Small Void Fraction Research Articles

Potential and design of imine-linked two-dimensional covalent organic framework membranes for Ethane/Methane separation

Membrane separation of CH4 and C2H6 is challenging due to the similar kinetic diameter and chemical affinity. In comparison with other crystalline porous materials, covalent organic frameworks (COFs) possess the advantages of low density, large surface area, tunable pore structure, versatile functionality, and so on, providing the COFs with great potential as separation membranes. Herein, we studied the membrane-based C2H6/CH4 separation performance of 80 two-dimensional imine-linked COFs. Monte Carlo and molecular dynamics simulations were employed to investigate the adsorption and diffusion of gas molecules in the COF membranes. 7 promising COF structures are screened by evaluating the membrane’s selectivity and permeability, and they are found to have small pore size (<20 Å), void fraction (<0.7), and surface area (<8000 m2·g−1) as well as high framework density (>200 kg·m−3). Pore size and framework density are demonstrated to be dominating factors in determining the selectivity and permeability. Based on two typical COFs, COF-LZU1 and NUS-15, grafting hydrocarbons onto the linker and introducing high-carbon additives into the pore can effectively improve the separation performance. Especially, with an appropriate additive concentration, the fullerenes can increase the C2H6/CH4 selectivity by>10 times. This work aims to explore the potential of COFs as membranes for C2H6/CH4 separation, and provides design strategies to optimize the separation performance, which are also applicable to other porous materials.

Development of All Metal Multi-plate-type Capacitive Void Fraction Sensor for gas-liquid two-phase flow

In the aerospace field, liquid hydrogen and oxygen which are “cryogenic fluid” are used as the propellant for rockets and hypersonic engines. These fluids become easily a gas-liquid two-phase flow in pipes and fluid machines. We have developed small capacitive void fraction sensors which can measure in real time, which is used to control the flow rate of cryogenic fuel and to understand typical two-phase flow phenomena such as the cavitation. The conventional capacitive sensors have a structure in which two electrodes sandwich a pipe made of dielectric material. However, the dielectric material has problems such as insufficient strength under high pressure condition and decreasing measurement accuracy due to changes in dielectric constant with temperature change. We developed a new capacitive sensor which consists of only metal parts to solve these problems. On the other hand, it may cause a decrease in measurement accuracy due to the structural limitation. This paper presents the results of the electric field analysis and the static fluid experiment using silicon oil and air to verify the accuracy of the sensor. The mean errors obtained by the analysis and the experiment are 3.47 % and 5.32 %, respectively, which almost satisfy the required specification. However, it was found that the accuracy is poor in some void fraction ranges, and the sensor needs to be improved.

Experimental study of the effect of pipe size on bubbly to beyond-bubbly flow regime transition

The two-phase flow regime and its transition is closely related to the flow channel geometry. In this study, the effect of pipe size on the bubbly-to-beyond-bubbly flow regime transition was experimentally investigated. Experiments focusing on air-water two-phase bubbly to slug flow regime transition using impedance meter and high-speed imaging were performed on 12.7 mm, 25.4 mm, and 50.8 mm inner-diameter vertical upward pipes. The flow conditions were finely selected and cover a range of superficial liquid and gas velocities of <jf> = 0.3–2.0 m/s and <jg> = 0.03–2.5 m/s. The flow regime identification was realized using a 1-D convolutional neural network-based learning algorithm, with the aid of 5 different data augmentation methods. Through the experimental results, the difference of the transition boundary between bubbly to cap-bubbly/slug flow among different pipes sizes was observed. The difference among the pipes is due to the pipe wall effect on the bubble distribution and relative motion. For a smaller diameter pipe, the transition happens in a smaller void fraction. A simple flow regime transition criterion is proposed regarding the wall effect based on Mishima-Ishii’s approach.

