Variation Of Void Fraction Research Articles

A New Capacitance Sensor for Measuring the Void Fraction of Two-Phase Flow Through Tube Bundles.

Evaluating the two-phase flow parameters across tube bundles is crucial to the analysis of vibration excitation mechanisms. These parameters include the temporal and local variation of void fraction and phase redistribution. Understanding these two-phase parameters is essential to evaluating the stability threshold of tube bundle configurations. In this work, capacitance sensor probes were designed using finite element analysis to ensure high sensor sensitivity and optimum response. A simulation-based approach was used to calibrate and increase the accuracy of the void fraction measurement. The simulation results were used to scale the normalized capacitance and minimize the sensor uncertainty to ±5%. The sensor and required conditioning circuits were fabricated and tested for measuring the instantaneous void fraction in a horizontal triangular tube bundle array under both static and dynamic two-phase flow conditions. The static calibration of the sensor was able to reduce the uncertainty to ±3% while the sensor conditioning circuit was able to capture instantaneous void fraction signals with frequencies up to 2.5 kHz.

Analysis of dense phase pneumatic conveying of fly ash using CFD including particle size distribution

In this paper, the flow of fly ash in fluidized dense phase pneumatic conveying through a section of pipeline having uniform pipe diameter has been modeled and analyzed using Computational Fluid Dynamics (CFD). Besides considering a single solid phase with the mean particle diameter, several solid phases based on the particle size distribution (PSD) have been incorporated in the developed CFD model. For the CFD simulation, particle size distribution with the Euler–Euler approach is adopted for analyzing the behavior of the flow of fly ash in the dense phase. The validity of the developed CFD model has been evaluated under different operating parameters of pneumatically fly ash flow in the horizontal pipeline. The results of the simulation demonstrated an excellent matching with experimental pressure drop data with an error margin of ±15%. Furthermore, variations of pressure drop, air velocity and void fraction along downstream of flow, and distributions of solid volume and void fractions spatial profiles have been presented and discussed. Then, the flow regimes have been investigated under the effects of conveying velocity, solids loading ratio and PSD. The numerical observation showed that the dense regime is mainly dominated by the behavior of large-sized particles while the fine particles control the dilute regime and there is intense contact between these two regimes through the intermediate regime.

Experimental study of vibration effects on local interfacial parameters in boiling flow

In this paper, the subcooled boiling flow experiments in a vertical annulus were conducted to study the effect of vibration on local interfacial characteristics. The local interfacial parameters including void fraction, bubble velocity, interfacial area concentration and Sauter mean diameter were measured by four-sensor conductivity probe. The time-averaged local parameters under vibration conditions were firstly measured and analyzed. It is found that the effects of vibration on time-averaged void fraction and interfacial area concentration are very limited. Therefore, we have developed the phase partitioning method to obtain the instantaneous variation of void fraction for further investigation. The instantaneous void fraction along the radial direction varies periodically with the vibration phase. The additional forces induced by vibration are analyzed and compared with each other. Among them, the tangential force is the dominant, which can account for the change tendency of void fraction under vibration. Besides, the variations of void fraction depend on different working conditions. The change tendencies of void fraction near the inner wall and outer wall are opposite in relatively low averaged void fraction regime while those in relatively high averaged void fraction regime are consistent.

Annular and spiral bubble nets: A simulation-focused analysis of humpback whale feeding strategies

Humpback whales can generate bubbly regions (called bubble nets) via their blowholes, which they appear to exploit via loud vocalizations for feeding purposes. We model this phenomenon as the acoustic excitation of an dilute bubble net of radially varying void fraction. A fully coupled phase-averaging approach is used to compute the bubble response and corresponding acoustics. We first assess the possibility of a sophisticated wave-guidance behavior of high-frequency whale vocalizations within the bubble net. For a small range of flow parameters, the reflections associated with the bubbly region result in an observable wave-guidance behavior, though even then these reflections disperse rapidly. In light of this, we also consider multiple whales surrounding the bubble net, each vocalizing towards its center. We show that for physically realistic configurations, including variations in the bubble net void fraction and number of whales, the bubble net can keep its core region substantially quieter than the exterior. Finally, we investigate the ability of spiral, rather than annular, geometries for keeping the bubble-free region quiet.

