Liquid Two-phase Flow Regimes Research Articles

Numerical Investigation Into the Gas–Liquid Two-Phase Flow Regime and Heat Transfer Characteristics in a Gravity Heat Pipe

Abstract In order to reveal the gas–liquid distribution, heat and mass transfer characteristics inside a gravity heat pipe, the bubble behavior and the flow regime transition during the phase-change process were examined by employing a copper-water heat pipe, with the length of 500 mm and Φ22 × 1.5 mm. The results indicate that in the process of phase change, the typical flow regimes of bubble flow, slug flow, and churn flow can be observed in the evaporator, and the presence of bubbles has an obvious disturbance on the flow field. In addition, the discontinuous liquid film plays an important role in the heat transfer mechanism in the condenser, which allows the vapor to contact the wall directly and reduces the heat transfer resistance. The temperature difference between the evaporator and the condenser can be reduced by adjusting the saturation temperature, so as to effectively improve the heat transfer performance of the heat pipe and contribute to the practical engineering design.

Numerical Gas–Liquid Two-Phase Flow Regime Identification in a Horizontal Pipe Using Dynamic Pressure Data

Gas–liquid two-phase flow is very common in industrial pipelines. Flow regime identification is the first step to design, analyze, and operate the gas–liquid system successfully. The purpose of this study is to develop a methodology for identification of a two-phase flow regime using post signal processing techniques, namely Fast Fourier Transform (FFT) and Probabilistic Density Function (PDF). Three different flow regimes were simulated in a 6 m horizontal pipe with a 0.050 m inner diameter. A Level-Set (LS) method coupled with Volume of Fluid (VOF) method is used to model the air–water interface. After validation of the numerical method, dynamic pressure readings were collected with the intent to identify the associated flow regimes by post-processing of these signals. It was concluded that dynamic pressure signals of different flow regimes show different characteristics (like dominant frequency, FFT amplitude, PDF location and PDF magnitude) in the time and frequency domains. These characteristics can be potentially used as differentiating factors to distinguish different flow regimes. This research is limited to stratified, slug, and annular flow in the horizontal pipe. This paper uses a new approach to identify the flow regime in a horizontal pipe by Fast Fourier Transform and Probability Density Function of dynamic pressure readings obtained by using numerical simulation.

Development of Gas-Liquid Flow Regimes Identification Using a Noninvasive Ultrasonic Sensor, Belt-Shape Features, and Convolutional Neural Network in an S-Shaped Riser.

The problem of classifying gas-liquid two-phase flow regimes from ultrasonic signals is considered. A new method, belt-shaped features (BSFs), is proposed for performing feature extraction on the preprocessed data. A convolutional neural network (CNN/ConvNet)-based classifier is then applied to categorize into one of the four flow regimes: 1) annular; 2) churn; 3) slug; or 4) bubbly. The proposed ConvNet classifier includes multiple stages of convolution and pooling layers, which both decrease the dimension and learn the classification features. Using experimental data collected from an industrial-scale multiphase flow facility, the proposed ConvNet classifier achieved 97.40%, 94.57%, and 94.94% accuracy, respectively, for the training set, testing set, and validation set. These results demonstrate the applicability of the BSF features and the ConvNet classifier for flow regime classification in industrial applications.

Signal optimization for recognition of gas–liquid two-phase flow regimes in a long pipeline-riser system

Accurate and fast recognition of multiphase flow regimes is an urgent requirement for the flow assurance of oil/gas pipelines. Experiments on gas–liquid flow regimes are conducted on a 1657 m horizontal pipeline system with a 16.7 m S-shaped riser. The flow regimes are classified as severe slugging, oscillating flow and stable flow, based on quantitative criteria of riser pressure difference. Data from the accumulation stage of severe slugging is selected for training the model, and a new optimization scheme is proposed for the recognition process from feature extraction and selection to model construction and testing. The influence of the measurement distance and location on the recognition rate is revealed, and a new set of signal evaluation and optimization criteria is proposed, covering the recognition rate (higher than 90%), reliability and practicality. An accurate recognition rate over 90% is yielded by using above-water signals with a sample duration of 6.2 s.

