Porous Inserts Research Articles

Joint Influence Of Surface Roughness And Permeability On Stability And Transition Of Mach = 2 Boundary Layer

Joint theoretical and experimental investigation of the influence of surface permeability and roughness on stability and laminar-turbulent transition of the supersonic flat-plate boundary layer at free-stream Mach number M = 2 have been performed. Good quantitative agreement of experimental data obtained with artificially generated disturbances performed on models with various porous inserts and calculations based on the linear stability theory has been achieved. At research of the joint effect of the model surface permeability and roughness on the boundary layer stability and transition, it is shown that, as a certain (critical) roughness value is reached, with the rough and porous coating thickness increase, the boundary layer stability diminishes and the laminar-turbulent transition is displaced toward the model leading edge.

Thermal characteristic of a laminar flow through a circular tube fitted with wire-net cylinder inserts

A novel type of porous media inserts, wire-net cylinder inserts, was developed to improve convection heat transfer in the present work. Heat transfer performance and pressure drop was numerically studied. Wire-net cylinder inserts with different spacer length (S/D=2.1, S/D=4.2, S/D=8.3) and different open area rate (OAR=0.33, 0.5 and 0.67) were investigated at the Reynolds number range of 290–2024. Liquid water was used as the working fluid. The results show that wire-net cylinder inserts have an excellent heat transfer performance in the laminar regime. This was mainly due to the increase in the velocity at the inlet and the high intensity of the fluid mixing between annular region and the core region. The open area rate has a great impact on the heat transfer rate. The best heat transfer rate resulting from inserts with P=0.2mm (OAR=0.5) is about 3.45 times that of smooth tube. The friction factor increases with the decrease in open area rate. Although inserts with P=0.4mm (OAR=0.67) have the lowest heat transfer rate, its friction factor is also the lowest. As a result, inserts with P=0.4mm achieve the best PEC values (performance evaluation criterion). Heat transfer performance of wire-net cylinder inserts is also affected by the spacer length. The heat transfer rate increases with the decrease in the spacer length. Although a short spacer length can result in a high friction factor, the PEC values are still very high. It is still valuable to adopt a short spacer length in the design of heat exchanger. Compared with other inserts, wire-net cylinder inserts are simple and can be fabricated easily at a low cost.

Optimal performance of an annular heat exchanger with a porous insert for a turbulent flow

The use of porous materials as a passive technique to intensify heat transfer has been well established. Yet, this is always accompanied by a non-desirable and costly effect that is a higher pressure drop. Adopting such a technique requires a compromise between hydrodynamics and thermal performances in order to use efficiently the available energy. In this study, we analyze the performance of an annular heat exchanger with a porous insert by means of the assessment of entropy generation rate due to fluid friction and heat transfer for a turbulent flow. The porous substrate is attached over the inner pipe. A modified κ-ε model is used to account for turbulence in the porous medium. Effects of several parameters such as the porous layer thickness, its permeability, its thermal conductivity and the inlet temperature difference are investigated and documented. It is highlighted that the porous insert with an optimal thickness allows a significant reduction of total entropy generation when it has a high effective thermal conductivity. Hence, this heat transfer enhancement technique has a great merit for thin porous inserts not exceeding about 40% of the annular gap.

First and second law analyses of nanofluid forced convection in a partially-filled porous channel – The effects of local thermal non-equilibrium and internal heat sources

Generation of entropy and transfer of heat during forced convection of a nanofluid through a partially-filled porous channel are investigated theoretically. The problem includes a fully developed flow in a channel with a central porous insert and under constant heat flux boundary condition. The system is assumed to be under local thermal non-equilibrium and the solid and nanofluid phases can feature internal heat generations. Darcy–Brinkman model of momentum transfer along with the two-equation thermal energy transport and two different fundamental porous-fluid interface models are utilised to analyse the heat transfer problem. Analytical expressions are developed for the temperature fields, Nusselt number and, the local and total entropy generations. The subsequent parametric study reveals the strong influences of the pertinent parameters and the utilised porous-nanofluid interface models. In keeping with others, the results show considerable increases in the Nusselt number with increasing the concentration of nanoparticles. Internal heat generations are demonstrated to have major effects on the heat transfer and entropy generation characterises of the system. Further, the existence of internal heat sources signifies the role of nanoparticles concentration in the thermal and entropic behaviours of the system. It is, also, shown that the choice of the porous-nanofluid interface model can significantly alter the predictions of the local and total entropy generations within the system. This appears to be, particularly, the case at low Biot numbers for which the system is significantly away from the local thermal equilibrium condition.

