Fluid Dynamic Conditions Research Articles

Numerical Fluid-Dynamic Optimization of Microchannel-Provided Porous Scaffolds for the Co-Culture of Adherent and Non-Adherent Cells

Computational fluid dynamic (CFD) techniques were used to optimize the microenvironment inside scaffolds for hematopoietic stem cell (HSC) culture in a perfusion bioreactor. These matrices are meant to be seeded with adherent bone marrow stromal cells and then co-cultivated with HSCs; the scaffold micro-architecture and the fluid-dynamic conditions have to be optimized to avoid non-adherent stem cells being dragged away while ensuring adequate nutrient supply. The insertion of longitudinal microchannels was tested as a tool to improve perfusion in a homogeneous porous scaffold. Models of microchannel-provided scaffolds, characterized by different values of geometric parameters concerning pores and channels, were built, and numerical fluid-dynamic and oxygen-transfer analyses were carried out. The results of the computations indicated that the microchannels created preferential paths for culture medium flow, causing low shear stresses and drag forces within the pores; meanwhile, they improved oxygen delivery by forcing its penetration into the scaffold bulk. In particular, an 85% porous, 3-mm-thick scaffold with 175-microm-diameter pores was considered; at a constant average drag force guaranteeing stem cell suspension inside this porous bulk, the addition of approximately 260-microm-diameter, 700-microm-spaced channels resulted in 34% higher oxygen partial pressure at the exit (approximately 135 vs 101 mmHg), maintaining a wall shear stress median value of approximately 0.14 mPa. The present work demonstrates the capacity of microchannel-provided scaffolds to ensure suitable conditions for HSC culture and shows that CFD methods are a valuable tool to retrieve significant clues for the design of the culture environment.

Concentration polarization analysis in self-supported Pd-based membranes

In this work, the concentration polarization phenomenon in hydrogen permeation through self-supported Pd-alloy membranes is evaluated by an opportune coefficient expressed as a function of the ratio of the flux calculated by means of a validated complex model considering all the elementary steps involved in the permeation [A. Caravella, G. Barbieri, E. Drioli, Modelling and Simulation of Hydrogen Permeation through Supported Pd-based membranes with a multicomponent approach, Chemical Engineering Science 63 (8) (2008) 2149–2160] and the one obtained by the Sieverts’ law utilizing the bulk driving force and hydrogen permeance. The polarization coefficient was evaluated as a function of several operating conditions: upstream hydrogen molar fraction ([0, …, 1]), total pressure of upstream ([200, …, 1000] kPa), total pressure of down-stream ([100, …, 800] kPa), temperature ([300, …, 500] °C), membrane thickness ([1, …, 150] μm), permeance ([0.1, …, 20] mmol m −2 s −1 Pa −0.5) and upstream fluid-dynamic conditions (Reynolds’ number). The analysis shows that the polarization effect can be relevant not only when using very thin membranes (1–5 μm ca.), but also when thicker ones (100 μm ca.) are operated in specific conditions. A validation of the analysis is provided by means of some experimental data from literature, finding a good agreement with them. The overall result of the paper is the development of so-called “polarization maps”, on which the influence of concentration polarization can be evaluated quantitatively in different conditions, providing a useful tool to reduce the uncertainties in the hydrogen purification equipment design.

Tapered microfluidic chip for the study of biochemical and mechanical response at subcellular level of endothelial cells to shear flow

A lab-on-a-chip application for the investigation of biochemical and mechanical response of individual endothelial cells to different fluid dynamical conditions is presented. A microfluidic flow chamber design with a tapered geometry that creates a pre-defined, homogeneous shear stress gradient on the cell layer is described and characterized. A non-intrusive, non-tactile measurement method based on micro-PIV is used for the determination of the topography and shear stress distribution over individual cells with subcellular resolution. The cellular gene expression is measured simultaneously with the shape and shear stress distribution of the cell. With this set-up the response of the cells on different pre-defined shear stress levels is investigated without the influence of variations in repetitive experiments. Results are shown on cultured endothelial cells related to the promoter activity of the shear-responsive transcription factor KLF2 driving the marker gene for green fluorescent protein.

