Increase In Void Fraction Research Articles

Theoretical modeling and experimental verification of percolation threshold with MWCNTs’ rotation and translation around a growing bubble in conductive polymer composite foams

Foaming is a potential method to prepare high-performance, lightweight, composite materials with enhanced electrical properties. Here, a theoretical model and its experimental verification are presented to assess the evolution of electrical percolation threshold and conductivity as a bubble grows during the foaming of conductive filler/polymer composites. The filler networks were changed by translation and rotation of filler particle during bubble growth that was presented by void fraction increase. The evolution of percolation threshold with filler network was estimated using a Monte Carlo approach. The model results showed that the percolation threshold decreased, and the electrical conductivity increased for void fraction values less than 20%, beyond which the percolation threshold started to increase. Furthermore, a series of polystyrene/multiwalled carbon nanotubes (PS/MWCNTs) composite foams were fabricated at various void fractions matching the parameters in modeling. At an optimum void fraction of approximately 17%, the percolation threshold of the foamed samples was decreased by 20%, from 0.6 vol% down to 0.48 vol%. Further increases in the void fraction increased the percolation threshold. The simulation prediction was in good agreement with the experimental results. The model offers a valuable tool to better understand the electrical behavior and can be used in the design of conductive polymer composite foams with rod-like conductive fillers.

Influence of hydrogen dilution and doping gas on the material properties of plasma-deposited polycrystalline silicon thin films

We studied the effects of precursor gas composition and the addition of dopant gas, PH3, on polycrystalline silicon films deposited using plasma-enhanced chemical vapor deposition. Spectroscopic ellipsometry measurement and modeling results revealed that the poly-Si films consist of a subsurface layer and a bulk layer. The sample deposited under greater hydrogen dilution exhibited a fully crystallized subsurface layer and a denser bulk layer. We also studied the effects of various dopant gas flow rates on structural and electrical properties of the poly-Si films. Ellipsometry, X-ray diffraction, and Raman analyses revealed that the film deposited using a large dopant-gas flow rate exhibits decreased grain size and increased void fraction. Furthermore, the doped poly-Si films show a trend of Raman peak shift, which was attributed to tensile stress. The stress of the doped poly-Si films increases with higher dopant-gas flow rate. Measurement of the temperature-dependent dark conductivity shows saturation of the electrical conductivity of heavily doped films.

Investigation of fluidized bed behaviour using electrical capacitance tomography

AbstractThe temporal and cross‐sectional distributions of particles in a 127 mm diameter fluidized bed have been obtained using a new generation, high‐speed electrical capacitance tomography. Two planes of eight electrodes were used and mounted at 160 and 660 mm from the gas distributor which was a 3 mm thick porous plastic plate (maximum pore size of 50‐70 μm). 3 mm diameter, nearly‐spherical polyethylene granules made up the bed. Experiments at sampling frequencies of 200‐2000 cross‐sections per second and gas superficial velocities from just below the minimum fluidization to 83% above minimum fluidization velocities were used. The time series of the cross‐sectional average void fractions have been examined both directly and in amplitude and frequency space. The last two used probability density functions and power spectral densities. The information gathered shows that the fluidized bed was operating in the slugging mode, which is not surprising given the size of the particles. It has been found that an increase in the excess gas velocity above the minimum fluidization velocity resulted in an increase in the mean void fraction, an increase in the length and velocity of the slug bubbles as well as the bed height, and a slight decrease in the slug frequency. The results are presented in a level of detail suitable for comparison with later numerical simulation.

Natural Circulation of a Fluid in a Thermosiphon Slightly Inclined to the Horizontal

A pronounced stratification of the temperature (density) of the fluid over the cross section of the heating zone occurs in a thermosyphon that is slightly inclined to the horizontal and has a high degree of water filling, which leads to the onset of natural circulation of the medium along the length of the thermosiphon. In this case, there is an upward flow of water or a steam–water mixture in the upper part of the thermosiphon and a downward flow in the lower part. In experiments with a full-scale thermosiphon, the natural circulation in question increased the axial heat transfer through nonboiling water along all thermosiphon zones by three to seven times as compared to the heat transfer in a vertical thermosiphon. The estimated mass flow rate of natural circulation along the heating zone ranged from 25 to 105 kg/(m2 s). The heat-transfer coefficient and friction factor are estimated at the boundary of countercurrent water flows induced by natural circulation. The conditions of poorer cooling of the thermosiphon heating zone due to steam separation at its upper generatrix are examined. A decrease in the mass filling of the thermosiphon, i.e., an increase in the average void fraction of the medium in it, successively leads to the onset of bubbling steam condensation within the transport section and then in the cooling zone, with a decrease in the length of the upper section of nonboiling water. A transition to film condensation of steam occurs at the limit along the entire length of the cooling zone with a countercurrent steam flow and a near-wall film of its condensate above the level of the steam–water mixture in the thermosiphon.

