Particle Volume Fraction Research Articles

Influence of Temperature on Properties and Dynamics of Gas-Solid Flow in Fluidized-Particle Tubular Solar Receiver

A fluidized particle single-tube solar receiver has been tested for investigating the gas-particle characteristics that enable the best operating conditions in a commercial-scale plant. The principle of the solar receiver is to fluidize the particles in a vessel – the dispenser – in which the receiver tube is plunged. The particles are flowing upward in the tube, irradiated over 1-meter height, by applying an overpressure in the dispenser. Experiments with a concentrated solar flux varying between 188 and 358 kW/m² are carried out, and the particle mass flux varied from 0 to 72 kg/(m²s). The mean particles and external tube wall temperatures in the irradiated zone are heated from the ambient to respectively 700°C and 940°C. It is shown that the temperature rise leads to a decrease of the particle volume fraction. Furthermore, a self-regulation of the system is evidenced with a short transient regime. This characteristic is essential from the operational viewpoint. The thermal efficiency of the receiver increases with the particle flow rate, reaching between 60 and 75% above 30 kg/(m²s). Several fluidization regimes are identified thanks to pressure signal analyses, like slugging, turbulent and fast fluidization, showing that regimes transitions are strongly affected by the temperature.

The influence of olivine settling on the formation of basaltic cumulates revealed by micro-tomography and numerical simulations

The speed at which crystals settle in magmatic reservoirs affects the solidification rate of magmas and their differentiation. Despite extensive prior work on the subject, most of our quantitative understanding of the process is still restricted to treating crystals as spherical particles and does not address the geometric complexities of natural crystals. Here, we use three-dimensional (3D) X-ray microcomputed tomography (X-μCT) observations on olivine crystals from Kīlauea Volcano (Hawai‘i) to document their highly intricate and variable geometries, and the textural growth relationships between the olivine crystals and their inclusions (melt, spinel, and fluid/vapor bubbles). Olivine crystals generally have clustered polyhedral or skeletal shapes, which reflect variable magmatic conditions (or growth rates) during their formation. The cumulative presence of spinel, melt, fluid, and vapor inclusions affects the density of the host crystals by up to 6% relative, and thus plays a limited role on modifying crystal settling rate. In contrast, the overall crystal shape plays a major role. We performed numerical simulations employing a finite element method to investigate the effect of crystal morphology on settling rate and the evolution of the particle volume fraction in a magmatic convective layer. We show that for all olivine geometries investigated, the settling velocity is highest when the long axis of the crystal is aligned with the flow direction of the melt. Increasing the aspect ratio of the olivine tends to decrease its settling velocity and results in an increase in the influence of its orientation on the terminal velocity (UT). Extrapolation of the simulation results to variable particle volume fractions (Φ = 0–0.5) indicates high crystal settling rates (UT = ∼9.7 × 10−6–1.4 × 10−5 m/s) that are used to estimate the timescales for the formation of olivine cumulates in natural melt-dominated basaltic systems. The formation of olivine cumulates is therefore rapid, potentially leading to the accumulation of a crystal layer at the bottom, where the frictional contacts between the crystals exert a rheological lock-up acting against further convection. Crystal accumulation in the locked layer (parametrized with the solid/liquid volume ratio in the reservoir) is a function of the reservoir size and crystal fraction, and takes a few years in small reservoirs (<1 km thick) and a few decades in larger reservoirs (several kms thick). We propose a calibration of olivine suspension timescales for mafic magma reservoirs (based on the knowledge of the particle volume fraction, reservoir height, and olivine morphology). This calibration is used to estimate the rate of cumulate build-up, and can help interpret crystal size distributions in the framework of crystal suspension times.

Experimental study of a dense stream of particles impacting on an inclined surface

We present measurements of the impact process of gravity-driven flows of particles against inclined surfaces for a range of particle loadings in the four-way coupled regime. These were investigated with particle image velocimetry (PIV) for a range of particle curtain thicknesses, volume fractions, velocities and mass flow rates using a series of hoppers of various aperture sizes, following impact on a steel plate at a range of angles from 15° to 75°. In addition, micro-shadowgraphy was used to provide new details of the region of impact, providing the first direct evidence of the formation of layer of particles that slide along the surface for sufficiently high particle volume fractions. Statistical measurements of particle impact and rebound velocities, together with rebound angle and coefficient of restitution, were obtained from the PIV analysis, which revealed that an increase in particle loading leads to increase in the probability that a layer of particles at the surface is formed at the point of impact. This layer acts to absorb energy for all impact angles, thereby influencing the rebound processes. For impact angles close to the normal direction the layer also increases the range of rebound angles, while it causes particles to slide along the surface as a chute-like flow for large inclination angles.