Experimental study on transient flow fluctuation characteristics in a 3 × 3 rod bundle under rolling condition

Abstract Experimental investigation on adiabatic transient flow fluctuation characteristics in a 3 × 3 rod bundles subjected to rolling motion in normal pressure and temperature was carried out. Single-phase flow as well as gas parameters under stagnant liquid condition were concerned. Ranges of liquid Reynolds number and gas flow rate were 657–3 328 and 0.079–0.2 m3/h, respectively. Rolling period and amplitude range from 8 s to 16 s and from 5° to 15°, respectively. Fluctuation amplitudes of transient parameters such as flow rate, pressure drop, void fraction and gas rising velocity were focused on this study. The results indicate that flow rate and frictional pressure drop present obvious fluctuations after rolling motion, and the fluctuation amplitudes decrease with the decrease of bypass valve opening. Flow rate reaches to peak at the positive rolling amplitude and it reaches to trough at the negative rolling amplitude, and there is no obvious phase difference between the fluctuation and rolling angle. Periodical fluctuation of gas flow rate was weakened with the increase of gas inlet pressure and valve opening under experimental conditions, while the variations of void fraction and gas rising velocity were limited. Transient parameter fluctuations correspond to the rolling period at low gas flow rate, whereas their periodical fluctuations are inconspicuous in relative high gas flow rate. Amplitudes of void fraction and gas rising velocity increase with increasing the rolling amplitude while the amplitudes of void fraction and gas rising velocity are nearly invariable with the rolling period no matter if gas flow rate oscillates.

Experimental and computational investigation on two-phase flow and heat transfer of highly subcooled flow boiling in vertical upflow

This study explores subcooled nucleate flow boiling of FC-72 in a rectangular channel having two opposite heating walls at different mass velocities and wall heat fluxes ranging from 42% to 45% of CHF. Experiments are performed in which constant and equal heat fluxes are supplied from the heating walls, with local wall temperatures measured at several axial locations and used to determine axial variations of local heat transfer coefficient. Additionally, detailed information on flow pattern and phase distribution is captured by high-speed video through the channel’s transparent sidewalls. Experimental values for key fluid flow and thermal parameters are compared to predictions of 2D computational simulations using ANSYS FLUENT. The computations are based on the multi-phase volume of fluid (VOF) model combined with an appropriate phase change model, and account for conjugate heat transfer along the heating walls. Computed transverse profiles and stream-wise variations of void fraction, flow velocity, and wall temperature are examined. The computed results show reasonable agreement with experimentally captured interfacial behavior as well as heat transfer parameters. Computed cross-sectional temperature profiles for the mixture show non-equilibrium subcooled boiling with steep temperature gradients near the heating walls and low temperature liquid in the core region. This study is concluded by exploring limitations of the 2D model used.

Numerical investigation of the effect of inlet subcooling on flow instabilities in a parallel channel natural circulation boiling system

Some of the modern reactor concepts are based on natural circulation boiling systems. These systems are prone to flow instabilities and parallel channel systems can exhibit peculiar types of flow oscillations. In the present work, the existence and the nature of instabilities in a parallel channel natural circulation test facility (PCNCTF) has been investigated through numerical simulations using the system code RELAP5/MOD3.2. The numerical model is validated using experimental data of PCNCTF. The effect of inlet subcooling on flow oscillations in parallel channels is studied for different system pressure and input powers. Time variations of mass flow rate and void fraction in the channels are examined and the nature of oscillations is analyzed using phase portraits and Fourier power spectra. It is found that there exists a particular range of inlet subcoolings (at a fixed system pressure and input power), above or below which instabilities occur. Mixed mode of oscillations at low degrees of inlet subcoolings and superimposed density wave oscillations are observed at high inlet subcoolings.

Frictional Drag Reduction: Review and Numerical Simulation of Microbubble Drag Reduction in a Channel Flow

The reduction of ship’s resistance is one of the most effective way to reduce emissions, operating costs and to improve EEDI. It is reported that, for slow moving vessels, the frictional drag accounts for as much as 80% of the total drag, thus there is a strong demand for the reduction in the frictional drag. The use of air as a lubricant, known as Micro Bubble Drag Reduction, to reduce that frictional drag is an active research topic. The main focus of authors is to present the current scenario of research carried out worldwide along with numerical simulation of air injection in a rectangular channel. Latest developments in this field suggests that, there is a potential reduction of 80% & 30% reduction in frictional drag in case of flat plates and ships respectively. Review suggests that, MBDR depends on Gas or Air Diffusion which depends on, Bubble size distributions and coalescence and surface tension of liquid, which in turn depends on salinity of water, void fraction, location of injection points, depth of water in which bubbles are injected. Authors are of opinion that, Microbubbles affect the performance of Propeller, which in turn decides net savings in power considering power required to inject Microbubbles. Moreover, 3D numerical investigations into frictional drag reduction by microbubbles were carried out in Star CCM+ on a channel for different flow velocities, different void fraction and for different cross sections of flow at the injection point. This study is the first of its kind in which, variation of coefficient of friction both in longitudinal as well as spanwise direction were studied along with actual localised variation of void fraction at these points. From the study, it is concluded that, since it is a channel flow and as the flow is restricted in confined region, effect of air injection is limited to smaller area in spanwise direction as bubbles were not escaping in spanwise direction.