Analysis of screw feeding of faceted particles by discrete element method

Reliable and consistent powder flow in screw feeders is of great interest to a wide range of industries, particularly for continuous manufacturing of pharmaceutical powders. However, analysis of flow of cohesive powders with sharp corners and edges, as commonly found in the case of crystalline solids, presents a great challenge due to complexity of shape and its influence on flow. In the present work, the influence of particle shape and cohesion on phenomena such as cohesive arching in hoppers and screw feeder pitches is analysed by numerical simulations using the Discrete Element Method, and their impact on the outlet mass flow rate is evaluated. Faceted and spherical particles with different cohesion levels are generated and allowed to settle in a hopper on top of a screw feeder. The screw is then rotated, thus feeding the particles through the barrel. Particle interactions are analysed numerically for the hopper region, a predominantly slow-flow regime, and for the pitches of the screw feeder, where a speed-dependent regime prevails.Paracetamol crystal shape is taken as a model faceted shape. Its parameters such as the coefficients of restitution and friction, needed for the simulations, are calibrated by experimental work.Transient arching occurs as the level of cohesion is increased. The frequency of formation and collapse of arches within the hopper region increases, and eventually, permanent arching is observed. Analysis of stress and strain rate in the screw barrel region shows that the shear stress is a weak function of the shear rate with a power index of around 0.3, which is independent of particle shape. The flow rate is influenced considerably by particle shape, whilst increased cohesion causes an increase in void fraction and affects transient arching.

Modeling mesoscale energy localization in shocked HMX, Part II: training machine-learned surrogate models for void shape and void–void interaction effects

Surrogate models for hotspot ignition and growth rates were presented in Part I, where the hotspots were formed by the collapse of single cylindrical voids. Such isolated cylindrical voids are idealizations of the void morphology in real meso-structures. This paper therefore investigates the effect of non-cylindrical void shapes and void-void interactions on hotspot ignition and growth. Surrogate models capturing these effects are constructed using a Bayesian Kriging approach. The training data for machine learning the surrogates are derived from reactive void collapse simulations spanning the parameter space of void aspect ratio (AR), void orientation ($\theta$), and void fraction ($\phi$). The resulting surrogate models portray strong dependence of the ignition and growth rates on void aspect ratio and orientation, particularly when they are oriented at acute angles with respect to the imposed shock. The surrogate models for void interaction effects show significant changes in hotspot ignition and growth rates as the void fraction increases. The paper elucidates the physics of hotspot evolution in void fields due to the creation and interaction of multiple hotspots. The results from this work will be useful not only for constructing meso-informed macro-scale models of HMX, but also for understanding the physics of void-void interactions and sensitivity due to void shape and orientation.

Surface topographical studies of glass fiber reinforced epoxy-ZnO nanocomposites

The objective of present research work is to investigate the surface morphology and surface microhardness of unidirectional E-glass fiber epoxy composites filled with varying amount of ZnO nanofiller content such as 1, 2, 3, 4 and 5 wt% respectively. ZnO nanofiller was added to the epoxy resin matrix in varying amount (wt%) using mechanical stirrer and followed by ultrasonication process. The laminate composites were fabricated using a compression molding press technique. Further, laminate composites were subjected to individual characterization and testing according to ASTM standards. The crystalline nature of ZnO nanofiller was studied using x-ray diffraction analysis (XRD) and surface morphology of ZnO nanofiller on the resin surface was examined by using a scanning electron microscope (SEM). The experimental test results revealed that addition of nanofiller content by 1, 2 and 3 wt% resulted in a gradual reduction of void fraction and thereafter increase in void fraction was observed with 4 and 5 wt% of ZnO loading. The surface microhardness results indicated a linear increment with increase in ZnO nanofiller loading from 1 to 5 wt%. Further, surface topography was studied with the help of atomic force microscopy (AFM), to obtain the surface roughness values. The surface roughness values increased with increase in ZnO wt% within the epoxy resin matrix. The results of the surface analysis of the fabricated composites indicate that at higher loading of ZnO nanofiller, there is formation of clusters and agglomerates of the nanofiller which reduces the nano-scale effects of the filler and nanofillers tend to behave as micro-fillers.