Numerical investigation of heat exchanger performances for a two engine aircraft in pusher configuration by an u-RANS code

This paper deals with the numerical analysis of the effect a heat exchanger has on the flow field in an oil cooling system for aerospace applications. To this end, an important upgrade of the UZEN in house CIRA Reynolds Averaged Navier–Stokes solver has been carried out to account for the pressure drop and heat rejection that usually are present in a heat exchanger. This upgrade is based on the addition of a source term in the momentum and energy equations of balance, respectively, whose values are updated during the simulation. The correspondent code modification needs a proper validation before any concrete application. At this aim, a two dimensional channel has been firstly studied in order to test the accuracy of the UZEN code. Then a three dimensional circular duct has been studied with a porous insert to validate the porous approach robustness. After this preliminary campaign of assessment, a new generation two engine aircraft has been studied with particular emphasis on its nacelle cooling system. Velocity and pressure are compared using different turbulence models with the well known Fluent ones. The more interesting parameter for such an investigation is the mass flow rate through the cooling ducts as computed by UZEN that show a good agreement with the Fluent one.

Influence of a Porous Insert on the Fluid Flow inside the Gasifier Shaft

Turbulent fluid flow inside the vertical gasifier shaft having the porous insert has been numerically analyzed. The effect of the porous medium structure on the fluid flow has been studied. The mathematical model has been formulated in dimensional primitive variables using the realizable k-ε turbulent model. The distributions of velocity and skin friction coefficient inside the gasifier shaft have been obtained. The results clearly show an essential effect of the porous medium structure on turbulent fluid flow.

A Diffusion-Based and Dynamic 3D-Printed Device That Enables Parallel in Vitro Pharmacokinetic Profiling of Molecules.

The process of bringing a drug to market involves many steps, including the preclinical stage, where various properties of the drug candidate molecule are determined. These properties, which include drug absorption, distribution, metabolism, and excretion, are often displayed in a pharmacokinetic (PK) profile. While PK profiles are determined in animal models, in vitro systems that model in vivo processes are available, although each possesses shortcomings. Here, we present a 3D-printed, diffusion-based, and dynamic in vitro PK device. The device contains six flow channels, each with integrated porous membrane-based insert wells. The pores of these membranes enable drugs to freely diffuse back and forth between the flow channels and the inserts, thus enabling both loading and clearance portions of a standard PK curve to be generated. The device is designed to work with 96-well plate technology and consumes single-digit milliliter volumes to generate multiple PK profiles, simultaneously. Generation of PK profiles by use of the device was initially performed with fluorescein as a test molecule. Effects of such parameters as flow rate, loading time, volume in the insert well, and initial concentration of the test molecule were investigated. A prediction model was generated from this data, enabling the user to predict the concentration of the test molecule at any point along the PK profile within a coefficient of variation of ∼ 5%. Depletion of the analyte from the well was characterized and was determined to follow first-order rate kinetics, indicated by statistically equivalent (p > 0.05) depletion half-lives that were independent of the starting concentration. A PK curve for an approved antibiotic, levofloxacin, was generated to show utility beyond the fluorescein test molecule.

Influence of porous carbon inserts on scaling up studies for performance enhancement on PEMFC

In order to reduce the effect of water flooding on larger active area of Proton Exchange Membrane Fuel cell (PEMFC), an enhanced flow distribution on the flow channel is mandatory. In this paper to mitigate the water flooding problem, experimental studies have been carried out on 25 cm2 and 70 cm2 active area of PEMFCs with various flow channel designs on the cathode flow plate namely serpentine, uniform and zigzag pin type; and flow channel with 2 mm cubical porous inserts on the uniform and zigzag pin types. The results show that the cathode flow field plate with zigzag positioned porous carbon inserts having 80–90% porosity has improved the current and power densities by 7% and 11.5% respectively, when compared to the conventional serpentine flow field plate for 25 cm2 PEMFC. The above procedure has been followed for scaling up studies from 25 cm2 to 70 cm2 PEMFCs for effective water management. It is concluded that the 70 cm2 PEMFC with zigzag positioned porous carbon inserts on the cathode flow field has increased the current and power densities by 8.7% and 20.6% respectively compared to the conventional serpentine flow field. The porous carbon inserts on the rib surface of the cathode flow field removes the accumulated liquid water from the rib surface through the capillarity of its porous structure, due to this the performance of PEMFC is enhanced in scaling-up studies with better water transport characteristics.