Numerical Study of Gas–Liquid Two-Phase Flow Regimes for Upward Flow in a Helical Pipe

This study presents numerical investigations of air–water two-phase flows for upward flow in a vertical helical pipe. The numerical simulations were done in accordance with the experimental work of...

Detection of the gas–liquid two-phase flow regimes using non-intrusive microwave cylindrical cavity sensor

Gas–liquid two-phase flow phenomenon occurs in various engineering applications and the measurement of it is important. A microwave sensor in the form of a cylindrical cavity has been designed to operate between 5 and 5.7 GHz. The aim is to analyse a two phase gas–liquid flow regime in a pipeline. LabVIEW software is utilised to capture the data, process them and display the results in real time. The results have shown that the microwave sensor has successfully detected the two-phase flow regimes in both the static and dynamic flow environments with reasonable accuracy. The study has also shown the independence of the technique and its accuracy to the temperature change (28–83 °C). Several flow regimes of the gas–liquid two-phase flow have been discussed. The system is also able to detect the stratified, wavy, elongated bubbles and homogeneous flow regimes.

Segregation and dispersion of binary solids in liquid fluidised beds: A CFD-DEM study

Liquid fluidised beds often operate with particles of different sizes and densities, encountering partial or complete segregation of solid particles at certain operating conditions. In this study, the segregation and dispersion of binary particle species of the same size but different densities in liquid fluidised beds have been investigated based on the analysis of computational fluid dynamics – discrete element method (CFD-DEM) simulation results.The vertical fluid drag force acting on the particles was found to be responsible for the particle segregation. The mechanisms governing the particle dispersion strongly depended upon the solid–liquid two-phase flow regime, which transited from pseudo-homogeneous to heterogeneous when the superficial liquid velocity reached a certain value. In the homogeneous or pseudo-homogeneous flow regime (Rep≤40, ∈L, ave≤0.74), particle collisions acted as the main mechanism that drove the dispersion of particles. However, after the system became heterogeneous, the magnitude of the vertical collision force decreased towards zero and correspondingly, the magnitude of the vertical fluid drag force was approaching that of the particle net weight force as the superficial liquid velocity increased. Therefore, in the heterogeneous flow regime (Rep>40, ∈L, ave>0.74), the local turbulence of the fluid flow and particle collisions (if there were any) were found to be the main mechanisms that drove the dispersion of particles in all directions. The dispersion coefficient of individual particles varied significantly throughout the system in the heterogeneous flow regime. The simulation results reasonably agreed with the experimental data and the prediction results by existing correlations.

Experimental identification of flow regimes in gas–liquid two phase flow in a vertical pipe

In this experimental study a new flow regime map for gas–liquid two-phase flow regime detection is presented based on interpretation of transient flow pressure signals and utilizing neural networks. In this regard, the pressure signals of an air–water two phase flow at different location on a vertical pipe with an inside diameter of 50 mm and a total length of 6 m are measured. Averaged pressure and pressure fluctuation data were analyzed in different flow regimes along the upriser pipe to recognize the effects of flow regimes on pressure response. A set of pressure parameters are introduced based on averaged pressure data to present meaningful criteria for flow regime identification. Pressure fluctuations also used for flow regime recognition by applying two common signal processing methods. The processed signals are used as the inputs of a feed forward back propagation neural network to predict the flow regime. The neural network outputs are then used to present a flow regime map. The proposed neural network flow map is compared with the experimental data by photographic flow visualization technique. This comparison shows an acceptable agreement between the neural network outputs and experimental results.

