Porous Layer Thickness Research Articles

Natural Convection in Differentially Heated Partially Porous Layered Cavities Filled with a Nanofluid

Natural convection heat transfer in a differentially heated and vertically partially layered porous cavity filled with a nanofluid is studied numerically based on double–domain formulation. The left wall, which is adjacent to the porous layer, is isothermally heated, while the right wall is isothermally cooled. The top and bottom walls of the cavity are thermally insulated. Impermeable cavity walls are considered except the interface between the porous layer and the nanofluid layer. The Darcy–Brinkman model is invoked for the porous layer which is saturated with the same nanofluid. Equations govern the conservation of mass, momentum, and energy with the entity of nanoparticles in the fluid filling the cavity and that are saturated in the porous layer are modeled and solved numerically using under successive relaxation upwind finite difference scheme. The contribution of five parameters are studied, these are; nanoparticle volume fraction ϕ (0–0.1), porous layer thickness Xp(0–0.9), Darcy number Da (10−7–1), aspect ratio A (1, 2, 4), and Rayleigh number Ra (103–106). The nanofluid is considered to be composed of copper nanoparticles and water as a base fluid. The results have shown that with the aid of a nanofluid, the convective heat transfer can be enhanced even at a low permeable porous medium. It is found that when Ra ≤ 105, there is a critical porous layer thickness Xp at which the Nusselt number is maximum. Otherwise, the Nusselt number Nu decreases rapidly with Xp. Correlations of Nu with the other parameters are established and tested for A = 2.

Enhanced Performance of Oxidation of Rosalva (9-decen-1-ol) to Costenal (9-decenal) on Porous Silicon-Supported Silver Catalyst in a Microstructured Reactor

The use of metal-assisted HF chemical etching as a convenient technique to produce a few microns thick porous layer in silicon microchannels was demonstrated. Gas phase selective oxidation of rosalva to its aldehyde (costenal) was performed in glass/silicon microstructured reactors at temperatures of 375–475 °C on silver catalyst which was deposited on both porous and flat silicon surface by sputter-coating. The effects of temperature (375–475 °C), rosalva concentration (1.17%–3.43%), O2 to rosalva ratio (0.5–20) and residence time on the reaction were investigated. The reactivity of rosalva on the porous silicon supported silver was 5.7–6.4 times higher than on the thin film silver catalyst at 450 °C. Furthermore, activation energy for the porous silicon supported silver was lower. Isothermal conditions in the microreactors allowed high conversion and selectivity to be achieved in a wide range of temperature and oxygen concentration. At typical reaction conditions (1.75% rosalva, O2/rosalva = 3, residence time 18 ms and 450 °C), conversion of 97% and selectivity of 95% to costenal was achieved, corresponding to a turnover frequency of 268 h−1.

Silver particles deposited on porous silicon as surface-enhanced Raman scattering active substrate

A simple, low-cost, and high-efficient method is used for the fabrication of surface-enhanced Raman scattering (SERS) substrates. Silver particles deposited on porous silicon are prepared as a highly efficient SERS substrate by direct immersion of porous silicon in silver solution. The SERS measured with rhodamine 6G as a target molecule is affected by the morphology of silver particles on the top of porous silicon layer. The effect of solution concentration, dipping time, and thickness of porous layer on the morphology of silver particle is investigated. Highly efficient SERS spectra are observed for substrates with porous layer thickness of about 3 μm and incubated in the 50 mM AgNO3 solution for 3 minutes. The SEM images of the substrates show that there are many small Ag particles with the size of a few nanometers among large Ag particles with the size of several microns.

Cocktails of Paste Coatings for Performance Enhancement of CuInGaS2 Thin-Film Solar Cells

To fabricate low-cost and printable wide-bandgap CuInxGa1-xS2 (CIGS) thin-film solar cells, a method based on a precursor solution was developed. In particular, under this method, multiple coatings with two pastes with different properties (e.g., viscosity) because of the different binder materials added were applied. Paste A could form a thin, dense layer enabling a high-efficiency solar cell but required several coating and drying cycles for the desired film thickness. On the other hand, paste B could easily form one-micrometer-thick films by means of a one-time spin-coating process but the porous microstructure limited the solar cell performance. Three different configurations of the CIGS films (A + B, B + A, and A + B + A) were realized by multiple coatings with the two pastes to find the optimal stacking configuration for a combination of the advantages of each paste. Solar cell devices using these films showed a notable difference in their photovoltaic characteristics. The bottom dense layer increased the minority carrier diffusion length and enhanced the short-circuit current. The top dense layer could suppress interface recombination but exhibited a low optical absorption, thereby decreasing the photocurrent. As a result, the A + B configuration could be suggested as a desirable simple stacking structure. The solar cell with A + B coating showed a highly improved efficiency (4.66%) compared to the cell with a film prepared by paste B only (2.90%), achieved by simple insertion of a single thin (200 nm), dense layer between the Mo back contact and a thick porous CIGS layer.