Two-way coupled CFD-DEM model of a pre-mixed abrasive water jet and its application to the investigation of abrasive motion characteristics

This paper uses the weight function to modify the particle volume fraction algorithm. The VOF model is modified by adding the particle phase volume fraction and momentum source to the control equation, the semi-resolved VOF-DEM model is established, and PTV experiment is used to verify the model. Based on this model, the flow and distribution characteristics of the abrasive are obtained: 1) The water-abrasive flow near the outlet of the convergent section of the nozzle has a significantly dense two-phase flow characteristic, and the collision between particles dominates particle flow. 2) The particles at the nozzle outlet are distributed in the radial direction of “n”, and with the increase of the stand-off distance of the jet, the particle phase volume transitions from “n” to “m” distribution. The research on the flow characteristics of particles can provide a basis for the nozzle design of pre-mixed abrasive water jets.

3-D numerical simulation of the heat transfer of a fluidized bed with a horizontal tube bundle and Geldart D particles

In the present study, a numerical simulation is performed to analyse hydrodynamic and heat transfer of a fluidized bed in a vertical channel. As a novelty, MP−PIC method is applied to 3-D tube bundles immersed in a fluidized bed with Geldart D particles. The main results are compared and validated using the experimental results published in the literature.In both tube bundle distributions, the lowest heat transfer coefficients were obtained in the top region of the tube, which represents approximately 58% of the time average heat transfer coefficient of the surface tube. This behaviour differs at the bottom and sides, where particles are continuously being replaced by other particles, resulting in low particle volume fractions and higher heat transfer coefficients.Analysing each tube arrangement for the cases with the same gas velocity and particle diameter, the results numerically obtained were consistent with the experimental findings. The present study found that the time average heat transfer coefficient for the staggered arrangement was approximately 6%, 5% and 1.6% higher than that in-line, for the particle diameter of 1400, 1600 and 1850 μm, respectively. Some discrepancies can be highlighted when comparing the experimentally and numerically obtained time average heat transfer coefficients, with maximum differences close to 18%, which correspond to the cases with the highest gas velocity and particle diameter (1.71 m/s, 1850 μm).

Accounting for spatial distribution in mean-field homogenization of particulate composites

Several analytical mean-field homogenization methods, which take into account the particle volume fraction, shape and orientation are readily available to estimate the effective properties of particulate composites. Models have also been proposed to account for the spatial distribution of the particles. The classical Ponte-Castañeda and Willis (PCW) model is based on a parametrization of the statistical distribution law, while the Interaction Direct Derivative (IDD) model associates a matrix cell to each inclusion, representative of close interactions. In the literature, the use of the IDD is commonly reduced to the particular case of the classical Mori and Tanaka (MT) scheme or to the aforementioned PCW model. The present study introduces an original approach to calibrate the IDD model, for 2D linear conductivity, based on representative images of the microstructure. The links between the models and the range of validity of the IDD model are discussed. Besides, an “IDD-based” PCW model and a two-step scheme are proposed for situations where the IDD estimate is inconsistent (lack of major symmetry). Finally, an image analysis method using Voronoï diagrams is implemented to define the cells associated to each inclusion and supply the models. The method is validated by comparisons between the obtained IDD and PCW estimates, the Mori–Tanaka (MT) model and benchmark full-field numerical simulations. Accounting for the inclusion distribution is seen to lead to better estimates, both qualitatively (by capturing anisotropic behaviors due to the sole distribution) and quantitatively. Possible extensions to elastic composites are discussed.

Investigation on Critical Characteristics of Edible Vegetable Oil by using Cuo Nanoparticles

This work demonstrates how the composition of the nanoparticles affects the thermal, electrical, and physical characteristics of vegetable oil (RBO). Cuo nanoparticles with particle volume fractions varying from 0.1 percent to 0.4 percent were injected one at a time into vegetable oil (RBO) to produce nanofluids (NFs) that affect the basic characteristics of the oil. It was able to evaluate characteristics including room temperature viscosity, firepoint, breakdown voltage (BDV), and flash point using IEC and ASTM standards. The breakdown voltage rises when the vegetable oil (RBO) oil is heated in the presence of nanoparticles. The size of the nanoparticles has changed the viscosity, and the flash and fire points have gone from 10°C to 15°C..

