Void Fraction Change Research Articles

Buoyancy Driven Flow with Gas-Liquid Coatings of Peristaltic Bubbly Flow in Elastic Walls

In this paper, liquid coatings of bubbly flow with peristaltic motion inside elastic walls is investigated. The proposed model is constructed using the two-fluid approach with the most distinctive collaboration among gas, fluid, pressure, and drag forces. Variation in pressure leads to a change in void fraction. The differential controlling conditions affected by the long wavelength of the peristaltic wave and the slow movement are taken into account. Analytical results of the simplified governing equations are obtained using the homotopy perturbation method (HPM). The features of the significant parameters are shown and examined graphically.

Laser Thermal Gradient Testing and Fracture Mechanics Study of a Thermal Barrier Coating

It is critical for thermal barrier coating (TBC) development that a testing method be used to understand the potential and limitation of a coating’s durability and integrity under gas turbine engine operating conditions. To this end, a TBC-coated button is tested using a laser high-heat flux facility. The ceramic coating is ZrO2-8 wt.% Y2O3 applied via the air plasma spraying process on top of a NiCoCrAlY bond coating and an Inconel alloy 617 substrate button of 25.4 mm diameter. The coated button is subject to precisely controlled laser heating on the top side (1150 °C) and a temperature gradient of 63.9 °C/mm through the button overall thickness. The coated button lasts 160.9 h or 570 cycles of laser heating. The void fraction change before and after the test, the thermal conductivity change during the laser test and the failure assessment are presented. After the test, significant horizontal cracks exist in the top coating close to the thermally grown oxide (TGO) layer and near the button center. Based on the cracks and the TGO layer geometry, the stress intensity factor and strain energy release rate are computed. The combined experimental and computational approach can lead to a TBC lifetime model.

Addressing Two-Phase Flow Maldistribution in Microchannel Heat and Mass Exchangers

Multiphase flow phenomena in single micro and minichannels have been widely studied. Characteristics of two-phase flow through a large array of microchannels are investigated here. An air–water mixture is used to represent the two phases flowing through a microchannel array representative of those employed in practical applications. Flow distribution of the air and water flow across 52 parallel microchannels of 0.4 mm hydraulic diameter is visually investigated using high-speed photography. Two microchannel configurations are studied and compared, with mixing features incorporated into the second configuration. Slug and annular flow regimes are observed in the channels. Void fractions and interfacial areas are calculated for each channel from these observations. The flow distribution is tracked at various lengths along the microchannel array sheets. Statistical distributions of void fraction and interfacial area along the microchannel array are measured. The design with mixing features yields improved flow distribution. Void fraction and interfacial area change along the length of the second configuration, indicating a change in fluid distribution among the channels. The void fraction and interfacial area results are used to predict the performance of different microchannel array configurations for heat and mass transfer applications. Results from this study can help inform the design of compact thermal-fluid energy systems.

Assessment of the CUPID Code for Bubbly Flows in Horizontal Pipes

Two-phase flow in a horizontal pipe has a pronounced feature; that is, two-phase-flow parameters are highly nonsymmetric because gravity is perpendicular to the mean flow direction. Thus, three-dimensional analysis is necessary for the accurate prediction of two-phase flow in a horizontal pipe, such as the hot leg and cold leg of a pressurized water reactor and the pressure tubes in a CANDU reactor. In this study, we simulated bubbly flows in horizontal pipes using the CUPID code, which adopts a two-fluid, three-field model for two-phase flow. In the preliminary calculations, it was found that the particle-averaged two-fluid momentum equation, rather than the standard two-fluid momentum equation, predicts a physically reasonable slip ratio and nondrag forces, except turbulent dispersion forces have negligible effects on the radial void distribution when the particle-averaged two-fluid momentum equation is used. Based on the results, we selected the physical models and computational mesh for subsequent code assessment using various bubbly flow experiments in horizontal pipes. The turbulent dispersion force model was improved to take into account the large void fraction change at the top. The results of the code assessment show good predictions for the axial pressure drop, liquid velocity, and turbulent kinetic energy profile and predict reasonably well the effects of jl and jg on two-phase-flow parameters. However, additional studies are needed for more accurate prediction of the nonsymmetric distribution of gas velocity and turbulent kinetic energy.