Application of gas/liquid two-phase flow in cross-flow microfiltration of oil-in-water emulsion; permeate flux and fouling mechanism analysis

The effects of gas–liquid two-phase flow regimes on permeate flux and fouling mechanisms were investigated in a gas sparging assisted microfiltration of oil-in-water emulsion. Different two-phase flow patterns were attained by introducing gas stream into the liquid phase. It was found that the permeate flux was increased due to disruption of the local deposited cake layer and concentration polarization as the gas velocity increased. Flux enhancement of up to 35% was observed at slug-flow pattern and it was also found that gas sparging is less efficient at high liquid velocities in which turbulence was high. Fouling mechanisms were examined through four individual blocking laws. Membrane resistance curves were used to find dominant fouling mechanism during filtration and as a result, cake formation showed the best agreement with the experimental data in flux decline prediction. The effect of oil droplets on the membrane surface, pore blockage, and accumulation of fouling layer were also analyzed through scanning electron microscopy images.

Flow regime transition for cocurrent gas–liquid flow in micro-channels

Experimental investigations of gas–liquid two-phase flow regimes in micro-channels were carried out in this work. Four distinctive flow patterns of slug flow, slug-annular flow, annular flow and parallel stratified flow were captured by a digital video recording (DVR) system and the transitions among different flow regimes were studied. The effect of fluid properties and wetting properties of the channel wall on the flow regime and flow pattern transition were studied as well. Novel empirical correlations for predicting flow pattern transitions during the steady gas–liquid flow in micro-channels have been developed with the fluid properties and the wetting properties of micro-channels incorporated, whereas the traditional flow criteria were based on gas and liquid superficial velocities only. The predictions of the present empirical models agree well with the experimental data.

Visual technique for detection of gas–liquid two-phase flow regime in the airlift pump

Simplicity of manufacturing and high reliability of airlift pumps have promoted these pumps to be used in different industries, such as petrochemical and oil industries, especially in oil recovery from dead wells. One of the main parameters affecting the performance of these pumps is two-phase flow regime in the main pipe of the pump. In this research, experimental data are utilized to investigate the influence of the flow regimes on the performance of an airlift pump. The data are obtained for air–water two-phase flow in a vertical pipe with a diameter of d = 50 mm and an aspect ratio of L/d = 120. In this study, the gas liquid upward two-phase flow regime in the upriser pipe is experimentally analyzed by the image analysis technique. The gas phase velocity is measured in two-phase flow from the data obtained by the high-speed camera. The three main flow regimes namely slug, churn and annular are visually detected in the airlift pump. The results obtained by the series of experiments show an improvement in the performance of the airlift pump in a slug flow regime in the pump's upriser pipe.

Recognition of gas–liquid two-phase flow patterns based on improved local binary pattern operator

A new method to pattern recognition of gas–liquid two-phase flow regimes based on improved local binary pattern (LBP) operator is proposed in this paper. Five statistic features are computed using the texture pattern matrix obtained from the improved LBP. The support vector machine and back-propagation neural network are trained to flow pattern recognition of five typical gas–liquid flow regimes. Experimental results demonstrate that the proposed method has achieved better recognition accuracy rates than others. It can provide reliable reference for other indirect measurement used to analyze flow patterns by its physical objectivity.

Gas–liquid two-phase flow patterns in rectangular polymeric microchannels: effect of surface wetting properties

Here we map gas–liquid two-phase flow regimes observed in polymeric microchannels with different wetting properties. We utilized video and confocal microscopy to examine two-phase flow patterns produced by parallel injection of air and water through a Y-shaped junction into a rectangular microchannel made of poly(dimethylsiloxane) (PDMS). We observed seven flow regimes in microchannels with hydrophobic walls, whereas only two flow patterns were identified in hydrophilic microchannels. Our study demonstrates that surface wettability has a profound influence on the spatial distribution of air and water moving in microchannels.