Void fraction measurement using concave capacitor based sensor – Analytical and experimental evaluation

Two-phase flow in pipelines is very common in the oil and gas industries. Void fraction is one of the important parameters in two-phase flow. Measurement of void fraction plays an important role in various areas of industries. Two types of measurement techniques are commonly used in electrical methods for macroscale range- variable capacitance type and variable conductance type. A comparatively low cost and non-invasive design of the concave capacitive sensor is analytically evaluated and experimentally validated to measure the void fraction inside the non-metallic pipe. The design is effective to reduce the fringing effect. The electrical model of the concave capacitance sensor is analyzed, which involves all the parasitic capacitances along with the cable capacitances. The capacitance of the sensor varies for the whole range of void fraction (0–100%). If the two phases are distilled water and air, the capacitance value varies from 94.0134 pF to 3.4377 pF experimentally for void fraction variation of 0–100%. Uncertainty analysis for the proposed sensor design along with the uncertainty budget is presented, for the whole range of measurement. The standard uncertainty is less than 0.97 pF for the whole range of measurement. The procedure for analysis is also discussed in this paper.

Pressure drop, void fraction and wave behavior in two-phase non-Newtonian churn flow

Many commonly used industrial fluids display non-Newtonian effects. Their rheology exerts a strong influence on flow structure, interface fluctuation, pressure drop, heat transfer and many other flow characteristics. Owing to the complexity of churn flow, knowledge of the flow characteristics with non-Newtonian fluids is not well documented in existing literature. In the present study, we employ the power-law model to describe the non-Newtonian fluid behavior and establish an analytical model to predict pressure gradient, void fraction and wave behavior in churn flow. One churn flow unit is carefully divided into two parts (the falling film region and the wave region) and analyzed separately. The results indicate that liquid viscosity significantly affects the variations of pressure gradient, void fraction, velocity profiles, film thickness and wave behavior. These findings will provide insight into the effect of viscosity on flow structures and benefit a better understanding of the non-Newtonian churn flow.

Experimental investigation of void fraction variation in subcooled boiling flow under horizontal forced vibrations

An experimental investigation of horizontal forced vibration effect on void fraction variation of subcooled boiling flow was carried out in this study. In order to simulate the fuel assembly subchannel of a boiling water reactor (BWR), an annular test section with inner and our diameters of 19.1 and 38.1mm was utilized for subcooled boiling tests under atmospheric pressure. The annular test section was attached to an eccentric-cam vibrator, which was driven by a low-speed motor and can produce horizontal forced vibrations with frequency up to 20Hz and maximum displacement of 22.2mm. The inlet liquid velocity and subcooling were set as vf,in=0.25–1.00m/s and ΔTSub=5–20°C. Different heat fluxes of q″=0.058–0.193MW/m2 were loaded through the center heater rod, and the void fraction and fluid temperature were measured during the tests under stationary (no vibration) and vibration conditions. Test results show that in the subcooled boiling region, the void fraction and fluid temperature can vary under horizontal forced vibrations, and the variation trends were presented in NZu-NSub and vf,in-〈α〉 plots. These variations can be explained by the potential changes of thermal boundary layers (TBL) and the heat transfer enhancement under vibrations. In addition, no significant change of void fraction and fluid temperature was found in or near the saturated boiling conditions under vibrations.

