Hydrodynamic Layer Research Articles

Effective Thermal Conductivity of Cyclohexane-Based Nanofluids Containing Cerium Dioxide Nanoparticles with Chemisorbed Organic Shell

In the present study, the effective thermal conductivity λeff of nanofluids containing metal oxide nanoparticles with a chemisorbed organic shell was investigated experimentally and theoretically. The model systems synthesized by a continuous-flow hydrothermal method consist of cyclohexane as organic base fluid and dispersed nearly spherical cerium dioxide (CeO2) core nanoparticles with a decanoic acid shell chemically attached to their surface. From the differences between the hydrodynamic diameters of the two core–shell nanoparticle types with (8.6 or 9.1) nm determined by dynamic light scattering (DLS) and the nearly spherical CeO2 core diameters obtained by analytical ultracentrifugation (AUC) and transmission electron microscopy (TEM), an estimation for the thickness of the entire hydrodynamic layer around the particle core in the range of about (1.1 to 1.3) nm could be deduced. Experimental data for λeff of the nanofluids and the thermal conductivity of the base fluid λbf were determined with a steady-state guarded parallel-plate instrument (GPPI) with an expanded (k = 2) relative uncertainty of 0.026 at atmospheric pressure over a temperature range from (283.15 to 313.15) K in steps of 10 K. The measurement results for the thermal-conductivity ratio λeff ·λbf–1 are independent of temperature and increase with increasing volume fraction of the CeO2 core nanoparticles up to about 0.023. It was found that the experimental results can be described by the Hamilton–Crosser model within their experimental uncertainties for all temperatures investigated.

Dynamic behavior and driving region of spray combustion instability in a backward-facing step combustor.

We numerically study the dynamic behavior and driving region of spray combustion instability in a backward-facing step combustor using analytical methodologies based on dynamical systems theory, symbolic dynamics, complex networks, and machine learning. The global dynamic behavior of a heat release rate field represents low-dimensional chaotic oscillations with deterministically aperiodic intercycle dynamics. Spray combustion instability is driven in the formation and separation region of a large-scale organized vortex induced by the hydrodynamic shear layer instability at the edge of the backstep. This region corresponds fairly to that of the hub in an acoustic-energy-flux-based spatial network. The feature importance in a random forest is valid for clarifying the feedback coupling of spray combustion instability.

Conformal Electrodeposition of Mesoporous Silica over High Aspect Ratio (AR>100) Nanomesh Electrodes

The high surface area provided by the nanopores of ordered mesoporous silica thin films enables a wide range of electrochemical applications such as sensors, catalysis, energy storage and ion-selective membranes. Further improvements are possible by coating such thin films on large surface area electrodes with high aspect ratios (> 100) to increase the active surface area. Atomic layer deposition (ALD) is often preferred to deposit conformal thin film oxides on high aspect ratio structures. Such vapor-phase depositions are limited by the available surface reactive sites to deposit thin films of thickness ranging from sub-nanometer to few tens of nanometer. However, there is a tradeoff in terms of deposition time/cycles to achieve quality films especially when high aspect ratios (>100) substrates are introduced. Electrochemistry offers an alternative, versatile method to conformally coat layers on 3D electrode surfaces. The electrochemically assisted self-assembly (EASA) of mesoporous silica, utilizes electrochemical reactions to locally increase the pH near the electrode surface, which in turn catalyzes the silica gelation and surfactant self-assembly, thus causing the controlled growth of mesoporous silica thin films [1]. Recently, our group has shown that good quality mesoporous silica films with thickness between 20-2000 nm could be achieved by controlling the hydrodynamic layer in a rotating disc electrode setup [2]. Moreover, EASA of mesoporous silica offers meso-channels that are aligned perpendicular to the underlaying conductive substrate which is optimal for mass-transport.In this follow-up work, we demonstrate a reliable, electrochemically controlled, conformal deposition of mesoporous silica onto large surface area nanomesh electrodes with an aspect ratio close to 100. Nanomesh electrodes developed in our group at imec, is built up of a regularly spaced (50 nm), inter-connected network of vertical and horizontal nanowires of diameter ~40 nm. These electrodes offer an 80x area enhancement (80 cm2 per geometric cm2) while maintaining a porosity close to 75%. We show that, the extent of silica deposition over these nanomesh electrodes can be electrochemically controlled from conformal coatings (~10 nm thick) uniformly coating the whole nanomesh architecture to complete fill and overfill of the nanomesh electrodes (~4 µm thick). Furthermore, excellent mass-transport properties of various silica coated nanomesh electrodes has been demonstrated using a ruthenium hexaammine redox probe. Finally, the area enhancement offered by the silica coated Ni nanomesh is demonstrated using the characteristic accumulation of a positively charged [Ru(NH3)6]3+ probe within the negatively charged silica nanochannels, which shows 55x increase for the amount of charge from the probe stored in the coated nanomesh, compared to a planar silica layer, consistent with the expected surface area enhancement of the 4 µm thick nanomesh electrodes.