Comparison of void fraction change and local shear stress fluctuation in channel flow

Comparison of void fraction change and local shear stress fluctuation in channel flow

Unified constitutive model with consideration for effects of porosity and its application to polyurethane foam

Abstract Polyurethane foam (PUF) is a popular insulation material and has crucial mechanical characteristics under compression in the principal loading direction, including nonlinear behavior dependent on strain rate and temperature, degradation of elastic modulus, and decrease of void fraction. Additionally, its insulation performance relies on the blowing agents in voids. These material characteristics enable the evaluation of the structural integrity and performance of PUF insulation systems, if the prediction of these characteristics and the relationship between change in void fraction and insulation performance are possible. In this paper, a unified constitutive model was proposed for this purpose. This model was verified with corresponding experimental results, and the validation showed that the material characteristics can be depicted successfully. Moreover, for the sake of applicability, the model was used in the evaluation of the insulation system in an LNG Carrier.

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.

Investigation on two-phase distribution in a vibrating annulus

Seismic vibration can induce additional interphase forces acting on bubbles and change flow characteristics in two-phase flow system. Local measurements of air-water two-phase flow parameters, namely void fraction, interfacial area concentration and Sauter mean diameter, were conducted in a vibrating annulus. The vibration acceleration ranged from 0.05g to 0.24g. Nine inlet flow conditions were selected, which covered bubbly and slug flows. No significant difference was observed in the time-averaged flow parameters between reference and vibration experiments. During vibration cycle, the radial two-phase flow distribution varied continuously with vibration phase. The change in non-dimensional line-averaged void fraction decreased with the increase of void fraction. As the vibration amplitude increased from 3.04mm to 10.08mm, the vibration effects on variations of local flow parameters were greatly enhanced with the mean relative variation (MRV) of void faction reached up to 55%. The study on MRV of local parameters showed that the vibration effects were strongly related to specific flow conditions. With the increase of superficial gas velocity and void fraction, the vibration effects were reduced and the MRV of void fraction decreased from 55% to 18%.

Experimental investigation of horizontal forced-vibration effect on air-water two-phase flow

In order to investigate the potential seismic vibrations effect on two-phase flow in an annular channel, experimental tests with air-water two-phase flow under horizontal vibrations were carried out. A low-speed eccentric-cam vibration module capable of operating at motor speed of 45–1200rpm (f = 0.75–20Hz) was attached to an annular channel, which was scaled down from a prototypic BWR fuel sub-channel with inner and outer diameters of 19.1mm and 38.1mm, respectively. The two-phase flow was operated in the ranges of 〈jf〉 = 0.25–1.00m/s and 〈jg〉 = 0.03–1.46m/s with 27 flow conditions, and the vibration amplitudes controlled by cam eccentricity (E) were designed for the range of 0.8–22.2mm. Ring-type impedance void meters were utilized to detect the area-averaged time-averaged void fraction under stationary and vibration conditions. A systematic experimental database was built and analyzed with effective maps in terms of flow conditions (〈jg〉-〈jf〉) and vibration conditions (E-f and f-a), and the potential effects were expressed by regions on the maps. In the 〈jg〉-〈jf〉 maps, the void fraction was found to potentially decrease under vibrations in bubbly flow regime and relatively lower liquid flow conditions, which may be explained by the increase of distribution parameter. Whereas and the void fraction may increase at the region closed to bubbly-to-slug transition boundary under vibrations, which may be explained by the changes of drift velocity due to flow regime change from bubbly to slug flows. No significant change in void fraction was found in slug flow regime under the present test conditions.

Gas-liquid bubbly flow structure in a vertical large-diameter square duct

An experimental study was performed on the local structure of atmospheric upward-moving air–water two-phase flow in a vertical large-diameter square duct with a cross-section of 100 mm × 100 mm and a height of 3 m by using a four-sensor probe at the axial position of z/DH = 29.2. The flow regime was simultaneously observed by a high speed camera at z/DH = 12.5. The four-sensor probe method classifying spherical and non-spherical bubbles was applied as a key measurement way to obtain local parameters such as 3-D bubble velocity vector, bubble diameter and interfacial area concentration, which are necessary for people to know the internal flow structures as well as local void fraction change. The local measurements were performed at 66 points within an octant symmetric triangular area of the cross-section, which agreed well with area-averaged void fractions from differential pressure gauges and the superficial gas velocity from air flow meters. Both the local void fraction and interfacial area concentration indicated radial core-peak and wall-peak distributions at low and high liquid flow rates respectively. Main flow component of the bubble velocity vector showed a power-law distribution shape with its center peak increasing with the liquid flow rate. The 2 components of the bubble velocity vector in the cross-section revealed that there exists a rotating secondary flow in the octant symmetric triangular area and the magnitude of the rotating secondary flow increases with the liquid flow rate. Some of constitutive correlations of drift-flux model for rectangular flow channels and large-diameter circular flow channels and interfacial area concentration correlations for small-diameter and large-diameter circular flow channels are selected and reviewed to study their predictabilities against the present experimental data. The comparison study shows that the drift-flux correlations by Hibiki and Ishii (2003) reasonably predict void fraction and the interfacial-area-concentration correlations by Hibiki and Ishii (2002) and Shen and Hibiki (2015) can be recommended for the prediction of interfacial area concentration in the bubbly flows in large-diameter square ducts.

