Critical Volume Fraction Research Articles

Order-disorder transition during shear thickening in bidisperse dense suspensions

Shear thickening of multimodal suspensions has proven difficult to understand because the rheology depends largely on the microscopic details of stress-induced frictional contacts at different particle size distributions (PSDs). Our discrete particle simulations below a critical volume fraction ϕc over a broad range of shear rates and PSDs elucidate the basic mechanism of order–disorder transition. Around the theoretical optimal PSD (relative content of small particles ζ1= 0.26), particles order into a layered structure in the Newtonian regime. At the onset of shear thickening, this layered structure transforms to a disordered one, accompanied by an abrupt viscosity jump. Minor increase in large-large particle contacts after the order–disorder transition causes apparent increase in radial force along the compressional axis. Bidisperse suspensions with less regular but stable layered structure at ζ1= 0.50 show good fluidity in the shear thickening regime. This work shows that in inertial flows where particle collisions dominate, order–disorder transition could play an essential role in shear thickening for bidisperse suspensions.

Interstage Performance and Power Consumption of a Multistage Mixed-Flow Electrical Submersible Pump in Gas–Liquid Conditions: An Experimental Study

Abstract The performance degradation of electrical submersible pump (ESP) caused by high gas volume fraction (GVF) has always been a common problem in the field of oil and gas production. Experiments are conducted to investigate the gas–liquid performances of pressurization, power, and efficiency of a 15-stage mixed-flow ESP self-designed and manufactured. A calculation method of gas–liquid useful power is proposed based on the hypothesis of isothermal compression, with the relative error of calculated results within ±2% between isothermal compression and polytropic compression. The rapid decrease of useful power is found to be the main reason for the rapid decrease of hydraulic efficiency as the pump-inlet gas volume fraction (IGVF) increases. Moreover, the interstage pressurization deterioration gradually weakens along the increasing direction of stage number. The calculation method of interstage gas volume fraction is proposed, and the gas–liquid pressurization of each booster stage is found to follow the same distribution with the variation of interstage gas volume fraction. Increasing the stage number can efficiently improve the critical gas volume fraction of interstage pressurization deterioration, as well as the pressurization boundary and maximum value of average single-stage gas–liquid pressurization. The prediction correlations of gas–liquid operating conditions of the maximum pressurization and the highest hydraulic efficiency are established, respectively, and the gas–liquid condition range of high-efficiency pressurization is discovered.

Experimental investigation and numerical analysis on the confined deflagration behavior of methane-air mixtures within the suppression of typical haloalkanes

To investigate the mechanisms of haloalkanes in promoting and inhibiting explosions in methane-air mixtures, the halon alternatives of CHF3, C3HF7, and C3H2F6 were selected as inhibitors. Experimental research was conducted in a 20-liter spherical explosion vessel to examine the promotional and inhibitory effects of these haloalkanes on methane-air mixtures. The adiabatic flame temperature, heat release rate, and concentrations of the key radicals were calculated. The results showed that C3H2F6, C3HF7, and CHF3 all exhibited a dual effect, initially promoting and subsequently inhibiting explosions in fuel-lean methane-air mixtures. The critical explosion suppression volume fractions of C3HF7 and CHF3 were 1.5 times and 2.25 times that of C3H2F6, respectively, for the suppression of methane explosion at Φ = 1.2. Furthermore, C3HF7 demonstrated a dual impact on the explosion of stoichiometric methane-air mixtures. From the perspective of reaction kinetics, for fuel-lean methane-air mixture (Φ = 0.8), a small volume of halogenated hydrocarbons led to an increase in the adiabatic flame temperature due to the heat release from critical fluorine-containing elementary reactions, thereby promoting the explosion. Under fuel-lean conditions, the introduction of a small volume of halogenated hydrocarbons led to an increase in the adiabatic flame temperature due to the heat release from critical fluorine-containing elementary reactions, thereby promoting the explosion. Simultaneously, these fluorine-containing reactions scavenged the key free radicals of H, O, and OH inhibiting the explosion process. These two effects competed with each other, ultimately determining the direction of the explosion process.