Diffusion Kinetics Theory of Removal of Assemblies’ Surface Deposits with Flushing Oil

The diffusion kinetics theory of cleaning assemblies such as combustion engines with flushing oil has been introduced. Evolution of tar deposits on the engine surfaces and in the lube system has been described through the erosion dynamics. The time-dependent concentration pattern related to hydrodynamic (sub)layers around the tar deposit has been uncovered. Nonlinear equations explaining the experimentally observed dependences for scouring the contaminants off with the oil have been derived and indicate the power law in time. For reference purposes, a similar analysis based on formal chemical kinetics has been accomplished. Factors and scouring parameters for the favor of either mechanism have been discussed. Any preference for either diffusion or chemical kinetics should be based on a careful selection of washing agents in the flushing oil. Future directions of studies are proposed.

Remarkable potential of Na-montmorillonite as a sustainable and eco-friendly material for flocculant studied in the standardized mixing flow

This present study focuses on the ability of montmorillonite, a ubiquitous and non-toxic clay which can be used as an eco-friendly flocculant. For this purpose, coagulation behavior of Polystyrene Amidine Latex (AL) particles with Na-montmorillonite was investigated by the application of standardized mixing method. The flocculation rate of AL particles was assessed as a function of Na-montmorillonite concentration, considering ionic strength and pH. The results showed an enhancement factor (β) ranging from 2 to 5 at an optimal concentration of Na-montmorillonite (∅9.8×10−7). The obtained enhanced rate is attributed to the adsorbing conformation of montmorillonite onto AL particles detected by the combined experiment of electrophoretic mobility and measurement of hydrodynamic layer thickness of clay on AL particles. The finding and established method will be ultimately useful to assess the effect of bio-surfactants or biopolymers which are usually present in microbial extracellular polymeric substances (EPS) interacting with environmental colloidal particles.

The axisymmetric flow of an MHD Powell–Eyring fluid with viscous dissipation and Newtonian heating condition: Keller‐box method

AbstractA special investigation on the heat transfer by applying viscous dissipation and considering the Newtonian heating condition in magnetohydrodynamic Powell–Eyring fluid has been attempted. It is regarded as the flow in an axisymmetrical direction over a radially stretching surface. The converted governing system of equations is solved to obtain a closed‐form solution using the Keller‐box technique. The skin‐friction coefficient's influence fully developed local Nusselt number is then presented graphically, and temperature profiles are sorted out for the pertinent parameters. The fluid overshoot towards the plate surface with rising magnetic field strength, hence, both the fluid velocity and the hydrodynamic border line layer thickness will fall, though, the skin‐friction coefficient will increase. Various relevant results of the energy indulgence have been discussed from heating and in view of viscous dissipation phenomena. The decision with minimal cases from previous studies in the literature received confirmation of the findings.