Efficient cooling system design by using different shaped nanoparticles and rotating cylinders in channel system for photovoltaic thermal management

Research focusing on improving the efficiency and performance of renewable energy systems requires the development of cooling systems (CSs) with effective solutions, including photovoltaic (PV) thermal management. The current work proposes a unique cooling method that uses double rotating cylinders (RCs) and different shaped alumina nanoparticles (NP; spherical, blade, brick, and cylindrical) in water for conductive panel’s cooling channel. The numerical study is performed for different values of Reynolds number (Re: 100–600), rotational Re (Rew: 0–20), size of the cylinders (Rc: 0.01H–0.2H), and solid particle volume fractions (svf: 0–0.03). It is observed that vortex size and numbers behind the cylinders can be controlled by varying Re and Rew. A 16 °C temperature drop of the panel is achieved at the highest Re at svf = 0.03. When rotations are active, the average Nu increases 99% and 37% for water and nanofluid (NF) at Rew = 20 and Re = 250 while temperature drops of 15 °C and 7.5 °C are obtained. The highest performance coefficient (PEC) is obtained as PEC = 1.38 when NF is used at Rew = 15. Large cylinders results in lower PEC values. When rotations are not active, the average Nu rises by about 26.8%, 13.5%, 12.8%, and 3.5% for cylindrical, blade, brick, and spherical-shaped particles while average cell temperature drops by about 5.5 °C, 4.5 °C, 4.3 °C, and 1 °C. The highest PEC value is obtained as 1.23 when cylindrical particles at the highest loading are used for stationary cylinders. Proper orthogonal-based model is considered for estimation of spatially varying panel temperature.

The Impact of the Weak Magnetic Field on Hydromagnetic Nanofluid Flow Via Divergent and Convergent Channels

This paper has studied the effect of a low-intensity of magnetism acting on hydromagnetics of nanoparticles (Silver-water) via divergent-convergent channels. The study aimed at determining the effect of Hartmann and Reynolds numbers, the distribution of energy in the system, and the volume fraction of nanofluid particles on the movement of nanofluid particles, the distribution of temperature, and the distribution of concentration of nanofluid particles. The governing equations were transformed to a linear system of the differential equations and numerical solutions were found using the collocation technique and MATLAB was used to generate the results. It is discovered, varying the Reynolds values decreases the distribution of temperature for divergent medium. Variation in Reynolds values augments the distribution of temperature for the shrinking walls. The observation shows that increasing Hartmann values reduces the velocity profile in both channels which are diverging –converging channels. This is because Lorentz intensity is generated by the magnetism that alters the movement of nanofluid flow hence reduction in the velocity distribution. The concentration of nanofluid reduces in both channels when the distribution of energy in the system is augmented. The distribution of temperature increases in both channels when the energy in the system is augmented. Variation in the distribution of energy facilitates the transferring of heat to nanoparticles hence the temperature profile of nanofluid is increased. The distribution of the velocity is constant when varying the energy intensity. The heat generation resulted in a variation of temperature and had minimum impact on the movement of the nanoparticles for both channels. The concentration of nanofluid is increased in the divergent channel when Reynolds values are increased. The reduction occurs in the concentration of nanofluid when Reynolds values are increased. As the distance between molecules becomes wider due to augmenting the energy, this results in a reduction of concentration distribution of the nanofluid in both channels. These research findings are applied in medical sciences, engineering, geophysics, and astrophysics.

Fully optimized second-order estimates for the macroscopic behavior and field statistics of particle-reinforced viscoplastic composites

This paper is concerned with the characterization of the macroscopic behavior and statistics for the distribution of the stress and strain-rate fields in composites consisting of random and isotropic suspensions of rigid spherical particles in power-law viscoplastic materials. For this purpose, use is made of the Fully Optimized Second-Order (FOSO) homogenization method (Ponte Castañeda, 2016) in combination with recently developed estimates (Kammer and Ponte Castañeda, 2022) for the macroscopic properties of the associated ‘linear comparison composite’ (LCC). Special attention is devoted to the method’s ability to account for the dependence of the homogenized properties of the nonlinear composite on the Lode angle (third invariant) of the applied loading. It is found that, while, for large particle volume fractions c, the effective flow stress is only weakly dependent on the Lode angle, for dilute volume fractions, the dependence on the Lode angle becomes more pronounced. In the ideally plastic limit, as c tends to zero, the effective yield stress is shown to depend linearly on c for axisymmetric shear, while this dependence becomes weaker with a non-analytic leading-order correction of O(c/lnc) for pure shear loading. This strong dependence on the Lode angle at dilute concentrations is shown to be due to significant differences in the local deformation patterns, which become strongly anisotropic and localize for pure shear conditions, but do not for axisymmetric shear. In turn, the FOSO homogenization method is able to capture the statistical features of these different deformation patterns by providing consistent estimates for the covariance tensor of the strain-rate field fluctuations in the matrix phase, which tend to become more strongly anisotropic for the pure shear case. As c increases, the shear bands are deflected by the randomly dispersed spheres leading to a more isotropic distribution of the stress and strain-rate fields, which is consistent with a weaker Lode angle effect. The estimates can also capture the effect of strong particle interactions, including the existence of a rigidity threshold where the macroscopic flow stress and field fluctuations blow up.