Coupled effects of circular and elliptical feed spacers under forced-slip on viscous dissipation and mass transfer enhancement based on CFD

Combining unsteady state shear strategies and a novel spacer geometry for spirally wound membrane modules may lead to greater flux enhancement than only optimising the spacer geometry. In this paper, CFD is employed to simulate the effect of different spacer profiles (circular and elliptical) and configurations (2- and 3-layer) on flux enhancement generated by unsteady forced-slip. Data analysis shows that a similar Strouhal number is observed for all spacer filaments under consideration, regardless of profile shape. Although a submerged spacer leads to relatively larger pressure losses, implementing forced-slip with that geometry results in the largest flux increase (40%) compared to other configurations under consideration. This is associated with the shedding of vortices near the membrane surface, coupled with vortex flow cross-over from regions of high concentration (near the membrane) to low concentration regions in the bulk. However, the flow instabilities that improve mass transfer are weakened in channels with smaller void fraction, leading to smaller flux enhancement. The results therefore suggest that the simple circular profile is preferred for maximising vortex-shedding-induced permeate flux enhancement.

Research of Water Packing Numerical Problem in Thermal Hydraulic System Code

Because of the discontinuous change in the compressibility between a two-phase mixture with a small void fraction and the pure liquid phase and the discrete method of the discrete momentum equation, the thermal hydraulic system program based on the two-fluid six-equation method may calculate a fictitious pressure peak when the steam is about to disappear from the volume or the water is about to fill the volume. In this paper, the thermal hydraulic system analysis program RELAP5 is used as a reference to analyze and mitigate the water packing problem. The detailed detection logic and mitigation scheme are given, and applied in the calculation and analysis of the Pryor’s Pipe Problem and the Condensation Test. The results show that the implementation of the water packing mitigation scheme can alleviate the pressure transient effects caused by the numerical method in the two-fluid program, which can significantly reduce the pressure peaks and avoid the emergence of severely distorted transient solutions. The water packing mitigation scheme slows down the peak of this fictitious pressure, which is helpful to improve the stability of program calculation. In view of this problem, this method can provide a reference for the development of the same type of system program and model optimization.

Mechanical and Microstructural Characterisation of Cooling Pipes for the Compact Muon Solenoid Experiment at CERN.

The Compact Muon Solenoid (CMS) is a particle physics experiment situated on the Large Hadron Collider (LHC) at CERN, Switzerland. The CMS upgrade (planned for 2025) involves installing a new advanced sensor system within the CMS tracker, the centre of the detector closest to the particle collisions. The increased heat load associated with these sensors has required the design of an enhanced cooling system that exploits the latent heat of 40 bar CO2. In order to minimise interaction with the incident radiation and improve the detector performance, the cooling pipes within this system need to be thin-walled (~100 μm) and strong enough to withstand these pressures. The purpose of this paper is to analyse the microstructure and mechanical properties of thin-walled cooling pipes currently in use in existing detectors to assess their potential for the tracker upgrade. In total, 22 different pipes were examined, which were composed of CuNi, SS316L, and Ti and were coated with Ni, Cu, and Au. The samples were characterised using computer tomography for 3D structural assessment, focused ion beam ring-core milling for microscale residual stress analysis, optical profilometry for surface roughness, optical microscopy for grain size analysis, and energy dispersive X-ray spectroscopy for elemental analysis. Overall, this examination demonstrated that the Ni- and Cu-coated SS316L tubing was optimal due to a combination of low residual stress (20 MPa axial and 5 MPa hoop absolute), low coating roughness (0.4 μm Ra), minimal elemental diffusion, and a small void fraction (1.4%). This result offers a crucial starting point for the ongoing thin-walled pipe selection, development, and pipe-joining research required for the CMS tracker upgrade, as well as the widespread use of CO2 cooling systems in general.