Critical consideration of gas bubbles to replace fat globules in acidified milk matrices

The influence of low gas volume fraction and bubble deformability on rheological properties and microstructure of acidified milk matrices was investigated; functionality was compared with that of fat globules. With increasing fat volume fraction, stiffness and yield stress of the acidified milk models increased, but decreased with increasing gas volume fraction. This was caused by the large size of the bubbles compared with the fat globules and different processing of the samples containing gas and fat, respectively. A mixture of 1% fat together with gas bubbles resulted in a smaller mean bubble size and thus stiffer bubbles. Simultaneously, an increase in stiffness and yield stress of the bubble-containing matrices was observed, although not to the same extent as for unfoamed matrices with increasing fat content. Beyond a critical gas volume fraction of 0.27, gas bubbles were no longer separated and interacted with other matrix components and themselves, weakening the structure.

Development of low carbon engineered cementitious composite (ECC) using nano lime calcined clay cement (nLC3) based matrix

The construction industry is responsible for around 5% of total CO2 emissions globally. As a result, recent research is focused on the sustainability aspects of construction materials. Engineered cementitious Composites (ECC) are high-performance fiber reinforced composites with improved ductility and tensile strength but utilize higher cement content, raising concerns about their sustainability. In this study, nano-lime and calcined clay are combined with cement to develop a high strength sustainable nano lime calcined clay cement (nLC3) based Engineered Cementitious Composite (ECC). Initially, the packing density model was employed to develop a high strength nLC3-based matrix by combining the particle packing models and chemical compatibility while keeping costs to a minimum. Micromechanical modeling was then applied to determine the critical volume of fibers for the novel matrix using single fiber pullout tests and matrix toughness tests. According to the micromechanical model, the critical volume fraction is found to be 1.93% for the nLC3 mix. This was further confirmed by casting dog bone samples containing 2% fibers by volume which confirmed strain hardening response. The uniaxial test results indicated a strength of around 5.85 MPa in tension and 51 MPa in compression with about 3% tensile strain. These results are comparable to conventional ECC and better than previously developed LC3 based ECC. This study reveals that nLC3-based ECC is a more sustainable composite as compared to conventional ECC mix without any compromise on strength and cost.

Experimental study on the fire control and smoke transportation using sealing strategy in tunnel

Sealing tunnel portals is a favorable approach to controlling tunnel fires. This paper experimentally studied the influence of portals-sealing on fire behavior in a small-scale tunnel. A series of characteristic parameters, including fuel burning rate, O2 and CO2 volume fraction, and ceiling smoke temperature in the tunnel were measured. Besides, the flame self-extinguishment time, smoke transportation characteristics and gas temperature field at different fire source locations and sealing conditions were systematically investigated. Results indicated that the critical equivalence ratio for methanol fuel converted to ventilation-controlled tunnel fires is within 0.783–0.923. The combustion state is influenced by fire location. Fires occurring near the tunnel entrance are more prone to self-extinguishment than those away from the tunnel entrance. The critical volume fraction of O2 for flame self-extinguishment ranges from 13.5% to 15.3% and decreases with increasing gas temperature. Moreover, tunnel sealing significantly impacts peak gas temperature values, but hardly changes the longitudinal distribution pattern of temperature. The longitudinal distribution model of gas temperature under the action of the strong plume in a confined space is developed. This study provides guidance for fire risk assessment during the tunnel sealing process and contributes to the scientific development of sealing strategies.

Rheology of sheared polyhedral granular materials in inclined flows

An investigation of frictional, Platonic solid-shaped particle flows on inclined planes is performed using the discrete element method, and the effects of particle angular shape on rheological properties are analyzed. Higher shear stresses at a specified depth of particle bed are obtained for more angular particles. As particle angularity increases, rapid surges in the coordination number and solid-phase stresses occur at a smaller critical solid volume fraction. The friction and dilatancy laws of polyhedral particle flows are significantly different from those of spherical particle flows, due to particle angularity. Nevertheless, by applying a specific rolling friction to the spherical particles, their rheological properties can match those of the polyhedral particles, indicating that the angular particle shape plays a similar role to the rolling friction in preventing particle rotation. Thus, the flows of spherical particles with a rolling friction incorporated can be used to mimic the flows of angular polyhedral particles. At last, a scaling law is adopted to describe rheological relations for various polyhedral particles based on a bulk friction coefficient, a dimensionless granular temperature, and an inertial number.

