Gas-liquid Annular Flow Research Articles

An empirical correlation for the entrainment fraction at the onset of annular flow based on 2006 CHF look-up table

The critical heat flux (CHF) of gas-liquid flow plays an important role in the safety of industrial equipment. At present, the liquid film dryout model is widely used for predicting CHF in gas-liquid annular flow. Most parameters in this model can be determined by some empirical correlations which are valid under different conditions. However, up to now, the entrainment fraction at the onset of annular flow is always assumed due to the lack of relevant experimental data. In this paper, the normalized data of the 2006 CHF look-up table (LUT) which has been adopted widely, especially in the nuclear industry, were used. Firstly, the empirical correlations, provided for the onset of annular flow and the limiting quality, were employed. In the valid pressure and mass flux range of these correlations, the selected database from LUT was confirmed. Secondly, the liquid film model was built. The entrainment fraction at the onset of annular flow was obtained when the calculated CHF by the model agreed with the corresponding value in LUT. A parallel look-up table for it was developed. Its correlation including the Weber and the liquid Reynolds number at outlet was proposed. The errors are mostly within ±30%. Finally, its reliability on the other conditions in LUT, which are beyond the valid range of the empirical correlations used for determining the database, was discussed. All the conditions whose errors are outside ±30% of the predictions by the provided correlation were marked in the tables.

Prediction of entrained droplet fraction in co-current annular gas–liquid flow in vertical pipes

The entrained droplet fraction is an important parameter in annular two-phase flow, as its correlations are key inputs in flow simulation codes for the prediction of pressure drop and critical heat flux or dryout. Investigators have stressed the importance of extending the validity range of current correlations so that more conditions are covered. This could be achieved for example by including fluids with higher viscosities, a wider range of operating pressures, and increase in the size of pipes used for experiments (most of the data in the literature are from pipes of 50mm diameter and below). In attempt to improve the latter, experiments were conducted in a 101.6mmgas–liquid flow loop at Cranfield University’s Oil and Gas Engineering Laboratory and data on the fraction of droplets were collected in the annular flow regime by measuring the film velocity, from which the droplet fraction was calculated. Comparison of the film velocity by this method and by a mass balance showed close agreement. A capacitance Wire Mesh Sensor was used for flow visualisation in order to distinguish between annular and churn flow. In order to arrive at an improved correlation, over 1300 data points were gathered from other published works. These include air–water studies where large pipes of up to 127mm in diameter were used. The others were from small-diameter pipes and for refrigerant, steam–water, air–water, and air–glycerine flows. Since in the annular regime, the gas flow entrains liquid droplets into the core, their presence alters the properties of the gas core. Therefore, accurate predictions are pivotal for the energy efficient design and operation of facilities in the petroleum and nuclear power industry. The correlation obtained here showed good agreement with the collected databank.

Numerical Simulation of Wellhead Back Pressure in Under-balanced Drilling

Abstract Based on the application of liquid-phase under-balanced drilling in a vertical well, this paper studies the wellhead back pressure (WBP) on annular gas-liquid two-phase flow characteristics and its impact on the law of gas cut speed. Annular gas-liquid two-phase flow transient calculation model and a model of gas cut during drilling are established in this study. And used to calculate the changing law of WBP under constant and variable bottom hole underbalanced. Results show that a risk of excessive gas cut or WBP exists when uncovering gas reservoir under constant underbalanced. The WBP needed for using variable negative pressure to keep gas cut speed unchanged remains stable after a period of growth. And is mainly affected by gas cut speed, formation permeability, time to apply WBP, and other factors. The simulation result is compared with the WBP value collected on site in real time, show that the changing trend is roughly the same.

气液环状流周向液膜测量传感器的优化设计

By taking the circumference liquid films of cross-sectional pipe in gas-liquid two-phase annular flow for measuring objectives,a novel visual sensor with virtual two-visual angles was designed and optimized by combining a single high-speed camera and planar reflectors.Based on the virtual binocular stereo vision principle,a measurement model for visual sensor with two-visual angles was established.To maximize the effective measurement angle and to obtain much liquid film flow informations,the model of the virtual double-view vision sensor was analyzed and its structure parameters were optimized by considering the field of view,sensor size,measuring distance and the optical path refraction of the pipe.The theory analyzing and experimental results indicate that the optimized sensor can get a view field closed to 300° effective circumference visual angle,which is far better than that using a single high-speed camera to capture directly.It provides a theoretical basis for the real-time measurement of the gas-liquid two-phase annular flow circumference liquid films through the doubleview vision sensor,and has great significance to study the film thickness and analyze the flow state of the annular flow.