Swarms of similar long-period earthquakes in the mantle beneath Mauna Loa Volcano

We present analyses of two swarms of long-period (LP) earthquakes at > 30 km depth that accompanied the geodetically observed 2002–2005 Mauna Loa intrusion. The first LP earthquake swarm in 2002 consisted of 31 events that were precursory and preceded the start of Mauna Loa inflation; the second LP swarm of two thousand events occurred from 2004–2005. The rate of LP earthquakes slowed significantly coincident with the occurrence of the December 26, 2004 M w 9.3 Sumatra earthquake, suggesting that the seismic waves from this great earthquake may have had a dynamic triggering effect on the behavior of Mauna Loa's deep magma system. Using waveform cross correlation and double difference relocation, we find that a large number of earthquakes in each swarm are weakly similar and can be classified into two families. The relocated hypocenters for each family collapse to compact point source regions almost directly beneath the Mauna Loa intrusion. We suggest that the observed waveform characteristics are compatible with each family being associated with the resonance of a single fluid filled vertical crack of fixed geometry, with differences in waveforms between events being produced by slight variations in the trigger mechanism. If these LP earthquakes are part of the primary magma system that fed the 2002–2005 intrusion, as indicated by the spatial and temporal associations between mantle seismicity and surface deformation, then our results raise the possibility that this magma system may be quite focused at these depths as opposed to being a diffuse network. It is likely that only a few locations of Mauna Loa's deep magma system met the geometric and fluid dynamic conditions for generating LP earthquakes that were large enough to be recorded at the surface, and that much of the deep magma transfer associated with the 2002–2005 intrusion occurred aseismically.

3D topography design of membranes for enhanced mass transport

This work describes the process of fabrication of 3D topography membranes and the fully quantitative characterisation of their topography using atomic force microscopy (AFM), small-angle light scattering (SALS), scanning electron microscopy (SEM) and polarizing optical microscopy (POM). The use of these membranes and the impact of the 3D membrane topography on the enhancement of mass transport during solute recovery (hexyl acetate) from a viscous room temperature ionic liquid 1- n-butyl-3-methyl-imidazolium tetrafluoroborate ([C 4mim] [BF 4]) by organophilic pervaporation is presented and discussed. A quantitative analyses of the results obtained allow to conclude that the use of the surface-modified membrane led to an increase of solute flux of 14%, which is well above the value of 4% that may be attributed to an augment of membrane surface area due to the increased membrane roughness. This increase of solute flux has to be granted to improved fluid dynamic conditions due to micro-turbulence at the membrane surface. This work illustrates how the design of three-dimensional relief microstructures at the membrane surface, with topographic modulation of features with characteristic dimensions, may lead to improved momentum and mass transport conditions at the membrane surface through promotion of local micro-turbulence.

Chemical Vapor Deposition Model of Polysilicon in a Trichlorosilane and Hydrogen System

The traditional polysilicon processes should be refined when addressing the low energy consumption requirement for the production of solar grade silicon. This paper addresses the fluid dynamic conditions required to deposit polysilicon in the traditional Siemens reactor. Analytical solutions for the deposition process are presented, providing information on maximizing the rate between the amount of polysilicon obtained and the energy consumed during the deposition process. The growth rate, deposition efficiency, and power-loss dependence on the gas velocity, the mixture of gas composition, the reactor pressure, and the surface temperature have been analyzed. The analytical solutions have been compared to experimental data and computational solutions presented in the literature. At atmospheric pressure, the molar fraction of hydrogen at the inlet should be adjusted to the range of 0.85–0.90, the gas inlet temperature should be raised within the interval of 673 and , and the gas velocity should reach the Reynolds number 800. The resultant growth rate will be between 6 and . Operation above atmospheric pressure is strongly recommended to achieve growth rates of at .

