Transition of plug to slug flow and associated fluid dynamics

Transition of plug to slug flow is associated with bubble detachment from elongated bubble tail or bubble entrainment inside the liquid slug. The mechanism responsible for this transition was earlier identified by Ruder and Hanratty (1990) and Fagundes Netto et al. (1999) based on the shape of the hydraulic jump observed at elongated bubble tail region. The transition mechanism reported by Ruder and Hanratty (1990) and Fagundes Netto et al. (1999) was only based on their flow visualization study. Plug to slug transition and associated dynamics of bubble detachment from the elongated bubble is analysed in the present paper using flow visualization and local velocity measurements. Experiments are reported for 13 different inlet flow conditions of air and water phases. Images of plug/slug flow structures are captured at a rate of 4000 FPS using FASTCAM Photron camera and the local values of axial liquid velocity are measured using LDV system synchronised with a 3D automated traverse system. LDV measurement of local liquid velocity in the liquid slug and liquid film establishes the reason for detachment of bubbles from the slug bubble tail.     

两相流中密度波现象的研究及进展

密度波（又称连续波、空隙度波、运动学波）是两相流中的特殊现象，本文评述两相流密度波理论的主要研究进展，这些理论不仅研究了线性密度波和非线性密度波的特性，还研究了气液两相流的流型转变与密度波的关系，流化床中气塞的形成和床层的塌陷现象与密度波的关系等．同时介绍试验和理论研究在密度波的波速、频率、稳定性和色散性等方面取得的主要结论．最后指出密度波的理论和试验研究尚需进一步解决的问题．     

The bubble-slug flow pattern transition and instabilities of void fraction waves

The propagation of spontaneous void fraction disturbances in a nitrogen-water flow has been studied through the statistical analysis of conductivity probe signals, for void fractions ranging from 0.1 to 0.5 and including the bubble-slug transition. The power spectral density function and the standard deviation of the void fraction have been computed for each probe, as well as the system phase factor (related to the wave velocity), the coherence function and the system gain factor between each pair of consecutive probes as functions of frequency. For bubble flow, the results are compatible with the results obtained by other authors. The transition from bubble to slug flow is associated with void fraction wave instabilities. Two kinds of instabilities seem to occur simultaneously: amplitude increase (system gain factor > 1) and wave-breaking.     

Large scale structure in gas-liquid mixture flows

Relatively slow variation in mixture void fraction in gas-liquid mixture flows are indicated by low pass filter averaging. The slow void fluctuations are found to have a regular characteristic frequency or scale in the churn flow regime or near the boundary with the dispersed bubble flow regime. These regular disturbances develop inherently in a vertical pipe flow in strength and in size and are not due to the method of flow mixing. There was no evidence of distinctive gas slugs in the flow, and the structures were identified as large clouds of bubbles which moved faster than the average velocity, growing in size and strength as they moved with the flow. The magnitude of the voidage fluctuations in the churn flow regime was on average 57% of the value for a slug flow. The large scale bubble clouds convect coherently over relatively long distances at up to 1.45 times the mean mixture flow velocity at a gas volume flow fraction of 0.4. In the bubble flow regime, the slow voidage variations were more random in scale and were only approx. 10% of the slug flow (maximum possible) value. However, even in the bubble flow regime, the disturbances convected coherently over relatively long distances at a velocity of approx. 1.1 times the mean mixture velocity.     

Two-phase flow patterns of a top heat mode closed loop oscillating heat pipe with check valves (THMCLOHP/CV)

This research is aimed at studying the two-phase flow pattern of a top heat mode closed loop oscillating heat pipe with check valves. The working fluids used are ethanol and R141b and R11 coolants with a filling ratio of 50% of the total volume. It is found that the maximum heat flux occurs for the R11 coolant used as the working fluid in the case with the inner diameter of 1.8 mm, inclination angle of ?90?, evaporator temperature of 125?C, and evaporator length of 50 mm. The internal flow patterns are found to be slug flow/disperse bubble flow/annular flow, slug flow/disperse bubble flow/churn flow, slug flow/bubble flow/annular flow, slug flow/disperse bubble flow, bubble flow/annular flow, and slug flow/annular flow.     

