Local Void Fraction Distribution Research Articles

An experimental investigation of void fraction characteristics of flow boiling in a 5×5 rod bundle channel

Flow boiling in the rod bundle channel is an important phenomenon in nuclear reactors. The local inhomogeneous void fraction distribution in the flow channel will affect the flow and heat transfer in two phase flow. A 5 × 5 rod bundle test section is designed and utilized in the experiments and a conductivity probe is utilized to measure the local void fraction of two phase flow. A remote-control stepping driving assembly is designed to drive the void probe. The local void fraction characteristics at the outlet of a 5 × 5 rod bundle channel during subcooled boiling and saturated boiling are experimentally studied when the system pressure is 2∼3 MPa. The outlet subcooling ranges from 0 °C to 18 °C. The mass flux is 2000 kg/m2s and the heat flux ranges from 186.3 kW/m2 to 320.8 kW/m2.The effects of thermal parameters on local phase interfacial parameters in subcooled boiling and saturated boiling are obtained. The research results are important for revealing the two phase flow mechanisms in rod bundle channels.

In situ measurements of void fractions and bubble size distributions in bubble curtains

We report the development of a novel measurement system designed to measure bubble properties in bubble curtains (i.e. planar bubble plumes) in situ alongside acoustical measurements. Our approach is based on electrical, contact-based needle sensors in combination with an optical system. The latter is used for calibration and validation purposes. Correcting for the insensitive distance of the needle tips yields very good agreement between the two approaches in terms of the local void fraction and bubble size distributions. Finally, the system is employed to study bubble plumes evolving from three different hose types. All hoses display consistent self-similar behaviour with spreading rates increasing with increasing gas flow. The spreading is further found to be significantly higher when the bubble plumes originated from a porous hose compared to the two other hose types featuring either discrete holes or nozzle elements.

Modeling of void fraction and relative velocity covariance for dispersed bubbly and intermittent flows in horizontal channels

The interfacial drag force governs the momentum exchange between gas and liquid phases and is a key parameter in predicting void fraction using a one-dimensional thermal-analysis code. The interfacial drag force is formulated by the product of the overall drag coefficient and the area-averaged relative velocity between two phases. The area-averaged relative velocity for gas-dispersed two-phase flows has been formulated with the aid of the distribution parameter, which expresses the covariance due to the distributions of void fraction and mixture volumetric flux. It has been pointed out that the term due to void fraction covariance is missing in the formulation of the area-averaged relative velocity adopted in one-dimensional thermal-analysis codes. This study has been conducted to develop void fraction and relative velocity covariance models for gas-dispersed two-phase flows in horizontal pipes. The database of local void fraction distributions for horizontal bubbly flows was established, and an artificial database for intermittent horizontal flows was modeled with assumed distributions of local void fractions. The developed void fraction covariance model for gas-dispersed horizontal two-phase flows could reproduce the data with the mean relative deviation (i.e., bias) of 8.33% and the mean absolute relative deviation (i.e., random uncertainty) of 21.4%. The developed relative velocity covariance model for gas-dispersed horizontal two-phase flows could reproduce the data with the mean relative deviation (i.e., bias) of -0.265% and the mean absolute relative deviation (i.e., random uncertainty) of 7.11%.