Identification of two-phase flow patterns in minichannel based on RQA and PCA analysis

In the paper it has been shown that the analysis of dynamics of void fraction changes in time can be used for identification of two-phase flow patterns. The two-phase flow (air–water) in horizontal square minichannel (3×3mm) has been analysed. The laser–phototransistor sensor signal was used for qualitative assessment of void fraction in minichannel. For data analysis there has been used the recurrence quantification analysis and the principal component analysis. The method of selection of coefficients resulted of recurrence quantification analysis has been proposed. Those coefficients describe the different aspects of two-phase flow patterns, but they are not independent variables describing the dynamics of two-phase flow. To obtain the set of independent variables characterising the dynamics of two-phase flow patterns the principal component method has been used. The proposed method allows us for the identification of following two-phase flow patterns: flow of micro-bubbles, flow of micro- and mini-bubbles, flow of micro- and mini-bubbles with confined bubbles, slug flow, stratified flow. The obtained results confirm that this type of analysis can be considered as an alternative way of identification of two-phase flow patterns in the minichannel.

Void Reactivity Coefficient Analysis during Void Fraction Changes in Innovative BWR Assemblies

The study of the void reactivity variation in innovative BWR fuel assemblies is presented in this paper. The innovative assemblies are loaded with high enrichment fresh UO2and MOX fuels. UO2fuel enrichment is increased above existing design limitations for LWR fuels (>5%). MOX fuel enrichment with fissile Pu content is established to achieve the same burnup level as that of high enrichment UO2fuel. For the numerical analysis, the TRITON functional module of SCALE 6.1 code with the 238-group ENDF/B-VI cross section data library was applied. The investigation of the void reactivity feedback is performed in the entire 0–100% void fraction range. Higher values of void reactivity coefficient for assembly loaded with MOX fuel are found in comparison with values for assembly loaded with UO2fuel. Moreover, coefficient values for MOX fuel are positive over 75% void fraction. The variation of the void reactivity coefficient is explained by the results of the decomposition analysis based on four-factor formula and neutron absorption reactions for main isotopes. Additionally, the impact of the moderation enhancement on the void reactivity coefficient was investigated for the innovative assembly with MOX fuel.

Evidence that filament fracture occurs in an ITER toroidal field conductor after cyclic Lorentz force loading in SULTAN

We analyzed the ITER TFEU5 cable-in-conduit conductor (CICC) after the full SULTAN conductor qualification test in order to explore whether Lorentz force induced strand movement inside the CICC produces any fracture of the brittle Nb3Sn filaments. Metallographic image analysis was used to quantify the change in void fraction of each sub-cable (petal); strands move in the direction of the Lorentz force, increasing the void space on the low force side of the CICC and producing a densification on the high force side. Adjacent strand counting shows that local increases in void space result in lower local strand–strand support. Extensive metallographic sampling unambiguously confirms that Nb3Sn filament fracture occurred in the TFEU5 CICC, but the filament fracture was highly localized to strand sections with high local curvature (likely produced during cabling, where strands are pivoted around each other). More than 95% of the straighter strand sections were free of filament cracks, while less than 60% of the bent strand sections were crack free. The high concentration of filament fractures on the tensile side of the strand–strand pivot points indicates that these pivot points are responsible for the vast majority of filament fracture. Much lower crack densities were observed in CICC sections extracted from a lower, gradient-field region of the SULTAN-tested cable. We conclude that localized filament fracture is induced by high Lorentz forces during SULTAN testing of this prototype toroidal field CICC and that the strand sections with the most damage are located at the petal corners of the high field zone.