Numerical study of the volume fraction and thermophysical properties of nanofluids in a porous medium

PurposeThe purpose of the paper is to conduct a numerical and experimental investigation into the properties of nanofluids containing spherical nanoparticles of random sizes flowing through a porous medium. The study aims to understand how the thermophysical properties of the nanofluid are affected by factors such as nanoparticle volume fraction, permeability of the porous medium, and pore size. The paper provides insights into the behavior of nanofluids in complex environments and explores the impact of varying conditions on key properties such as thermal conductivity, density, viscosity, and specific heat. Ultimately, the research contributes to the broader understanding of nanofluid dynamics and has potential implications for engineering and industrial applications in porous media.Design/methodology/approachThis paper investigates nanofluids with spherical nanoparticles in a porous medium, exploring thermal conductivity, density, specific heat, and dynamic viscosity. Studying three compositions, the analysis employs the classical Maxwell model and Koo and Kleinstreuer’s approach for thermal conductivity, considering particle shape and temperature effects. Density and specific heat are defined based on mass and volume ratios. Dynamic viscosity models, including Brinkman’s and Gherasim et al.'s, are discussed. Numerical simulations, implemented in Python using the Langevin model, yield results processed in Origin Pro. This research enhances understanding of nanofluid behavior, contributing valuable insights to porous media applications.FindingsThis study involves a numerical examination of nanofluid properties, featuring spherical nanoparticles of varying sizes suspended in a base fluid with known density, flowing through a porous medium. Experimental findings reveal a notable increase in thermal conductivity, density, and viscosity as the volume fraction of particles rises. Conversely, specific heat experiences a decrease with higher particle volume concentration.xD; xA; The influence of permeability and pore size on particle volume fraction variation is a key focus. Interestingly, while the permeability of the medium has a significant effect, it is observed that it increases with permeability. This underscores the role of the medium’s nature in altering the thermophysical properties of nanofluids.Originality/valueThis paper presents a novel numerical study on nanofluids with randomly sized spherical nanoparticles flowing in a porous medium. It explores the impact of porous medium properties on nanofluid thermophysical characteristics, emphasizing the significance of permeability and pore size. The inclusion of random nanoparticle sizes adds practical relevance. Contrasting trends are observed, where thermal conductivity, density, and viscosity increase with particle volume fraction, while specific heat decreases. These findings offer valuable insights for engineering applications, providing a deeper understanding of nanofluid behavior in porous environments and guiding the design of efficient systems in various industrial contexts.

Viscosimetric squeeze flow of suspensions.

The rheology of particle suspensions has been extensively explored in the case of a simple shear flow, but less in other flow configurations which are also important in practice. Here we investigate the behavior of a suspension in a squeeze flow, which we revisit using local pressure measurements to deduce the effective viscosity. The flow is generated by approaching a moving disk to a fixed wall at constant velocity in the low Reynolds number limit. We measure the evolution of the pressure field at the wall and deduce the effective viscosity from the radial pressure drop. After validation of our device using a Newtonian fluid, we measure the effective viscosity of a suspension for different squeezing speeds and volume fractions of particles. We find results in agreement with the Maron-Pierce law, an empirical expression for the viscosity of suspensions that was established for simple shear flows. We prove that this method to determine viscosity remains valid in the limit of large gap width. This makes it possible to study the rheology of suspensions within this limit and therefore suspensions composed of large particles, in contrast to Couette flow cells which require small gaps.

