Large Agglomerates Research Articles

Design of super‐tough and antibacterialPPR/nano‐ZnOcomposites based on the excellent dispersion ofZnOparticles

AbstractHerein, polyvinyl pyrrolidone (PVP) dispersants are used to disperse and isolate zinc oxide (ZnO) particles. A high‐pressure spray device is used to dry the dispersed nano‐fillers quickly to achieve the structure of PVP chains wrapping and isolating ZnO particles. Scanning electron microscopy shows that most of the size of the agglomerated ZnO nanoparticles in polypropylene random (PPR, obtained by random copolymerization of propylene and ethylene) is maintained below 100 nm, and only a small amount of large agglomerates with a particle size less than 400 nm. An excellent dispersion of nano‐ZnO fillers prepared by this method can greatly improve the toughness of PPR and endow it with good antibacterial properties. When the content of the ZnO nanoparticles is 3 wt%, the notched impact strength of PVZ3 increases to 48.85 kJ m−2(unbroken), which is 3.6 times than that of pure PPR. In addition, 3 phr ZnO nanoparticles give PPR excellent antibacterial properties, with an antibacterial rate of 99.9% (Escherichia coliandStaphylococcus aureus). The value of antibacterial activity (E. coli) of PVZ3 reached more than 6.0, which is nearly three times that of PZ.

Understanding the Effects of CoAl2O4 Inoculant Additions on Microstructure in Additively Manufactured Inconel 718 Processed Via Selective Laser Melting

The effect of varying amounts of CoAl2O4 inoculant ranging from 0 to 2 wt pct on the microstructure evolution of Inconel 718(IN718) fabricated by selective laser melting (SLM) was evaluated. Characterization of the as-built microstructure revealed that addition of CoAl2O4 resulted in a modest degree of grain refinement with a slight increase in microstructural anisotropy. Increasing the total CoAl2O4 content beyond 0.2 wt pct resulted in severe agglomeration of the non-metallic particles and the formation of slag inclusions measuring up to 100 μm in size present in the as-built microstructure. In addition to large agglomerates, the inoculant was chemically reduced to form a fine dispersion of submicron-sized Al2O3 particles throughout the IN718 matrix. The fine dispersion of oxides significantly hindered grain recrystallization during the post-fabrication heat treatment due to a Zener pinning effect. The findings from this study indicate in order to effectively utilize CoAl2O4 as a grain refining inoculant for additive manufacturing, the process parameters need to be optimized to avoid agglomeration of the non-metallic particles and other process-related defects.

Controlling the selective distribution of hydrophilic silica nanoparticles in polylactide/ethylene-co-vinyl-acetate blends via tailoring the OH surface concentration of silica

In this short communication, we report a novel route to control the selective distribution of hydrophilic silica (SiO2) in polylactide/ethylene-co-vinyl-acetate (PLA/EVA) blends via tailoring the OH surface concentration of silica (often expressed as silanol number). Interestingly, SiO2 nanoparticles with low silanol number are mainly distributed in the EVA phase; those with medium silanol number are evenly dispersed in the EVA phase and between PLA and EVA phase; while those with high silanol number are primarily accumulated at the phase interface in the form of large agglomerates. The silanol number induced dispersion and distribution of SiO2 could be due to the regulated interactions between SiO2, EVA and PLA. Furthermore, the introduction of uniformly interfacial localized SiO2 imparts the blends a significantly improved impact toughness and well maintained strength and modulus. This work demonstrates that tuning the silanol number is a universal platform to manipulate the selective distribution of SiO2 toward a good stiffness-toughness balance in SiO2 filled immiscible polymer blends.

Kinematics of cohesive and elongated particulate materials in a vertical axis mixer

Cohesive elongated particles are ubiquitous in some industrial particle and powder processing operations; however, only a few studies investigated the kinematics of such particles using a discrete element method (DEM). Therefore, in this study, a DEM-based model was used to investigate the kinematics of cohesive elongated particles agitated in a vertical axis mixer. A contact-area-proportional model accounting for sphero-cylindrical particle geometry was employed for modeling particle–particle and particle–boundary cohesion. Generally, the flow patterns of the elongated particles in the vertical axis mixer remain stable as the particle cohesion level increases from no cohesion to low cohesion. However, flow pattern differences appear when the particle cohesion level increases to high cohesion, including the formation of large agglomerates, particle adhesion to the drum base and sidewall, and a significant heap height increase owing to the large angle of repose which high cohesion particles could maintain. Overall increasing the particle cohesion level increases the mass flow rate of particles over the blade, emphasizing the importance of particle cohesion of the bed. At the condition of high cohesion, the variation of the flow rate over the blade is significantly higher than that for no cohesion or low cohesion.

