Void Fraction Of Two-phase Flow Research Articles

An optimised and novel capacitance-based sensor design for measuring void fraction in gas–oil two-phase flow systems

ABSTRACT This study explores a new electrode configuration for measuring the void fraction of two-phase flows using capacitance-based sensors. The proposed method is considered ‘skewed’ because of its unique geometric shape, and the performance of the proposed sensor was evaluated and improved via multiple simulations using the COMSOL Multiphysics software. The simulations encompass three different flow patterns, stratified, annular and homogeneous, whose performance themselves were verified in a previous study. The influences of geometric properties and parameters on the sensitivity of the proposed sensor were evaluated to determine an optimal configuration. Furthermore, the sensitivity distribution on different void fractions of the sensor was analysed for various void fractions in different flow patterns. Additionally, the proposed configuration was also compared alongside double-ring and concave sensors to determine the overall sensitivity. At 2.11 pF, the overall sensitivity of the proposed sensor was significantly higher than that of the other sensors. It is worth mentioning that the measurement precision of multiphase flow meters is of high importance, particularly in petroleum industry because of the oil price and also the high amount of transported products.

Void Fraction of Two-Phase Flow of Air – Emulsion of Water and Oil in Horizontal Mini Pipe

Two-phase flow is the most straightforward flow of multi-phase flow. The void fraction is one of the significant parameters affecting the two-phase flow pattern on both small and mini channels. The flow pattern will be significantly sure, and fluctuations in pressure will change. If the pressure fluctuations are very high, it will affect the piping network system or channel and cause damage. Somebody can find the application of this case of damage to mini heat exchangers, mini boilers, and other small and mini equipment. This paper presents a significant correlation between void fractions and flow patterns. This study used a glass pipe with a length of 160 mm and a diameter of 1.6 mm with a horizontal position. The working fluids used are an emulsion of water and coconut oil with a concentration of 350 mg/dl and 500 mg/dl. The superficial velocity is gas (JG) = 0.08 m/s to 74.6 m/s, and the superficial velocity of the Liquid (JL)= 0.04 m/s to 4.15 m/s. Data measurements use a high-speed camera and are processed using MATLAB R2014a software. The research results showed that the difference in the concentration of the emulsion of water and coconut oil sufficiently affected the value of the void fraction significantly. The void fraction is because the increased concentration of such emulsions affects the increase in the viscosity value of the Liquid. The increase in the viscosity of the Liquid affects the speed of the Liquid tends to decrease. Under these conditions, the increasing superficial velocity of the gas affects the value of the growing void fraction.

Experimental investigation of the void fractions of refrigerants R32 and R1234yf in a 1 mm diameter horizontal channel using a capacitance-based method

The void fraction of two-phase flow in the 1 mm of channel is investigated in this study. The small channels, including micro to mini-channel, are widely used for developing miniaturized air conditioning systems. In terms of carbon emission reduction, optimizing the refrigerant charging amount based on sufficient information and understanding of the characteristics of the two-phase flow is a noteworthy challenge. The void fraction is a parameter of the two-phase flow that is essential for determining the heat transfer coefficient, modeling the pressure drop, and predicting the refrigerant charging amount. Therefore, the precise measurement and prediction of void fraction in small channels for various refrigerants are required. However, thus far, the void fraction characteristics of small channels have not been actively investigated because of the limited volume inside a single small channel and the relatively high error of the quick-closing valve (QCV) method for small channels. This study proposes a capacitance-based sensor as an alternative method for void fraction measurement of small channels. The newly designed void fraction sensor was fabricated with 7.8 % uncertainty under compensation for manufacturing limitations. The void fractions of refrigerants R32 and R1234yf flowing through smooth small horizontal channels under adiabatic conditions (inner diameter: 1 mm, mass flux: 300–600 kg m−2s−1, saturation temperature: 20–30 °C, vapor quality: 0.025–0.900) were measured. The measurement results were compared with ten existing prediction correlations of five classifications, and the correlations that optimally predicted the void fractions of R32 and R1234yf in the small horizontal channel were presented.