Investigation of slug flow characteristics in the valley of a hilly-terrain pipeline

The objective of this study is to improve the current phenomenological understanding of slug flow characteristics over an entire hilly-terrain section, and in particular, the slug initiation mechanism at the lower dip.The experimental part of this study revealed that five possible flow behavior categories exist along a hilly-terrain section. In these categories, the flow behavior at the dip is coupled with flow conditions of the upstream downhill section. This qualitative classification was superimposed on steady-state flow pattern maps for the upstream downhill section in an attempt to relate the qualitative flow behavior at a dip to the flow pattern maps through the flow behavior in the downhill section.Statistical analyses of mean slug length, maximum slug length, slug frequency, and slug length variation across the hilly-terrain pipeline revealed that slug length distribution characteristics change across a symmetrical hilly-terrain pipeline. Physical modeling of the slug initiation mechanism and the characteristics of initiated slugs at the lower dip indicated two main mechanisms, namely, wave growth and wave coalescence initiation mechanisms. The initiated “pseudo slugs” or slug characteristics of each mechanism differ significantly with respect to frequency, length, liquid holdup and velocity. It was observed that pseudo slugs initiated by the wave coalescence mechanism have velocities less than the mixture velocity due to gas blowing through the slug body.     

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

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

二维水平通道内流动沸腾换热的格子Boltzmann模拟

利用格子Boltzmann方法模拟二维水平通道内水的流动沸腾过程,获得不同壁面过热度下流型特点和不同因素对换热过程的影响规律。结果表明,随着壁面过热度升高,流道内流型依次经历从泡状流、弹状流到反环流的转变,平均热流密度和平均换热系数先增大后减小。入口流速降低会使流道内出现受限气泡流,核态沸腾受到抑制。提高入口流速能够有效促进气泡脱离,壁面平均换热系数随入口流速增大而增大,但增长速率有所减小。减小通道宽度有利于汽化现象发生,核态沸腾得到强化,壁面平均换热系数有所提高。     

Experimental study on hydrodynamic characteristics of upward gas–liquid slug flow

To clarify the impacts of the hydrodynamic boundary layer and the diffusion boundary layer in the near wall zone on gas–liquid two-phase flow induced corrosion in pipelines, the hydrodynamic characteristics of fully developed gas–liquid slug flow in an upward tube are investigated with limiting diffusion current probes, conductivity probes and digital high-speed video system. The Taylor bubble and the falling liquid film characteristics are studied, the effects of various factors are examined, and the experimental results are compared with the data and models available in literature. The length of Taylor bubble, the local void fraction of the slug unit and the liquid slug, the shear stress and mass transfer coefficient in the near wall zone, are all increased with the increase of superficial gas velocity and decreased with the increase of superficial liquid velocity, whereas the length of liquid slug and the liquid slug frequency are changed contrarily. The alternate wall shear stress due to upward gas–liquid slug flow is considered to be one of the major causes for the corrosion production film fatigue cracking. A normalized formula for mass transfer coefficient is obtained based on the experimental data.     

The interrelation between void fraction fluctuations and flow patterns in two-phase flow

A fast response, linearized X-ray void measurement system has been used to obtain statistical measurements in normally fluctuating air-water flow in a rectangular channel. It is demonstrated that the probability density function (PDF) of the fluctuations in void fraction may be used as an objective and quantitative flow pattern discriminator for the three dominant patterns of bubbly, slug, and annular flow. This concept is applied to data over the range of 0.0 to 37 m/sec mixture velocities to show that slug flow is simply a transitional, periodic time combination of bubbly and annular flows. Film thicknesses calculated from the PDF data are similar in magnitude in both slug and annular flows. Calculation of slug length and residence time ratios along with bubble lengths in slug flow are also readily obtainable from the statistical measurements. Spectral density measurements showed bubbly flow to be stochastic while slug and annular flows showed periodicities correlatable in terms of the liquid volume flux.     