Numerical study of heat transfer enhancement in a pipe filled with porous media by axisymmetric TLB model based on GPU

Numerical simulations have been carried out to investigate the heat transfer enhancement for fluid flow in a pipe partially filled with porous media. The new axisymmetric lattice Boltzmann model is used to calculate the fluid flow and heat transfer characteristics in a pipe filled with porous media. The proposed algorithm is implemented on the Graphics Processing Unit (GPU) using NVIDIA’s CUDA for a high calculating speed. Finally, the effects of several parameters, such as porous layer thickness, Darcy number and porosity, on thermal conductivity efficiency are investigated. It is found that controlling the thickness of the porous media can significantly improve heat transfer performance and the influence of porosity is very small both on flow field and temperature field when other parameters are fixed.

A Study of Hot Corrosion of Fe-Cr Alloys Induced by a Thin Film of Fused (Na,K)2SO4 by Electrochemical Impedance Spectroscopy

AbstractTo understand the hot corrosion mechanism of materials it is of significance to establish electrochemical measurements under a thin film of fused salts. In this paper, the hot corrosion behavior of Fe-Cr alloys containing 5, 10 and 25wt% Cr, respectively, in the presence of a fused film of 0.9Na2SO4-0.1K2SO4 (mole fraction) at 1173 K in air has been investigated by electrochemical impedance spectroscopy using a two-electrode system. The Nyquist plots for the corrosion of both Fe-5Cr and Fe-10Cr are composed of a very small semicircle at high frequencies and a line at low frequencies indicating that the corrosion is controlled by the diffusion of oxidants in the fused salt film. The alloys were severely corroded, forming a thick porous corrosion layer containing large amounts of non-protective precipitated Fe oxide particles. Conversely, the Nyquist plots for the corrosion of Fe-25Cr are composed of double capacitive loops, with significantly larger impedance values, which are associated with the formation of a protective Cr2O3 scale. Based on the experimental diffusion impedance results and the theoretic solubility and diffusion coefficient of oxygen in molten salts, respectively, a theoretical value of the diffusion flux of molecular O2 has been calculated.

A Thermoelectric Generator Using Porous Si Thermal Isolation

In this paper we report on a thermoelectric generator (TEG) using thermal isolation provided by a thick porous Si layer locally formed on the Si wafer and thermocouples composed of p-doped polycrystalline Si/Al. The “hot” contacts of the thermocouples lie on the porous Si layer, while the “cold” contacts lie on bulk crystalline Si. A housing was also designed and fabricated in order to transfer any external temperature change on the “hot” contacts of the thermocouples, the “cold” contacts being isolated from the “hot” contacts by a thick resist layer. The fabrication of the sensing element (Si die) is fully compatible with batch Si processing. The output power of the thermoelectric generator depends on the porous Si isolation layer thickness, porosity, structure and morphology. For a mesoporous Si layer of 60% porosity and a macroscopic temperature differential of 10 K, an output power of 0.39 μW/cm2 was measured for a 50 μm thick porous Si layer.

Hierarchical porous titania/carbon nanotube nanocomposite photoanode synthesized by controlled phase separation for dye sensitized solar cell