Pressure Drop and Void Fraction of Two-Phase Flow (Air-Water) in Grooved Vertical Pipes

Pressure Drop and Void Fraction of Two-Phase Flow (Air-Water) in Grooved Vertical Pipes

Design and construction of an LSTM-powered high sampling rate dual-beam gamma densitometer for real-time measurement of the two-phase flow void fraction

In this work, an intelligent gamma densitometer gauge for non-intrusive real-time measurement of the two-phase flow void fraction has been designed and constructed. It has been implemented in a horizontal pipe over a wide range of gas and liquid superficial velocities. Moreover, a Long Short-Term Memory (LSTM) neural network has been used for analyzing the data sequence from a dedicated high sampling rate dual-beam gamma densitometer. This approach considerably reduced the complexity of the data processing and regression operations while providing a high capability to identify and compensate for the errors caused by intermittent, and inhomogeneous flow regimes in a real-time mode. A comprehensive sensitivity analysis has been performed to adjust the network parameters for the most accurate void fraction prediction in all studied flow regimes including slug, plug, wavy, stratified, bubbly, and semi-annular. The experimental evaluations revealed that considering a sampling time of a few seconds, the optimized model approximated the actual data with an R-squared of 0.98, and almost all measurements were carried out with the maximum absolute error of 10%. The high accuracy and relatively fast response of the proposed method make it a reliable technique for developing a high-performance real-time multiphase flow meter.

Void fraction measurement of lubricant air-oil two-phase flow using a near infrared optical-fiber spectrometer associated with partial least squares regression models

Motivated by recent research on two-phase flow void fraction measurement, this paper presents a study on void fraction measurements of a lubricant air-oil two-phase flow using a near infrared (NIR) optical-fiber spectrometer associated with partial least squares (PLS) regression models. To overcome the measurement inaccuracy due to interface scattering, this study used PLS regression models to analyse the spectrum of air-oil two-phase flow. First, an NIR optical-fiber spectrometer experimental system was developed. Second, a flow regime analysis was conducted for the bubble flow transition inside the experimental system. Finally, the PLS regression model validations, the void fraction measurement accuracies, and the uncertainties of the experimental system were evaluated and discussed, and the different PLS regression models were compared in detail. The results indicate that the NIR optical-fiber spectrometer combined with the PLS regression model can achieve void fraction measurements of lubricants in air-oil two-phase flow, with a maximum squared correlation coefficient (R 2) of 0.981, a minimum Type A uncertainty of 0.039% and a minimum expanded uncertainty of 0.077%.

Experimental and theoretical study on fluidelastic instability of tube bundles subjected to cross-flow in the parallel direction

Fluidelastic instability is an important security issue for the steam generator tube bundles subjected to cross-flow. Lots of studies have been conducted on the fluidelastic instability of the tube bundles in the transverse direction. Recently, the streamwise fluidelastic instability of the tube bundles in cross-flow has been observed in some experimental studies. However, the mechanism of the fluidelastic instability in the parallel is not clear. Thus, in this paper, experimental and theoretical studies were performed to investigate the fluidelastic instability of a rotated triangular tube array subjected to cross-flow in the parallel direction. The influences of the boundary constraint condition and the void fraction of two-phase flow on the threshold of the streamwise fluidelastic instability of the tube bundles were discussed.

The Study on Void Fraction Prediction of Gas-liquid Two Phase Flow Based on Convolutional Neural Network

For the gas-liquid two phase flow in the horizontal pipeline, at the center angle the void fraction of the different liquid phases is calculated with the finite element simulation software, and then a soft measurement model of the void fraction is established. By comparing with traditional recursive augmented least squares (RELS), particle swarm optimization (PSO), and simulated annealing-based PSO, the void fraction soft measurement model is identified and calculated separately. The segmentation optimization results of PSO based on simulated annealing have higher accuracy and stability than RELS and PSO, but as the number of center angles increases, the relative accuracy and stability of the system will deteriorate. And the characteristic is not conducive to the calculation and analysis of data results. By combining the actual model, the convolutional neural network weight update algorithm is added to the LSTM, and the RNN-LSTM convolutional neural network is used to predict the void fraction of the second half the region. It improves the effect of RNN gradient problem on learning ability and improves learning ability. Through comparison, it is found that the convolutional neural network based on RNN-LSTM has a better prediction effect, improves the accuracy and stability of the system, and provides a new method for the measured void fraction of twophase flow.