Effect of fluid velocity and particle size on the hydrodynamic diffusion layer thickness

The aim of this study was to determine the thickness of the hydrodynamic diffusion layer (hHDL) of three poor water-soluble compounds under laminar fluid flow using a single particle dissolution technique. The single particle dissolution experiments were performed in a flowing aqueous medium using four different fluid velocities (v), ranging from 46 to 103 mm/s. The particles used had an initial radius (r) of 18.8 to 52.3 μm. The determined hHDL values were calculated from both dissolution experiments and computational fluid dynamics (CFD) simulation. In this study, single particle dissolution experiments gave, with one exception, hHDL values in the range of 2.09 to 8.85 µm and corresponding simulations gave hHDL values in the range of 2.53 to 4.38 µm. Hence, we found a semi-quantitative concordance between experimental and simulated determined hHDL values. Also, a theoretical relation between the dependence of hHDL on particle radius and flow velocity of the medium was established by a series of CFD simulations in a fluid velocity range of 10–100 mm/s and particle size (radius) range of 5–40 µm. The outcome suggests a power law relation of the form hHDL∝r3/5v-2/5. In addition, the hHDL seems to be independent of the solubility, while it has a diffusion coefficient dependence. In conclusion, the hHDL values were determined under well-defined conditions; hence, this approach can be used to estimate the hHDL under different conditions to increase the understanding of the mass transfer mechanisms during the dissolution process.

Fluid-solid coupled simulation on thermal management of hydrodynamic gas foil bearing with an axial cooling throughflow

The axial throughflow cooling is a realistic solution for the purpose of thermal management in the hydrodynamic gas foil bearings operating at ultra-high rotational speed and mini bearing clearance. In order to illustrate the effects of axial throughflow cooling within the fluid-solid coupled heat transfer mode on the aerodynamic and thermal behaviors of hydrodynamic gas foil journal bearing, a steady three-dimensional fluid-solid coupled numerical simulation is performed in the present study by assuming that the journal-bearing works in a steady-state condition with a fixed eccentricity ratio of 0.9. The results show that the shearing-induced circumferential flow is significantly dominant in the hydrodynamic air film layer. At both edges of the foil bearing section, local suction flow into the bearing clearance and local leakage flow out the bearing clearance happen simultaneously at different circumferential positions. The increase of axial throughflow mass flowrate plays an effective role on improving the cooling capacity. However, it should be noteworthy that the bleeding air pressure is prompted in a nearly quadratic relationship along with the increase of axial throughflow mass flowrate. Due to the non-uniformity nature of viscous-shearing heat generation inside the hydrodynamic gas foil bearing with a big eccentricity, the temperature on the shaft and top foil surfaces takes on a non-uniform distribution. The peak temperature generally has a 20 K–30 K difference with respect to the mean temperature.

Observation of temporal change of the hydrodynamic layer thickness of adsorbed polyelectrolytes on spherical particles by single-particle-tracking method

The temporal behavior of adsorbed polymers is important in the analysis of colloid–polymer systems. The single-particle-tracking method provides a reliable approach to acquiring information on the adsorbed polymer layer on a spherical particle. Imaging, tracking, and analysis of the Brownian motion of a single colloidal particle with/without adsorbed polymers provided the trajectory of the Brownian motion and the layer thickness of adsorbed polymers. The temporal decrease in the thickness of an adsorbed polyelectrolyte layer was monitored within 2 h. A polyelectrolyte with a high charge density formed a thick layer on the particle surface and the layer thickness rapidly decreased. A polyelectrolyte with a low charge density formed a thinner layer, and its thickness decreased slightly. The transient adsorption behavior in a system undergoing flocculation can be used for the analysis of adsorption and flocculation in colloid–polymer systems.

Vancomycin-Loaded Furriness Amino Magnetic Nanospheres for Rapid Detection of Gram-Positive Water Bacterial Contamination.

Bacterial pathogens pose high threat to public health worldwide. Different types of nanomaterials have been synthesized for the rapid detection and elimination of pathogens from environmental samples. However, the selectivity of these materials remains challenging, because target bacterial pathogens commonly exist in complex samples at ultralow concentrations. In this study, we fabricated novel furry amino magnetic poly-L-ornithine (PLO)/amine-poly(ethylene glycol) (PEG)-COOH/vancomycin (VCM) (AM-PPV) nanospheres with high-loading VCM for vehicle tracking and the highly efficient capture of pathogens. The magnetic core was coated with organosilica and functionalized with cilia. The core consisted of PEG/PLO loaded with VCM conjugated to Gram-positive bacterial cell membranes, forming hydrogen bonds with terminal peptides. The characterization of AM-PPV nanospheres revealed an average particle size of 56 nm. The field-emission scanning electron microscopy (FE-SEM) micrographs showed well-controlled spherical AM-PPV nanospheres with an average size of 56 nm. The nanospheres were relatively rough and contained an additional 12.4 nm hydrodynamic layer of PLO/PEG/VCM, which provided additional stability in the suspension. The furry AM-PPV nanospheres exhibited a significant capture efficiency (>90%) and a high selectivity for detecting Bacillus cereus (employed as a model for Gram-positive bacteria) within 15 min, even in the presence of other biocompatible pathogens. Moreover, AM-PPV nanospheres rapidly and accurately detected B. cereus at levels less than 10 CFU/mL. The furry nano-design can potentially satisfy the increasing demand for the rapid and sensitive detection of pathogens in clinical and environmental samples.