Incorporation of carbon nanotubes (CNT) in titania photoanode of dye sensitized solar cell (DSC) with hierarchical porous structure synthesized by controlled phase separation was studied. The hierarchical porous composite photoanode contained mesopores of 15nm and macropores of 280nm with surface roughness of about 15.4nm. The Raman spectroscopy confirmed the presence of CNTs beside titania anatase phase. Also, the transmission electron microscopy revealed that a very thin titania layer was formed on CNTs after impregnation with titania sol. The I–V characteristics of the DSCs with and without CNTs showed that incorporation of CNTs in photoanode led to an increase in current density and efficiency, which was due to the increase in surface area and hence dye adsorption, increase in density of states in titania and charge injection, and increase in charge collection and electron lifetime. The electrochemical impedance spectroscopy exhibited lower series resistance, lower charge injection resistance, and higher electron lifetime in DSCs containing CNTs. Addition of CNTs in photoanode also affected the critical thickness of porous layer with which the maximum efficiency could be obtained. Furthermore, the open circuit voltage and fill factor of the DSCs was independent of thickness when TiO2/CNT composite was used. The optimum amount of CNTs was found to be 0.32wt%, which led to the short circuit current density of 10.12mA/cm2, open circuit voltage of 680mV, fill factor of 56%, and efficiency of 3.85%.

Design, analysis and optimization of porous silicon microcavity based on silicon-on-insulator at optical communication wavelengths

With optimized parameters and the optimized sequence structure from simulations, proper porous silicon microcavity (PSM) based on silicon-on-insulator (SOI) wafer, constituted of a Fabry–Perot cavity between distributed Bragg reflectors (DBRs), has been achieved. Just as expected, the resonance wavelengths of these two oxidized PSMs are located separately at 1328nm and 1560nm, which are very close to the optical communication wavelengths of 1310nm and 1550nm. The effects of tuning resonance wavelength were shown by composition of Fabry–Perot cavity integrated on each thickness of porous silicon layer. Since the preparation technology of optical devices with these working wavelengths has been well developed, it is expected that such a design could be of great potential in realizing integrated optical devices. Compared with various optical devices on SOI wafer, PSM based on SOI by electrochemical anodization is simple and low-cost. Moreover, a sharp resonance in the reflectance spectrum means a high Q factor which predicts an interesting possibility in sensor application on SOI wafer.

Design optimization of porous fibrous material for maximizing absorption of sounds under set frequency bands

In this paper, a methodology is proposed for designing porous fibrous material with optimal sound absorption under set frequency bands. The material is assumed to have a rigid frame and a hexagonal arrangement of fibers, and the analytical model derived by Johnson, Champoux and Allard (“JCA model”) is used to investigate the influences of the micro-structural parameters (fiber radius r and gap w) on sound absorption performance, and the macro-acoustic parameters used in JCA model is determined via finite element analysis for the hexagonal micro-structure. Moreover, a mathematical model is constructed to obtain the optimized micro-structure design, with fiber radius and gap as design parameters and average absorption performance of the porous fibrous material under set frequency band as target. Utilizing the constructed optimization model, the microstructure parameters are derived with optimal sound absorption under low frequency (20⩽f<500Hz), medium frequency (500⩽f<2000Hz) and high frequency (2000⩽f<15,000Hz), respectively. On top of that, for a given thickness of porous fibrous material layer, the analytical relationship between fiber radius and optimal porosity under set frequency bands is constructed.

Preparation and kinetic performance assessment of thick film 10–20 μm open tubular silica capillaries in normal phase high pressure liquid chromatography

It is well-known that the open tubular column design basically can offer very high plate numbers. Experimental realization has however not kept pace with theoretical considerations, lacking efficient methods for the deposition of a thick film porous layer within a microbore capillary. A previously published sol–gel synthesis method was extended from 20μm to 10μm inner diameter fused silica capillaries and the resulting columns were compared to a monolithic reference capillary in terms of kinetic performance at pressure maximum. Column permeability was investigated and pressure/flow-diagrams were obtained with a 400-fold permeability gain for the open tubes. Structural characterizations regarding layer thickness and surface porosity were carried out and displayed via scanning electron microscopy and nitrogen sorption analysis. Chromatographic results in normal phase mode at elevated mobile phase flow rate reveal the intrinsic performance potential of this column format when it comes to kinetic performance limitation plots, which were constructed for all columns prepared and compared to the monolithic silica reference capillary.