Local multiphase flow characteristics of a severe-service control valve

For safety-critical industrial applications, severe-service valves are often used, and the conditions during operations can be either single phase or multiphase. The design requirements for valves handling multiphase flows can be very different to the single-phase flow and depend on the flow regime within valves. The variation in flow conditions during the operation of such valves can have a significant effect on performance, particularly in oil and gas applications where multiphase behaviour can rapidly change within the valve causing unwanted flow conditions. Current practices in designing and sizing such valves are based solely on global phase properties such as pressure drop of the bulk fluid across the valve and overall phase ratio. These do not take into account local flow conditions, as with multiphase fluids, the flow behaviour across the valve becomes more complex. In this work, a well-validated computational fluid dynamics (CFD) model is used to locally and globally quantify the performance characteristics of a severe service valve handling multiphase gas and liquid flow. Such flows are frequently encountered in process equipment found in vital energy industries e.g. process and oil & gas. The CFD model was globally validated with benchmark experiments. Two valve opening positions of 60% and 100% were considered each with 5, 10, and 15% inlet air volume fractions to simulate real life conditions. The results show that while the non-uniformity in pressure field is along expected lines, there is severe non-uniformity in the local air, water and void fraction distributions within the valve trim. To quantify the phase non-uniformities observed, an equation for the distribution parameter was defined and used to calculate its value in each localised quarter within the trim. Phase velocity and void fraction data extracted from the CFD results were also used to obtain relationships for the local void fraction distribution and flow coefficient. The detailed investigation that has been carried out allows for local flow characteristics to be determined and embedded in sizing methodology for severe-service control valve systems with multiphase gas and liquid flow.

CFD simulation focusing on void distribution of subcooled flow boiling in circular tube under rolling condition

A numerical investigation of void fraction distribution of subcooled flow boiling in circular tube under rolling condition has been performed based on the RPI method. In this paper, the rolling motion is considered by UDF (user-defined function) and coupled with CFD code to realize the rolling motion. Numerical simulation is verified by comparing the simulation result with experimental result. The CFD simulation results under both static and rolling conditions agree well with the experimental data. Different working conditions are studied in this paper, and detail analysis is presented to discuss the effect of working conditions on the void distribution. With different rolling periods, it is found that the local void fraction distribution is different. When the rolling period is bigger than 4 s, the local maximum void fraction decreases with the increasing period. When the rolling period is less than 4 s, the local maximum void fraction increases with the increasing period. For the working conditions with different rolling amplitudes, the maximum local void fraction increases with increasing amplitude. Based on the analysis of flow field and phase distribution, the local two-phase flow heat transfer coefficient fluctuates periodically with the rolling motion.

Experimental study of local void fraction distribution in bubble flow in rod assembly

In this work an experimental study of local void fraction distribution in square arrangement rod bundle channel 3 * 3 under bubble flow regime was carried out. Measurements were performed with conductivity probe help, which was introduced into the flow through an angular or side rod at an angle of 15° to the flow. This method made it possible to determine the local void fraction close to the walls of the rods and in the narrow gap between them. Measurements have shown that a dome-shaped profile of local void fraction is realized in the gap between the side rode and the central one, while a saddle-shaped distribution of the void fraction is observed on the line connecting the central and angular rods. Also the effect of the spacer grid on the distribution of the gas phase in the channel was shown.

Experimental study on local interfacial parameters in upward air-water bubbly flow in a vertical 6 × 6 rod bundle

This paper presents a complete database of the local two-phase flow parameters for upward adiabatic air-water two-phase flows in a vertical 6 × 6 rod bundle flow channel. A total of 16 flow conditions were selected and investigated at 16 measuring points in the octant triangular measuring region of a cross section at the position with height-to-diameter ratio of 149 (z/DH = 149) in the rod bundle experiment. The local void fraction, interfacial area concentration (IAC), bubble diameter and bubble velocity vector were measured by using the four-sensor optical probe measuring technique. In the measured cross-sectional distributions of local void fraction and IAC, both core-peaking and wall-peaking distribution shapes were observed and the distribution pattern trended to change from the core-peaking shape to the wall-peaking shape with the increase of superficial liquid velocity (〈jf〉) and the decrease of superficial gas velocity (〈jg〉). The radial distributions of local bubble diameter are nearly flat, which explains the similarity of the radial local void fraction and IAC distributions. The measured local bubble diameters increase with the increase of 〈jg〉 and the decrease of 〈jf〉. The bubbles move along the rod bundle flow channel with their velocity component in the main flow direction (vgz) showing a typical radial core-peaking profile and their velocity components in the cross section showing a significant movement from the central region to the channel box wall region especially under high superficial liquid velocities. The cross-sectional area-averaged results of the local parameters were obtained by a measuring-point-based graphical integration. The obtained area-averaged void fraction and IAC are compared with the predictions of the drift-flux model with its available 2 frequently-used distribution parameter and drift velocity correlations and the 2 recently-developed IAC correlations respectively. The applicability of the 2 drift-flux correlations and the 2 IAC correlations to the rod bundle flow channel is evaluated and analyzed.