Stability of packed bed thermoclines

Abstract Packed bed thermoclines have attracted considerable interest as an economical method for storing large amounts of thermal energy. They are a constituent part of a range of proposed thermo-mechanical energy storage systems, such as Adiabatic Compressed Air Energy Storage (ACAES) and Pumped Thermal Energy Storage (PTES). The low cost of the thermal storage media (crushed rock or gravel) means that even with the cost of the required compression and expansion equipment, these systems potentially have a lower Levelised Cost of Storage than batteries, especially for grid scale storage. Packed bed thermoclines rely on a stratified temperature gradient (thermal front) between heated material at the top and cooler material at the bottom. The stability of this thermal front can affect the exergetic efficiency of the store. We present a simple criterion for the stability of a thermal front and show that during discharge of a hot store, a small cold perturbation in the thermal front can develop into a cold tunnel that propagates ahead of the main thermal front. By contrast, the presence of a small hot perturbation at the thermal front prior to charging with hot gas is shown to be quickly dissipated. We also calculate a theoretical critical perturbation size required for a cold tunnel to develop ahead of the thermal front. Below this size transverse thermal diffusion is able to dissipate perturbations before they can develop. Three dimensional Computational Fluid Dynamics simulations are used to accurately visualise thermal front instabilities and also to quantify their effect on the exergetic efficiency of a cycling thermal store. Adding a small high void fraction region near the bottom of the thermal store caused a significant disruption of the thermal front on each discharge cycle and resulted in a 4.5% increase in the exergy loss rate. Low void fraction adjacent to the walls of the thermal store, which typically occurs during packing, caused a more significant 63% increase in the exergy loss rate relative to a uniformly packed thermal store.

Study of bubble-induced turbulence in upward laminar bubbly pipe flows measured with a two-phase particle image velocimetry

In the present study, focusing on characterizing the bubble-induced agitation (turbulence), spatially varying flow statistics of gas and liquid phases in laminar upward bubbly flows (Reynolds number of 750) with varying mean void fraction are investigated using a two-phase high-speed particle image velocimetry. As the flow develops along the vertical direction, bubbles with small-to-moderate void fractions, which were intentionally distributed asymmetrically at the inlet, migrate fast and show symmetric distributions of wall or intermediate peaking. Meanwhile, the mean liquid velocity saturates relatively slowly to a flat distribution at the core region. Despite small void fractions considered, the bubbles generate a substantial turbulence, which increases with increasing mean void fraction. Interestingly, it is found that the mean vertical velocity, bubble-induced normal stress in radial direction, and Reynolds stress profiles match well with those of a single-phase turbulent flow at a moderate Reynolds number (e.g., 104), indicating the similarity between the bubble-induced turbulence and wall-shear-generated turbulence in a single-phase flow. Previously suggested scaling relations are confirmed such that the mean bubble rise velocity and bubble-induced normal stress (in both vertical and radial directions) scale with mean volume void fraction as a power of −0.1 and 0.4, respectively. Finally, based on the analysis of measured bubble dynamics (rise in an oscillating path), a theoretical model for two-phase turbulent (Reynolds) stress is proposed, which includes the contributions by the non-uniform distributions of local void fraction and relative bubble rise velocity, and is further validated with the present experimental data to show a good agreement with each other.

Effect of bubble’s arrangement on the viscous torque in bubbly Taylor-Couette flow