Grain size and void formation in Mg alloy AZ31

The present study examines void formation during tensile loading in coarse and fine-grained AZ31 magnesium alloy using tensile testing, scanning electron microscopy and micro X-ray tomography techniques. Reducing the grain size from 30 to 4.5 μm doubles the total tensile elongation. At failure, the fine-grained material displays a higher volumetric number density of voids compared to the coarse-grained counterpart. Large voids with lower sphericity are considerably more prevalent in the coarse-grained material. Depending on the grain size, two different void nucleation mechanisms could be distinguished; failure at twin or grain boundaries or at second-phase particles. The dominant mechanism in the fine-grained material is failure at second-phase particles. The void volume fractions at failure were comparable in both materials. We propose that the present results can be understood in terms of the effect of grain size on the rate at which the void fraction grows with strain. The larger voids formed in the large grain sized samples lead to a more rapid increase in void fraction with strain and thus failure ensues at lower strains.

The Limerick bubbly flow rig: Design, performance, hold-up and mixing pattern

As Euler-Euler CFD simulations of bubbly flows suffer from uncertainties due to the many underpinning models, there is an obvious need of accurate experimental data for validation. With this in mind, a new bubbly flow test rig was built to be operated with and without liquid co-flow, with bubble size as uniform as possible in the range 4–7mm, and with a very even horizontal bubble distribution. We designed the gas sparging system such that we can also produce an essentially bi-modal bubble size distribution. The column consists of two square sections to allow for studying the mixing of two originally separated bubbly flows with either the same or a different bubble size. The bubbles are produced from 2×196 needles, bubble sizes are determined with high-speed imaging and with a simple acoustical method, overall volume fractions in the column by means of air chamber pressure measurements. Overall volume fractions are presented as a function of gas and liquid flow rates, with slip velocity mostly increasing with increasing void fraction. First results are obtained on (a) producing bi-model bubble size distributions and the pertinent volume fractions in the column, and (b) flow patterns in the case of unequal aeration.

Two-phase flow-induced instability and nonlinear dynamics of a single tube in tube bundles in the transverse direction

Two-phase cross-flow-induced instability and chaotic motions of a single tube in a rotated triangular tube array impacting on loose support was studied using an analytic model. The Hopf bifurcation velocities of the tube bundles within void fraction of the two-phase flow ranging from 0% to 80% were first investigated. The theoretical results have indicated that, with increasing void fraction, the tube bundles may become more stable verified by our previous experimental studies. To explore the post-instability dynamics characteristics of a single tube in tube bundles, a Runge-Kutta scheme was used to solve the nonlinear governing equations in five void fraction conditions. The numerical results that when the flow pitch velocity increases beyond the Hopf bifurcation velocity, the limit cycle motion occurs. And, the amplitude of the limit cycle motion grows until chaotic motion occurs at a sufficiently high flow pitch velocity. The effect of the void fraction on the global dynamics of a single tube in tube bundles system was also investigated.

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

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

Influence of Newtonian and non-Newtonian fluid behaviour on void fraction and bubble size for a gas-liquid flow of sub-millimeter bubbles at low void fractions

This study compares the effect of Newtonian and non-Newtonian blood-mimicking test liquids on void fraction and bubble size distribution (BSD) for the case of gas–liquid bubbly flow. Along with this comparison, the novelty of the present work is extended to the examined conditions, which combine low void fractions (<10−1) with bubbles smaller than 1 mm, resembling bubbly flow in human bloodstream during Decompression Sickness. Such conditions, however, can be also found in common two-phase applications such as subcooled flow boiling in macro-channels. Experiments are performed in co-current upward bubbly flow. Void fraction and BSD are determined by a non-intrusive electrical impedance technique and by image analysis of bubbly flow images, respectively. Xanthan gum is employed (0, 150 and 1000 ppm) to impart non-Newtonian (shear-thinning) behaviour to the baseline aqueous glycerol/NaCl test liquid. The role of surface tension (by adding Triton X-100) and gas/liquid superficial velocities (Usg, Usl) on acquired data is studied as well. Results demonstrate that increase of bubble size and decrease of void fraction is primarily due to the non-Newtonian liquid character rather than the increase of dynamic viscosity caused by the addition of high Xanthan gum concentration. The addition of surfactant (Triton X-100) decreases bubble size and increases void fraction. Furthermore, bubble size and void fraction increase with Usg and decrease with Usl. Void fraction signal fluctuations show a strong dependence on bubble size. Furthermore, the performance of an empirical equation that predicts average bubble diameter from void fraction statistical quantities is tested and seems to be unaffected by the Newtonian/non-Newtonian liquid behaviour. Of particular interest are the intense peaks noticed in void fraction signals for the highest examined Xanthan gum concentration (1000 ppm) and the lowest examined liquid superficial velocity (2.89 cm/s). These peaks correspond to voluminous bubbles clusters rising among isolated bubbles, which do not appear at higher liquid velocities.