Behaviour of agglomerates formed by liquid injection in fluidized beds

AbstractFormation of agglomerates in fluidized beds can cause operating problems, such as excessive stripper shed fouling, which can lead to premature unit shut down. The focus of this study is to reach a better understanding of how agglomerates move through a fluidized bed to improve the Fluid Cokers and minimize the risk of agglomerates reaching regions where they cause problems. Two experimental methods were used: first, a Gum Arabic binder solution was injected into a two‐dimensional (2D) fluidized bed under conditions that simulate agglomerate formation in Fluid Cokers, and the mass and density of recovered agglomerates were measured; second, a new 2D radioactive particle tracking (RPT) method was developed to track the motion of model agglomerates. The fluidized bed had a steel wall, resulting in significant and non‐uniform radiation absorption. The 2D RPT system was, thus, calibrated by placing the source at 290 locations in the bed, for each fluidization velocity. Since bubble flow patterns greatly affect agglomerate motion and segregation, a tribo‐electric method was used to determine bubble flow distribution in the fluidized bed. The RPT and liquid injection methods gave similar vertical distributions of agglomerates. The main results were that increasing the fluidization velocity reduced segregation, and larger agglomerates were more likely to segregate. There was a strong correlation between the bubbles and agglomerates flow patterns: establishing an asymmetric flow pattern by injecting more fluidization gas in one side of the bed greatly reduced agglomerate segregation.

Studies on Aluminum Agglomeration and Combustion in Catalyzed Composite Propellants

Composite propellants are tested using the quench particle collection bomb (QPCB) for the pressure ranging from 2 to 8 MPa to estimate the particle size distribution of aluminum agglomerates from quenched combustion residues emerged out from the burning surface. The major ingredients included in the propellants are ammonium perchlorate (AP), aluminum (Al), hydroxyl-terminated polybutadiene (HTPB), and toluene diisocyanate (TDI). Five propellant compositions are considered in this study; two of them are mixed with catalysts. Propellant formulation variables like the coarse AP/fine AP ratio, total solid loading, catalyst percentage, and aluminum content are varied to assess their effects on the aluminum agglomeration process at different pressures. Unburnt aluminum in agglomerates is continuously getting combusted as they move out from the propellant burning surface. Large agglomerates comprise both Al2O3 and unburnt aluminum. The majority of agglomerates are spherical in shape, and the sizes vary from 31 to 115 $$\mu$$ m for non-catalyzed propellants and from 28 to 136 $$\mu$$ m for catalyzed propellants over the tested pressure conditions. These results can give further insight into the aluminum agglomeration process of catalyzed and non-catalyzed propellants and also affect the choice of the propellant ingredient percentage aimed at reducing aluminum agglomeration, which causes two-phase flow losses of thrust and slag accumulation in full-scale solid rocket motors.

Effect of amount and distribution of primary TiC on the wear behavior of a 12%Cr–3%C white iron under dry sliding conditions’

From this study, titanium additions of 1, 3 and 5% were added to a 12%Cr–3%C white iron, and their effects on the microstructure, hardness and sliding wear were analyzed. The experimental irons were melted in a 10 kg capacity vacuum induction furnace and cast into wedge metallic molds to analyze different solidification thicknesses. The alloys were characterized by optical and electronic microscopy, and X-ray diffraction. Bulk hardness was measured in the as-cast conditions and after a destabilization heat treatment at 900 °C for 30 min. Sliding wear tests (block-on-ring) were undertaken for different thicknesses according to the ASTM G77 standard in both as-cast and heat-treated conditions under a load of 52 N. The results show that, high titanium additions caused a decrease in the carbon content in the alloy and that some carbon was also consumed to form primary TiC during solidification; this in turn decreased the eutectic M7C3 carbide volume fraction and promoted a more martensitic matrix. Bulk hardness changed from 53 HRC (as-cast) to 65 HRC (as-heat treated); however, the best wear behavior was observed for the 3%Ti iron. It was found that for such amount of titanium, a good combination of austenite/martensite matrix reinforced with primary TiC carbides was obtained. While for higher titanium amounts (5%) large agglomerates of TiC particles were segregated to the eutectic zone leaving the matrix unprotected; this phenomenon was particularly observed for thicker sections. After heat treatment, the precipitation of secondary carbides occurred within the matrix, which improved the wear resistance of most irons; however, the best behavior was observed again 3%Ti iron. These results are explained in terms of the obtained microstructure; particularly in the well distribution of primary TiC carbides within the matrix.