Computational and experimental case study on the instability of water with gold, silver and platinum nanoparticles

Research in the field of thermal enhancement is going to increase due to their diverse applications in the field of chemical industries and engineering domains. Nanofluids deal with the concentration of nano-sized particles that act as base fluids, such as vegetable oil, vinegar, ethylene glycol, water, etc. In the current study, the hydro-dynamic or MHD (Magneto-hydrodynamic) instability of nanofluid in which nanoparticles (gold, silver and bentonite) are considered to encounter wastewater treatment. Here, the special nature of the magnetic field known as the transverse magnetic field is used in the current analysis. Reynold’s number with various parameters is analyzed for the stability of wastewater. Nanoparticles dispersed due to the effect of magnetic effect in a base fluid (water) are employed. This experiment is done by using Reynold operations, it is observed that first, the fluid flow is not stable and adding a nanoparticle flow to be more stable. The critical Reynolds number, temperature, time, pressure, and volume fraction of nanoparticles are estimated and given numerically in tables and graphs. The instabilities of nanofluids that contain nanoparticles with water as the based fluid for different embedded physical parameters are compared and discussed using growth rate and neutral graphs.

A cyclic GTN model for ultra-low cycle fatigue analysis of structural steels

Ultra-low cycle fatigue (ULCF) is a critical concern in the seismic design of steel structures due to its adverse effects on the ductility and energy dissipation capacity of steel connections. The Gurson-Tvergaard-Needleman (GTN) model, a well-accepted micromechanical fracture model, cannot be applied directly to ULCF analysis, as it requires proper estimation of the effects of cyclic loadings on the ductile fracture of materials. In this paper, a cyclic GTN (C-GTN) model was proposed for the ULCF prediction of materials, in which the evolution of microvoids during ULCF loadings was addressed for tensile and compressive half load cycles, respectively, and the effect of the cumulative plastic strains on the material’s resistance to ductile fracture was considered by the reduction of the critical void volume fraction for void coalescence. Then, a VUMAT subroutine was programmed for the C-GTN model, in which the cyclic hardening behavior of materials was simulated with the Voce-Chaboche model. Finally, the C-GTN model was calibrated for the G20Mn5QT cast steel based on previous tests on notched round bar specimens of the material. The applicability of the C-GTN model and the VUMAT subroutine were validated in the ULCF analysis of double-hole plate specimens of the G20Mn5QT cast steel.

Morphological Evolution of Hybrid Block Copolymer Particles: Toward Magnetic Responsive Particles.

The co-assembly of block copolymers (BCPs) and inorganic nanoparticles (NPs) under emulsion confinement allows facile access to hybrid polymeric colloids with controlled hierarchical structures. Here, the effect of inorganic NPs on the structure of the hybrid BCP particles and the local distribution of NPs are studied, with a particular focus on comparing Au and Fe3O4 NPs. To focus on the effect of the NP core, Au and Fe3O4 NPs stabilized with oleyl ligands were synthesized, having a comparable diameter and grafting density. The confined co-assembly of symmetric polystyrene-b-poly(1,4-butadiene) (PS-b-PB) BCPs and NPs in evaporative emulsions resulted in particles with various morphologies including striped ellipsoids, onion-like particles, and their intermediates. The major difference in PS-b-PB/Au and PS-b-PB/Fe3O4 particles was found in the distribution of NPs inside the particles that affected the overall particle morphology. Au NPs were selectively localized inside PB domains with random distributions regardless of the particle morphology. Above the critical volume fraction, however, Au NPs induced the morphological transition of onion-like particles into ellipsoids by acting as an NP surfactant. For PS-b-PB/Fe3O4 ellipsoids, Fe3O4 NPs clustered and segregated to the particle/surrounding interface of the ellipsoids even at a low volume fraction, while Fe3O4 NPs were selectively localized in the middle of PB domains in a string-like pattern for PS-b-PB/Fe3O4 onion-like particles.