混合翼付きグリッドスペーサが垂直円管内気液環状二相流の液膜厚さに及ぼす影響

混合翼付きグリッドスペーサが垂直円管内気液環状二相流の液膜厚さに及ぼす影響

Physical understanding of gas-liquid annular flow and its transition to dispersed droplets

Transformation from annular to droplet flow is investigated for co-current, upward gas-liquid flow through a cylindrical tube using grid based volume of fluid framework. Three transitional routes, namely, orificing, rolling, and undercutting are observed for flow transformation at different range of relative velocities between the fluids. Physics behind these three exclusive phenomena is described using circulation patterns of gaseous phase in the vicinity of a liquid film which subsequently sheds drop leading towards transition. Orifice amplitude is found to grow exponentially towards the core whereas it propagates in axial direction in a parabolic path. Efforts have been made to fit the sinusoidal profile of wave structure with the numerical interface contour at early stages of orificing. Domination of gas inertia over liquid flow has been studied in detail at the later stages to understand the asymmetric shape of orifice, leading towards lamella formation and droplet generation. Away from comparative velocities, circulations in the dominant phase dislodge the drop by forming either a ligament (rolling) or a bag (undercut) like protrusion in liquid. Study of velocity patterns in the plane of droplet dislodge reveals the underlying physics behind the disintegration and its dynamics at the later stages. Using numerical phase distributions, rejoining of dislodged droplet with liquid film as post-rolling consequences has been also proposed. A flow pattern map showing the transitional boundaries based on the physical mechanism is constructed for air-water combination.

Effects of Mixing-vane Attached to Grid Spacer on Pressure Drop and Deposition Rate in BWR Simulated Channel

This paper describes the experimental study on the effects of grid spacer with mixing-vane (MV) on gas-liquid two-phase annular flow in a vertical circular pipe of 16 mm i.d. In order to know the effects, grid spacers with and without MV were inserted in turn into the test channel. Furthermore, in order to know the effects of inclination angle of the mixing-vane to main flow, two inclination angles, i.e., 30° (MV30) and 20° (MV20) were tested. Pressure drop of flow through the spacer and liquid droplets deposition rate in the section downstream from the spacer were measured with a pressure transducer and double liquid film extraction method, respectively. Experimental data revealed that deposition mass transfer coefficient and the pressure drop were higher for the spacer with MV than that without MV. Regarding the effects of the inclination angle of MV, the vane of MV30 gave higher deposition mass transfer coefficient than MV20 although it increased the pressure drop.

Investigations of mass transfer in annular gas-liquid flow in a microreactor

Abstract The paper presents an investigation of mass transfer in gas-liquid annular flow in a microreactor. The microreactor had a meandered shape with a square cross-section of the channel (292×292 μm, hydraulic diameter 292 μm) and 250 mm in length. The rate of CO2 absorption from the CO2/N2 mixture in NaOH (0.1 M, 0.2 M, 0.7 M, 1.0 M and 1.5 M) water solutions was measured. Two velocities of gas flow and two velocities of liquid flow were used. In two cases a fully developed annular flow at the beginning of the channel was observed, whilst in two cases annular flow was formed only in about 2/3 of the microchannel length. Based on the measurements of CO2 absorption rate, the values of volumetric liquid - side mass transfer coefficients with the chemical reaction were determined. Then physical values of coefficients were found. Obtained results were discussed and their values were compared with the values predicted by literature correlations.

Measurement of Circumferential Liquid Film Based on LIF and Virtual Stereo Vision Sensor

Gas-liquid annular flow is widely used in many industrial applications such as petroleum, chemical, and nuclear engineering. The feature parameters of liquid film in the annular flow are of great significance to understand the flow characteristics and measure the flow precisely. For the annular flow, the circumferential features of liquid film are more important than the axial features to acquire abundant flow structures and reveal the flow mechanism. In the paper, a measurement platform based on the laser-induced fluorescence (LIF) and virtual stereo vision sensor is presented. The virtual stereo vision sensor comprises a high-speed camera and two optical reflection sets, which can acquire the liquid film from two views simultaneously and reconstruct the features of liquid film. Image processing techniques are proceeded with to extract the feature parameters of liquid film; then the circumferential flow characteristic can be reconstructed by views transformation and fusion. The flow characteristic based on the thickness distribution is analysed. The experimental results show that the method is valid and effective, which can give a more detailed and accurate description for the liquid film in annular flows.