Sustainable Synthesis of Bimetallic Single Atom Gold‐Based Catalysts with Enhanced Durability in Acetylene Hydrochlorination

Gold single-atom catalysts (SACs) exhibit outstanding reactivity in acetylene hydrochlorination to vinyl chloride, but their practical applicability is compromised by current synthesis protocols, using aqua regia as chlorine-based dispersing agent, and their high susceptibility to sintering on non-functionalized carbon supports at >500 K and/or under reaction conditions. Herein, a sustainable synthesis route to carbon-supported gold nanostructures in bimetallic catalysts is developed by employing salts as alternative chlorine source, allowing for tailored gold dispersion, ultimately reaching atomic level when using H2 PtCl6 . To rationalize these observations, several synthesis parameters (i.e., pH, Cl-content) as well as the choice of metal chlorides are evaluated, hinting at the key role of platinum in promoting a chlorine-mediated dispersion mechanism. This can be further extrapolated to redisperse large gold agglomerates (>70nm) on carbon carriers into isolated atoms, which has important implications for catalyst regeneration. Another key role of platinum single atoms is to inhibit the sintering of their spatially isolated gold-based analogs up to 800 K and during acetylene hydrochlorination, without compromising the intrinsic activity of Au(I)-Cl active sites. Accordingly, exploiting cooperativity effects of a second metal is a promising strategy towards practical applicability of gold SACs, opening up exciting opportunities for multifunctional single-atom catalysis.

Direct imaging evidences of metal inorganic contaminants traced into cigarettes

Today, environmental health research on toxicological adverse effects of metal-inorganic materials diffused by cigarettes represents a new challenge for assessing new health risks directly related to the critical chemical-size features of the particles. Therefore, morpho-chemical analyses of hazardous particles become critical in response to the distinctive assumptions about the origin, evolution, and coexisting phases. Here, we report a detailed investigation through direct microscopy imaging of metal-inorganic contaminants for one traditional and two heat-not-burn commercial cigarettes of three different brands. Chemical-size studies revealed the critical presence of heavy metal-inorganic nanostructured microparticles on both paper and filter components of the cigarette, before and after smoking. The direct experimental imaging evidenced on how hazardous particles evolved in mass-size forming coexisting multi-phases of large agglomerate because of the persistence and accumulative effect of the heating puffing. The estimated porosity of the unsuitable engineered filters validated the allowed migration of micrometric pollutants independently from their intrinsic size-shape property. Furthermore, the inappropriate design of the filters made it an adverse sponge reservoir capable of collecting all possible hazardous chemical agents potentially toxic. These substantial results strongly support experimentally the tremendous effect of the smoke capable of transporting and manipulating a high amount of elusive particles, as a particles heat carrier.

Layer-by-Layer Spray-Coating of Cellulose Nanofibrils and Silver Nanoparticles for Hydrophilic Interfaces