Ductile fracture prediction of HPDC aluminum alloy based on a shear-modified GTN damage model

In this paper, we investigate how the shear-modified Gurson-Tvergaard-Needleman (GTN) model can be used to reveal the effect of manufacturing-process-induced porosity on the scatter of ductile fracture properties of a high-pressure die-casting aluminum alloy. The GTN model exhibits great advantages in predicting the variation of failure strain/displacement by altering the initial void volume fraction (f0), compared with uncoupled ductile damage models. For a specific metallic material, one unique set of model parameters is usually determined from experiments, which leads to the fact that only a deterministic failure strain/displacement can be obtained under a fixed porosity. To overcome this shortcoming of the shear-modified GTN model, a novel parameter calibration scheme is proposed and validated in the present study. Following the physical background of the GTN model, the twelve material parameters of the shear-modified GTN model have been determined based on a combination of macroscopic mechanical tests covering a wide range of stress states and micromechanics-based unit cell simulations. Different from the existing calibration strategies, the simulation results of unit cells embedded with a spherical void have revealed a mutual dependence between the f0 and critical void volume fraction (fc). By conducting interrupted shear tests, the evolution of secondary nucleated void volume fraction was identified according to the statistical results of the fractured brittle silicon particles. The remaining parameters have been further iteratively determined. In the end, the effectiveness of the proposed parameter determination approach is confirmed based on the accurate prediction of stochastic ductile failure properties (both the global failure displacements and the local failure patterns) in different fracture tests. Given the proposed parameters identification strategy, the GTN material models have been enriched to predict the ductile failure behavior of metallic materials, which possess relatively high porosities and more pronounced scattering in failure properties.

Pickering phase change slurries

HypothesisPhase change slurries (PCS) have emerged as a promising class of oil-in-water emulsions for energy applications, but stability remains an issue. Pickering phase change slurries (PPCS) stabilized solely by nanoparticles could offer enhanced stability. We hypothesize that stability in PPCS can be achieved by tuning environmental variables of salinity and temperature. ExperimentsA paraffin-based PPCS stabilized using fumed silica nanoparticles was developed and assessed under varying NaCl concentrations (up to 150 mM) and temperatures (up to 70 °C). Extended-DLVO modeling, confocal, and cryogenic electron microscopy analyzed the silica-paraffin interactions. Rheological experiments examined the impact of effective volume fraction, thermal expansion, and salinity on the viscosity and shear stability of PPCS. The stability of the resulting formulation was assessed under high pressure and temperature conditions. FindingsIncreased salinity did not change the packing density of the silica at the oil–water interface (82% ± 6%) but did increase the adsorbed layer thickness and network formation, enhancing the formulation's resistance to shear-induced instability. A critical volume fraction of 0.51 ± 0.01 was identified, beyond which viscosity increased significantly. The resulting formulations remained stable under high pressures and temperatures, regardless of salinity. These findings offer insights into the variables affecting PPCS properties, assisting in designing stable PPCS formulations for diverse applications.

Strain hardening magnesium-silicate-hydrate composites with extremely low fiber dosage of 0.5% by volume

Formulation of strain hardening cementitious composites typically engage 2% or more fiber by volume, resulting in higher cost and difficult processing. This study presents the development of strain hardening magnesium-silicate-hydrate composite with an extremely low fiber volume fraction of 0.5% via micromechanics-guided design approach. The developed composite demonstrated a tensile strain capacity of 7.2% with a tensile strength of 2.24 MPa, and a compressive strength of 86.1 MPa. The fiber/matrix interfacial bond was characterized using single fiber pullout test. The microstructural characterization of fiber surface and fiber tunnel in the matrix was carried out to understand the fiber/matrix interface properties. The micromechanics-based assessment of critical fiber volume fraction required to achieve strain hardening was also conducted. The material sustainability of the developed composite was evaluated and compared with existing Portland cement-based strain hardening cementitious composites, and strategies to further reduce embodied carbon and primary energy were proposed.