Cracking of an austenitic stainless steel lance pipe in a lime calcining plant

Kiln of a lime plant, a part of an integrated steel plant, converts limestone (CaCO3) into lime (CaO) by heating at a temperature of around 900°C using coke oven gas as a fuel. Coke oven gas is a by-product of the coke plant, which contains mainly hydro-carbons. The coke oven gas is lanced into the kiln through a number of pipes attached to the kiln. The lance pipes are cracking prematurely within 6months of their service against an expected service life of 3years. The analysis of cracking of the lance pipe has been presented. The investigation includes visual observation, chemical analysis, characterization of microstructures using optical and scanning electron microscopes, EDS analysis, and measurement of hardness. The lances are found to be made of AISI 302 grade of austenitic stainless steel. Visual observation shows longitudinal cracks on the pipe wall associated with irregular yellowish surface having multiple pits. Analysis of the deposit on the pipe wall shows the presence of sulphur. Microstructural examination shows intergranular corrosion with thick porous layer of scale on the surface. Microstructural and EDS analysis indicate sulphidation and chromium carbide networks along the grain boundary as well as sigma phases within the grains. Sulphidation, sensitization due to grain boundary carbides and sigma phases led to intergranular corrosion causing cracking of the pipe wall.

Effect of thickness of nonplatinum cobalt-tetramethoxyphenylporphyrin-based catalyst layer on the selectivity of O2 electrochemical reduction

Kinetics of O2 reduction at CoPP/XC72-catalyst in acid electrolyte is studied by the rotating disc electrode method. The data obtained with rotating ring-disc electrode over the 1-to-26 μm thickness range showed the effect of the porous layer thickness on the selectivity of O2 reduction reaction. Composition of the catalyst active centers is discussed, their number estimated.

Low thermal conductivity porous Si at cryogenic temperatures for cooling applications

Porous Si thermal conductivity was determined in a wide temperature range from 20 to 350 K using the steady-state direct current method and a subsequent finite element method. The method was applied to a 40 µm thick porous Si layer of ∼60% porosity, formed on p-type Si of resistivity 1–10 Ω cm. It was found that in the temperature range 20–80 K the thermal conductivity of the studied porous Si layer was more than four orders of magnitude lower than that of bulk crystalline Si, due to phonon confinement in the nanostructured skeleton composing porous Si. The temperature distribution as a function of distance from a heater on the porous Si layer and as a function of depth from the porous Si/metal interface was also calculated for different values of the applied electric power and for two different temperatures, 20 and 290 K, using a combination of experimental results and simulations. The effectiveness of local thermal isolation from the Si substrate by a thick highly porous Si layer in a wide temperature range was demonstrated, showing that this material is challenging for use in Si micro-cooling devices.

Instability modes of a two-layer Newtonian plane Couette flow past a porous medium

We explore the salient features of the different instability modes of a pressure-driven two-layer plane Couette flow confined between a moving wall and a Darcy-Brinkman porous layer. A linear stability analysis of the conservation laws leads to an Orr-Sommerfeld system, which is solved numerically with appropriate boundary conditions to identify the time and length scales of the instability modes. The study reveals that the movement of the confining wall together with the slippage at the porous-liquid interface originating from the flow inside the porous layer can stimulate a pair of finite-wave-number shear modes in addition to the long-wave interfacial mode of instability. The shear modes dominate the interfacial mode, especially when the frictional influence at the liquid layers is smaller due to the movement of the confining plate or due to the larger slippage at the porous-liquid interface. The shear modes are found to be present under all combinations of the viscosity μ(r) and thickness h(r) ratios of the liquid layers. This is in stark contrast to the two-layer flow confined between nonporous plates where the interfacial (shear) mode is observed only when μ(r)>h(r)(2) (μ(r)<h(r)(2)). Interestingly, the strength of one of the shear modes is found to increase with the velocity of the bounding moving plate, whereas the other shear mode gains strength in the presence of highly porous, permeable, and thick porous layers. The results reported can be of significant importance in the microscale two-phase flows where the presence of a bounding porous layer or moving wall can expedite the intermixing of layers to improve the multiphase mixing, heat and mass transfer, and emulsification characteristics.

Water jet impingement boiling from structured-porous surfaces

Structured-porous surfaces were created by diffusion bonding a single layer or multiple layers of highly conductive copper wire mesh screen onto plain heat transfer base surfaces. Flow boiling heat transfer experiments were then conducted with a liquid water jet impinging onto those porous copper surfaces under atmospheric pressure. For comparison, experiments of jet impinging on a plain copper surface were also conducted. The effect of jet Reynolds number, jet inlet subcooling, and porous layer thickness on boiling heat transfer were explored. Surface ageing and data repeatability were also discussed. The results show that a base surface heat flux of 674W/cm2 was reached at a superheat of 5K and a jet Reynolds number of 10,000 with a four-layer 200-mesh test surface. Compared to jet impinging on a plain copper surface under identical working conditions, a fourfold performance enhancement was obtained in increasing heat transfer from the base surface.