Simulation of food drying: FEM analysis and experimental validation

The aim of the present work is the formulation of a theoretical model describing the simultaneous transfer of momentum, heat and mass occurring in a convective drier where hot dry air flows under turbulent conditions around a food sample. The proposed model does not rely on the specification of interfacial heat and mass transfer coefficients and, therefore, represents a general tool capable of describing the behavior of real driers over a wide range of process and fluid-dynamic conditions. The system of non-linear unsteady-state partial differential equations modeling the behavior of a cylindrical-shaped vegetable sample in a drier, has been solved by using finite elements method. It has been observed that air characteristics influence drying performance only when external resistance to mass transfer is the rate controlling step. An experimental study was undertaken which shows very good agreement between model predictions and experimental results.

Thermoelectric Power Conversion System Combined with LNG Vaporizer

A conceptual design of the thermoelectric power conversion system combined with open rack type LNG (liquefied natural gas) vaporizer to make use of cold heat of LNG is presented. The system performance analysis has been made based on the thermoelectric module performance data obtained at the cryogenic thermoelectric (CTE) test rig which could realize temperature and fluid dynamic condition of the open rack type LNG vaporizer. Conventional bismuth-telluride thermoelectric modules were tested, however, each module is encapsulated in the stainless steel container to achieve water proof. Electricity production cost evaluation of the system is also discussed.

Zeolite Formation from Expanded Perlite Using Hydrothermal Treatment with Alkaline Solutions of NaCl: A Kinetic Study of the Process

Amorphous natural silicoaluminates, such as expanded perlite, transform themselves in three-dimensional crystalline species, exhibiting effects of the alkaline solutions with NaCl. In this paper the kinetics and the effect of the reaction conditions over the crystallization process are studied.Na(OH) solutions 15%, 18%, and 20 % w/v, and NaCl solutions from 0 % up to 30 % w/v were used. Temperature range was from 80ºC to 95 ºC. The effect of the solid/liquid relationship and the fluid dynamic conditions were studied. The rate of the zeolite formation process depends on the concentration of Na(OH) / NaCl solutions and on the reaction temperatures.The fluid dynamic conditions and the solid/liquid relationships have no effect over the crystallization process in the studied range. The effect of the NaCl is to increase the reaction rate independently of the kind of obtained zeolite, which is associated with the relationship Si/Al.Powder X – ray diffraction (XRD) analysis and scanning electron microscopic (SEM) were employed to study the zeolite formation process in order to elucidate the mode of zeolite crystallization.The mechanism implies the formation of units of nuclei of growth, and the population of these units is controlled by NaCl concentrations. The nucleation and growth Avrami model properly describes the process of formation of the crystalline species. It is found that the parameters k and n are linked respectively with the conditions and the reaction mechanism.

Clarification of blood orange juice by ultrafiltration: analyses of operating parameters, membrane fouling and juice quality

Blood orange juice was clarified by cross-flow ultrafiltration (UF) using tubular polyvinylidenefluoride (PVDF) membranes. In experimental tests performed according to the total recycle mode the effect of transmembrane pressure (TMP), axial feed flow-rate and temperature on permeate fluxes was studied. The clarified juice was produced in experimental tests carried out according to the batch concentration mode working in optimal operating and fluid dynamic conditions. A theoretical evaluation of the fouling effects on flux performance was performed by using a modified form of the differential equation used to describe classical dead-end filtration. The fouling mechanism during cross-flow UF was identified by estimation of the model parameters according to a nonlinear regression optimization procedure. The theoretical predictions, in terms of permeate flux as a function of time, resulted in a good agreement with the experimental data. Analysis of the results revealed that, in the fixed operating conditions of TMP and temperature, the fouling mechanism evolves from a partial to a complete pore blocking condition in dependence of the axial velocity. The quality of the samples coming from the UF process was evaluated in terms of: total soluble solids (TSS), suspended solids (SS), total antioxidant activity (TAA), ascorbic acid, flavonones and anthocyanins content. The resulting clarified orange juice was highly similar to the initial juice except for insoluble solids which were concentrated in the retentate stream. In the permeate of the process a low reduction of the TAA (1.5%) was observed with respect to the fresh juice.

