Binary Particle Size Distribution Research Articles

A comparison of mechanistic models for the combustion of iron microparticles and their application to polydisperse iron-air suspensions

Metals can serve as carbon-free energy carriers, e.g., in innovative metal-metal oxide cycles as proposed by Bergthorson (Prog. Energy Combust. Sci., 2018). For this purpose, iron powder is a suitable candidate since it can be oxidized with air, exhibits a high energy density, is non-toxic and abundant. Nevertheless, the combustion of iron powder in air is challenging especially with respect to flame stabilization which depends on the particle size distribution and the morphology of the iron microparticles among other factors. Models for the prediction of reaction front speed in iron-air suspensions can contribute to overcoming this challenge. To this end, three different models for iron particle oxidation are integrated into a laminar flame solver for simulating such reaction fronts. The scientific objective of this work is to elucidate the influence of polydispersity of the iron particles on the reaction front speed, which is still not satisfactorily understood. In a systematic approach, cases with successively increasing complexity are considered: The investigation starts with single particle combustion and then proceeds with iron-air suspensions prescribing binary particle size distributions (PSDs), parametrized generic PSDs, and an exemplary PSD measured for a real iron powder sample. The simulations show that, dependent on the kinetic particle model, the particles’ thermal inertia, and the PSD, particles undergo thermochemical conversion in a sequential manner according to their size and every particle fraction exhibits an individual combustion environment (surrounding gas phase temperature and oxygen concentration). The local environment can be leaner or richer than the overall iron-to-air ratio would suggest and can be very different from single particle experiments. The contribution of individual particle fractions to the overall reaction front speed depends on its ranking within the PSD. The study further demonstrates, that although the three particle models show good agreement for single particle combustion, they lead to very different reaction front speeds. This is due to the different ignition behavior predicted by the particle models, which is shown to strongly influence the reaction front characteristics. Overall, the present work illustrates the complex relationship between characteristics of single particle ignition and combustion, polydispersity, and properties of reaction fronts in iron-air suspensions.

Numerical study of heat transfer for gravity-driven binary size particle flow around circular tube

As the primary working medium in moving bed heat exchangers, solid bulk materials are composed of multiple size particles. To clarify the effect of particle size distribution (PSD) on heat transfer, the discrete element method (DEM) is used to study the flow and heat transfer of binary size particle flow around a circular tube. The heat transfer model for multiple size particles is developed and coupled with DEM method. The particle flow characteristics with PSD width are analyzed in detail. The results show that particle size segregation, competitive flow and flow instability are the unique characteristics of binary size particle flow around a tube. Wider PSD will enhance these characteristics and have a positive effect on particle renewal in the stagnation zone, which could decrease the local heat resistance. The total heat transfer coefficient between particle flow and tube wall increases at most by 9.73% as PSD is wider. • Binary size particle flow around a circular tube is studied with DEM method. • Binary size particle flow can form local particle size segregation around tube. • Binary size particles can decrease the stagnation zone above the tube. • A wider binary particle size distribution can increase the flow instability. • A wider binary particle size distribution can enhance heat transfer performance.

Investigation of the experiment and simulation on TiH2 powder packing by a novel irregular 3D model

Density and porosity have an important influence on powder metallurgy products, and it is feasible to predict the tap density of powder by simulating the packing of powder. Analysis models of spherical and simple geometric have been developed to predict the packing density of the powder. In this study, a novel particle model with complex three-dimensional (3D) morphology was established to simulate the packing process of titanium hydride powder with binary particle size distribution. The results have indicated that when the particle size ratio and the volume ratio of fine powder to coarse powder were 1:3 and 1:1, respectively, the sintered sample reaches its optimum sintered density of 99.24%. Compared with these samples sintered by single-scale powder, the size and distribution of residual pores reduced significantly with the application of mixed powders, which is beneficial to the mechanical properties of powder metallurgy products.

Charge Carriers in Commercial Photocatalysts: Fractal Kinetics and Effect of \u201cInert\u201d Additives

High photon conversion efficiencies and low-cost materials are crucial for implementing photocatalytic solutions in large scale applications. To improve the conversion efficiencies, knowledge on the lifetime and dynamics of photogenerated charge carriers is fundamental, for which transient absorption spectroscopy has provided important insights. Regarding the cost of the materials, one approach to decrease it is mixing the photocatalyst with cheaper additives while avoiding a significant loss in its activity. In this short review we summarize the findings of our group dealing with both aspects. A new model to fit transient absorption decays, based on fractal kinetics, is discussed. The model has been applied to the transient decays of TiO2 mixtures with binary particle size distributions and of TiO2 mixtures with a priori inert additives. In both cases, the insights obtained from the model were crucial to explain the photocatalytic activities of the mixtures.