Local properties of vertical mercury-argon two-phase flow in a circular tube under transverse magnetic field

Experimental data are presented in this paper on the profiles of local void fraction, bubble impaction rate, bubble velocity and its spectrum, and also bubble length and its spectrum, of mercury-argon two-phase slug flow flowing upwards in a vertical circular tube in the presence of a transverse magnetic field. Decrease in void fraction and increase in bubble velocity are significant when the magnetic flux density is larger than $\text{0.3～0.4}$$\text{T}\text{(Ha ? 100)}$. This effect is discussed by analyzing the bubble size distribution. Recovery of local void fraction profile in the downstream of an obstacle and diffusion of void injected from only one nozzle in the presence of magnetic field are also discussed.     

壁面处气泡在静止流场和高速水流中溃灭过程的计算仿真

通过数值仿真计算,模拟近壁面以及附壁面气泡在静止流场和高速水流中的溃灭过程,研究气蚀作用机理.结果表明:气泡与壁面的距离和水流的速度影响其溃灭时间;附壁面气泡在高速水流中完全溃灭的时间最短,而在静止流场中最长,远离壁面将增加气泡的不稳定性;当气泡距离壁面一定距离溃灭时,射流不能直接作用于壁面,壁面承受冲击波的最大压力远小于气泡溃灭中心的压力;当气泡溃灭中心在壁面时,射流直接作用于壁面产生微小而严重的点破坏,而冲击波则使材料产生交变应力,造成环形破坏;当气泡在高速水流中溃灭时将产生逆流斜向射流,这可能是水力机械过流部件产生鱼鳞坑和波纹状破坏的主要原因.     

On the interaction between two consecutive elongated bubbles in a vertical pipe

The motion of elongated air bubbles in a vertical pipe filled with water is studied quantitatively using video imaging of the flow and subsequent digital image processing of the recorded sequence of images. Experiments are carried out to determine the influence of the separation distance between two consecutive bubbles (liquid slug length) upon the behavior of the trailing bubble in vertical slug flow. The details of the trailing bubble acceleration and merging process are observed and the instantaneous parameters of the trailing bubble, such as its shape, velocity, acceleration, etc., are measured as a function of the separation distance. The leading bubble is found to be unaffected by the trailing elongated bubble.     

On the interaction of Taylor bubbles rising in two-phase co-current slug flow in vertical columns: turbulent wakes

An experimental study on the interaction between Taylor bubbles rising through a co-current flowing liquid in a vertical tube with 32 mm of internal diameter is reported. The flow pattern in the bubble's wake was turbulent and the flow regime in the liquid slug was either turbulent or laminar. When the flow regime in the liquid slug is turbulent (i) the minimum distance between bubbles above which there is no interaction is 5D-6D; (ii) the bubble's rising velocity is in excellent agreement with the Nicklin relation; (iii) the experimental values of the bubble length compare well with theoretical predictions (Barnea 1990); (iv) the distance between consecutive bubbles varied from 13D to 16D and is insensitive to the liquid Reynolds number. When the flow regime in the liquid slug is laminar (i) the wake length is about 5D-6D; (ii) the minimum distance between bubbles above which there is no interaction is higher than 25D; (iii) the bubble's rising velocity is significantly smaller than theoretical predictions. These results were explained in the light of the findings of Pinto et al. (1998) on coalescence of two Taylor bubbles rising through a co-current liquid. Received: 2 February 2000 / Accepted: 15 March 2001     

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

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

An updated three-zone heat transfer model for slug flow boiling in microchannels

This work proposes a novel physics-based model for the fluid mechanics and heat transfer associated with slug flow boiling in horizontal circular microchannels to update the widely used three-zone model of Thome et al. (2004). The heat transfer model has a convective boiling nature and predicts the time-dependent variation of the local heat transfer coefficient during the cyclic passage of a liquid slug, an evaporating elongated bubble and a vapor plug. The capillary flow theory, extended to incorporate evaporation effects, is applied to estimate the bubble velocity along the channel. A liquid film thickness prediction method also considering bubble proximity effects, which may limit the radial extension of the film, is included. The minimum liquid film thickness at dryout is set to the channel wall roughness. Theoretical heat transfer models accounting for the thermal inertia of the liquid film and for the recirculating flow within the liquid slug are utilized. The heat transfer model is compared to experimental data taken from three independent studies. The 833 slug flow boiling data points cover the fluids R134a, R245fa and R236fa, and channel diameters below 1 mm. The proposed evaporation model predicts more than 80% of the database to within ±30%. It demonstrates a stronger contribution to heat transfer by the liquid slugs and correspondingly less by the thin film evaporation process compared to the original three-zone model. This model represents a new step towards a complete physics-based modelling of the bubble dynamics and heat transfer within microchannels under evaporating flow conditions.     