Void fraction distribution in a bisectional bubble column reactor

This article presents the distribution of the local void fraction (α) in a mock‐up bisectional bubble column with a diameter of 0.63 m. Owing to the lack of such data in the literature, α was investigated experimentally using a microresistivity (R) probe and a phase discrimination procedure based on the probe signal. The two‐phase mixture that consisted of air and tap water was measured at 342 nodes in the vertical half‐section plane of the column. Relatively small differences between the volume‐integrated local values of α and the measured total gas holdups showed reasonably good agreement under all conditions. Experimental data were used for validation of a bubble column numerical model for a low hydrodynamic regime with commercial computational fluid dynamics (CFD) code. The difference between the CFD calculated total gas holdup and the experimentally measured mean value is 8.8%, while some differences in the local void fraction distributions were found in the lower part of the column. © 2019 American Institute of Chemical Engineers AIChE J, 65: 1186–1197, 2019

Air-water two-phase bubbly flow across 90° vertical elbows Part II: Modeling

Following Part (I) of the current study, which presents the experimental results of the elbow effects on two-phase flow parameters in bubbly flow, Part (II) develops models and correlations to predict the evolution of these parameters across and downstream of 90° vertical-upward and vertical-downward elbows. To quantify the length requires for the effects of the elbows to dissipate (or the dissipation length), the strength of elbows is defined as the variance of the local void fraction distribution. The axial development of the elbow-strength is modeled by an exponential function of the axial development length based on the experimental data. Then, the dissipation length of the elbow is determined by characterizing the evolution of the elbow-strength parameter. The elbow-strength parameter is also used to correlate the void-weighted bubble velocity and covariance terms in the interfacial area transport equation (IATE). The two-phase pressure drop across vertical elbows is modeled with a modified Lockhart-Martinelli correlation which considers the additional pressure drop induced by elbows. To evaluate the above developed models and correlations, they are implemented into the IATE applicable to the elbow-influenced region. The established IATE together with the available IATE of different flow orientations in straight channels are implemented to predict the interfacial area transport from the vertical-upward to horizontal to vertical-downward two-phase flow across elbows. It is found that the interfacial area concentration predictions are in good agreement with the experimental data with an average absolute percent difference of ±6% throughout the test section. The individual contributions to the interfacial area concentration transport due to each source and sink term in the IATE are discussed, demonstrating that the models and correlations developed for the two-phase flow parameters in elbow regions are reliable.

Gas-liquid mixing performance, power consumption, and local void fraction distribution in stirred tank reactors with a rigid-flexile impeller

In this work, the mixing performance of gas–liquid was characterized by the largest Lyapunov exponent (LLE), relative power demand (RPD), power number NP and local void fraction in a rigid-flexible impeller reactor equipped with a tube sparger. The effects of impeller types, gas flow rates, agitation speed, flexible piece lengths, axial spacing distances lh and angles θ were investigated. Meanwhile, global variable analysis (GVA) based on the mixing index (MI) was proposed to characterize the mixing process. The results show that the increases in flexible piece length, agitation speed, and gas flow rate could enhance the mixing degree. Meanwhile the reductions of axial spacing, and angle were also able to improve the mixing performance of gas–liquid two phases to a certain extent. These findings were in conformity with the results of investigation into MI. The results of this study could provide theoretical guidance for the optimal design and engineering scale-up of STRs.