Equilibrium and non‐equilibrium gas–liquid two phase flow in long and short pipelines following a rupture

The two‐phase flow following the blowdown of pipeline carrying flashing liquid is numerically investigated by using thermodynamic equilibrium and non‐equilibrium models. Model equations are solved numerically by the finite volume method. The values of fluxes at cell boundaries are obtained by AUSM+‐up. To obtain proper values for the coefficients of dissipation, both single phase liquid and two phase shock tube problems are investigated. The transient release from the pressurized pipeline is studied for two cases of long and short pipes. Comparison of the predictions against experimental data reveals non‐equilibrium model performs a little better than equilibrium model in the prediction of temporal variations of pressure and void fraction of the long pipe. However, equilibrium model totally overestimates pressure and void fraction of the short pipe. The relative error of equilibrium model in the prediction of pressure variation with time exceeds 50% and it is 20% for non‐equilibrium model. © 2017 American Institute of Chemical EngineersAIChE J, 63: 3214–3223, 2017

Development of three-dimensional thermal-hydraulic analysis code for steam generator with two-fluid model and porous media approach

Steam generator (SG) is one of the major components in nuclear power plants, thus, it is crucial to analyze its thermal-hydraulic characteristics for the economical and safe operation. Various analytical codes have formulated for the research of the secondary side of SG. However, due to its complex geometry, the previous analytical codes have certain limitations in accurately predicting the thermal-hydraulic characteristics at secondary side. In this paper, a specialized code named THAC-SG (Thermal-hydraulic Analysis Code-Steam Generator) has been developed after summarizing main feature of widely-used steam generator three-dimensional thermal-hydraulics calculation codes. The finite volume method is used for the computation of two-phase flow and natural circulation at secondary side, and the complex structure was simulated by porous-media model. This paper takes the SG of Guangdong Daya Bay Nuclear Power Station (GNPS) as the verification model. The code can simulate the dynamically varying void fraction, pressure and temperature in the secondary side. The accuracy and reliability of this code has been validated and can be used for thermal-hydraulic analysis of steam generator under various operating conditions.

Experimental Study on Interfacial Area Transport of Two-Phase Flow under Vibration Conditions

An experimental study on air-water two-phase flow under vibration condition has been conducted using double-sensor conductivity probe. The test section is an annular geometry with hydraulic diameter of 19.1 mm. The vibration frequency ranges from 0.47 Hz to 2.47 Hz. Local measurements of void fraction, interfacial area concentration (IAC), and Sauter mean diameter have been performed along one radius in the vibration direction. The result shows that local parameters fluctuate continuously around the base values in the vibration cycle. Additional bubble force due to inertia is used to explain lateral bubble motions. The fluctuation amplitudes of local void fraction and IAC increase significantly with vibration frequency. The radial distribution of local parameters at the maximum vibration displacement is specifically analyzed. In the void fraction and IAC profiles, the peak near the inner wall is weakened or even disappearing and a strong peak skewed to outer wall is gradually observed with the increase of vibration frequency. The nondimensional peak void fraction can reach a maximum of 49% and the mean relative variation of local void fraction can increase to more than 29% as the vibration frequency increases to 2.47 Hz. But the increase of vibration frequency does not bring significant change to bubble diameter.

Influence of pipeline modeling in stability analysis for severe slugging

In this paper a numerical linear stability analysis is performed to a mathematical model for the two-phase flow in a pipeline-riser system. Most of dynamic models and stability criteria are based on a simplified lumped parameter pipeline, where it is assumed that the void fraction variations can be neglected. As a pipeline with a constant void fraction is not able to describe the flow pattern transition or void fraction propagation waves, the choice of pipeline model influences the stability map. Three different models for the pipeline are considered: (a) A lumped parameter model with constant void fraction; (b) A lumped parameter model with time dependent void fraction; (c) A distributed parameter model, with void fraction dependent on time and position.The results show that the constant void fraction model overestimates the unstable region, pushing the near horizontal branch of the stability boundary beyond the stratified-intermittent transition boundary for high buffer lengths and invalidating the model assumptions. The distributed parameter model is capable of describing void fraction waves along the pipeline under the restriction of a stratified flow pattern, with an additional stabilizing effect, being less sensitive to high buffer lengths and not crossing the stratified-intermittent transition in the nearly horizontal branch. The lumped parameter model with variable void fraction behaves close to the distributed model, but predicting bigger unstable regions.The behavior of the different pipeline models is explained based on the basic assumptions that led to many semi-empirical stability criteria. It is concluded that a distributed parameter model is recommended, with a void fraction correlation that includes the transition from stratified to intermittent flow pattern taken into account in the problem statement.