Instability of hagen-poiseuille flow for axisymmetric mode

An investigation is described for instability problem of flow through a.pipe of circular cross section. As a disturbance motion, we consider an axisymmetric nonlinear mode. An associated amplitude or modulation equation has been derived for this perturbation. This equation belongs to the diffusion type. The coefficient of it can be negative with Reynolds number increasing, because of the complex interaction between molecular diffusion and convection. The negative diffusion, when it occurs, cause a concentration and focusing of energy within the decaying slug, acting as a role of reversing natural decays.     

Holdup of the liquid slug in two phase intermittent flow

A physical model for the prediction of gas holdup in liquid slugs in horizontal and vertical two phase pipe slug flow is presented. This model can also be used to yield the transition between elonganted bubbles and slug flow within the intermittent flow pattern. In addition a previously published model for predicting the stable slug length in vertical upward slug flow (Taitel et al. 1980) is extended here for the case of horizontal slug flow.     

Hydrodynamic and mass transfer characteristics of slug flow in a vertical pipe with and without dispersed small bubbles

In this work, we present a numerical study to investigate the hydrodynamic characteristics of slug flow and the mechanism of slug flow induced CO₂ corrosion with and without dispersed small bubbles. The simulations are performed using the coupled model put forward by the authors in previous paper, which can deal with the multiphase flow with the gas–liquid interfaces of different length scales. A quasi slug flow, where two hypotheses are imposed, is built to approximate real slug flow. In the region ahead of the Taylor bubble and the liquid film region, the presence of dispersed small bubbles has less impacts on velocity field, because there are no non-regular intensive disturbance forces or centrifugal forces breaking the balance of the liquid and the dispersed small bubbles. In the liquid slug region, the strong centrifugal forces generated by the recirculation below the Taylor bubble lead to the effect of heterogeneity, which makes the profile of the radial liquid velocity component sharper with higher volume fraction of dispersed small bubbles. The volume fraction has a maximum value in the range of r/R = 0.5–0.6. Meanwhile, it is usually higher than 0.35, which means that larger dispersed bubbles can be formed by coalescences in this region. These calculated results are in good agreement with experimental results. The wall shear stress and the mass transfer coefficient with dispersed small bubbles are higher than those without dispersed small bubbles due to enhanced fluctuations. For short Taylor bubble length, the average mass transfer coefficient is increased when the gas or liquid superficial velocity is increased. However, there may be an inflection point at low mixture superficial velocities. For the slug with dispersed small bubbles, the product scales still cannot be damaged directly despite higher wall shear stress. In fact, the alternate wall shear stress and the pressure fluctuations perpendicular to the pipe wall with high frequency are the main cause for breaking the product scales.     

Void fraction measurement of bubble and slug flow in a small channel using the multivision technique

Using the multivision technique, a new void fraction measurement method was developed for bubble and slug flow in a small channel. The multivision system was developed to obtain images of the two-phase flow in two perpendicular directions. The obtained images were processed—using image segmentation, image subtraction, Canny edge detection, binarization, and hole filling—to extract the phase boundaries and information about the bubble or slug parameters. With the extracted information, a new void fraction measurement model was developed and used to determine the void fraction of the two-phase flow. The proposed method was validated experimentally in horizontal and vertical channels with different inner diameters of 2.1, 2.9, and 4.0 mm. The proposed method of measuring the void fraction has better performance than the methods that use images acquired in only one direction, with a maximum absolute difference between the measured and reference values of less than 6%.