Simulation of Taylor Flow in Capillaries Based on the Volume-of-Fluid Technique

Taylor flow, a flow regime characterized by Taylor bubbles separated by liquid slugs that do not contain entrained micro bubbles, is a predominant gas−liquid two-phase flow regime in capillaries and minichannels (channels with hydraulic diameters in the 0.1−1 mm range), and it occurs in monolithic catalytic converters and other multiphase reactors. Taylor flow regime is morphologically relatively simple and has been modeled in the past using computational fluid dynamics (CFD) methods. However, most of the past CFD models have either assumed a fixed gas−liquid interfacial geometry or have modeled the gas−liquid interphase movement based on the method of spines, which imposes some restrictions on the free movement of the interface. In this study, we examine the feasibility of CFD modeling of the Taylor flow regime in capillaries by using the volume-of-fluid (VOF) technique for the motion of the gas−liquid interphase. It is shown that such a model predicts well the experimental data and empirical correlation...

On gas–liquid two-phase flow regimes in microchannels

On gas–liquid two-phase flow regimes in microchannels

A neural network to correct mass flow errors caused by two-phase flow in a digital coriolis mass flowmeter

Coriolis mass flow meters provide accurate measurement of single-phase flows, typically to 0.2%. However gas–liquid two-phase flow regimes may cause severe operating difficulties as well as measurement errors in these flow meters. As part of the Sensor Validation (SEVA) research at Oxford University a new fully digital coriolis transmitter has been developed which can operate with highly aerated fluids. This paper describes how a neural network has been used to correct the mass flow measurement for two-phase flow effects, based entirely on internally observed parameters, keeping errors to within 2%. The correction strategy has been successfully implemented on-line in the coriolis transmitter. As required by the SEVA philosophy, the quality of the corrected measurement is indicated by the on-line uncertainty provided with each measurement value.

Experimental study on gas–liquid two-phase flow regimes in rectangular channels with mini gaps

Experimental investigations were conducted for adiabatic air/water two-phase flow in vertical rectangular channels. Three test sections with the narrow gaps of 0.3, 0.6 and 1.0 mm respectively were studied. The flow regimes were observed by using a high-speed video-camera and were identified by examining the video images. The flow regimes in channels with the gaps of 1.0 and 0.6 mm are similar to those found by previous studies, and can be classified into bubbly, slug, churn and annular flow. With decreasing the channel gaps, the transition lines for bubbly-slug, slug-churn and churn-annular flow shift to left in the flow regime maps with the superficial gas velocity as the horizontal coordinate and the superficial liquid velocity as the vertical coordinate. However, the flow regimes in the channel with the gap of 0.3 mm are dramatically different from the previous studies. Bubbly flow was never observed even at very low gas flow velocity. Cap-bubbly flow can exist in the channels stably. Due to the increased influences of the surface tension and friction shear stress in the mini channels, the liquid droplets adhere on the wall surface and are pushed by the flowing gas. The flow regimes can be classified into cap-bubbly, slug-droplet, churn and annular-droplet flow.

Gas–liquid two-phase flow regimes in rectangular channels with mini/micro gaps

An adiabatic concurrent vertical two-phase flow of air and water in vertical rectangular channels (12×260 mm) with narrow gaps of 0.3, 0.6–1.0 mm was investigated experimentally. Flow regimes were observed by using a CCD camera and were identified by examining the video images. The flow regimes for gaps of 1.0 and 0.6 mm were found to be similar to those in the existing literature which can be classified into bubbly flow, slug flow, churn–turbulent flow and annular flow. With the decrease of the channel gap, the transition from one flow regime to another occurs at smaller gas flow rates. However, flow regimes for micro-gaps of 0.3 mm or less are quite different from the previous studies: bubbly flow was never observed even at very low gas flow rates. Due to the increased influence of the surface tension force and the frictional shear stress in channels with a micro-gap, the liquid droplets adhered on the wall surface and were pushed by the gas phase. Flow regimes in these micro-gaps can be classified into cap–bubbly flow, slug–droplet flow, churn flow and annular–droplet flow. A previous model has been extended to predict the flow regime transitions from bubbly flow to slug flow, slug flow to churn flow using the bubble rising velocity and the increased frictional coefficient for rectangular channels. A new criterion has been developed to predict the transition of the annular flow. Comparisons of our prediction results with experimental data are discussed for gaps larger than 0.6 mm. With micro-gaps of 0.3 mm or smaller, a new theory needs to be developed.