Structural transition in a ferroelectric nano-dispersed cholesteric liquid crystal

ABSTRACT We present a theoretical study of the orientational and electrical behaviors of a cholesteric liquid crystal dispersed with ferroelectric nanoparticles. We assume a soft planar coupling between the liquid crystal molecules and the nanoparticles. We consider two spiral structures in the ferroelectric nano-dispersed system under an external electric field. This assumption is due to the fact that the director and the polarization vector would have different pitches of spiral structure. We study the behavior of the average polarization and the pitch of the helical structure as a function of the field strength. The impact of ferroelectric nanoparticles on cholesteric-nematic phase transition is investigated by calculating the critical electric field. The influence of field strength and material parameters on the phase transition is also discussed. The calculations are based on a developed continuum theory and a modified form of free energy for the helical supramolecular structure. The influence of nanoparticle volume fraction on the helix unwinding of both spiral structures is studied. It is found that an electric field with a sufficiently high strength causes an increase in the pitch of the helical structure of polarization. We obtain a critical volume fraction of nanoparticles, after which the pitch of the polarization helical structure differs from the director pitch.

Dimensionless unified modelling of macroscale electric properties of conductive fibre networks in different regimes

The electric properties of conductive fibre networks, which can be representative of carbon nanotube networks, are investigated by means of numerical simulations and a dimensionless unified macroscale model is developed. Fibres are represented as rigid chains of discrete element method particles and we compress systems of fibres in a representative volume element periodic in three dimensions. In the used electric resistor network method, the particles are used as discretisation points to construct a system of linear equations linking the particle conductivities to their local electric potentials and in turn allowing for predictions on the electric macroscale properties of the fibre system. A carried out dimensional analysis suggests suitable scaling laws to unify different macroscale fibre systems in two different regimes - a percolating and a conductive regime. The dimensionless macroscale conductance is found to depend on the percolation threshold and percolation probability. It is moreover found that the critical solid volume fraction for rigid fibres is not only dependent on the fibre aspect ratio, but can also depend on the compression height of the system. Additionally, correlations between transmittance and solid volume fractions are found allowing for possibly simple solid volume fraction estimations in experiments.

Micromechanical characterization of a reduced-activation ferritic/martensitic steel by instrumented indentation and scratch methods

Micromechanical properties of a reduced-activation ferritic/martensitic steel CLF-1 were characterized by nanoindentation tests with Berkovich indenter, microhardness tests with Vickers and Knoop indenters, and microscratch tests with Berkovich and spherical indenters to investigate the effects of different indenters. Elastic modulus and indentation hardness obtained by various methodologies such as energy-based and displacement-based approaches are consistent with those obtained by Oliver & Pharr method. The reasonable fracture toughness of CLF-1 steel can also be obtained by energy-based nanoindentation method based on the deterioration of indentation hardness with the critical void volume fraction of 0.49.

Strengthening mechanism of NiCoAl alloy induced by nanotwin under Hall-Petch effect

Grain boundary plasticity dominates the mechanical behavior of materials, and the introduction of intragranular nanotwins can achieve coordination between strength and ductility in metals and alloys. In this effort, nanotwins with different volume contents are introduced in NiCoAl alloys to explore the strengthening mechanism under the Hall-Petch effect from perspectives such as grain boundary energy, grain rotation angle, and grain coarsening. The results indicate that the introduction of nanotwins with critical volume fraction effectively inhibits grain boundary migration and grain combination, and improves the strength of the alloy by reducing grain boundary energy and hindering grain rotation. Meanwhile, it leads to a change in the strengthening mechanism from the combined effects of dislocation movement, grain boundary migration and deformation twinning to the dual strengthening of local movement of dislocations within the grain and Lomer-Cottrell locks. The latter hinders the slip of local dislocations within the grain and improves the mechanical property. In addition, based on the average grain size and atomic mean square displacement after high-temperature annealing, the stability mechanism of nanotwins in the alloy can obtain an insight from the aspect of atomic motion. This article explains the Hall-Petch strengthening mechanism from a more detailed and interesting point and provides insights into the strengthening mechanism of nanotwins under the Hall-Petch effect.