Effect of viscous dissipation on the heat transfer in sinusoidal profile finned dissipators

In the present work, the efficiency of finned heat dissipators cooled by laminar flow is studied. The analysis is carried out by varying certain sizing parameters in correspondence with different viscous dissipation conditions. The velocity distribution in the fluid and the temperature distribution in the dissipator and in the fluid are determined by means of a finite element method. The model allows to study the variation of the heat transfer coefficient due to the fluid dynamic conditions imposed by the fin profile. Lastly, a comparison between the optimum fin shapes obtained under different viscous dissipation effects is carried out.

Lab Tests for the Remediation of a PCE Polluted Site by Means of HRC Reactant

The injection of reactants is a remediation technology that may be applied both in the vadose zone and in groundwater. In this work a new reagent is examined for the remediation of the vadose zone and groundwater of an industrial site polluted by perchloroethylene (PCE). The tested reactant is the HRC (Hydrogen Release Compound, Regenesis, USA), a reducing agent which enhances the biological and chemical dechlorination of PCE. Some laboratory column and batch tests were performed on soil coming from the polluted site applying different fluid dynamic conditions, with the aim to evaluate the efficiency of the hypothesized remediation technology on the aquifer and the vadose zone and the eventual degradation products deriving from PCE or HRC. The performed tests confirmed HRC efficiency in PCE remediation. The modelling of the injected reactant both in the aquifer and in the vadose zone was performed.

Effects of fluid dynamic induced shear stress on fungal growth and morphology

Industrial applications of filamentous microorganisms become more and more relevant for example for the production of pharmaceuticals. Therefore, pellet morphology is often requested. Knowledge about pellet growth behaviour at different fluid dynamic conditions is important, because many production processes are influenced by morphology. Morphology itself is influenced by the fluid dynamic conditions in a bioreactor. In the present study, pellet growth of Aspergillus niger AB1.13 was studied under different fluid dynamic conditions. A new experimental set-up was designed to guarantee comparability and reproducibility. Kinetic parameters of pellet growth were correlated with energy input and the maximum shear rate. The present work shows that kinetic parameters depend on the fluid dynamic conditions. Additionally, it could be demonstrated that for model development a correlation with the local maximum shear rate is of relevance especially in relation to transferability to other reaction systems with different set-up.

Ultrafiltration of kiwifruit juice: Operating parameters, juice quality and membrane fouling

Fresh depectinised kiwifruit juice has been clarified by ultrafiltration (UF) process on laboratory scale. In experimental tests performed according to the total recycle mode the effect of transmembrane pressure (TMP), axial flow-rate and temperature on permeation flux has been studied. The results showed that flux increased with temperatures from 20 to 30 °C and with axial feed flow rate from 300 to 700 l/h. The flux-pressure curves showed no increase in permeate fluxes for TMP values higher than 90 kPa (TMP lim). Clarified kiwifruit juice has been produced in experimental tests carried out according to the batch concentration mode working in optimal operating and fluid dynamic conditions. The quality of clarified juice has been analysed in terms of total antioxidant activity (TAA), content of ascorbic acid, suspended solids, turbidity and viscosity. The UF process permitted a good level of clarification reducing totally the suspended solids and the turbidity of the fresh juice. In the permeate a 16% reduction of ascorbic acid was observed with respect to the fresh juice; however, the reduction of the TAA was lower than 8%. Cake layer and irreversible fouling resistances gave a minimum contribution to the total resistance (2.23% and 2.75%, respectively) while the contribution of the reversible fouling was more significant (29.4%). A good restore of the hydraulic permeability of the membrane (about 96% of the initial one) was observed after a cleaning treatment performed by using alkaline and acid detergents. Thus, the flux decline during UF could be ascribed to fouling layers formed by a combination of suspended particles and adsorbed macromolecular impurities.