Silver nanoparticles (AgNPs) and AgNP-based composite materials have attracted growing interest due to their structure-dependent optical, electrical, catalytic, and stimuli-responsive properties. For practical applications, polymeric materials are often combined with AgNPs to provide flexibility and offer a scaffold for homogenous distribution of the AgNPs. However, the control over the assembly process of AgNPs on polymeric substrates remains a big challenge. Herein, we report the fabrication of AgNP/cellulose nanofibril (CNF) thin films via layer-by-layer (LBL) spray-coating. The morphology and self-assembly of AgNPs with increasing number of spray cycles are characterized by atomic force microscopy (AFM), grazing-incidence small-angle X-ray scattering (GISAXS), and grazing-incidence wide-angle X-ray scattering (GIWAXS). We deduce that an individual AgNP (radius = 15 ± 3 nm) is composed of multiple nanocrystallites (diameter = 2.4 ± 0.9 nm). Our results suggest that AgNPs are assembled into large agglomerates on SiO2 substrates during spray-coating, which is disadvantageous for AgNP functionalization. However, the incorporation of CNF substrates contributes to a more uniform distribution of AgNP agglomerates and individual AgNPs by its network structure and by absorbing the partially dissolved AgNP agglomerates. Furthermore, we demonstrate that the spray-coating of the AgNP/CNF mixture results in similar topography and agglomeration patterns of AgNPs compared to depositing AgNPs onto a precoated CNF thin film. Contact-angle measurements and UV/vis spectroscopy suggest that the deposition of AgNPs onto or within CNFs could increase the hydrophilicity of AgNP-containing surfaces and the localized surface plasmon resonance (LSPR) intensity of AgNP compared to AgNPs sprayed on SiO2 substrates, suggesting their potential applications in antifouling coatings or label-free biosensors. Thereby, our approach provides a platform for a facile and scalable production of AgNP/CNF films with a low agglomeration rate by two different methods as follows: (1) multistep layer-by-layer (LBL) spray-coating and (2) direct spray-coating of the AgNP/CNF mixture. We also demonstrate the ability of CNFs as a flexible framework for directing the uniform assembly of AgNPs with tailorable wettability and plasmonic properties.

In vitro digestion of food grade TiO2 (E171) and TiO2 nanoparticles: physicochemical characterization and impact on the activity of digestive enzymes.

Titanium dioxide is a food additive that has raised some concerns for humans due to the presence of nanoparticles. We were interested in knowing the fate of TiO2 particles in the gastro-intestinal tract and their potential effect on digestive enzymes. For this purpose, we analysed the behaviour of two different food grade TiO2 samples (E171) and one nano-sized TiO2 sample (P25) through a standardized static in vitro digestion protocol simulating the oral, gastric and intestinal phases with appropriate juices including enzymes. Both E171 and P25 TiO2 particles remained intact in the digestive fluids but formed large agglomerates, and especially in the intestinal fluid where up to 500 μm sized particles have been identified. The formation of these agglomerates is mediated by the adsorption of mainly α-amylase and divalent cations. Pepsin was also identified to adsorb onto TiO2 particles but only in the case of silica-covered E171. In the salivary conditions, TiO2 exerted an inhibitory action on the enzymatic activity of α-amylase. The activity was reduced by a factor dependent on enzyme concentrations (up to 34% at 1 mg mL-1) but this inhibitory effect was reduced to hardly 10% in the intestinal fluid. In the gastric phase, pepsin was not affected by any form of TiO2. Our results hint that food grade TiO2 has a limited impact on the global digestion of carbohydrates and proteins. However, the reduced activity specifically observed in the oral phase deserves deeper investigation to prevent any adverse health effects related to the slowdown of carbohydrate metabolism.

Phosphorylcholine-Grafted Molecular Bottlebrush-Doxorubicin Conjugates: High Structural Stability, Long Circulation in Blood, and Efficient Anticancer Activity.

Controlling the particle structure of tumor-targeting nanomedicines in vivo remains challenging but must be achieved to control their in vivo fate and functions. Molecular bottlebrushes (MBs), where brush side chains are densely grafted from a main chain, have recently received attention as building blocks of polymer-based prodrugs because their rigid structure would be expected to demonstrate high structural stability in vivo. Here, we synthesized a poly(methacryloyloxyethyl phosphorylcholine) (pMPC)-grafted molecular bottlebrush (PCMB) conjugated with a cancer drug, doxorubicin (DOX), via an acid-cleavable hydrazone bond. A pMPC-based linear polymer (LP) conjugated with DOX was also prepared for comparison. We confirmed the lack of structural transition in the PCMB between before and after conjugation with DOX using small-angle light and X-ray scattering techniques, whereas the structure of LP was significantly influenced by DOX conjugation and transformed from a random-coil structure to a large agglomerate via hydrophobic interactions among DOXs. Although PCMB-DOX and LP-DOX showed comparable tissue permeability, pharmacokinetics, and ability to accumulate in tumor tissues, the antitumor efficacy of PCMB-DOX was better than that of LP-DOX. This was presumably due to the formation of LP-DOX agglomerates. The diffusion of cleaved DOX would be restricted in the hydrophobic core of the agglomerate, resulting in the DOX release at the tumor site being compromised. In contrast to LP-DOX, DOX release from PCMB-DOX was not compromised after accumulation in tumor tissues because it did not form such an agglomerate, resulting in the strong antitumor effect. We have demonstrated the potential of MBs as building blocks of drug carriers and believe that these findings can contribute to the design of polymer-based nanomedicines.