Experimental study on two-phase flow natural circulation in a core catcher cooling channel for EU-APR1400 using air-water system

Ex-vessel core catcher cooling system driven by natural circulation is designed using a full scaled air-water system. A transparent half symmetric section of a core catcher coolant channel of a pressurized water reactor was designed with instrumentations for local void fraction measurement and flow visualization. Two designs of air-water top separator water tanks are studied including one with modified ‘super-step’ design which prevents gas entrainment into down-comer. In the experiment air flow rates are set corresponding to steam generation rate for given corium decay power. Measurements of natural circulation flow rate, spatial local void fraction distribution and re-wetting time near the top wall are carried out for various air flow rates which simulate boiling-induced vapor generation. Since heat transfer and critical heat flux are strongly dependent on the water mass flow rate and development of two-phase flow on the heated wall, knowledge of two-phase flow characteristics in the coolant channel is essential. Results on flow visualization showing two phase flow structure specifically near the high void accumulation regions, local void profiles, rewetting time, and natural circulation flow rate are presented for various air flow rates that simulate corium power levels. The data are useful in assessing the cooling capability of and safety of the core catcher system.

Local characteristic of horizontal air–water two-phase flow by wire-mesh sensor

Two-phase flow widely exists in many industries. Understanding local characteristics of two-phase flow under different flow conditions in piping systems is important to design and optimize the industrial process for higher productivity and lower cost. Air–water two-phase flow experiments were conducted with a 16×16 conductivity wire-mesh sensor (WMS) in a horizontal pipe of a multiphase flow facility. The cross-sectional void fraction time series was analysed by the probability density function (PDF), which described the void fraction fluctuation at different flow conditions. The changes and causes of PDFs during a flow regime transition were analysed. The local structure and flow behaviour were characterized by the local flow spectrum energy analysis and the local void fraction distribution (horizontal, vertical and radial direction) analysis. Finally, three-dimensional transient flow fluctuation energy evolution and characteristic scale distribution based on wavelet analysis of air–water two-phase flow were presented, which revealed the structural features of each phase in two-phase flow.

Experiments in Cap-Bubbly Two-Phase Flows for Two-Group IATE Development

In the two-group interfacial area transport equation (IATE) used to calculate the interfacial area concentration (ai), bubbles are categorized into two groups. Namely, group-I consists of spherical/distorted bubbles, and group-II consists of cap/slug/churn-turbulent bubbles. Robust models for the major bubble interaction mechanisms that cause the transition from purely one-group to two-group flows are essential to the dynamic closure of the two-fluid model with the two-group IATE. Therefore, the present study seeks to establish an experimental database in cap-bubbly flows that highlights this transition to support model development. A four-sensor conductivity probe is used to obtain measurements of local time-averaged two-phase flow parameters, including the void fraction and ai, in vertical-upward air-water two-phase flows in a 5.08-cm pipe. Four flow conditions are investigated at ⟨jf⟩ = 2 m/s with increasing ⟨jg⟩ to study the generation and growth of group-II bubbles. Characteristic features of the local void fraction and ai distributions are discussed. Additionally, axial development of area-averaged void fraction and ai that is indicative of exchange between the bubble groups is presented.

Experimental characterization of vertical gas–liquid pipe flow for annular and liquid loading conditions using dual Wire-Mesh Sensor