Numerical simulation of heat transfer and entropy generation due to the nanofluid natural convection with viscous dissipation in an inclined square cavity

In this work, the effects of viscous dissipation on heat transfer and entropy generation of the nanofluid natural convection in an inclined square cavity are numerically investigated. A fractional-step semiimplicit finite difference algorithm based on the projection method on a staggered grid is proposed to solve the laminar natural convection problem, which has the advantage of larger time steps. The square cavity is filled with a nanofluid composed of (Cu) copper nanoparticles and water. and the viscous dissipative behavior of the mixture flow is not negligible. The studied parameters are Rayleigh number Ra ( 10 4 and 10 5 ), Eckert number Ec ( 0 − 2 ), the volume fraction of solid particles ϕ ( 0 − 0.06 ), and inclination angle of square cavity α ( 0 − π / 2 ). The Prandtl number is fixed to Pr = 6.2 . The results show that at any inclination angle, the increase in viscous dissipation leads to weakened heat transfer on the hot wall, enhanced heat transfer on the cold wall, and weakened flow in the square cavity. For the base solution without nanoparticles, the Eckert number has the greatest impact, with its effects on the average Nusselt number on the hot wall, maximum streamfunction, and average Nusselt number on the cold wall being ∼13, 0.15, 18.69%, and at Ra = 10 4 , and 12, 0.5, 28.87% at Ra = 10 5 , respectively. Research on entropy generation shows that as Eckert number increases, the entropy generation due to heat transfer increases, while due to fluid friction decreases. As the Rayleigh number increases, the effect of viscous dissipation increases. As the inclination angle increases, the effects of volume fraction and Eckert number weaken. Adding solid particles can effectively weaken the effect of viscous dissipation.

Implementation and Validation of a Numerical Method for Concentrated Suspensions in Large Flows Based on the Particle Diffusion Equation

This work presents the development of an OpenFOAM solver aimed at correctly predicting dynamics of concentrated suspensions when subjected to non-uniform shear flows. The newly implemented solver is able to predict the behavior of a heterogeneous mixture whose characteristics depend on the solid particle local concentration. To simulate such behavior, the conservation equation expressing the time variation of the particle volume fraction has been implemented in OpenFOAM; this was achieved by modifying a pre-existing solver, pimpleFoam, which discretizes the Navier–Stokes system of equation through the PIMPLE algorithm. As a first step, the formulation of the momentum equation has been adapted to correctly solve cases with non-Newtonian fluids. Successively, the Krieger’s correlation has been used to model the viscosity variation in the domain to take in account heterogeneous particle distributions. Finally, the iterative cycle for the solution of the migration equation has been included within the time loop. The above-mentioned code has been successfully validated by comparing the numerical results with the measured data provided by experiments reported in literature.

Synthesis of aluminium (Al) and alumina (Al2O3)-based graded material by gravity casting

Abstract In the present work, aluminium alloy-based composites were developed for automobile applications. Aluminium-based functionally graded material (FGM) was developed by adding 2.5, 5 and 7.5 wt% of nanoparticles of alumina. The composites were made using gravity casting in an open hearth furnace with the surface temperature of FGM maintained at 750, 850 and 1,000°C. The microstructure of the samples was studied using scanning electron microscopy and particle distribution. The particle distribution was higher at the bottom in all compositions, which can be attributed to more solidification time. As the wt% of the Al2O3 increased, the volume fraction of particles also increased from the top surface to the bottom surface of the samples. After adding 7.5 wt% of Al2O3 and heating up to 1,000°C led to the grain refinement of the alloy. The increase in hardness from the top surface to the bottom surface of the sample verified the development of FGM. Due to increase in the solidification temperature, better reinforcement was observed in the developed FGM.

A Novel Shrink–Expand–Shrink Method for Modeling Composites with Ultrahigh Volume Fractions of Pre-Graded and Gradient-Distributed Particles

An advanced shrink–expand–shrink method is proposed in this paper for efficiently modeling concrete-like particle-reinforced composites with ultrahigh volume fraction of aggregates. The gradation of the aggregates can be pre-given and the aggregate spatial distribution can be nonuniform. By this method, the shrunk aggregates are first generated in the model space, and then expanded to jostle each other, afterwards they are shrunk again to normal size to obtain the final mesostructure models. Any high volume fractions of particles even more than 90% can be easily achieved by adjusting the shrinkage level during this process. Besides, a gradient distribution algorithm is established to conform to the aggregate segregation during the actual pouring process, and the corresponding periodic boundaries can also be generated to quickly create large specimens. Finally, the compression processes of polymer concrete with different aggregate packing densities are calculated via a finite element method. The shrink–expand–shrink method and the corresponding numerical models have memorable importance in the performance analysis and material design of particle-reinforced composites.