E-13 垂直円管流路内の混合翼付グリッドスペーサが気液環状二相流に及ぼす影響

E-13 垂直円管流路内の混合翼付グリッドスペーサが気液環状二相流に及ぼす影響

Study of formation and development of disturbance waves in annular gas–liquid flow

Wavy structure of downward annular gas–liquid flow with liquid entrainment in 15 mm pipe was studied using high-speed laser-induced fluorescence technique. Measurements were performed near the inlet, which was organized as a tangential slot. Spatiotemporal records of film thickness were obtained over the first 100 mm of the pipe length in order to investigate formation and initial stages of development of the disturbance waves. It was shown that for high enough gas and liquid flow rates disturbance waves appear and start to dominate in the wavy structure of liquid film within the area of interrogation. Disturbance waves were found to be formed due to coalescence of small high-frequency waves appearing at the inlet. Similar mechanisms of formation of large waves with fast ripples on them were observed far downstream for waves near transition to entrainment and for ephemeral waves in flow regimes with entrainment. Significant individual acceleration of disturbance waves at the initial stage of their development was observed. Spectral analysis has shown strong energy transfer from high to low frequencies, which is in agreement to the proposed mechanism of waves formation.

Imaging of gas–liquid annular flows for underbalanced drilling using electrical resistance tomography

The underbalanced drilling technique, which is also known as managed-pressure drilling, is playing an important role in oil and gas sector, as it reduces common conventional drilling problems such as minimal drilling rates and formation damage, differential sticking and lost circulation. Flow regime monitoring is one of the key topics in annular multiphase flow research, particularly for underbalanced drilling technique. Prediction of the prevailing flow regime in an annulus is of particular importance in the design and installation of underbalanced drilling facilities. Especially, for establishing a suitable pressure-drop model based on the characteristics of the active flow regime. The methods of flow regime prediction (or visualisation) in an annulus that are currently in use are very limited, this is evidently due to poor accuracy or they are simply not applicable to underbalanced drilling operation in practice. Therefore, this paper presents a monitoring method, in which Electrical Resistance Tomography (ERT) is used to rapidly image the prevailing flow regime in an annulus with a metallic inner pipe. Experiments were carried out using an air–water flow loop with a test section 50mm diameter flow pipe. The two-phase air–water flow regimes are visualised in the upward vertical annulus with a radius ratio (r/R) 0.4. This paper highlights the visualisation results of only three flow regimes, namely bubble flow, transitional bubble-slug flow and slug flow. The flow regimes are visualised through axial images stacked from 50mm diameter-pixels of 2D tomograms reconstructed with the Conjugate Gradient Method (SCG). Gas volume fraction profiles within the annular flow channel are also illustrated. The profiles are extracted using the Modified Sensitivity coefficient Back-Projection (MSBP) method with a sensitivity matrix generated from a realstic phantom in the finite element method software. The results are compared with visual observations (e.g. photographs) of the active flow regime at the time of ERT measurements.

Error analysis and liquid film thickness measurement in gas–liquid annular flow

The parameters measurement of liquid film in gas–liquid two-phase annular flow is of great significance for the study of heat and mass transfer and flow characteristics. The paper focuses on the thickness measurement and flow evolution analysis of the liquid film in downward gas–liquid two-phase annular flow. Compared with the traditional measurement methods, the method combined the laser-induced fluorescence (LIF) technique with the high-speed photography is developed to measure the liquid film thickness and analyze the flow distribution. Firstly, the geometric optical path model of the organic glass pipeline was established based on the Gauss optics to recognize the real and virtual film images and quantitatively estimate the measurement linearity. And then the digital image processing technology was developed to achieve non-intrusive measurement of liquid film thickness. Finally, the film thickness distribution in downward gas–liquid two-phase annular flow was further analyzed. The method can avoid the interference with the flow field. Meanwhile, the measurement results by experiment show that the film thickness manifests stochastic distribution, which corresponds to the features of randomness and small flow in the process of liquid film flow.

403 Effects of grid spacer with mixing vane on gas-liquid two-phase annular flow

403 Effects of grid spacer with mixing vane on gas-liquid two-phase annular flow

ICOPE-15-1066 Effects of surface tension on pressure drop of gas-liquid annular flows in a vertical small diameter pipe

ICOPE-15-1066 Effects of surface tension on pressure drop of gas-liquid annular flows in a vertical small diameter pipe

Experimental Investigation of Horizontal Gas–Liquid Stratified and Annular Flow Using Wire-Mesh Sensor