An experimental investigation of the interactions between bubbles, coherent motion, and viscous drag in a Taylor-Couette flow with the outer cylinder at rest is presented. The cylinder radii ratio η is 0.91. Bubbles are injected inside the gap through a needle at the bottom of the apparatus. Different bubbles sizes are investigated (ratio between the bubble diameter and the gap width ranges from 0.05 to 0.125) for very small void fraction (α ≤ 0.23%). Different flow regimes are studied corresponding to Reynolds number Re based on the gap width and velocity of the inner cylinder, ranging from 6 × 102 to 2 × 104. Regarding these Re values, Taylor vortices are persistent leading to an axial periodicity of the flow. A detailed characterization of the vortices is performed for the single-phase flow. The experiment also develops bubbles tracking in a meridian plane and viscous torque of the inner cylinder measurements. The findings of this study show evidence of the link between bubbles localisation, Taylor vortices, and viscous torque modifications. We also highlight two regimes of viscous torque modification and various types of bubbles arrangements, depending on their size and on the Reynolds number. Bubbles can have a sliding and wavering motion near the inner cylinder and be either captured by the Taylor vortices or by the outflow areas near the inner cylinder. For small buoyancy effect, bubbles are trapped, leading to an increase of the viscous torque. When buoyancy induced bubbles motion is increased by comparison to the coherent motion of the liquid, a decrease in the viscous torque is rather observed. The type of bubble arrangement is parameterized by the two dimensionless parameters C and H introduced by Climent et al. [“Preferential accumulation of bubbles in Couette-Taylor flow patterns,” Phys. Fluids 19, 083301 (2007)]. Phase diagrams summarizing the various types of bubbles arrangements, viscous torque modifications, and axial wavelength evolution are built.

Cabling Technology of &lt;formula formulatype="inline"&gt;&lt;tex Notation="TeX"&gt;$\hbox{Nb}_{3}\hbox{Sn}$&lt;/tex&gt; &lt;/formula&gt; Conductor for ITER Central Solenoid

Under the International Thermonuclear Experimental Reactor (ITER) project, the Japan Atomic Energy Agency (JAEA) is procuring all of the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductors for the Central Solenoid (CS). The CS consists of six vertically stacked modules. The height and outer diameter of the CS are approximately 13 m and 4 m, respectively. The CS has a circular five stage cable. All of approximately 43 km of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn CS cables will be manufactured in Japan. Before mass-production start, the jacketed cable conductors should be tested in the SULTAN facility in Switzerland to confirm their superconducting performance. The original cabling design had relatively long twist pitches and is referred to as the normal twist pitch (NTP) conductor. The NTP conductor test results revealed decreasing the current sharing temperature (Tcs) with increasing number of electro-magnetic (EM) load cycles. Therefore, a short twist pitch (STP) design was proposed and the STP conductors were also tested. The STP conductor results showed that the Tcs is stable during EM cyclic load tests. Because the conductors with STP have a smaller void fraction in the cable area than those with NTP, a higher compaction ratio during cabling is required and the possibility of damage on strands increases. The STP cable technology was developed in collaboration among Japanese cabling suppliers and JAEA. Several key technologies will be described in this paper.

Electron Transport Dynamics in TiO2 Films Deposited on Ti Foils for Back-Illuminated Dye-Sensitized Solar Cells

In this study, we examine the electron transport dynamics in TiO2 films of back-illuminated dye-sensitized solar cells. The TiO2 films are fabricated using electrophoretic deposition (EPD) and the conventional paste-coating (PC) of TiO2 nanoparticles on Ti-foil substrates. Intensity-modulated photocurrent spectroscopy reveals that red-light irradiation is more efficient than blue-light irradiation for generating photocurrents for back-illuminated cells. A single trapping-detrapping diffusion mode, without trap-free diffusion, reveals the electron transport dynamics involved in the backside illumination. The closely-packed EPD films exhibit a shorter electron transit time than does the loosely packed PC films. The porosity dependence of the electron diffusion rate is consistent with the 3D percolation model for metallic solid spheres. The EPD films possess longer electron lifetimes because of their smaller void fraction, which suppresses recombination with electrolytes. The EPD cells, which feature rapid electron transport and suppressed recombination in the TiO2 films, exhibit a maximum power conversion efficiency of 7.1%, which is higher than that of PC cells (6.0%). Because the distance between electron injection and collection is close to the film thickness and the transport lacks trap-free diffusion, the performance of back-illuminated cells is more sensitive to TiO2 film thickness and porosity than the performance of the front-illuminated cells. This study demonstrates the advantages of EPD-film architecture in promoting charge collection for high power conversion.