The effect of the clearance restriction on post-instability of the tube bundles in two-phase cross-flow

This paper investigated the two-phase cross-flow-induced vibration of the tube bundles considering the effects of the tube support structure. A mathematical model of the tube bundles subjected to two-phase flow and the clearance restriction was developed. Based on this model, the vibration responses and bifurcation diagrams of the tube bundles in 20%, 40%, 60%, and 80% void fractions were calculated. The effects of the symmetric and asymmetric clearance restrictions on the nonlinear dynamics of the tube bundles were discussed. The results show that the two-phase flow void fraction has a great effect on the nonlinear dynamic characteristics of the tube bundles with clearance restriction. With increasing of the flow pitch velocity, the original equilibrium position could become unstable, and a new stable equilibrium will be generated within the four void fraction conditions. At the new stable equilibrium position, the nonlinear dynamics characteristics of the tube bundles system are much more complicated.

Void Fraction Prediction Method in Gas–Liquid Flow through Channel Packed with Open-Cell Metal Foams

This paper reports the results of a study concerned with air–water and air–oil two-phase flow in channels packed with open-cell metal foams. The research was conducted in horizontal channel with an internal diameter of 0.02 m and length of 2.61 m. The analysis applied three metal foams with pore density 20, 30, and 40 PPI and porosity typical for industrial applications, changing in the range of 92–94%. The experimental data were used to develop a new method for predicting void fraction in two-phase gas–liquid flow in channels packed with metal foams. A new gas void fraction calculating method based on drift-flux model was developed. This model gives a correct representation of changes in the gas void fraction value and good prediction accuracy. The average relative error in calculating the air void fraction in two-phase flow is less than 13%, and 86% of experimental points is characterized by an error less than 20%.

Capacitance sensor for measuring void fraction in small channels

The paper presents a method for the measurement of void fraction in two-phase flow of liquid and gas phases in small channels by means of capacitance sensor. It was assumed arbitrarily that the small channels are those of diameter less than 10 mm. The capacitance sensor investigated in this paper consisted of two electrodes mounted on the outer wall of a glass pipe, and an electronic circuit for signal detection. The capacitance of two electrodes depends on the mean electric permittivity of the medium between them. The mean permittivity is a function of permittivity of the channel material and the percentage of both phases of different dielectric constants: liquid and gas. In this paper, the void fraction in two-phase flow was determined from frequency deviation of a high-frequency oscillator, in which the capacitance sensor was connected to a resonant circuit with an inductance in parallel. The advantage of this method over other methods based on the direct capacitance measurement (for example, LCR bridge) is that the intermediate steps for the determination of void fraction from the capacitance changes are omitted, and the void fraction can be directly determined from the calibration curve. Changes in the capacitance of electrodes for various void fractions have been determined numerically from the distribution of electric field in this geometry.

Determination of void fraction in a bed of solid particles by the method of flow cutoff

The paper presents a new method for determining the void fraction of two-phase flow with a condensable vapor phase. This method relies on the known technique of blocking the flow and allows measuring void fraction in the pore space of solid particle packings. The paper presents the results of experiments with packed beds of steel balls with diameters of 2 mm and 4 mm, 250 mm high. A comparison with some techniques for calculation of void fraction for empty vertical pipes is made.

Semitheoretical formulation of annular flow void fraction using the principle of minimum entropy production

The two-phase flow void fraction is a critical parameter for characterising the pressure drop as well as heat and mass transfer capability of the working fluid within thermal systems, the accurate estimation of which drives heat exchanger design and control optimisation. A semitheoretical expression for the void fraction of two-phase flows, also applicable to small-sized channels, is obtained from an analytical study based on the principle of minimum entropy production and the introduction of empirical coefficients to be fitted to experimental data available in the open literature. These coefficients embody the importance of the simplified physical terms of this formulation while recovering the accuracy loss owing to nonlinear phenomena, heat and mass transfer, and three-dimensional effects. By accounting for surface tension, this model generalises previous theories and describes the influence of smaller-sized channels in terms of the stable void fraction. This mathematical framework can be used to summarise data covering different refrigerants, channel diameters, and operating conditions.