Modeling of Polyolefin Polymerization in Semibatch Slurry Reactors: Experiments and Simulations

In this work, a well‐known single‐particle model (the multigrain model) is applied to simulate size growth and morphology evolution of polyethylene particles obtained through a slurry phase, catalytic polymerization in semibatch reactor. The model explicitly accounts for diffusion limitations within the polymer particle and between the particle and the solvent, as well as void fraction variations due to the particle growth. Catalyst behavior is described assuming a two‐site model, a good compromise between accuracy and simplicity, along with a conventional kinetic scheme, whose kinetic parameters are determined by fitting the model predictions to the experimental data. Polymer molecular weight is evaluated from population balances, solved through the method of moments. The semibatch reactor is modeled by means of fundamental mass conservation equations. Simulations show that the proposed model is able to describe not only the polymer properties of interest but also the evolution of the average particle size, thus providing a comprehensive overview of the system behavior. The model is further validated through regression‐free simulation of additional sets of experimental data, including materials with a bimodal molar mass distribution: the good agreement also found in this case confirms the robustness of the model and the estimated parameter values.image

Experimental and computational investigation of vertical upflow condensation in a circular tube

This study explores the condensation of FC-72 in vertical upflow both experimentally and computationally. An axisymmetric 2-D computational model is used to predict variations of void fraction, condensation heat transfer coefficient, wall temperature and temperature profile across the liquid film. The computed results are shown to effectively capture the observed complex flow characteristics during flooding and climbing film conditions, including the annular film’s interfacial waviness, formation of liquid ligaments along the film’s interface, and breakup of liquid masses from these ligaments that are either re-deposited onto the film or entrained in the vapor core before moving towards the centerline. The model also shows good agreement with measured spatially averaged condensation heat transfer coefficients and wall temperatures. The predicted temperature profiles across the flow area successfully capture an appreciable temperature gradient at the liquid–vapor interface and saturation temperature in the vapor core.

Experimental and Numerical Investigation of Combined Sensible/Latent Thermal Energy Storage for High-Temperature Applications.

Combined sensible/latent heat storage allows the heat-transfer fluid outflow temperature during discharging to be stabilized. A lab-scale combined storage consisting of a packed bed of rocks and steel-encapsulated AlSi(12) was investigated experimentally and numerically. Due to the small tank-to-particle diameter ratio of the lab-scale storage, void-fraction variations were not negligible, leading to channeling effects that cannot be resolved in 1D heat-transfer models. The void-fraction variations and channeling effects can be resolved in 2D models of the flow and heat transfer in the storage. The resulting so-called bypass fraction extracted from the 2D model was used in the 1D model and led to good agreement with experimental measurements.

Experimental and numerical investigation of combined sensible–latent heat for thermal energy storage at 575 °C and above

The design, testing, and modelling of a high-temperature thermocline-type thermal energy storage (TES) are presented. The TES concept uses air as the heat-transfer fluid and combines sensible and latent heat for stabilizing the discharging outflow air temperature. A 42kWhth lab-scale prototype of 40cm diameter was fabricated, containing a 9cm high layer of encapsulated phase change material (AlSi12) on top of a 127cm high packed bed of sedimentary rocks with a mean diameter of about 3cm. A two-phase transient heat transfer model of the thermal storage cycle was numerically formulated and experimentally validated with measured thermoclines during charging and discharging obtained with the lab-scale prototype. The thermal inertia of the experimental setup and the radial variation of void fraction due to the small tank-to-particle diameter ratio affected the validation process. The outflow air temperature during discharging was stabilized around the melting temperature of AlSi12 of 575°C. The thermal losses stayed below 3.5% of the input energy for all the experimental runs.

Interaction between free-surface, two-phase flow and total pressure in hydraulic jump

A hydraulic jump is characterised by intense turbulent flow patterns and substantial flow aeration. The flow turbulence, at both macroscopic and microscopic scales, interacts with the air entrainment process and the free-surface. A series of simultaneous measurements of the free-surface fluctuations, jump toe oscillations, void fraction and total pressure variations allowed for an investigation of the interactions between these characteristics. Experiments were conducted for a range of Froude numbers from 3.8 to 8.5. The total pressure measurements were justified for the air–water flow characterisation of the flow region with a positive time-averaged velocity. The interactions between roller surface deformation, air entrainment and diffusion, velocity variation, flow bulking, and the associated total pressure field modulation highlighted different flow regions, hence flow patterns, in the roller. The jump toe oscillation was found closely linked to the air entrapment at the toe and velocity variation in the shear flow. The instable total pressure distribution was primarily associated with the free-surface fluctuation for the bubbly roller region and with the velocity re-distribution for the lower shear region underneath. The present work provides new information on the physical characteristics of hydraulic jumps and a comprehensive insight into the nature of such complex turbulent two-phase flow.