Characteristics of starch-based biodegradable composites reinforced with date palm and flax fibers

The aim of this work is to study the behavior of completely biodegradable starch-based composites containing date palm fibers in the range from 20 to 80wt%. Hybrid composites containing date palm and flax fibers, 25wt% each, were also examined. The composites were preheated and then hot pressed at 5MPa and 160°C for 30min. SEM investigation showed strong adhesion between fibers and matrix. Density measurements showed very small void fraction (less than 0.142%) for composites containing up to 50wt% fiber content. Increasing fiber weight fraction up to 50wt% increased the composite static tensile and flexural mechanical properties (stiffness and strength). Composite thermal stability, water uptake and biodegradation improved with increasing fiber content. The present work shows that starch-based composites with 50wt% fibers content have the optimum mechanical properties. The hybrid composite of flax and date palm fibers, 25wt% each, has good properties and provides a competitive eco-friendly candidate for various applications.

Magnetic susceptibility based magnetic resonance estimation of micro-bubble size for the vertically upward bubbly flow

The approach originally developed for the Nuclear Magnetic Resonance analysis of stable micro-bubbles is applied to studies of vertical bubbly flows. A very fast dispersion (diffusion) of water in bubbly flows extends the fast diffusion limit down to short (2–10ms) measurement times, permitting the use of the simplified analytical expression to extract the micro-bubble size information both in bulk and spatially resolved. The observed strong bubble-induced reduction in T2∗ necessitates the use of very short encoding times and pure phase encoding methods to accurately measure the void fraction. There was an expected underestimation of bubble sizes at faster flow rates due to the limitations of the theory derived for small bubble sizes and non-interacting spherical bubbles (low void fractions and slow flow rates). This approach lends itself to studies of bubbly flows and cavitating media characterized by small bubble sizes and low void fractions.

Experimental study of thermo-fluid dynamic effect in He II cavitating flow

We have conducted a series of experiments of superfluid helium (He II) cavitating flow driven by a bellows pump by applying flow visualization method and PIV (Particle Image Velocimeter), and through the measurements of the temperature and the pressure depressions. In this study the experimental results were examined in particular focusing on the thermodynamic effect by referring to the thermodynamic parameter Σ introduced by Brennen. It is found that the thermodynamic effect is negligible in cavitating flows of He II with small void fraction, in accordance with the Σ prediction. On the other hand, in the case of large void fraction the magnitude of the temperature depression is finite and there is apparent temperature dependence in its variation. These features indicate that He II also behaves as a cryogenic fluid in cavitating flow, in which the thermodynamic effect is noticeable.

Influencing Parameters Analysis of Solar Energy Regenerator Performance

The paper studied the effect of running and structural parameters on solar energy regenerator performance, in which pebble was storage material. Comparing simulated values to tested values of outlet air temperature, good agreement was achieved, which proved the model was correct. Taking average regenerative power as target function, the simulated results showed that the larger inlet air temperature, the larger average regenerative power. The smaller void fraction, the larger average regenerative power. The larger cross section velocity, the larger average regenerative power.

Impact of cross section libraries on CASMO-4E and CASMO-4 results

A detailed comparison between the code CASMO-4 and its extended version CASMO-4E has been made. In addition to the standard library, CASMO-4E calculations have been performed also with its extended libraries. The differences are significant enough to be considered when choosing the library to be used for a particular problem. The differences in the multiplication factor k ∞ range up to several hundred pcm depending on the void history, burnup and other parameters. The differences in fuel temperature or void coefficients are smaller especially at small void fraction and low burnup. At large void and low burnup CASMO-4E with the standard library gives significantly different results than the other combinations. The microscopic cross sections show small differences when calculated with the same library but clear differences due to the extended libraries.