EXPERIMENTAL INVESTIGATION OF THE PERFORMANCE OF FIVE LAYERED GAS TO GAS HEAT EXCHANGER WITH A POROUS MEDIUM

This paper presents the performance of a simple five-layers-type gas to gas heat exchanger, which is simple to construct, easy to maintain and has low space requirements, under different laminar gaseous flow rates and temperatures. The merits and drawbacks of the use of porous material inserts to improve the performance of this type of heat exchanger are investigated. An experimental work was carried out to investigate the influence of the insertion of porous segments with different thicknesses on heat transfer enhancement and the associated pressure drop as these are the many determining factors in most engineering applications. During these experiments the volume flow rates of the hot and cold air streams were varied such that the volume flow rate of both of the streams was the same in any of the experimental runs. These experiments were conducted for different values of hot air inlet temperature, while keeping the cold air inlet temperature nearly constant. Also the results obtained in this work was compared with the corresponding results obtained previously using a similar three layered heat exchanger under similar operating hydro-thermal conditions. The investigations showed that the thermal performance of the heat exchanger due to the two additional layers was pronouncedly improved on the expense of an increase in pressure drop of much lower cost.

Making Hollow Cylindrical Products of High-Porosity Silicon Nitride by the Centrifugal Forming of Granules of a Thixotropic Thermoplastic Slip

This article describes features of the manufacture of high-porosity silicon semifinished products in the form of hollow cylinders by the warm centrifugal casting of elements obtained from cutting up castings of a silicon-based thermoplastic slip. Silicon nitride obtained by plasmochemical synthesis is added to the slip to impart the necessary rheological properties. Products in the form of hollow cylinders with walls made of a highly porous silicon nitride material are obtained from the semifinished products by removing the binder through sublimation and then subjecting the semifinished products to reaction sintering in nitrogen. The areas of application proposed for the material and the hollow products obtained from it: porous partitions for the filtration and combustion of natural fuels in power plants; porous inserts for the local reinforcement of die-cast products made of light metals.

Radial Bearings with Porous Elements

This paper presents the analytical model of the radial bearing of the finite length with the two-layer porous insert obtained on the basis of the dimensionless equations of movement of a viscous incompressible lubricant in an operating clearance and in porous layers of a bearing sleeve and continuity equations as well. We are analyzing the case when permeability of porous layers on the border of section of a two-layer insert accepts one value. The lubricant moves in a circumferential direction through a bore in a bearing body and the subsequent filtration occurs through insert pores. Permeability anisotropy of porous layers is taken into consideration in radial and circumferential directions.As a result of the given task, we have obtained the field of speeds and pressure in porous layers and in the lubricant layer. Analytical dependences for components of a vector of supporting force and the friction moment have been found, and we have also defined the loading factor and friction factor. Moreover, in our calculations we have revealed and used the parameter characterizing specificity of lubrication feeding in a circumferential direction. It is proved that the rational mode of operation is reached by a design of a two-layer porous insert given when the resistance factor increases more intensively and from the parameter ψ, and from a relative eccentricity ɛ in comparison with the bearing having a single-layer insert. Hence, anisotropy of permeability of porous layers in a circumferential and radial direction has a defined possibility of practical application of this new model.

Microchannels enhanced by porous materials: Heat transfer enhancement or pressure drop increment?