COMPARISON OF BLOOD DAMAGE PREDICTIONS FROM FLUID MEASUREMENTS AND COMPUTATIONS TO IN VITRO AND IN VIVO OBSERVATIONS IN A CENTRIFUGAL BLOOD PUMP

The accurate prediction of the blood damage caused by fluid shearing is necessary for the design of blood contacting devices. It has been shown that fluid dynamics can be used to predict both hemolysis and thrombosis. Numerous studies of the fluid dynamic conditions for hemolysis and activation of the clotting cascade exist and have been used to estimate the damage potential of devices. Still, these predictions are not generally accurate and reliable because of 1) the high variability of reported threshold values, 2) the fact that estimates of turbulent stresses (Reynolds stresses) are often compared to threshold values of viscous stress and 3) the lack of studies that have shown that relations developed for simple flows are effective in damage predictions of complicated flows. A graphical summary of available data is presented. Estimates of damage based on measurement and numerical simulation of the flow within a centrifugal blood pump are compared to levels of damage from animal implant experiments. The effectiveness of prediction based on comparing shear and velocity fields to published relations is evaluated. Results: While the application of commonly used relations between shear stress, exposure time, and blood damage would predict extensive damage in this pump, actual measured damage is negligible. It is suggested that revised empirical fits to the available data be used along with consideration of turbulent length scales in the flow and that the distinction between viscous and Reynolds stresses must be maintained.

Computational Fluid Dynamics: Hemodynamic Changes in Abdominal Aortic Aneurysm After Stent-Graft Implantation

The aim of this study was to demonstrate quantitatively and qualitatively the hemodynamic changes in abdominal aortic aneurysms (AAA) after stent-graft placement based on multidetector CT angiography (MDCT-A) datasets using the possibilities of computational fluid dynamics (CFD). Eleven patients with AAA and one patient with left-side common iliac aneurysm undergoing MDCT-A before and after stent-graft implantation were included. Based on the CT datasets, three-dimensional grid-based models of AAA were built. The minimal size of tetrahedrons was determined for grid-independence simulation. The CFD program was validated by comparing the calculated flow with an experimentally generated flow in an identical, anatomically correct silicon model of an AAA. Based on the results, pulsatile flow was simulated. A laminar, incompressible flow-based inlet condition, zero traction-force outlet boundary, and a no-slip wall boundary condition was applied. The measured flow volume and visualized flow pattern, wall pressure, and wall shear stress before and after stent-graft implantation were compared. The experimentally and numerically generated streamlines are highly congruent. After stenting, the simulation shows a reduction of wall pressure and wall shear stress and a more equal flow through both external iliac arteries after stenting. The postimplantation flow pattern is characterized by a reduction of turbulences. New areas of high pressure and shear stress appear at the stent bifurcation and docking area. CFD is a versatile and noninvasive tool to demonstrate changes of flow rate and flow pattern caused by stent-graft implantation. The desired effect and possible complications of a stent-graft implantation can be visualized. CFD is a highly promising technique and improves our understanding of the local structural and fluid dynamic conditions for abdominal aortic stent placement.

New sea spray generation function for spume droplets

With increasing wind forcing and development of wind waves, more and more sea spray droplets are produced on the sea surface. On the basis of now available observational data from field and laboratories with 10‐m wind speeds ranging from 8 to 41 m/s, it is shown that the traditional approach of using wind speed only fails to describe the increase of spume droplet production, owing to the neglect of effect of wave state. Instead, a nondimensional parameter RB = u*2/ωpν called the windsea Reynolds number (Toba et al., 2006) is very good for characterizing the observational data from laboratories and field, where u* is the friction velocity of air, ωp the angular frequency spectral peak of wind waves, and ν is the kinematic viscosity of air. The windsea Reynolds number RB represents the coupling effect of wind forcing and wind wave state, and can be regarded as a measure of fluid dynamical conditions at the air‐sea boundary layer. A new sea spray generation function for spume droplets is proposed as a function of RB. We conclude that spume droplets begin to be produced as RB exceeds 103. The effects of sea spray droplets on air‐sea transfers are also estimated with the new model. It is found that the heat and momentum fluxes induced by sea spray droplets become comparable to the interfacial fluxes by bulk formulas when RB is greater than 105 and 106, respectively.