Research on dynamical overflow characteristics of a vertical H2S‐containing natural gas well

AbstractThis work aims to explore the overflow characteristics of a vertical H2S‐containing natural gas well. A two‐phase flow model for H2S‐containing natural gas well combining with a transient temperature prediction model was established to simulate the overflow process of a vertical H2S‐containing natural gas well. The model was validated by reproducing the field data of Well Longhui #2. The effects of H2S content, mud displacement, drilling fluid density, geothermal gradient, and reservoir permeability on the overflow characteristics of a vertical H2S‐containing natural gas well were studied and analyzed in this work. Results indicate that bubble, slug, and churn flows constitute the main flow patterns in the whole overflow process. The higher the H2S content is, the more obviously the gas void fraction increases. The phase change position of H2S is closer to the wellhead at lower H2S content. An increase in mud displacement indicates the decreases in overflow time. As drilling fluid density increases, the release position of H2S moves up, and the overflow time and shut‐in casing pressure increase. The initial gas void fraction is higher and the gas invasion volume will be larger in gas reservoirs with higher permeability. As the reservoir permeability increases, the shut‐in casing pressure rises while the overflow time declines. With higher geothermal gradient, the wellbore temperature tends to be higher at the same depth, leading to an increase in the H2S solubility. The gasification starting position is further away from the wellhead at higher geothermal gradient. The results of this work could provide important theoretical basis and technical guidance for drilling engineers to reduce a blowout risk during drilling of H2S‐containing natural gas well.

Nonlinear dynamics of a cantilevered tube with clearance constraint subject to two-phase cross-flow

The main objective of this study is to determine the effects of two-phase cross-flow and nonlinear clearance constraint on the dynamic characteristics of steam generator tube bundles. The equation of motion of a cantilevered tube with nonlinear clearance constraint is established utilizing quasi-static fluid force of two-phase flow. The analytical model, after Galerkin discretization and Runge-Kutta algorithm numerical solution, exhibits several interesting behaviors of bifurcations and chaotic motions with increasing two-phase flow velocities after fluidelastic instability occurs. Numerical solutions show that, the bifurcation velocities of Hopf, pitchfork, period-doubling are delayed and the amplitudes of tube vibration at the corresponding bifurcation points become larger with increasing void fraction of two-phase. Moreover, the clearance size, constraint stiffness and asymmetry of clearance support have remarkable effects on nonlinear dynamic behavior. Larger clearance size, constraint stiffness and asymmetry lead to more complex nonlinear behavior, including distorted break along bifurcation track, inverse bifurcation and wider range of chaotic motions, so that some typical bifurcation points become indiscernible.

Study of structural stability of copper crystal with voids from molecular dynamics simulations

We perform molecular dynamics simulations to understand the melting mechanism in Cu crystals (modeled using Embedded Atom Potential) with spherical voids of radii 0.5, 0.6 and 1.0 nm. Along with these systems defect-free Cu and a Cu crystal with layered void is also studied. We analyse crystallinity in these systems using Q6 bond-orientational order parameter, Lindemann index, radial distribution functions and coordination numbers. At 1 atm, all crystals melt between 1700 and 1800 K and melting temperature decreases with increase in void fraction. It is also found that at 900 K voids disintegrate and become a part of the crystal and between 900 and 1700 K crystals amorphize losing long-range order.

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.