Calculation of the Void Fraction and Void Coefficient in an Aqueous Homogeneous Reactor

The aqueous homogeneous reactor has assumed some importance in recent years as a potential medical isotopes production system. The kinetic behavior of such systems depends on the rate of generation of the radiolytic gas bubbles and the associated reactivity void coefficient. In this work we describe a method based on perturbation theory, and a simple description of bubble production, for deriving a value of the void coefficient of reactivity. It is shown that, in the small void fraction limit, the void coefficient is dependent only on the system properties and does not depend on power level or the bubble properties. Values are given for the void coefficient for a range of parameters assuming that the voids are distributed in three different ways, i.e., uniformly, proportional to power, and equal to the distribution due to buoyancy. The results are in reasonable agreement with values obtained by others, using more detailed methods, based on the reactors SILENE and MIPR.

Nonlinear Wave Equations for Pressure Wave Propagation in Liquids Containing Gas Bubbles (Comparison between Two-Fluid Model and Mixture Model)

Based on the unified theory by the present authors (Kanagawa et al., J. Fluid Sci. Tech., 5, 2010), the Korteweg-de Vries-Burgers (KdVB) equation and the nonlinear Schrodinger (NLS) equation with an attenuation term for weakly nonlinear waves in bubbly liquids are re-derived from a system of bubble-liquid mixture model equations composed of the conservation equations of mass and momentum, the Keller equation for bubble dynamics, and supplementary equations. We show that the re-derived KdVB equation and NLS equation are essentially the same as those derived from a system of two-fluid model equations except for the coefficients of nonlinear, dissipation, and dispersion terms. The differences in these coefficients are studied in detail, and we find that for the case of KdVB equation, the mixture model is valid only for sufficiently small initial void fractions. On the other hand, for the case of NLS equation, the range of validity of the mixture model depends on not only the initial void fraction but also the wavenumber concerned.

Drag reduction by microbubbles in a turbulent boundary layer

An experimental characterization of the turbulent boundary layer over a flat plate in the presence of small amounts of microbubbles is performed. The average diameter of the injected bubbles is comparable with the local Kolmogorov lengthscale, and the bulk void fraction C¯ is approximately 0.1%. The velocity field of the liquid phase, as well as the bubble characteristics, is acquired by optical techniques. Even at the small void fraction typical of this investigation, the interaction between microbubbles and turbulence leads to significant modifications of the underlying flow field. In accordance with previous investigations, the main global effect consists in a reduction of the magnitude of the viscous drag. This has been checked with preliminary tests conducted in a towing tank, as well as by inferring the wall stress from the boundary layer velocity profile measured in a laboratory facility at the same conditions. Here, the local wall stress is found to drop by approximately 25%. An analysis of the turbulent statistics shows that the reduction in drag is accompanied by a substantial reduction of the momentum flux throughout most of the inner region and by a decrease of the characteristic dimension of the turbulent scales involved in the production of kinetic energy. The distribution of turbulent kinetic energy among different scales is found to be substantially altered in the two flows. In particular, an enhancement of the energy content at smaller scales is observed in the bubbly case. These effects are discussed in relation with the decrease of the coherence of near-wall flow structures induced by the bubble forcing.

気泡を含む液体中の圧力波伝播の非線形波動方程式 : 二流体モデルと混合体モデルとの比較(流体工学,流体機械)

Based on the unified theory by the present authors (T. Kanagawa, et al., J. Fluid Sci. Tech., 5, 2010), the Korteweg-de Vries-Burgers (KdVB) equation and the nonlinear Schrodinger (NLS) equation with an attenuation term for weakly nonlinear waves in bubbly liquids are re-derived from a system of bubble-liquid mixture model equations composed of the conservation equations of mass and momentum, the Keller equation for bubble dynamics, and supplementary equations. We show that the re-derived KdVB equation and NLS equation are essentially the same as those derived from a system of two-fluid model equations except for the coefficients of nonlinear, dissipation, and dispersion terms. The differences in these coefficients are studied in detail, and we find that for the case of KdVB equation, the mixture model is valid only for sufficiently small initial void fractions. On the other hand, for the case of NLS equation, the range of validity of the mixture model depends on not only the initial void fraction but also the wavenumber concerned.