Newly synthesized Ionic Liquids as potent lubricants and additives to existing lubricant oils

Ionic Liquids are promising candidates for next generation green lubricants. We have synthesized 39 Tetraalkylammonium Alkyl Ether Carboxylate Ionic Liquids and tested them for their lubricant capabilities. We measured friction coefficient to assess the transition from the boundary to the hydrodynamic lubrication, the hydrodynamic area and the minimum friction value. Some Ionic Liquids are capable of forming a hydrodynamic layer fully separating two specimens. Compounds with short C-chain of the cationic part show poor tribological behaviour. Similarly, increasing the PO-degree of the anionic part lowers the lubrication power. An increase of the C-chain length improves the tribological behaviour, i.e. the minimum friction value becomes lower. This is due to the formation of a uniform tribolayer of the long-chain carboxylic acids. Higher viscosity of the Ionic Liquids results in low friction coefficients and the development of a hydrodynamic layer. This is due to a strong hydrodynamic pressure, which is formed by the more viscous compound. Addition of small amounts of Ionic Liquids to low performance oils increases their capability to from tribolayers and thus improves their lubricant capability.

Analysis of the influence of dust formation on technological processes in the production of PET

Possible methods of dust formation during the production of polyethylene terephthalate granulate and its preparation for molding are considered. The features and patterns of dust formation during drying in an active hydrodynamic layer at different speeds and temperatures are revealed.

On the equilibrium and stability of a fluidized bed as a thermodynamic system

The transfer of the dispersed layer into a fluidized state makes it possible to intensify the drying process. The small size of the particles leads to an increase in the surface of their contact with the coolant at a relatively low hydrodynamic resistance. Other positive qualities of fluidization are listed, which is very important when carrying out exothermic processes. We studied the behavior of the fluidized bed during the drying process. The curve of fluidization of beet chips is shown. The suspended state of the material began when the forces of the hydrodynamic layer were equal to the weight of all its particles per unit area of the cross-section of the working chamber. The region of existence of the fluidized bed is marked. In this area, the flow was relatively equilibrium (fluidized). On the surface of the layer, small waves were observed with different frequencies and amplitudes of oscillations, as well as with spontaneous fluctuations. This mode of operation was achieved as a result of the study of the structures of the support - gas distribution grid and the drying chamber. The flow velocity profile in the working chamber is investigated. An efficient equalization of velocities with the help of flat stamped grids has been established. The results were confirmed by the spectra of the flow in the drying chamber. Oscillations on the free surface of a fluidized bed are considered. The Euler equation was written, which made it possible, as a result of various transformations, to obtain a formula for calculating the oscillation frequency of the fluidized bed. The studies carried out made it possible to establish the regimes of pseudo-fluidization, to a certain extent minimizing the heterogeneity of the layer, which is of significant practical importance. However, the operating parameters need to be adjusted depending on the type of material to be dried and other indicators. The research results do not obscure the general provisions of nonequilibrium thermodynamics. The fluidized bed cannot be in an equilibrium state, since the transfer of substances is obvious: energy, mass and momentum. It is correct to regard the fluidized bed as unstable. Small and spontaneous fluctuations always exist in the layer. The absence of conditions for their decay becomes a condition for the instability of the process.