Fluid flow and forced convection heat transfer around a solid cylinder wrapped with a porous ring

Convective heat transfer from cylinders embedded in porous media is important for many engineering applications. In the present study, flow-field and heat transfer around a cylinder embedded in a layer of homogenous porous media is investigated numerically. The range of Reynolds and Darcy numbers are chosen to be 1–40 and 1×10−8–1×10−1, respectively. Volume averaged equations are used for modeling transport phenomena within the porous layer and conservation laws of mass, momentum, an energy are applied in the clear region. A comprehensive parametric study is carried out and effects of several parameters, such as porous layer thickness and permeability as well as the Darcy and Reynolds numbers on flow-field and heat transfer characteristics are studied. Finally an optimization process is conducted in order to determine the optimal thickness and porosity of the porous layer resulting in the lowest heat transfer from the cylinder. The numerical results indicate that, in the presence of a porous layer around the cylinder, the wake length increases with decreasing the Darcy number while the critical radius of insulation increases.

Instabilities of a confined two-layer flow on a porous medium: An Orr–Sommerfeld analysis

Instabilities of a pressure driven two-layer Poiseuille flow confined between a rigid wall and a Darcy–Brinkman porous layer are explored. A linear stability analysis of the conservation laws leads to an Orr–Sommerfeld system, which is solved numerically with appropriate boundary conditions to identify the time and length scales of the instabilities.fde The study uncovers the coexistence of twin instability modes, (i) long-wave interfacial mode—engendered by the viscosity stratification across the interface and (ii) finite wave number shear mode—originating from the inertial stresses, for almost all combinations of the viscosity (μr) and thickness (hr) ratios of the liquid layers. The presence of the porous layer reduces the frictional influence on the films, which significantly alters the length and time scales of the shear mode while the interfacial mode remains dormant to this effect. This is in stark contrast to the two-layer flow confined between non-porous plates where an unstable interfacial (shear) mode is observed when μr>hr2(μr<hr2). The study reveals that strength of the shear mode, (a) increases with porosity, (b) initially increases and then becomes constant with porous layer thickness, (c) initially increases then reduces with increase in permeability, and (d) reduce with increase in the stress jump coefficient at the porous–liquid interface. Moreover, the gravity expedites the destabilization of both the modes in the inclined channels as compared to the similar non-inclined channels. The parametric study presented can find important applications in enhancing heat and mass transfer, mixing, and emulsification especially in the microscale flows.

Pore Formation in InP Anodized in KOH: Effect of Temperature and Concentration

The effect of both temperature and electrolyte concentration on the formation of porous layers in InP is explored. Pore width, porous layer thickness and porosity decrease with increasing temperature and show a minimum at intermediate KOH concentrations, increasing as concentration is either increased or decreased from that value. The variations in pore width and layer thickness are correlated: thinner pores appear to limit layer thickness, presumably by influencing mass transport throughout the porous network. A three step model of charge transfer at the semiconductor/electrolyte interface is used to explain these observations.

Mesoscopic Ni particles and nanowires by pulsed electrodeposition into porous Si

We report in this article on the formation of mesoscopic Ni particles and filling of continuous Ni nanowires into porous Si layers of thickness in the range of 0.5–4 μm with anisotropic vertical pores of average diameter in the range of 30–45 nm using pulsed electrodeposition from a Ni salt solution. The effect of pulse duration, number of pulses, and total process time on pore filling was investigated for porous Si with different porosities and porous Si layer thicknesses in the above thickness range. Scanning and transmission electron microscopy were used to characterize the samples. It was found that pore filling starts with Ni nucleation and nanoparticle formation at different points of the pore walls along the whole pore length and continues with nanoparticle coalescence to form continuous Ni nanowires that completely fill the pores. The mechanism involved in pore filling is particle nucleation and diffusion-controlled growth of Ni nanoparticles that coalesce to nanowires. From the beginning of the process, a metal film starts to form on the porous Si surface, and its thickness increases with increasing the process time. However, the presence of this film does not impede further pore filling and nanowire formation into the pores. This supports further the diffusion-controlled growth mechanism. Finally, it was demonstrated that full pore filling and continuous Ni nanowire formation were also achieved under direct current electrodeposition, and the results are quite similar to those obtained with pulsed electrodeposition when the same total deposition time is used in both cases.