Reactor performance optimization by the use of a novel combined pellet reflecting both catalyst and adsorbent properties

A mole based pellet model has been formulated to describe the evolution of species mole fraction, pressure, velocity, total concentration, temperature and mole diffusion flux in porous pellets for the steam methane reforming (SMR), sorption-enhanced steam methane reforming (SE-SMR) and desorption processes. In the case of molar based pellet model, a rigorous analysis reveals that both the mass- and mole averaged velocities occur in the equation system. On the other hand, the mass based counterpart contains only the mass averaged velocity. The effect of velocity definition inconsistency is elucidated in this work. The gas–solid reactions taking place within the reactive particles for the SE-SMR process, make the sorption micro-particles grow during the reaction process due to the solid formation, thus the pellet void-fraction is reduced as the reaction proceeds. This effect may reduce the effective reaction rate due to increased diffusion limitations. The changes in void-fraction is taken into account in the proposed pellet model for the SE-SMR- and desorption processes. The reaction efficiency factors determining the diffusion limitations of different reactions in the SMR-, SE-SMR- and desorption processes are calculated by the proposed pellet model, and used in the pseudo-homogeneous fixed bed reactor model. The effects of pressure, steam to carbon ratio, inlet temperature variation and inlet gas flow rate on the reactions were studied on the fixed bed reactor level. The objective of this work is to perform a numerical study elucidating the effects of the operational conditions by introducing a one-pellet design; one pellet holding both catalytic and sorbent properties, with application to the SE-SMR process in the fixed bed reactor. It is assumed that the carbonation reaction is kinetics controlled, neglecting the product layer diffusion effects. However, the porosity changes due to the solid product formation are considered. The later effect influences on the internal limitations. The SE-SMR process is not completely chemical kinetics controlled, as the internal diffusion limitations are significant due to the porosity variation. The approximate values of the efficiency factors for carbonation reaction is around 0.81 and for the desorption it is around 0.017. The mole based pellet- and fixed bed reactor models were used to evaluate the potential of carbon formation across the pellet and in the bulk gas phase of the fixed bed reactor. There is no solid carbon formation within the pellet or in the bulk phase of the reactor with steam to carbon ratio of 3.5.

Modification and application of the system analysis code ATHLET to trans-critical simulations

During the loss of coolant accident (LOCA) of supercritical water cooled reactor (SCWR), the pressure in the reactor system will undergo a rapid decrease from supercritical to subcritical condition. This process is called trans-critical transients, which is of crucial importance for the LOCA analysis of SCWR. Using the current version of system code (e.g. ATHLET, REALP), calculation will be terminated due to the abrupt change of void fraction across the critical point (22.064MPa). To solve this problem, a pseudo two-phase method is proposed by introducing a fictitious region of latent heat (enthalpy of vaporization hfg∗) at pseudo-critical temperatures. A smooth transition of void fraction can be realized by using liquid-field conservation equations at temperatures lower than the pseudo-critical temperature, and vapor-field conservation equations at temperatures higher than the pseudo-critical temperature. Adopting this method, the system code ATHLET is modified to ATHLET-SC mod 2 on the basic of the previous version ATHLET-SC mod 1 modified by Shanghai Jiao Tong University. When the fictitious region of latent heat is kept as a small region, the code can achieve an acceptable accuracy. Moreover, the ATHLET-SC mod 2 code is applied to simulate the blowdown process of a simplified model. The results achieved so far indicate a good applicability of the new modified code for the trans-critical transient.

A Two Property Catalyst/Sorbent Pellet Design for the Sorption-Enhanced Steam–Methane Reforming Process: Mathematical Modeling and Numerical Analysis

In the present study, the performance of a combined catalyst/sorbent pellet design for the sorption-enhanced steam– methane reforming process has been numerically investigated. The mathematical pellet model formulated for the process describes the evolution of species mole fractions, pressure, total concentration, temperature, mass and heat fluxes, and convection within the voids of the porous pellet. The mass diffusion fluxes are described according to the Maxwell– Stefan and dusty gas models and the effective diffusivities are described employing the parallel pore model. In addition, models proposed in the literature for void fraction changes and product layer diffusion resistance are adopted.The effectiveness factor is a convenient parameter used in modeling and simulations of chemical reactors indicating the relative importance of diffusion and reaction limitation. In this study, the simulation results presented in terms of effectiveness factors reveal the following noticeable findings for the two pellet diameters analyzed: (i) Knudsen diffusion is significant for the operation conditions set for the simulation study. Hence the pore size distribution within the pellet is significant. (ii) Structural changes within the pellet affects the pellet capacity.