Surface defects formation mechanism of Mg2Si/Al composites by micro cutting

Mg2Si/Al composites have wide application prospects in aerospace and automotive industries. However, many defects often remain on the machined surface after micro cutting. In this paper, the formation mechanism of surface defects of Mg2Si/Al composites by micro cutting was studied by finite element (FE) simulations and machining experiments. Considering the real microstructure characteristics of Mg2Si particles, 2D FE simulations of Mg2Si/Al composites in micro cutting were carried out and then verified by corresponding cutting experiments. The simulation results and experimental data reveal the relationship between damage and deformation behavior of Mg2Si particles and formation mechanism of surface defects. In addition, the effects of cutting parameters such as depth of cutting (DoC), cutting speed and volume fraction of particles on residual defects of machined surfaces were studied. The results show that with lower DoC and higher cutting speed, the residual defects and surface roughness are significantly reduced, especially machining the composites with smaller particle size and lower volume fraction. The research findings provide a theoretical basis for reasonable selection of machining parameters and improving the machinability of Mg2Si/Al.

Numerical computation of magnetic field with melting heat and homogeneous-heterogeneous chemical reaction effects on oblique stagnation flow of variable viscosity micropolar Fe3O4 nanofluids

The complex micro-structural characteristics of electro-conductive sol gel materials require simultaneous consideration of magneto-hydrodynamics, micro-rheology and also physico-chemical phenomena. In this editorial, a mathematical model is therefore developed to simulate the steady-state, oblique (non-orthogonal) stagnation flow of electro-conductive micropolar magneto-nano-liquid flow impacting on an extending horizontal plane under the impact of transverse magnetic field. To capture the sophisticated physico-chemistry, the simultaneous presence of homogeneous and heterogeneous chemical reactions is considered. Viscosity depending upon temperature is taken into consideration with Reynolds’ exponential model. Tiwari-Das and Maxwell-Garnett nano-liquid models are deployed which modifies density, thermal conductivity and electrical conductivity with volume fraction of nano-sized particles.

Effect of particle volume fraction on wear behavior in Al–SiC MMC coated on DIN AlZnMgCu1.5 alloy

Abstract In this study, DIN AlZnMgCu1.5 alloy surface (Al + SiC) was coated with metal matrix composite (MMC) by using hot press sintering method (HPSM). Al was used as matrix material and SiC powders were used as reinforcing material in the coating process on DIN AlZnMgCu1.5 alloy surface. Al/SiC MMC coating was produced at 600 °C under 120 MPa pressure and with varying SiC content (5, 10 and 15 vol.%). Optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) were used to examine the microstructure, elemental analysis and phase structure of both the coating zone and the transition zone between the substrate and the coating. The hardness was measured and a dry sliding linear reciprocating wear test was run to determine the mechanical properties of the coating layer. Consequently, the coefficient of friction (COF) and wear volume were determined. OM and SEM images showed a homogeneous distribution of SiC particles and a less porous structure. The hardness of the MMC coating increased with increasing SiC content. Also, the numerical analysis of the wear test simulation was done based on Archard’s law. The results of both wear tests showed that the volume loss values were consistent with each other and the amount of wear significantly reduced by increasing the rate of SiC reinforcement.

Preferential accumulation of finite-size particles in near-wall streaks

The preference for particles to accumulate at specific regions in the near-wall part is a widely observed phenomenon in wall-bounded turbulence. Unlike small particles more frequently found in low-speed streaks, finite-size particles can accumulate in either low-speed or high-speed streaks. However, mechanisms and influencing factors leading to the different preferential concentration locations still need to be clarified. The present study conducts particle-resolved direct numerical simulations of particle-laden turbulent channel flows to provide a better understanding of this seemingly puzzling behaviour of preferential accumulation. These simulations cover different particle-to-fluid density ratios, particle volume fractions, particle sizes and degrees of sedimentation intensity. We find that the large particle size is the crucial factor that results in particles accumulating in high-speed streaks. Large particles not only are difficult to be conveyed by the quasi-streamwise vortices to low-speed streaks but also can escape from the near-wall region before moving spanwisely out from high-speed streaks. The sedimentation effect allows particles to gather closer to the channel wall and stay longer in the near-wall regions, reinforcing the sweeping mechanism of quasi-streamwise vortices that transport particles from high- to low-speed streaks. As a result, sedimenting particles tend to accumulate in the low-speed streaks.