Effect of particle size distribution on the behavior of alkali and alkaline earth metals in solid-recovered fuel combustion in a circulating fluidized bed

ABSTRACTIn this paper, the effect of the particle size distribution (PSD) of the bed material on alkali and alkaline earth metal behavior in circulating fluidized bed combustion of solid-recovered fuel was studied. The Sauter mean diameter, of the PSDs studied: Gaussian-, flat-, binary-, and narrow distributions, was 200 µm. The concentration of both alkali and alkaline earth metals was found to decrease with increasing particle size, both on the surface and the cross-section, except for the surface concentration for calcium (Ca) which showed the opposite trend for the Gaussian and flat PSDs, increasing from approximately 1.7%-w to above 3%-w. The total surface concentration varied from 2.1% for the binary PSD to 2.6% for the flat, while the cross-section concentration varied from 0.4% for the Gaussian PSD to 0.7% for the binary. Ca is less volatile than the other elements studied making it less prone to vaporize and leave the circulating fluidized beds with the flue gases and it was also the most abundant element in the fuel. The experimental results presented here indicate smaller variation in potassium, sodium, and magnesium behavior between the PSDs than for Ca and that particle size changing phenomena, such as attrition, influence it as well.

Photocatalytic activity and charge carrier dynamics of TiO2 powders with a binary particle size distribution.

The effects of the particle size distribution on the charge carrier dynamics and the photocatalytic activity of mixed titanium dioxide (TiO2) powder samples were investigated in this work. Instead of the synthesis of the small semiconductor particles, the binary particle size distributions of the powders were obtained by mixing commercially available TiO2 powders with different particle sizes. The pure anatase samples (average diameters: 7, 20, and 125 nm, respectively) were created via ultrasound treatment and discreet drying. The photocatalytic activity of the powder samples was assessed by the degradation of nitric oxide (NO) and acetaldehyde in the gas phase. Furthermore, the charge carrier kinetics was determined using transient absorption spectroscopy following pulsed laser excitation. Importantly, a recently published model based on fractal dimensions was used to fit the transient signals of the photo generated charge carriers in the TiO2 powder samples. The effects of the particle size on the acetaldehyde degradation could be explained by the formation of agglomerates, which reduce the available surface area of smaller particles. The fast oxidation of acetaldehyde on the surface of TiO2 by direct hole transfer was further independent of the observed charge carrier lifetimes on the microsecond time scale. The photocatalytic NO degradation, on the other hand, increased for samples containing larger amounts of small particles. The corresponding photonic efficiencies correlated well with the charge carrier lifetimes determined by the time-resolved studies. Hence, it was concluded that a long charge carrier lifetime generally leads to higher fractional conversions of NO. The employed fractal fit function was proved to be beneficial for the kinetic analysis of charge carrier recombination in TiO2, in direct comparison with a second order fit function.

Epoxy Composites with Added Aluminum with Binary Particle Size Distribution for Enhanced Dielectric Properties and Thermal Conductivity

Three kinds of hybrid aluminum (Al) particles with binary particle size distribution, i.e., [2 μm/50 μm], [2 μm/18 μm] and [18 μm/50 μm], were added in epoxy (EP) to prepare hybrid Al/EP composites with enhanced dielectric properties and thermal conductivity for embedded capacitor applications. The dielectric permittivity, dissipation factor, and thermal conductivity of three types of hybrid Al/EP composites were investigated as a function of relative volume fraction of smaller-size Al of hybrid Al particles (Vs) at a total filler content of 60 wt.%, respectively. The results indicate that dielectric permittivity and thermal conductivity of the hybrid Al/EP mainly depend on two factors, such as the type of hybrid filler and the Vs. The maximum dielectric permittivity of 48 appears at Vs = V18μm/V(18μm+50μm) = 35%. While, the above two factors have a negligible influence on the dissipation factor, which is as low as 0.022. The highest thermal conductivity of 1.28 W/m K is obtained at Vs = V18μm/V(18μm+50μm) = 50%. The maximum thermal conductivity for three hybrid systems shifts towards lower Vs with decreasing the size ratio of a larger Al to a smaller one.

Particle size distribution in CPFD modeling of gas–solid flows in a CFB riser

A computational particle fluid dynamics (CPFD) numerical method to model gas–solid flows in a circulating fluidized bed (CFB) riser was used to assess the effects of particle size distribution (PSD) on solids distribution and flow. We investigated a binary PSD and a polydisperse PSD case. Our simulations were compared with measured solids concentrations and velocity profiles from experiments, as well as with a published Eulerian-Eulerian simulation. Overall flow patterns were similar for both simulation cases, as confirmed by experimental measurements. However, our fine-mesh CPFD simulations failed to predict a dense bottom region in the riser, as seen in other numerical studies. Above this bottom region, distributions of particle volume fraction and particle vertical velocity were consistent with our experiments, and the simulated average particle diameter decreased as a power function with riser height. Interactions between particles and walls also were successfully modeled, with accurate predictions for the lateral profiles of particle vertical velocity. It was easy to implement PSD into the CPFD numerical model, and it required fewer computational resources compared with other models, especially when particles with a polydisperse PSD were present in the heterogeneous flow.

