regressed condensation heat transfer correlation of the stratified-wavy flow regime is experimentally obtained. The dispersibility of the experimental data is inside the limits of ±22%. Combined with the criteria of flow pattern transitions, the correlations can be used for the design of a condenser with 3-D micro-fin tubes.     

Breakup of liquid jets from non-circular orifices

The purpose of this investigation is to study the effect of the orifice geometry on liquid breakup. In order to develop a better understanding of the liquid jet breakup, investigations were carried out in two steps—study of low-pressure liquid jet breakup and high-pressure fuel atomization. This paper presents the experimental investigations conducted to study the flow behavior of low-pressure water jets emanating from orifices with non-circular geometries, including rectangular, square, and triangular shapes and draws a comparison with the flow behavior of circular jets. The orifices had approximately same cross-sectional areas and were machined by electro-discharge machining process in stainless steel discs. The liquid jets were discharged in the vertical direction in atmospheric air at room temperature and pressure conditions. The analysis was carried out for gage pressures varying from 0 to 1,000 psi (absolute pressures from 0.10 to 6.99 MPa). The flow behavior was analyzed using high-speed visualization techniques. To draw a comparison between flow behavior from circular and non-circular orifices, jet breakup length and width were measured. The flow characteristics were analyzed from different directions, including looking at the flow from the straight edges of the orifices as well as their sharp corners. The non-circular geometric jets demonstrated enhanced instability as compared to the circular jets. This has been attributed to the axis-switching phenomenon exhibited by them. As a result, the non-circular jets yielded shorter breakup lengths as compared to the circular jets. In order to demonstrate the presence of axis-switching phenomenon in square and triangular jets, the jet widths were plotted along the axial direction. This technique clearly demonstrated the axis switching occurring in square and triangular jets, which was not clearly visible unlike the case of rectangular jets. To conclude, non-circular geometry induces greater instabilities in the liquid jets, thereby leading to faster disintegration. Thus, non-circular orifice geometries can provide a cheaper solution of improving liquid breakup and thus may enhance fuel atomization as compared to the precise manufacturing techniques of drilling smaller orifices or using costly elevated fuel injection pressure systems.     

Nonlinear instability of an annular liquid sheet exposed to gas flow

Nonlinear instability and breakup of an annular liquid sheet has been modeled in this paper. The liquid sheet is considered to move axially and is exposed to co-flowing inner and outer gas streams. Also, the effect of outer gas swirl on sheet breakup has been studied. In the developed model a perturbation expansion method has been used with the initial magnitude of the disturbance as the perturbation parameter. This is a comprehensive model in that other geometries of planar sheet and a coaxial jet can be obtained as limiting cases of very large inner radius and inner radius equal to zero, respectively. In this temporal analysis, the effect of liquid Weber number, initial disturbance amplitude, inner gas-to-liquid velocity ratio, outer gas-to-liquid velocity ratio and outer gas swirl strength on the breakup time is investigated. The model is validated by comparison with earlier analytical studies for the limiting case of a planar sheet as well as with experimental data of sheet breakup length available in literature. It is shown that the linear theory cannot predict breakup of an annular sheet and the developed nonlinear model is necessary to accurately determine the breakup length. In the limiting case of a coaxial jet, results show that gas swirl destabilizes the jet, makes helical modes dominant compared to the axisymmetric mode and decreases jet breakup length. These results contradict earlier linear analyses and agree with experimental observations. For an annular sheet, it is found that gas flow hastens the sheet breakup process and shorter breakup lengths are obtained by increasing the inner and the outer gas velocity. Axially moving inner gas stream is more effective in disintegrating the annular sheet compared to axially moving outer gas stream. When both gas streams are moving axially, the liquid sheet breakup is quicker compared to that with any one gas stream. In the absence of outer gas swirl, the axisymmetric mode is the dominant instability mode. However, when outer gas flow has a swirl component higher helical modes become dominant. With increasing outer gas swirl strength, the maximum disturbance growth rate increases and the most unstable circumferential wave number increases resulting in a highly asymmetric sheet breakup with shorter breakup lengths and thinner ligaments.     

Probabilistic mapping of adiabatic horizontal two-phase flow by capacitance signal feature clustering

In order to better quantify two-phase flow regime transitions, a sensor is developed which measures the electric capacitance of two-phase flows. A large number of experiments are done with air–water flow in a 9 mm ID horizontal tube. Based on a multivariate analysis, the most suitable sensor signal parameters are selected for building a flow regime classifier. This classifier is based on a fuzzy c-means clustering algorithm together with a regression technique. The output of the algorithm is used to create a probabilistic flow regime map. A comparison between a visual classification based on high speed camera images and the outcome of the flow regime classifier shows a remarkable agreement. The flow regime transitions are further quantified and discussed based on the probabilistic information and the sensor signal characterization. Probabilistic mapping makes it possible to combine flow regime dependent correlations in the two-phase flow models for heat transfer and pressure drop with smooth and appropriately quantified transitions from one flow regime to another.     