Characterization of membrane distillation modules and analysis of mass flux enhancement by channel spacers

The first aim of this work is to present a method in order to characterize a direct contact membrane distillation (DCMD) module, namely a method to determine the value of the parameters included in a commonly accepted transport model. The model includes membrane and module design characteristic parameters which have the same values regardless of feed used. The characterization method proposed here, includes DCMD measurements (both, water flux and evaporation efficiency measurements) using pure water as feed, and air–liquid displacement measurements. The latter are used to extract previous structural information about the membrane used. The method is applied to characterize two DCMD modules with different designs. The first module has open flow channels while the second one includes a spacer within the flow channels. The transport model together along with the obtained characteristic parameter values adequately predicts the water flux measured with both modules when different operating conditions were imposed and aqueous solutions of different solutes were used as feed. The results given here are for NaCl and sucrose feed solutions. They show that the membrane distillation performance is highly dependent on the design of membrane module, the specific separation being performed and the operating conditions (besides membrane characteristics). The second aim of this work is to study the water flux enhancement induced by the improvement of the fluid-dynamic conditions within the module with spacer in relation to the module with open channels. The dependence of flux enhancement on operating conditions (average temperature, temperature difference between feed and permeate solutions, type of solute, and feed concentration) is analysed. In the range of operating variables studied, the systems with high vapour pressure polarization have water fluxes especially sensitive to the fluid-dynamic changes induced by the spacer.

Flow boiling of pure fluids: local heat transfer and flow pattern modeling through artificial neural networks

A new modeling technique based on neural networks as a universal function approximator has been applied to study the local representation of flow boiling heat transfer at varying fluid dynamics conditions along the tube, i.e., moving through different flow pattern regions. After subdivision of the experimental data into subsets homogeneous for flow conditions, specific heat transfer equations have been heuristically got. Such specific equations have been validated and compared with the global MLFN heat transfer coefficient representation for the same target fluid as from a former work. The study has been extended to four fluids for which suitable data were available from the literature. The representation accuracies of the specialized heat transfer models are very high, even if such a method is not in general suggested due to the greater complication for both the equations development and for their practical use. It was furthermore shown that a global high accuracy flow boiling heat transfer equation can be developed also without any relation with local fluid dynamics conditions. A new modeling technique has been furthermore proposed for the fluid dynamics conditions along the tube. The linking of the picture of the actual flow condition directly with a conventional continuous real number, the so-called SF factor, allows to convert the flow condition into a numerical variable which can be turned into a continuous fluid specific function through a further MLFN technique. Limitedly to the few available literature SF data the method presents high performance and coherence for the whole SF surface. The present SF MLFN equation and the more advanced literature flow pattern model get comparable results, in spite of the limited data base. It is also shown that a flow boiling heat transfer model is not intrinsically flow pattern dependent, because a flow condition representation is never required for a global heuristic heat transfer coefficient modeling.

Influence of mechanical stress and surface interaction on the aggregation of Aspergillus niger conidia

Productivity of fungal cultures is closely linked with their morphologic development. Morphogenesis of coagulating filamentous fungi, like Aspergillus niger, starts with aggregation of conidia, also denominated as spores. Several parameters are presumed to control this event, but little is known about their mode of action. Rational process optimization requires models that mirror the underlying reaction mechanisms. An approach in this regard is suggested and supported by experimental data. Aggregation kinetics was examined for the first 15 h of cultivation under different cultivation conditions. Mechanical stress was considered as well as pH-dependent surface interaction. Deliberations were based on a two-step aggregation mechanism. The first aggregation step is only affected by the pH-value, not by the fluid dynamic conditions in the bioreactor. The second aggregation step, in contrast, depends on the pH-value as well as on agitation and aeration induced power input. For the given experimental set-up, agitation had much more influence than aeration. In addition, hyphal growth rate was determined to be the driving force for the second aggregation step.