Low-temperature synthesis and characterization of PVP-capped FeAu nanoparticles

Abstract

Study on thermal conductive BN/novolac resin composites

In this study, γ-aminopropyltriethoxysilane-treated boron nitride (BN) particles were used to modify novolac resin. The effect of varying the BN concentration, particle size, and hybrid BN fillers with the binary particle size distribution on the thermal conductivity of the composites was investigated. Scanning electron microscopy (SEM) imaging showed homogeneously dispersed treated BN particles in the matrix. Furthermore, the thermal conductivity increased as the BN concentration was increased. This behavior was also observed when the filler size was increased. Experimentally obtained thermal conductivity values agree with the predicted data from the Maxwell–Eucken model well at less than 70 wt.% BN loading. A larger particle size BN-filled novolac resin exhibits a higher thermal conductivity than a smaller particle size BN-filled one. The combined use of 0.5 and 15 μm particles with a mass ratio of 2:1 achieved the maximum thermal conductivity among the testing systems. The thermal resistance properties of the composites were also studied.

Effects of binary particle size distribution on minimum pick-up velocity in pneumatic conveying

Minimum pick-up velocities ( U pu ) for entrainments of particle mixtures having binary particle size distributions (PSD) are measured in a horizontal pneumatic-conveying line using the weight-loss method. Geldart's groups A, B, and C glass beads having diameters of 400, 170, 40, and 5 μm are used. Variations in U pu as a function of particle mass fraction ( m) are examined. The capability of empirical correlations of monodisperse U pu in predicting U pu of binary mixtures is investigated. For group B particle mixtures (i.e. 400 & 170 μm), the particles are entrained separately resulting in linear U pu variations with m, which is accurately predicted by the monodisperse U pu correlation. For mixtures involving group A and B particles (i.e. 170 & 40, 400 & 40 μm), the two particles are collectively entrained resulting in U pu that vary non-linearly with m and that cannot be predicted by the correlation. For mixtures involving group B and C particles (i.e. 400 & 5, 170 & 5 μm), U pu are comparable to that of the monodisperse group B particles, therefore they are accurately predicted by the correlation. The significant impacts of binary PSD on U pu found presently indicates that PSD effects on particle entrainment process warrants further investigations.

Effects of binary particle size distribution on the fluid dynamic behavior of fluidized, vibrated and vibrofluidized beds

In this work the effects of particle size distribution on the dynamics and segregation patterns in fluidized, vibrated and vibrofluidized beds were investigated. A binary particle size distribution and a reference one composed of glass spheres with a mean Sauter diameter of 2.18×10-3 m were tested. The experimental setup consisted basically of a circular glass chamber with a height of 0.50 m and a diameter of 0.114 m, operated in the fluidized bed mode (G = 0.00), in either vibrated or vibrofluidized bed modes (G = 2.00). The pressure drops in the fluidized and vibrofluidized beds were not significantly affected by the binary particle size distribution. Well-defined segregation patterns occurred in fluidized and vibrated beds with small particles concentrating at the top and large particles at the bottom in the first situation and the reverse in the second one. Segregation patterns in vibrofluidized beds depended on the values of vibration parameters. Segregation in vibrofluidized and vibrated beds was minimized by operating at a high amplitude of vibration.

Three-dimensional, rapid shear flow of particles with continuous size distributions

Rapid granular flows occur in nature and industry and often contain particles of many sizes. Over the last two decades, significant theoretical and experimental effort has been directed at rapid granular flows with monodisperse or binary particle-size distributions. In contrast, the behavior of rapid granular flows with more than two particle sizes has received only limited attention. The particle-size distributions in many natural and industrial granular flows may be represented as continuous distributions (e.g., Gaussian or lognormal distributions), providing incentive for the investigation of rapid granular flows with these particle-size distributions. As an extension of previous work for two-dimensional simulations of rapid shear flows with Gaussian and lognormal particle-size distributions, this work is directed at three-dimensional flows with continuous size distributions. Event-driven, discrete-particle (“molecular dynamic”) simulations are employed for the three-dimensional simple shear flow of smooth, spherical particles with Gaussian and lognormal particle-size distributions. The results parallel those found previously in two dimensions and demonstrate the effect of distribution width on the stress tensor and granular energy.