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

Flow regime transition criteria are of practical importance for two-phase flow analyses at reduced gravity conditions. Here, flow regime transition criteria which take the frictional pressure loss effect into account were studied in detail. Criteria at reduced gravity conditions were developed by extending an existing model from normal gravity to reduced gravity conditions. A comparison of the newly developed flow regime transition criteria model with various experimental datasets taken at microgravity conditions showed satisfactory agreement. Sample computations of the model were performed at various gravity conditions, such as 0.196, 1.62, 3.71 and 9.81 m/s² corresponding to micro-gravity and lunar, Martian and Earth surface gravity, respectively. It was found that the effect of gravity on bubbly–slug and slug–annular (churn) transitions in a two-phase flow system was more pronounced at low liquid flow conditions, whereas the gravity effect could be ignored at high mixture volumetric flux conditions. While for the annular flow transitions due to flow reversal and onset of droplet entrainment, higher superficial gas velocity was obtained at higher gravity level.     

Numerical prediction of droplet dynamics in turbulent flow,using the level set method

In the present article, the droplet dynamics in turbulent flow is numerically predicted. The modelling is based on an interfacial marker-level set (IMLS) method, coupled with the Reynolds-averaged Navier–Stokes (RANS) equations to predict the dynamics of turbulent two-phase flow. The governing equations for time-dependent, two-dimensional and incompressible two-phase flow are described in both phases and solved separately using a control volume approach on structured cell-centred collocated grids. The topological changes of the interface are predicted by applying the level set approach. The kinematic and dynamic conditions on the interface separating the two phases are satisfied. The numerical method proposed is validated against a well-known computational fluid dynamics problem. Further, the deformation and breakup of a single droplet either suddenly moved in air or exposed to turbulent stream are numerically investigated. In general, the developed numerical method demonstrates remarkable capability in predicting the characteristics of complex turbulent two-phase flows.     

Visualized study on specific points on demand curves and flow patterns in a single-side heated narrow rectangular channel

A simultaneous visualization and measurement study on some specific points on demand curves, such as onset of nucleate boiling (ONB), onset of significant void (OSV), onset of flow instability (OFI), and two-phase flow patterns in a single-side heated narrow rectangular channel, having a width of 40 mm and a gap of 3 mm, was carried out. New experimental approaches were adopted to identify OSV and OFI in a narrow rectangular channel. Under experimental conditions, the ONB could be predicted well by the Sato and Matsumura model. The OSV model of Bowring can reasonably predict the OSV if the single-side heated condition is considered. The OFI was close to the saturated boiling point and could be described accurately by Kennedy’s correlation. The two-phase flow patterns observed in this experiment could be classified into bubbly, churn, and annular flow. Slug flow was never observed. The OFI always occurred when the bubbles at the channel exit began to coalesce, which corresponded to the beginning of the bubbly–churn transition in flow patterns. Finally, the evolution of specific points and flow pattern transitions were examined in a single-side heated narrow rectangular channel.     

Flow patterns of n-hexadecane–CO2 liquid–liquid two-phase flow in vertical pipes under high pressure

To promote a better understanding of liquid–liquid two-phase flow behavior, particularly under high pressure, flow patterns of n-hexadecane–CO₂ liquid–liquid two-phase upward flow in vertical stainless steel pipes were experimentally investigated. Observations were made in two 0.0015 m I.D. pipes of different lengths (0.068 m and 0.5 m) under high pressure varying from 10.3 to 29.6 MPa using a high pressure visualization system. The total flow rate was fixed at 2.0 × 10⁻⁶ m³/min, while the flow rate ratio (φ) varied from 0.05 to 19. Bubbly flow, plug flow, slug flow, annular flow, and near-one-phase flow regions were found in both pipes, while stratified flow was observed only in the 0.068 m pipe. Flow pattern maps were constructed in the flow rate ratio versus pressure graph, which demonstrates significant impacts of flow rate ratio, pipe length, and pressure on flow patterns. These impacts are discussed in detail. To the authors’ best knowledge, this work is the first attempt to observe complex liquid–liquid two-phase flow behavior with flow pattern transitions under high pressure, and contributes to a better understanding of liquid–liquid two-phase flow behavior.