A numerical study of a two property catalyst/sorbent pellet design for the sorption-enhanced steam–methane reforming process: Modeling complexity and parameter sensitivity study

In this study the performance of a combined catalyst/sorbent pellet design for the sorption-enhanced steam–methane reforming process has been investigated. Different mathematical model complexities have been studied and parameter sensitivity analyses have been performed. The mathematical pellet model formulated for the process describes the evolution of species mole fractions, pressure, total concentration, temperature, fluxes, and convection within the voids of the porous pellet. The effective diffusivites are described according to the parallel pore and random pore models, and the mass diffusion fluxes are described according to the Maxwell–Stefan and dusty gas models. Moreover, models proposed in the literature for void fraction changes, product layer diffusion resistance, and degeneration due to multiple carbonation/calcination cycles are employed. The simulated pellet effectiveness factor is a convenient parameter frequently used in modeling and simulations of chemical reactors indicating the relative importance of diffusion and reaction limitations. Thus, in this study, the effectiveness factor behavior due to different mathematical modeling assumptions and model parameter values is elucidated. The combined pellet performance is promising compared to the conventional two-pellet design. For further improved modeling and simulations of the pellet, the characterizing of the pore size distribution is important because of the Knudsen diffusion mechanism. Moreover, the reduction in void fraction with the CaO conversion, product layer growth, and degeneration due to multiple cycles are all important effects in the SE-SMR process because these mechanisms influence on the cycle life time, reaction rate, and capture capacity. For industrial applications, the pellet is only of interest in the capture kinetic controlling step, i.e. before the process becomes controlled by the product layer diffusion resistance.

기공닫힘부 폐쇄정도 결정을 위한 평가방법 연구

This paper presents methods for analyzing the extent of cylindrical-shaped void closure. In addition, a quantitative relationship between change in void fraction and height reduction ratio of a compressed specimen is proposed. The height reduction ratio, number of deformation steps and billet rotation were chosen as key process parameters influencing the void closing behavior, namely, the changes in void shape and size during hot open die forging of a large ingot. The extent of void closure was analyzed from microscopic observations and estimated from tensile test results. The tensile strengths of specimens with closed voids and those without were compared for various reduction ratios in height. The results confirmed that void closure occurs at reduction ratios greater than 30 %. The void closing behavior could be expressed as a hyperbolic tangent function of reduction ratio in height, number of paths, and billet rotation. The knowledge presented in this paper could be helpful for optimizing deformation paths in open die forging processes.

Prediction of the Behavior of a Liquid‐Fluidized Bed of Inert Particles Used for Separation by Density

AbstractA liquid‐fluidized bed of inert particles was used to separate a pure object from a mixture. One (binary solid‐liquid‐fluidized bed) or two (tertiary solid‐liquid‐fluidized bed) types of objects with relatively large‐sized particles were immersed in an inert‐particle bed, and the bed behavior was observed for different liquid velocities. The void fraction and apparent density of the inert‐particle suspension were predicted by considering the effect of the change in object position for different liquid velocities. The prediction method, which considers the change in the minimum fluidization velocity, accurately expressed the changes in the void fraction and the apparent density of the bed with the position of the objects in the bed. Using this method, the liquid velocity required to separate a certain kind of object from a mixture can be predicted.

ICONE19-43786 THERMAL HYDRAULIC TEST OF ADVANCED FUEL BUNDLE WITH SPECTRAL SHIFT ROD (SSR) FOR BWR : EFFECT OF THERMAL HYDRAULIC PARAMETERS ON STEADY STATE CHARACTERISTICS

Japanese national project of next generation light water reactor (LWR) development started in 2008. Under this project, spectral shift rod (SSR) is being developed. SSR, which replaces conventional water rod (WR) of boiling water reactor (BWR) fuel bundle, was invented to enhance the BWR's merit, spectral shift effect for uranium saving. In SSR, water boils by neutron and gamma-ray direct heating and water level is formed as a boundary of the upper steam region and the lower water region. This SSR water level can be controlled by core flow rate, which amplifies the change of average core void fraction, resulting in the amplified spectral shift effect. This paper presents the steady state test results of the base geometry case in SSR thermal hydraulic test, which was conducted under the national project of next generation LWR. In the test, thermal hydraulic parameters, such as flow rate, pressure, inlet subcooling and heater rod power are changed to evaluate these effects on SSR water level and other SSR characteristics. In the test results, SSR water level rose as flow rate rose, which showed controllability of SSR water level by flow rate. The sensitivities of other thermal hydraulic parameters on SSR water level were also evaluated. The obtained data of parameter's sensitivities is various enough for the further analytical evaluation. The fluctuation of SSR water level was also measured to be small enough. As a result, it was confirmed that SSR's steady state performance was as planned and that SSR design concept is feasible.