Stratified and annular gas–liquid flow patterns are commonly encountered in many industrial applications, such as oil and gas transportation pipelines, heat exchangers, and process equipment. The measurement and visualization of two-phase flow characteristics are of great importance as two-phase flows persist in many fluids engineering applications. A wire-mesh sensor (WMS) technique based on conductance measurements has been applied to investigate two-phase horizontal pipe flow. The horizontal flow test section consisting of a 76.2 mm ID pipe, 18 m long was employed to generate stratified and annular flow conditions. Two 16 × 16 wire configuration sensors, installed 17 m from the inlet of the test section, are used to determine the void fraction within the cross section of the pipe and determine interface velocities between the gas and liquid. These physical flow parameters were extracted using signal processing and cross-correlation techniques. In this work, the principle of WMS and the methodology of flow parameter extraction are described. From the obtained raw data time series of void fraction, cross-sectional mean void fraction, time averaged void fraction profiles, interfacial structures, and velocities of the periodic structures are determined for different liquid and gas superficial velocities that ranged from 0.03 m/s to 0.2 m/s and from 9 m/s to 34 m/s, respectively. The effects of liquid viscosity on the measured parameters have also been investigated using three different viscosities.

An experimental characterization of liquid films in downwards co-current gas–liquid annular flow by particle image and tracking velocimetry

The hydrodynamics of downwards gas–liquid annular flows and falling films in a pipe were studied experimentally using simultaneous planar laser-induced fluorescence and a combination of particle image and particle tracking velocimetry techniques. The investigated conditions covered the range of liquid and gas Reynolds numbers: ReL=306–1532 and ReG=0–84600. The results presented in this paper concern: (i) information on the local and instantaneous velocity fields underneath the interfacial waves and the appearance of recirculation zones within the liquid films under certain conditions, and (ii) mean velocity, velocity fluctuation rms and kinetic energy profiles within the liquid films. The results indicate that large waves contain multiple recirculation zones, which may play an important role in the gas and liquid phase entrainment mechanisms as well as in the mass and momentum transfer from the near-wall region towards the gas–liquid interface.

Analysis of spatial and temporal evolution of disturbance waves and ripples in annular gas–liquid flow

Wavy structure of liquid film in downward annular gas–liquid flow is studied with high-speed laser-induced fluorescence technique. Film thickness measurements are resolved in both longitudinal distance and time with high spatial and temporal resolution. A method is developed to identify the characteristic lines of individual disturbance waves. Change of frequency of disturbance waves with downstream distance is modelled based on the obtained distributions of the disturbance waves by velocity and separation time. Using obtained characteristic lines further investigation of ripples’ properties is performed in reference system moving with the disturbance wave. As a result, velocities, amplitudes and frequencies of ripples are measured with relation to the distance from disturbance waves. Fast ripples travelling over crests of disturbance waves and slow ripples travelling over back slopes of disturbance waves and over the base film are studied separately. Besides, the average length of crest and back slope of disturbance waves were measured.

Experimental and modelling studies of gas–liquid vertical annular flow through a diverging section

Gas–liquid annular flow through a vertical circular pipe is well-understood and detailed phenomenological models exist in the literature. In the present work, the case of flow through a diverging section is studied experimentally and theoretically. Experiments have been carried out in air–water flow through a vertical diverging pipe section with a diameter ratio of 1.5 and 2.0. Pressure profiles have been recorded upstream, across and downstream of the diverging section for the cases of sudden expansion and gradual expansion with included half-angles of 8° and 15° for the diverging section. These show that the pressure variation is characterized by a strong pressure recovery downstream of the expansion which is in turn influenced by the smoothness of the expansion and the interfacial friction. The non-dimensionalized pressure loss across the expansion, which has been determined by extrapolating the pressure variations upstream and downstream, is found to vary systematically with the diameter ratio, the angle of the expansion and the ratio of superficial liquid-to-gas Reynolds numbers. A method, which uses the equilibrium annular flow model and an empirically determined expansion pressure loss coefficient, has been developed for the prediction of the pressure variation across the diverging section.

An experimental characterization of downwards gas–liquid annular flow by laser-induced fluorescence: Flow regimes and film statistics

Downwards co-current gas–liquid annular flows were studied experimentally and characterized. An advanced optical laser-based measurement technique, namely Planar Laser-Induced Fluorescence (PLIF), was used for the visualization of the annular flow over a range of liquid Reynolds numbers ReL=306–1532 and gas Reynolds numbers ReG=0–84600. Four distinct flow regimes, namely the ‘dual-wave’, ‘thick ripple’, ‘disturbance wave’ and ‘regular wave’ regimes, have been identified based on qualitative information and a consequent quantitative analysis that provided information on the film thickness, interface and wave statistics, and gas entrainment into the liquid film. The mean film thickness data are generally in good agreement with previous studies. Evidence suggests that the turbulent gas phase affects strongly the shape of the interface, and that the mechanism for gas entrainment into the liquid film is strongly reliant on the existence of large-amplitude waves.