Analysis of initial stage of colloidal particles flocculation induced by different degree branching polyelectrolytes

Flocculation performance of polyelectrolytes is affected by their molecular branching. To unravel the mechanisms involved, we performed flocculation experiments of negatively charged polystyrene latex particles using three cationic polyacrylamide-based polyelectrolytes differing only in the degree of molecular branching. In the experiment, a simple linear chain molecule and two branched polyelectrolytes were tested. The initial rate of flocculation under the condition of standardized mixing, the value of resulted electrophoretic mobility of particles coated with polyelectrolytes, and their hydrodynamic layer thickness by means of particle tracking of Brownian motion were monitored to compare the effect of branching degree. The rate of flocculation was found to increase with an increase in the degree of branching, as corresponding to the hydrodynamic adsorbed layer thickness. These results suggested the importance of the relaxation process polymer chain from coil-like solution conformation to flattened adsorbed state on the surface of colloidal particles. Branched polymers retain their shape after arriving on the surface due to the presence of branches that disturb smooth movement resulting in slow relaxation on the surface of the colloidal particle. Subsequently, forming thick adsorbed layer and enhances the rate of flocculation. Thus, explaining the commonly observed trend where branched polymers achieve better flocculating abilities than linear ones.

Role of extended surfaces on the enhancement of quenching performance

High heat transfer rate is essential in several industrial applications, including, nuclear reactors, storage and transport of cryogenic liquid, and metal forming, which deal with the rapid cooling of various materials. Rapid cooling of surfaces at temperatures significantly higher than the boiling point of the coolant is often restricted by formation of a vapor layer around the surface that retards the rate at which heat can be dissipated. In the present work we report on the effect of finned structures of millimetric length scale on the enhancement of heat transfer performance during quenching. The fins are observed to destabilize hydrodynamic vapor layer during film boiling. We show that the quench duration reduces by ~3.8 times and the maximum heat flux (MHF) increases by nearly 230% for the finned sample as compared to the smooth samples. The quench duration and heat transfer rate are shown to be dependent on spacing between fins and the optimum fin spacing is determined to yield the highest MHF and minimum quench time.

Study on Darcy-Forchheimer Flow and MHD Boundary Layer Flow with Heat Transfer Characteristics of Williamson Nanofluid Over Curved Stretching Surface.

Principle targets of this paper are to consider the Darcy-Forcheimer and magneto hydrodynamic limit layer stream with warmth and mass exchange of Williamson nanofluid over a bended extending surface. To create the stream, a non-straight bended extending surface is utilized. Likewise the capacities thermophoretic dispersion and irregular movement are created in the stream. The pertinent overseeing limit layer fractional differential conditions are changed into common differential conditions by utilizing existing comparability changes. These nonlinear coupled normal differential conditions, subject to the fitting limit conditions, are then addressed by utilizing bvp4c strategy. The impacts of the actual boundaries on the stream, heat move and nanoparticle fixation qualities of the issue are introduced through diagrams and are talked about in detail. The skin rubbing coefficient, neighborhood Nusselt number and Sherwood numbers are figured. In light of these plots the ends are given, and got results are tried for their exactness.

Temporal Changes of Adsorbed Layer Thickness and Electrophoresis of Polystyrene Sulfate Latex Particles after Long Incubation of Oppositely Charged Polyelectrolytes with Different Charge Densities.

The different desorption concepts of the two polyelectrolytes PTMA5M and PTMC5M, which have similar molecular weights and differ in the charge density on the polystyrene sulfate latex (PSL) particles by 25 times, and with various charge densities in a long incubation, were systematically investigated based on hydrodynamic adsorbed layer thickness (δH) and electrophoretic mobility (EPM) under two ionic strengths in the present study. Herein, in the case of highly charged polyelectrolyte PTMA5M, desorption continued for 4 h and re-adsorbing proceeded after a longer incubation time higher than 4 h. Meanwhile, in the case of lowly charged polyelectrolyte PTMC5M, an adsorption–desorption equilibrium was suggested to take into account the unchanging of both δH and EPM.