Review of measurement techniques for void fraction of two-phase flow through annulus

This paper presents a state of the art measurement and imaging systems used to determine the Gas Void Fraction (GVF) of two-phase flow through annuli. Several research works were conducted for GVF measurement of two and three-phase fluids flowing through full circular pipelines but only few of them targeted the annuli. The highly restricted access and the low volume within which the multiphase fluid flows through in annuli make the instrumentation design challenging. This is especially true if non-intrusive sensors are required to not disturb the flow, which is the case, for instance, if an accurate CFD modeling of annulus flow is sought out of these measurements. Motivated by the numerous industrial applications of annuli, several researchers have suggested various corresponding non-invasive GVF measurement devices to better understand the behavior of the annulus flow and also to determine the production rate. Many of these techniques are based on electrical conductivity measurements using one or several pairs of electrodes. Various configurations of the electrodes were suggested and this paper highlights that none of them can cope with all kinds of flow patterns. Imaging techniques using wire mesh and electrical resistance tomography were also suggested and the initial corresponding results were very encouraging. This paper provides a critical review of each of these techniques and highlights the advantages, disadvantages, and limitations of each one. Hence, it can be a valuable research reference for either new scientists in the area of instrumentation to design a sensor targeting annulus flow or to experienced engineers and researchers to keep in touch with the most recent research advances in the area.

Numerical and experimental analysis of the correlation between EIT data eigenvalues and two-phase flow phase fraction

Electrical impedance tomography (EIT) is an imaging technique with advantages of non-intrusiveness, low-cost and high temporal resolution, which is promising for multiphase flow instrumentation. However, it produces smooth images with low spatial resolution where the interface between phases cannot be distinguished and from which the phase fraction cannot be estimated correctly. In this article, an eigenvalue analysis of EIT raw data is used to estimate the void fraction, i.e. the phase area ratio in 2D, without reconstructing images. For a given EIT sensor, each acquisition frame is represented by an impedance matrix whose eigenvalues are computed after normalization. The main characteristics of the eigenvalue distribution for different two-phase flow patterns within a cylindrical pipe are analyzed numerically. The behaviors of the leading eigenvalue and of the sum of the absolute values of the following ones are assessed as functions of the void fraction. This leads to an estimation of the two-phase flow void fraction based on the EIT sensor configuration. The presented numerical results highlight the existing correlation between the eigenvalues and the void fraction for the phase distribution patterns considered. These simulation results are compared with experimental static tests for validation.

Two-phase flow void fraction estimation based on bubble image segmentation using Randomized Hough Transform with Neural Network (RHTN)

The International Atomic Energy Agency (IAEA) has been encouraging the use of passive cooling systems in new designs of nuclear power plants. Next nuclear reactor generations are intended to have simpler and robust safety resources. Natural Circulation based systems hold an undoubtedly prominent position among these. The study of limiting conditions of these systems has led to instability behavior analysis where many different two-phase flow patterns are present. Void fraction is a key parameter in thermal transfer analysis of these flow instability conditions. This work presents a new method to estimate void fraction from images captured of an experimental two-phase flow circuit. The method integrates a set of Artificial Neural Networks with a modified Randomized Hough Transform to make multiple scans over acquired images, using crescent-sized masks. This method was called Randomized Hough Transform with Neural Network (RHTN). Each different mask size is chosen according with bubble sizes, which are the main ‘objects of interest’ in this image analysis. Images are segmented using fuzzy inference with different parameters adjusted based on acquisition focus. Void fraction calculation considers the volume of the imaged geometrical section of flow inside cylindrical glass tubes considering the acquisition depth-of-field used. The bubble volume is estimated based on geometrical parameters inferred for each detected bubble. The image database is obtained from experiments performed on a vertical two-phase flow circuit made of cylindrical glass where flow-patterns visualization is possible. The results have shown that the estimation method had good agreement with increasing void fraction experimental values. RHTN has been very efficient as bubble detector with very low ‘false-positive’ cases (<0.004%) due robustness obtained through integration between Artificial Neural Networks with Randomized Hough Transforms.

Oscillation Frequency &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$LC$ &lt;/tex-math&gt; &lt;/inline-formula&gt;-Based Sensor for Characterizing Two-Phase Flows in Energy Systems

The need for two-phase flow measurement in the power generation industry has been significantly increased over the last few years. This is mainly because the reliable measurements of the two-phase flow parameters, such as void fraction, phase velocity, and flow pattern identification, are important for accurate modeling and/or in the operation of energy systems. Although many two-phase flow sensors were recently developed, challenges in measuring two-phase flow characteristics using a simple and inexpensive sensor remain unresolved. Therefore, extensive research efforts that were spent in designing accurate two-phase flow sensor that does not require complex software solving an inverse problem are currently under development worldwide. In this paper, a multichannel, high-resolution capacitance sensor system for two-phase flow void fraction measurements was developed for slightly conductive and non-conductive fluids. Inductance–capacitance ( $LC$ ) metering circuit is designed to relate the change in the measured resonance frequency to the change in capacitance registered by the sensor. The narrow band filtering effect of the $LC$ circuit allows for the system to be more resistant to background noise. Three sensor electrode configurations were designed in order to provide more information on the flow behavior in the piping system, including the bubble velocity, flow distribution, and time signal of void fraction for different flow patterns. Both static and dynamic measurements were carried out, and the sensor operation was validated using a high-speed imaging system.