Experimental determination of electrical and thermo-physical characteristics of dielectric nanofluids based on volume fraction change

Dielectric insulating fluids perform critical tasks such as electrical insulation and cooling functions in power system equipment. Improving the thermal properties of insulating fluids extends the service life of power equipment, while improving their dielectric properties ensures a reliable and safe electricity supply. Therefore, studies on enhancing the thermal and dielectric qualities of dielectric fluids using nanoparticles have become more popular in recent years. In this study, the dielectric and thermo-physical properties of mineral oil-based SiO2 nanoparticle suspensions, namely nanofluids, have been investigated. An approximately 25.6% enhancement in AC breakdown voltage level has been obtained at a 0.05% critical volume fraction. It has been noted that the level of the AC breakdown voltage decreases when the critical volume fraction is exceeded. The thermal conductivity of the prepared nanofluid has changed logarithmically in the positive direction according to the rising number of nanoparticles per unit volume. Thermal conductivity has increased by around 8.55% for the critical volume fraction value, whereas thermal diffusivity has increased by approximately 22%. Similarly, when the number of particles in the nanofluid rises, the viscosity increases nonlinearly. The viscosity increase, which is undesirable for heat transfer, has been determined to be around 10.64% for the crucial volume fraction. These findings indicate that dielectric nanofluids hold significant potential for the future. The SEM analysis, on the other hand, explains the difficulties of assuring the long-term stability of nanofluids in the volume fraction with the highest AC dielectric breakdown voltage.

Micromechanical characterization of microwave dielectric ceramic BaO–Sm 2O 3–5TiO 2 by indentation and scratch methods

Mechanical characterization of dielectric ceramics, which have drawn extensive attention in wireless communication, remains challenging. Micromechanical properties with microstructures of dielectric ceramic BaO-Sm₂O₃-5TiO₂ (BST) were assessed by nanoindentation, microhardness, and microscratch tests under different indenters, along with X-ray diffraction, scanning electron microscopy, and Raman spectroscopy. Accurate determination of elastic modulus (i.e., 260 GPa) and indentation hardness (i.e., 16.2 GPa) of brittle BST ceramic by instrumented indentation technique requires low loads with little indentation-induced damage. Elastic modulus and indentation hardness were analyzed by different methodologies such as energy-based and displacement-based approaches, and elastic recovery of Knoop imprint. Consistent values (about 3.1 MPa·m^1/2) of fracture toughness of BST ceramic were obtained by different methods such as Vickers indenter-induced cracking method, energy-based nanoindentation approaches, and linear elastic fracture mechanics-based scratch approach with a spherical indenter, demonstrating successful applications of indentation and scratch methods in characterizing fracture properties of brittle solids. The deterioration of elastic modulus or indentation hardness with the increase in indentation load is caused by indentation-induced damage, and can be used to determine fracture toughness of material by energy-based nanoindentation approaches, and the critical void volume fraction is 0.27 (or 0.18) if elastic modulus (or indentation hardness) of the brittle BST ceramic is used. The fracture work at the critical load corresponding to the initial decrease in elastic modulus or indentation hardness can also be used to assess fracture toughness of brittle solids, opening new venues of application of nanoindentation test as a means to characterize fracture toughness of brittle ceramics.

Dynamic recrystallization behavior and processing maps of 5CrNiMoV steel during hot deformation

Hot deformation tests of 5CrNiMoV steel were performed at deformation temperatures of 700 to 870 °C and strain rates of 0.001 to 0.1 s−1 using the DIL 805D thermomechanical simulator. The critical strain and volume fraction models of Dynamic Recrystallization (DRX) were constructed based on the work hardening theory. The results showed that the critical strain of DRX decreases with increasing deformation temperature and decreasing strain rate, which implies that DRX occurs easily at higher temperatures and lower strain rates. The average DRX grain size model was established to predict grain size changes during hot deformation. Based on the hot processing maps that were constructed using the Dynamic Material Model (DMM) and microstructure observation, the optimum hot working parameters for 5CrNiMoV steel are a deformation temperature of 800 °C–870 °C, a strain rate of 0.001–0.05 s−1.