Innovative Bioactive Ag-SiO2/TiO2 Coating on a NiTi-Shape Memory Alloy: Structure and Mechanism of Its Formation.

In recent years, more and more emphasis has been placed on the development and functionalization of metallic substrates for medical applications to improve their properties and increase their applicability. Today, there are many different types of approaches and materials that are used for this purpose. Our idea was based on a combination of a chemically synthesized Ag-SiO2 nanocomposite and the electrophoretic deposition approach on a NiTi shape memory substrate. As a result, silver-silica coating was developed on a previously passivated alloy, which was then subjected to sintering at 700 °C for 2 h. The micrometer-sized coat-forming material was composed of large agglomerates consisting of silica and a thin film of submicron- and nano- spherical-shaped particles built of silver, carbon, and oxygen. Structurally, the coatings consisted of a combination of nanometer-sized silver-carbonate that was embedded in thin amorphous silica and siloxy network. The temperature impact had forced morphological and structural changes such as the consolidation of the coat-forming material, and the partial coalescence of the silver and silica particles. As a result, a new continuous complex ceramic coating was formed and was analyzed in more detail using the XPS, XRD, and Raman methods. According to the structural and chemical analyses, the deposited Ag-SiO2 nanocomposite material’s reorganization was due to its reaction with a passivated TiO2 layer, which formed an atypical glass-like composite that consisted of SiO2-TiO2 with silver particles that stabilized the network. Finally, the functionalization of the NiTi surface did not block the shape memory effect.

Asphaltenes in asphalt: Direct observation and evaluation of their impacts on asphalt properties

Over the years, different hypotheses and speculations have been developed on asphaltenes, but their presence in asphalt has never been directly observed. In this study, asphaltenes in nine types of asphalt that vary in crude oil source and aging states are examined through scanning transmission electron microscopy (STEM) and atomic force microscopy (AFM). No solvent casting is used in sample preparation; hence, the natural state of the asphaltenes is preserved. Asphaltene particles are also separated and examined for their morphology and toughness. The impacts of asphaltene content and morphology on the zero shear viscosity (ZSV) and derived ductility of asphalt are investigated. STEM images reveal that asphaltenes in non-aged asphalt are generally well dispersed, with occasional formation of large agglomerates, and both the size of well-dispersed asphaltenes and the abundance of the agglomerates are dependent on asphalt source. Rod-shaped, crystal-like asphaltene particles can be found in both artificially-aged and field-aged asphalts. A probe into asphaltene particles using AFM suggests that the toughness of the particles is low. Rheological test results indicate that asphaltene content plays a predominant role in determining the relative ZSV (ratio between the system ZSV and the liquid phase ZSV), and asphaltene morphology likely plays a secondary role. Conversely, both the content and morphology of asphaltenes likely play major roles in determining the derived ductility of asphalt.

Two polyphenols with diverse mechanisms towards amyloidosis: differential modulation of the fibrillation pathway of human lysozyme by curcumin and EGCG