One-dimensional drift-flux correlation for vertical upward two-phase flow in large size concentric and eccentric annuli

In view of the practical importance of predicting void fraction in two-phase flow, a correlation based on the drift-flux model is often utilized in many industrial fields. To use the drift-flux type correlation, two drift-flux parameters such as distribution parameter and void fraction weighted mean drift velocity should be modeled accurately. Since the drift-flux parameters are susceptible to channel size, they are modeled differently depending on whether the channel size is characterized as small (or mini), medium or large. Although drift-flux type correlations with modeled distribution parameter and drift velocity for medium size channels with various geometries have been well-developed, limited drift-flux correlations are available for large size channels. Two-phase flow characteristics in a large size channel are quite different from those in a medium size channel at low flow conditions. Large slug bubbles observed in a medium size channel do not form in a large size channel due to the presence of surface instability. As large slug bubbles disintegrate into smaller cap bubbles, the drift velocity is increased. Complex two-phase flow characteristics in a large size channel also affect the distribution parameter. This study successfully models the distribution parameter and drift velocity for two-phase flow in a vertical concentric annulus. The drift-flux correlation with modeled distribution parameter and drift velocity is validated by existing data taken in large size annuli. The applicability of the drift-flux correlation to two-phase flow in a vertical eccentric annulus is also examined by existing data.

Numerical simulation method of ultrasonic wave propagation in gas-liquid two-phase flow of deepwater riser

Abstract The ultrasonic transmission monitoring in risers is a promising method for the early detection of gas kick in deepwater drilling. It is difficult to obtain satisfactory results using only theoretical calculation because the ultrasonic wave propagation in the multiphase flow in risers is very complicated. In addition, the key parameters such as bubble shape and void fraction of two-phase flow are hard to be accurately controlled and measured. In view of the above-mentioned facts, this paper aims to analyze the principle and physical model of ultrasonic monitoring of gas kick in risers, and to establish a numerical simulation model based on COMSOL platform. Through an analysis of the numerical simulation results, it was found that the time domain diagram of the received signals contains longitudinal wave, transverse wave, surface wave and coda wave. The cross-correlation function was used to quantitatively analyze the waveform of the received signals under different void fraction conditions. It also suggested that the direct wave was insensitive to the void fraction while the coda wave was sensitive to the void fraction, which was verified through frequency domain analysis. By establishing the experimental system, the attenuation coefficient was used to verify the numerical simulation results. After comparing and analyzing data, this paper argued that the numerical simulation method established in the project can be used as an effective supplement to experiments, and it can be used to calculate and analyze the ultrasonic propagation and attenuation under conditions, such as different bubble sizes and densities, and bubble non-uniform distribution, which can not be accurately and quantitatively measured or controlled in the experiments.

On the relationship between pipe accelerance and the void fraction of internal two-phase flow

This work presents an experimental study on the forced vibration response of horizontal tubes subjected to internal gas–liquid two-phase flow. Vertical and horizontal polarizations of the flexural tube conveying mixtures of air and water are investigated. Firstly, the accelerance frequency response functions FRFv and FRFh, which represent the relationship between the acceleration in the vertical and horizontal axes and the corresponding forces measured in these directions, respectively, were obtained from different two-phase flow conditions in a wide frequency range (1–1600 Hz). Significant changes in the accelerance FRF’s occur with void fraction. As expected, it was observed the increase in the natural frequencies of pipe and the increase of magnitude of the FRF’s as a result of the reduction of the mass of liquid added to the tube with increasing void fraction of two-phase flow. The differences between the FRFv and the FRFh due to the asymmetry of the horizontal two-phase flow are investigated. Finally, the horizontal tube is harmonically excited at specific frequencies determined from the FRF’s. Relationships between the pipe accelerance and the volumetric quality, as well as the relationships between the pipe accelerance and the void fraction of two-phase flow are obtained in the vertical and horizontal directions for different excitation frequencies. The results show that the pipe accelerance measured in the vertical and horizontal directions can be used to determine the void fraction and the volumetric quality of two-phase flow.