The analytical heat transfer performance study (i.e. HTP, introduced as the ratio of the heat transfer rate enhancement to the pressure drop increment) for microchannels enhanced by a porous material is performed. The Darcy–Brinkman equation of motion and the two-energy equation model for heat transfer within the porous material (i.e. the local thermal non-equilibrium condition, LTNE) are used to find the Poiseuille (Po) and Nusselt (Nu) numbers in the slip-flow regime (i.e. when the Knudsen number, Kn, <0.1). To cover a wide range of heat exchangers, effects of hydrodynamical characteristics of the porous material (i.e. porosity, ϕ, and Darcy number, Da), the effective thermal conductivity ratio (k), the interphase convective heat exchange between the two phases (Bi), the velocity-slip and temperature-jump coefficients (α and β), the dimensionless thermal resistance of the walls (Rw), and the internal heat generations within the solid and fluid phases of the porous material (ωf and ωs) are considered. HTP maps are presented for a wide variation range of all pertinent parameters to design or to select a micro-heat exchanger according to the heat transfer enhancement with respect to the pressure drop cost. Results reveal that the HTP may effectively be increased by inserting rarefied porous inserts. The present analysis suggests the implementation of porous inserts within flow passages especially when 0.001<Kn<0.1 (the slip-flow regime).

Thermal analysis of solar central receiver tube with porous inserts and non-uniform heat flux

In this paper, enhancement for convection heat transfer of turbulent flow in a solar central receiver tube with porous medium and non-uniform circumferential heat flux was numerically investigated. A new method was introduced to build different porous medium configurations in a unified grid system. Four kinds of enhanced receiver tubes (ERTs) with different porous insert configurations were modeled to optimize the performance of ERT. Furthermore, parameters including filling ratio of porous medium, thermal conductivity ratio (thermal conductivity of porous medium versus that of working fluid), porosity and Reynolds number were analyzed. The results showed that ERT partially filled with porous medium has better heat transfer performance than that fully filled with porous medium. The configuration of porous insert for optimal thermal or thermo-hydraulic performance is interactively affected by all the parameters discussed in this paper. The thermal conductivity ratio is the most crucial parameter to the thermal or thermo-hydraulic performance of ERT. The value of thermal conductivity ratio should be greater than 100 to obtain a good thermo-hydraulic performance. The ERTs with horizontal cylindrical segment shaped porous inserts and hollow cylinder shaped porous inserts are proposed because they can obtain optimal thermal or thermo-hydraulic performance.

Internalization of bevacizumab by retinal endothelial cells and its intracellular fate: Evidence for an involvement of the neonatal Fc receptor

Bevacizumab is one of the VEGF-binding proteins that are established in clinical practice to treat various ocular diseases. In view of therapeutic long-term application, potential accumulation of the antibody in retinal cells gave reason for safety concerns. Internalization of considerable amounts of bevacizumab by retinal endothelial (REC) and pigment epithelial cells has been observed which may affect their important functions. Therefore we investigated the transport and intracellular localization of bevacizumab in immortalized bovine REC (iBREC) in detail, considering possible roles of vesicles and receptors mediating uptake and intracellular transport.By performing transcytosis assays with iBREC monolayers cultivated on porous membrane inserts, we demonstrated that bevacizumab was transported efficiently through a tight monolayer from the lower to the upper chamber or vice versa. When added to the lower chamber in excess, the internalized antibody was transported through the cells, but it was also recycled to be set free at the same side of the cell into a bevacizumab-free environment. The rates of both processes strongly depended on the concentration of fetal bovine serum (FBS) in the environment. This observation is important because in vivo REC might be exposed to varying amounts of serum, e.g. in patients with macular edema. FBS also affected the intracellular localization of bevacizumab as shown by analyses of subcellular fractions and direct immunofluorescence staining. When iBREC were cultivated in low-serum medium, most of the antibody was found in the fraction of cytoskeleton proteins and spots of high intensity of bevacizumab-specific staining close to the nuclei were observed. Cultivation in medium with FBS resulted in internalized bevacizumab predominately found in the membrane/organelle fraction in addition to its weaker association with proteins from the cytoskeleton and uniform staining of the cell. Bevacizumab-specific staining close to the cytoskeleton proteins α-tubulin or vimentin was also observed. Accumulation and association of the antibody with the cytoskeleton induced by serum reduction could be reversed by subsequent FBS addition. In uptake and transport of bevacizumab vesicles and binding to a receptor seems to be involved: Internalization was strongly temperature-dependent which ruled out paracellular passage and a fraction of the internalized bevacizumab was associated with early endosomes. Protein A inhibited transcytosis and affected intracellular localization suggesting a key role of the neonatal Fc receptor (FcRn). Interestingly, FcRn expression was decreased when iBREC were cultivated without FBS.Our results suggest this pathway of bevacizumab uptake and transition through iBREC: Independent of serum, bevacizumab is taken up through a nonspecific mechanism. The subsequent sorting into transport vesicles depends on the presence of serum as regulator of FcRn expression. Without sufficient amounts of the receptor being expressed, a likely obstructed exocytosis results in intracellular accumulation and an increased association with cytoskeleton proteins. Interaction of substantial amounts of bevacizumab with the cytoskeleton may be the reason for under these conditions suppressed migration of iBREC. If long-term therapies by intravitreal injection lead to accumulation of bevacizumab in REC in vivo and potentially harmful consequences, will have to be revealed by future investigations.