In gas well production, liquid is produced in two forms, droplets entrained in the gas core and liquid film flowing on the tubing wall. For most of the gas well life cycle, the predominant flow pattern is annular flow. As gas wells mature, the produced gas flow rate reduces decreasing the liquid carrying capability initiating the condition where the liquid film is unstable and flow pattern changes from fully cocurrent annular flow to partially cocurrent annular flow. The measurement and visualization of annular flow and liquid loading characteristics is of great importance from a technical point of view for process control or from a theoretical point of view for the improvement and validation of current modeling approaches. In this experimental investigation, a Wire-Mesh technique based on conductance measurements was applied to enhance the understanding of the air-water flow in vertical pipes. The flow test section consisting of a 76mm ID pipe, 18m long was employed to generate annular flow and liquid loading at low pressure conditions. A 16×16 wire configuration sensor is used to determine the void fraction within the cross-section of the pipe. Data sets were collected with a sampling frequency of 10,000Hz. Physical flow parameters were extracted based on processed raw measured data obtained by the sensors using signal processing. In this work, the principle of Wire-Mesh Sensors and the methodology of flow parameter extraction are described. From the obtained raw data, time series of void fraction, mean local void fraction distribution, characteristic frequencies and structure velocities are determined for different superficial liquid and gas velocities that ranged from 0.005 to 0.1m/s and from 10 to 40m/s, respectively. In order to investigate dependence of liquid loading phenomenon on viscosity, three different liquid viscosities were used. Results from the Wire-Mesh Sensors are compared with results obtained from previous experimental work using Quick Closing Valves and existing modeling approaches available in the literature.

Influence of impeller diameter on local gas dispersion properties in a sparged multi-impeller stirred tank

Vertical distributions of local void fraction and bubble size in air–water dispersion system were measured with a dual conductivity probe in a fully baffled dished base stirred vessel with the diameter T of 0.48m, holding 0.134m3 liquid. The impeller combination with a six parabolic blade disk turbine below two down-pumping hydrofoil propellers, identified as PDT+2CBY, was used in this study. The effects of the impeller diameter D, ranging from 0.30T to 0.40T (corresponding to D/T from 0.30 to 0.40), on the local void fraction and bubble size were investigated by both experimental and CFD simulation methods. At low superficial gas velocity VS of 0.0077m·s−1, there is no obvious difference in the local void fraction distribution for all systems with different D/T. However, at high superficial gas velocity, the system with a D/T of 0.30 leads to higher local void fraction than systems with other D/T. There is no significant variation in the axial distribution of the Sauter mean bubble size for all the systems with different D/T at the same gas superficial velocity. CFD simulation based on the two-fluid model along with the population balance model (PBM) was used to investigate the effect of the impeller diameter on the gas–liquid flows. The local void fraction predicted by the numerical simulation approach was in reasonable agreement with the experimental data.

Experimental investigation of the effect of 90° standard elbow on horizontal gas–liquid stratified and annular flow characteristics using dual wire-mesh sensors

Fluid flowing through pipelines often encounters fittings such as elbows. Although it is true that two-phase flow patterns observed in elbows are qualitatively the same as those seen in straight pipes, the presence of a pipe elbow can modify relative positions and local velocities of the two phases as they are subjected to forces in addition to those encountered in a straight pipe. That redistribution can affect pressure drop values, chemical inhibitor concentration and distribution to the top of the pipe, as well as the erosion pattern occurring from solid particles such as sand that is entrained in oil and gas transportation pipelines. In this work, a wire-mesh sensor technique based on conductance measurements of void fraction was applied to investigate two-phase pipe flow through a standard elbow. The horizontal flow test section, consisting of a 76.2mm ID, 18m long pipe, was employed to generate stratified-wavy and annular flow conditions. Two 16×16 wire-mesh configuration sensors were positioned either 0.9m upstream or 0.6m downstream of the bend. The experiments were conducted at superficial liquid velocities equal to 0.03m/s and 0.2m/s and superficial gas velocities that ranged from 9m/s to 34m/s. The effects of liquid viscosity on the measured parameters are also investigated using two different viscosities of 1and 10cP. Stratified–slug transition, stratified wavy and annular flow patterns were observed visually in the clear section placed upstream of the wire-mesh sensors. Analysis of time series void fraction data from the dual wire-mesh sensors allows the determination of mean void fraction, local time average void fraction distribution, liquid phase distribution around the tube periphery, interfacial structure velocities, as well as probability density function characteristic signatures within the cross-section of pipe before and after the elbow. The results indicate that the distribution of gas and liquid phases and interfacial velocities are significantly altered even 20 diameters downstream of the elbow.