The effect of two widely used polyphenols, curcumin and EGCG was investigated on the amyloid fibrillogenesis of the well-characterized model protein human lysozyme (HuL), associated with non-neuropathic systemic amyloidosis, towards exploring their efficacy as modulators of HuL amyloid aggregation and toxicity and unravelling their mechanism of action. Curcumin exerts its inhibitory influence towards HuL fibrillation by interacting with the prefibrillar and fibrillar intermediates resulting in complete suppression of fibrillation at ∼200 µM and effectively disaggregates preformed fibrils of HuL. EGCG on the other hand suppresses fibrillation only upto 70% at ∼400 µM, modulates the pathway towards large, β-sheet rich amyloid fibril-like aggregates and modifies the preformed fibrils into similar type of large, clustered aggregate assemblies. The overall surface hydrophobicity and cytotoxicity of HuL is significantly reduced not only in the presence of curcumin but also EGCG, despite the latter forming large agglomerates, which could be accounted for by the dense and highly clustered nature of aggregates rendering their surface less exposed and thus less amenable to interact with cellular entities thereby causing reduced cellular toxicity. This study highlights the differential mechanisms employed by curcumin and ECCG in modulating the fibrillation pathway of HuL and illustrates the importance of overall modulation of fibrillation towards a general reduction in toxicity, rather than specifically focusing only on inhibition of fibrillation. This study also demonstrates how two widely different polyphenols employ disparate mechanisms to modulate the fibrillation pathway of a single protein and yet converge towards a common effect of alleviation of cytotoxicity.

Studies about the design of magnetic bionanocomposite

AbstractMagnetic nanocomposites are a class of smart materials that have attracted recent interest as drug delivery systems or as medical implants. A new approach toward the biocompatible nanocomposites suitable for remote melting is presented. It is shown that magnetite nanoparticles (MNPs) can be embedded into a matrix of biocompatible thermoplastic dextran esters. For that purpose, fatty acid esters of dextran with adjustable melting points in the range of 30–140 °C were synthesized. Esterification of the polysaccharide by activation of the acid as iminium chlorides guaranteed mild reaction conditions leading to high-quality products as confirmed by Fourier-transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopy as well as by gel permeation chromatography (GPC). A method for the preparation of magnetically responsive bionanocomposites (BNCs) was developed consisting of combined dissolution/suspension of the dextran ester and hydrophobized MNPs in an organic solvent followed by homogenization with ultrasonication, casting of the solution, drying and melting of the composite for a defined shaping. This process leads to a uniform distribution of MNPs in BNC as revealed by scanning electron microscope (SEM). Samples of different geometries were exposed to high-frequency alternating magnetic field (AMF). It could be shown that defined remote melting of such biocompatible nanocomposites is possible for the first time. This may lead to a new class of magnetic remote-control systems, which are suitable for controlled release applications or self-healing materials. BNCs containing biocompatible dextran fatty acid ester melting close to human body temperature were prepared and loaded with Rhodamine B (RhB) or green fluorescent protein (GFP) as model drugs to evaluate their potential use as drug delivery system. The release of the model drugs from the magnetic BNC investigated under the influence of a high-frequency AMF (20 kA/m at 400 kHz) showed that on-demand release is realized by applying the external AMF. The BNC possessed a long-term stability (28 d) of the incorporated iron oxide particles after incubation in artificial body fluids. Temperature-dependent mobility investigations of MNP in the molten BNC were carried out by optical microscopy, magnetometry, alternating current (AC) susceptibility, and Mössbauer spectroscopy measurements. Optical microscopy shows a movement of agglomerates and texturing in the micrometer scale, whereas AC susceptometry and Mössbauer spectroscopy investigations reveal that the particles perform diffusive Brownian motion in the liquid polymer melt as separated particles rather than as large agglomerates. Furthermore, a texturing of MNP in the polymer matrix by a static magnetic field gradient was investigated. First results on the preparation of cross-linkable dextran esters are shown. Cross-linking after irradiation of the BNC prevents melting that can be used to influence texturing procedures.

Subcooled liquid boiling: details of the mechanism and phenomenological description

Using high-speed filming (with a frame frequency up to 100 kHz) the important details of the subcooled liquid boiling mechanism are specified, which allow to conclude that Snyder-Bergles phenomenological model describes the process with the maximal likelihood. This model accounts for the main subprocesses of the subcooled liquid boiling (evaporation of the liquid microlayer located under the bubble near the dry spot boundary, vapor condensation at the bubble dome, liquid inflow to the microlayer). It corresponds to the modern ideas regarding to boiling mechanism and qualitatively well meets the experimental data. On the basis of the conducted experiments the description of several subprocesses of the model is defined more exactly: nucleation sites deactivation, het removal from the bubble dome by means of unsteady heat conduction, bubble size evolution as a result from the balance of masses of the evaporating and condensing vapors. It was shown that large vapor agglomerates appear in the flow due to small vapor bubbles coalescence with a heat flux density increase. Formation of dry patches under the agglomerates was studied. These dry patches are precursors of the boiling crisis. Superheating of the surface under dry patches immediately leads to the heating surface burnout.