Analysis of Global and Local Hydrodynamic Instabilities on a High-Speed Jet Diffusion Flame via Time-Resolved 3D Measurements

Measurements of 3D flame edge dynamics were made on a high-speed jet diffusion flame to assess the global/local hydrodynamic instability. The flame was generated by issuing high-speed ethylene (Uj = 170 m/s) into a low-speed vitiated hot coflow (Uc = 1.5 m/s), resulting in a hydrodynamic shear layer instability at the interface between combustion products and ambient flow. The measurements used a high-speed camera combined with nine-headed fiber endoscopes to simultaneously collect both the soot radiation and chemiluminescence projections of the flame from nine views, based on which 3D flame edges were obtained via computed tomography at 15 kHz. The measurements clearly capture the time-varying, 3D instantaneous flame edge structures with fine-scale corrugations, enabling the observation of small-scale vortices' evolution. The flame edge deformations induced by those vortices were calculated globally and locally to infer the relationship between global and local flame edge oscillations. Results show that various local oscillation frequencies exist at different locations along the flow direction for such a highly sheared flame. They are dominated by the periodical formation and motion of various localized, small-scale vortices. The local oscillations are much severer than the global oscillation, indicating a self-suppression of the instabilities between these local oscillations. The suppression mechanism is attributed to the constructive and destructive interference behavior of the local disturbances. The global oscillation of the flame edge turns out to be the linear superposition of local oscillations at different locations.

Soret, viscous dissipation, and thermal radiation effects on MHD free convective flow of Williamson liquid with variable viscosity and thermal conductivity

AbstractThe flow model of heat and mass transport of a Williamson liquid through a porous stretching sheet with radiation, viscous dissipation, Soret effect, and chemical reaction has been explored. The motion starts from the slot to the free stream. The present study is unique, because it examines the flow of a Williamson fluid under the influence of variable viscosity and thermal conductivity. The Williamson fluid term as added to the momentum and energy equation is considered in a nonlinear form as compared with other studies in literature. The flow model is a set of coupled highly nonlinear partial differential equations that are simplified and lead to coupled nonlinear total differential equations by employing sufficient similarity variables. The simplified equations are later solved by utilizing the spectral homotopy analysis method. Our experiment shows that the injected variable viscosity, together with thermal conductivity, has a great impact on the fluid profiles. An increase in the Williamson parameter (β) leads to a decrease in the thickness of the hydrodynamic thermal layer. Our numerical calculations were compared with earlier published work, and they were discovered to be correct.

Aggregate-Free Deposition of Highly Organized Nanocomposite Silica Thin Films on Conductive Substrates By Electrochemically Induced Templated Sol-Gel Synthesis

The synthesis of ordered nanocomposite or nanoporous silica thin films by template-assisted sol-gel processes offers interesting applications in various fields including catalysis, sensors, membranes and energy. The electrochemically assisted self-assembly method allows the growth of highly ordered vertically aligned nanoporous silica thin films on conductive substrates. This orientation, optimized for high mass-transport capabilities, is difficult to obtain by classical approaches. The method involves submerging the conductive substrate in a pre-hydrolyzed precursor liquid followed by the application of a cathodic potential, thereby causing the electrogeneration of hydroxide ions. The pH increase in the diffusion layer near the electrode surface catalyzes the sol-gel formation and the result is a silica thin film that grows on the conductive substrate. The addition of cetyltrimethylammonium bromide (CTAB) micelle templates leads to the formation of a highly ordered nanostructure.A fundamental limitation of the method as reported in the literature is that the maximum layer thickness which can be obtained is rather limited due to the formation of aggregate by-products. In our contribution we show that diffusion layer thickness control is essential to obtain aggregate-free layers. Silica formation is catalyzed by OH- formed in the electrode’s diffusion layer, which quickly grows up to hundreds of microns under stationary conditions, thereby leading to composite thin films as well as precipitation of aggregates. By using a rotating disc electrode, the hydrodynamic layer is controlled, enabling aggregate-free film growth over a wide thickness range. Additionally, thin film coatings on high aspect ratio micropillar structures were demonstrated. Parameters such as precursor age, deposition temperature and deposition current were systematically investigated and the deposits were characterized using SEM, ellipsometry, TEM, FT-IR and an electrochemical redox probe. Using these techniques, we demonstrated the controlled growth of aggregate-free, uniform and nanostructured thin films with high mass-transport capabilities and thicknesses of 20 nm up to 15 µm, thereby significantly expanding the range of 50-150 nm as reported in literature. These results are explained using a single hydrodynamic layer model. Figure 1