Robust, ultra-insulative and transparent polyethylene-based hybrid silica aerogel with a novel non-particulate structure

Aerogels derived from pre-polymerized vinyl trimethoxy silane (VTMS) precursor with nano-size particles are known to exhibit outstanding mechanical and insulation properties. However, the density reduction has been limited by the poor connectivity. This paper presents an innovative technology to generate a new class of VTMS-based hybrid silica aerogels that possess outstanding non-particulate, reticulated structure and superior properties. This technology relies on spinodal decomposition instead of conventionally exploited binodal decomposition, which leads to a particulate structure. This new aerogel technology has significantly increased the void fraction of the pre-polymerized VTMS-based aerogel, which could not be achieved previously using binodal decomposition. The increased void fraction in the form of nano-pores with an average pore size of 21.75 nm nullifies the gas thermal conductivity effectively. Another consequence of the non-particulate structure is decreased processing time by removing the aging step. These improvements are due to the non-particulate structure’s increased connectivity produced by spinodal decomposition. This novel structure was then compared to a particulate counterpart aerogel of the same material derived from the conventional binodal decomposition of the pre-polymerized VTMS precursor. To further decrease the processing cost, a lower molecular-weight polymeric precursor was synthesized under milder polymerization conditions. The effects of the polymeric precursor’s molecular weight on the mechanical and thermal properties of the aerogel created via spinodal decomposition were also investigated.

Investigation on gas-liquid two-phase flow centrifugal pump performances for different rotational speeds

Performance characteristics of a centrifugal pump under gas-liquid mixture are presented, using a direct coupled single-stage, single-suction centrifugal pump. Both experimental and numerical simulations comparison are carried out, for three different rotational speeds and different inlet gas volume fractions, the results of which are presented, based on dimensionless coefficients from similarity laws. The numerical results show that good agreement is obtained with experimental data at nominal rotational speed for several flow coefficients. It is found that the running of the pump is near the sudden break down of the present pump when the inlet void fraction is below 7%. However, numerical results are less sensitive to rotational speed effects compared with experiment ones; the influence of decreasing rotational speed on pump performances is more and more pronounced when inlet gas void fraction increases and flow coefficient decreases. Froude number effects are taken into account in order to explain part of these differences.

Three dimensional void distribution measurement of salt-water pool-boiling in 5 × 5 bundle geometry with X-ray CT system

Boiling of sea water may occur in a pressure vessel of light water (nuclear) reactors to flood the nuclear fuel as an accident management procedure. Another salt water is borated water, which will be injected into the reactor core as a neutron absorber to avoid recriticality. Boiling behavior of such salt water including these mixtures is a key to gain the confidence of accident strategy to cool down the reactor core during accidental conditions. Pool boiling experiments were conducted with a simulated 5 × 5 fuel-rod bundle with three different fluids: water, condensed (two and half times higher concentration) sea water and its mixture solution of sea water and borated water. Three-dimensional void-fraction distributions in the rod bundles were quantified by the high-energy X-ray CT system with a linear accelerator. There are no significant differences in void fraction distributions between condensed sea water and mixture solution. The void-fraction has a peak at the center on horizontal plane for all the fluids. The two salt-waters shift boiling incipience toward downstream and decrease void swell level so that vertical void-fraction profiles of the salt waters are steeper than that of water. This is because created bubbles in the salt waters were smaller than those in water. The spacer has a mixing effect to increase void fraction at upstream of the spacer and homogenize the void fraction on the horizontal plane.

Role of Hydrogen Flow Rate on Microstructure of a-Si:H(n) Films: Spectroscopic Ellipsometry Studies

The influence of hydrogen flow rate on the microstructure and optical properties of phosphorous doped hydrogenated amorphous silicon (a-Si:H(n)) films is studied through spectroscopic ellipsometry (SE). A series of a-Si:H(n) films were deposited using radio frequency plasma enhanced chemical vapour deposition (RFPECVD) technique by varying the hydrogen flow rate (HFR) in the range of 30–80 SCCM. The SE measurements showed a shift in peak position of pseudo dielectric function of these films from 3.6 eV (for pure a-Si:H) to 4.1 eV with a shoulder peak at 3.4 eV (nanocrystallites embedded a-Si:H) as the hydrogen flow rate was increased. A systematic increase in crystalline fraction along with a decrease in void fraction was observed as HFR was increased up to 70 SCCM during deposition. The results confirm that hydrogen dilution helps in improving the microstructure of films and favours the formation of strong Si–H bonds. Beyond 70 SCCM of HFR, a large flux of hydrogen resulted in an increase in void fraction and disorder in the films.