Notes on the Phenomenological Model of Subcooled Liquid Boiling

It is shown that the Snyder–Bergles model seems to be the best phenomenological one while describing the process of forced boiling of the liquid considerably subcooled relative to the saturation temperature. This model most likely (without contradictions) accounts for the main physical subprocesses that accompany subcooled liquid boiling (liquid microlayer evaporation below the bubble, vapor condensation at the bubble dome, liquid inflow to the microlayer), meets modern concepts on the boiling mechanism, and agrees with the experimental data. Based on experiments at the Joint Institute for High Temperatures, Russian Academy of Sciences (JIHT RAS) using high-speed video recording (at a rate of up to 100 thousand frames per second) and other studies, some subprocceses incorporated into the model are refined. They are as follows: deactivation of the nucleation sites, heat removal from the bubble dome via unsteady heat conduction, and the bubble size evolution in time as the balance of masses of the evaporated and condensed vapor. It is shown that the main mechanism through which heat from the bubble dome is transferred to the surrounding liquid is unsteady heat conduction, the intensity of which is inversely proportional to the square root of the bubble existence time. The evolution of bubble sizes is caused by the balance of masses of the evaporating liquid microlayer and steam condensing on the bubble dome. With high heat flux densities supplied to the heating surface and low liquid subcooling values, the imbalance shifts toward the evaporation side, as a result of which conditions may emerge for a growth of large bubbles and their subsequent merging and occurrence of large vapor agglomerates in the coolant flow.

Evolution of EGR cooler deposits under hydrocarbon condensation: Analysis of local thickness, roughness, and fouling layer density

Stringent new automobile emission standards have prompted extended the use of exhaust gas recirculation (EGR) systems in additional areas of engine maps, resulting in the operation of engines with exhaust gas flows having higher concentrations of particulate matter and hydrocarbons. These new operating conditions promote the formation of fouling layers inside the EGR cooler, owing to the accumulation of soot agglomerates and their interactions with hydrocarbon condensates. Hence, this study analyses the effects of hydrocarbons on the formation and evolution of deposits that grow on a ribbed surface. Using an experimental layout, an exhaust flow with soot agglomerates and a representative species of hydrocarbon vapour typically encountered in EGR flows are generated. The particulate matter of the exhaust stream is categorised using transmission electron microscopy images, while the deposits generated are analysed at different instants of the fouling process through tests of different durations. The effects of a high HC content on the fouling are assessed through the measurement of the local fouling layer thickness and the local roughness of the deposit. The outlet gas temperature and thermal efficiency are monitored during the fouling process; additionally, the density of the fouling layer is evaluated. The results reveal that, considering higher hydrocarbon concentrations, the thickness of the fouling layer decreased in regions near the outlet section of the deposit during the early stages of the deposit formation. When the large agglomerates generated under a high HC content hit the fouling layer on the windward side of the ribs, they modify the surface texture of the deposit, resulting in higher roughness values. Moreover, the analysis of the deposit density reveals that when hydrocarbon condensation occurs during the early phases of the fouling process, the density of the deposit can be 1.9 times higher than those of deposits generated in longer-duration tests.

Effect of successive sprays on liquid distribution in fluidized beds

In processes such as Fluid Coking™, agglomerate formation should be minimized since it reduces the yield of valuable products, and degrades operability because of the fouling of internals. An experimental model, consisting of an aqueous solution of gum arabic with a dye, has been successfully developed to simulate the formation of agglomerates in the Fluid Coking™ process, where bitumen is sprayed into a fluidized bed of coke particles The particles wetted by a spray could be predicted by assuming that all the particles in the wake of bubbles formed from the tip of the spray jet have been wetted by the injected liquid. The transfer of liquid from particles wetted with the spray to dry bed particles was relatively ineffective, as the number of wet particles increased by only 50%. With successive liquid injections, the proportion of the liquid trapped in agglomerates increases in latter injections: large agglomerates from earlier injections accumulate above the grid and are carried by gas bubbles into the spray jet cavity, where they seed fresh agglomerates.