On The Influence Of Porous Coating Thickness On Supersonic Boundary Layer Stability

Theoretical and experimental investigation of the influence of porous-coating thickness on the stability of the supersonic flat-plate boundary layer at free-stream Mach number M = 2 have been performed. Good quantitative agreement of experimental data obtained with artificially generated disturbances performed on models with various porous inserts and calculations based on the linear stability theory has been achieved. It is shown that the increase of the porous-coating thickness leads to the boundary layer destabilization.

Heat transfer and second law analyses of forced convection in a channel partially filled by porous media and featuring internal heat sources

This paper provides a comprehensive study on the heat transfer and entropy generation rates in a channel partially filled with a porous medium and under constant wall heat flux. The porous inserts are attached to the walls of the channel and the system features internal heat sources due to exothermic or endothermic physical or physicochemical processes. Darcy-Brinkman model is used for modelling the transport of momentum and an analytical study on the basis of LTNE (local thermal non-equilibrium) condition is conducted. Further analysis through considering the simplifying, LTE (local thermal equilibrium) model is also presented. Analytical solutions are, first, developed for the velocity and temperature fields. These are subsequently incorporated into the fundamental equations of entropy generation and both local and total entropy generation rates are investigated for a number of cases. It is argued that, comparing with LTE, the LTNE approach yields more accurate results on the temperature distribution within the system and therefore reveals more realistic Nusselt number and entropy generation rates. In keeping with the previous investigations, bifurcation phenomena are observed in the temperature field and rates of entropy generation. It is, further, demonstrated that partial filling of the channel leads to a substantial reduction of the total entropy generation. The results also show that the exothermicity or endothermicity characteristics of the system have significant impacts on the temperature fields, Nusselt number and entropy generation rates.

Fluid shear stress upregulates placental growth factor in the vessel wall via NADPH oxidase 4.

Placental growth factor (PLGF), a potent stimulator of arteriogenesis, is upregulated during outward arterial remodeling. Increased fluid shear stress (FSS) is a key physiological stimulus for arteriogenesis. However, the role of FSS in regulating PLGF expression is unknown. To test the hypothesis that FSS regulates PLGF expression in vascular cells and to identify the signaling pathways involved, human coronary artery endothelial cells (HCAEC) and human coronary artery smooth muscle cells were cultured on either side of porous Transwell inserts. HCAEC were then exposed to pulsatile FSS of 0.07 Pa ("normal," mimicking flow through quiescent collaterals), 1.24 Pa ("high," mimicking increased flow in remodeling collaterals), or 0.00 Pa ("static") for 2 h. High FSS increased secreted PLGF protein ∼1.4-fold compared with static control (n = 5, P < 0.01), while normal FSS had no significant effect on PLGF. Similarly, high flow stimulated PLGF mRNA expression nearly twofold in isolated mouse mesenteric arterioles. PLGF knockdown using siRNA revealed that HCAEC were the primary source of PLGF in cocultures (n = 5, P < 0.01). Both H2O2 and nitric oxide production were increased by FSS compared with static control (n = 5, P < 0.05). N(G)-nitro-l-arginine methyl ester (100 μM) had no significant effect on the FSS-induced increase in PLGF. In contrast, both catalase (500 U/ml) and diphenyleneiodonium (5 μM) attenuated the effects of FSS on PLGF protein in cocultures. Diphenyleneiodonium also blocked the effect of high flow to upregulate PLGF mRNA in isolated arterioles. Further studies identified NADPH oxidase 4 as a source of reactive oxygen species for this pathway. We conclude that FSS regulates PLGF expression via NADPH oxidase 4 and reactive oxygen species signaling.