Characterization of the dissipation of elbow effects in bubbly two-phase flows

This study develops a method to characterize the elbow-region for vertical-upward-to-horizontal air–water bubbly two-phase flows through a 90° elbow with a non-dimensional centerline radius of curvature, RC/D, of three. The dissipation of the elbow-effects in the horizontal region downstream of the elbow is characterized by the change in the local void fraction distribution. To quantify the dissipation characteristics, the elbow-strength, S, is defined as the inverse of the second moment of the local void fraction distribution. It is found that the elbow-strength decreases exponentially with increasing development length in the elbow-region. The slope of decrease of the elbow-strength in the elbow-region is determined by the dissipation parameter, β. The dissipation parameter determines the development length required for the elbow-effects on the two-phase flow parameters to become negligible. It is found that the dissipation parameter is a linear function of the mixture Reynolds number for the flow conditions investigated in the present study.

Experimental study and numerical simulation of local void fraction in cold-gassed and hot-sparged stirred reactors

Local void fraction distributions of different superficial gas velocities at 24, 54, 68, and 81°C were measured with an electric conductivity probe in a fully baffled, dished base stirred vessel of 0.48m diameter holding 0.145m3 of liquid. A 6-elliptical-blade disk turbine (HEDT) was used as the bottom impeller to disperse the incoming gas, with two down pumping 4-blade hydrofoil impeller (WHD) above. Results showed that the averaged local void fraction at of 81°C was about 20% lower than that at ambient temperature, though the profiles were similar in both cases. The local void fraction increased with increasing superficial gas velocity. The standard Eulerian–Eulerian formulation of the k–ε turbulence model with multiple frames of the reference (MFR) was used in the simulation. The population balance model (PBM) combined with MUSIG model was implemented in the commercial CFX code. The simulated local void fraction profiles at N=4, 6, and 8s−1 at the same superficial velocity of Vs=0.0156m/s and at the location of r/R=0.85 are in reasonable agreement with the measurements.

B121 4x4ロッドバンドル内気液二相気泡流に関する研究(OS9 熱・流動)

Although area-averaged void fractions in subchannels have been measured frequently in literatures, distribution of local void fraction in a rod bundle have been rarely reported. Little is also shown on bubble motion in a rod bundle. Distributions of local void fraction and bubble velocity in a 4x4 rod bundle were, therefore, measured using a conductance probe in this study. Liquid velocity was also measured using LDV. Experimental results showed that the distributions of the void fractions in the inner and side subchannels depend not only on lift force acting on bubbles but also on geometrical interaction between the bubble and the rod wall. The relative velocity between bubbles and liquid phase in the subchannel possesses non-uniform distribution over the cross-section. This is caused by increase in drag force acting on bubbles due to bubble deformation induced by interaction between the bubbles and the rod wall. (author)

Local void fraction and bubble size distributions in cold-gassed and hot-sparged stirred reactors

Vertical distributions of local void fraction, bubble size and gas–liquid interfacial area in air–water dispersions at 24 and 81 °C have been measured with a dual electric conductivity probe in a fully baffled dished base stirred vessel of 0.48 m diameter holding 0.145 m 3 liquid. The agitator was a hollow blade dispersing turbine below two up-pumping hydrofoils. The vertical distribution of the void fraction in the hot conditions is similar to that at ambient temperature though the void fraction is significantly lower in the hot system. The vertical distributions of bubble size show maxima with large bubbles above the bottom impeller, near the top impeller and close to the free surface. With given operating conditions, the overall Sauter means bubble size in the hot systems appears to be about 21% greater than when cold. Estimates of the local interfacial area show a maximum just above the level of the top impeller.