In vitro chondrogenic commitment of human Wharton's jelly stem cells by co-culture with human articular chondrocytes

Wharton's jelly stem cells (WJSCs) are a potential source of transplantable stem cells in cartilage-regenerative strategies, due to their highly proliferative and multilineage differentiation capacity. We hypothesized that a non-direct co-culture system with human articular chondrocytes (hACs) could enhance the potential chondrogenic phenotype of hWJSCs during the expansion phase compared to those expanded in monoculture conditions. Primary hWJSCs were cultured in the bottom of a multiwell plate separated by a porous transwell membrane insert seeded with hACs. No statistically significant differences in hWJSCs duplication number were observed under either of the culture conditions during the expansion phase. hWJSCs under co-culture conditions show upregulations of collagen type I and II, COMP, TGFβ1 and aggrecan, as well as of the main cartilage transcription factor, SOX9, when compared to those cultured in the absence of chondrocytes. Chondrogenic differentiation of hWJSCs, previously expanded in co-culture and monoculture conditions, was evaluated for each cellular passage using the micromass culture model. Cells expanded in co-culture showed higher accumulation of glycosaminoglycans (GAGs) compared to cells in monoculture, and immunohistochemistry for localization of collagen type I revealed a strong detection signal when hWJSCs were expanded under monoculture conditions. In contrast, type II collagen was detected when cells were expanded under co-culture conditions, where numerous round-shaped cell clusters were observed. Using a micromass differentiation model, hWJSCs, previously exposed to soluble factors secreted by hACs, were able to express higher levels of chondrogenic genes with deposition of cartilage extracellular matrix components, suggesting their use as an alternative cell source for treating degenerated cartilage. Copyright © 2015 John Wiley & Sons, Ltd.

Optimization of Porous Insert Configuration in a Central Receiver Tube for Heat Transfer Enhancement

In this paper, the heat transfer enhancement for convection heat transfer of turbulent flow in a central receiver tube filled with porous medium under non-uniform circumferential heat flux was numerically investigated. The effects of some parameters of porous medium (layout, thermal conductivity and porosity) and the Reynolds number (Re) on the thermal and thermo-hydraulic performance were discussed. The results showed that the enhanced receiver tube (ERT) with down-filling porous inserts and in-filling porous inserts have good thermal performance when the ratio of thermal conductivity of porous medium to working fluid (λs/λf) is less than 1,000. The ERT with out-filling porous inserts and up-filling porous inserts have good thermo-hydraulic performance when λs/λf >100. The porosity (ɛ) and Re also affect the thermal and thermo-hydraulic performance, the Nusselt number (Nu) and performance evaluation criteria (PEC) of heat transfer enhancement under constant pumping power of most kinds of ERTs decrease with the increase of ɛ, but the PEC of the ERT with in-filling porous inserts increases with the increase of ɛ. The Nu of all kinds of ERTs increases with the increase of Re, but the PEC decreases with the increase of Re.

Thermal management improvement of an air-cooled high-power lithium-ion battery by embedding metal foam

Effect of embedding aluminum porous metal foam inside the flow channels of an air-cooled Li-ion battery module was studied to improve its thermal management. Four different cases of metal foam insert were examined using three-dimensional transient numerical simulations. The effects of permeability and porosity of the porous medium as well as state of charge were investigated on the standard deviation of the temperature field and maximum temperature inside the battery in all four cases. Compared to the case of no porous insert, embedding aluminum metal foam in the air flow channel significantly improved the thermal management of Li-ion battery cell. The results also indicated that, decreasing the porosity of the porous structure decreases both standard deviation of the temperature field and maximum temperature inside the battery. Moreover, increasing the permeability of the metal foam drops the maximum temperature inside the battery while decreasing this property leads to improving the temperature uniformity. Our results suggested that, among the all studied cases, desirable temperature uniformity and maximum temperature were achieved when two-third and the entire air flow channel is filled with aluminum metal foam, respectively.