Search

Northern Chile has excellent conditions to develop concentrated solar power projects. Although solar irradiation makes a significant contribution to production in the region, solar thermal projects need some support mechanisms. This study focuses on the best combinations of solar incentives and financial parameters to have lowest government cost and maximum levelized cost of electricity reduction. Key findings of this paper showed that debt fraction and discount rate illustrated meaningful sensitivities on both LCOE and government cost. ITC, PTC, and DM as tax credit and PBI as cash incentives had the best effectiveness, and reduced LCOE better than IBI and STR. The effectiveness of ITC, PTC, PBI, and DM was independent of financial parameters even though STR and IBI showed dependency. Although cash incentives had no limits to reduce LCOE, tax credit incentives reached maximum values, which meant that their impacts were limited. As cash incentives, PBI showed better results when it was compared to IBI. Maximum values of ITC maintained the same for different installed costs, while it changed for PTC. Finally, it was obtained that tax credit incentives were more meaningful at higher PPA price although PBI made more sense in lower PPA prices.

Desert areas are the favorable geographical locations for desired solar resource and temperature variations for improving the performance of hybrid photovoltaic-thermoelectric generator (HPV-TEG) systems. Therefore, economic feasibility analysis of HPV-TEG system is carried out under real environment and market conditions for the Atacama Desert, Chile. The thermal, electrical and economic models of HPV-TEG system are developed and analyzed in MATLAB. Five different possible scenarios are considered for economic feasibility based on energy losses, system costs, nominal efficiencies of TEG and photovoltaic module and their contribution in the economic feasibility of HPV-TEG system is identified and payback period for all scenarios is determined at minimum and maximum PV temperatures for Atacama Desert including residential and industrial electricity prices. The results showed that with existing market costs and TEG efficiency, HPV-TEG system could not be economically competitive with photovoltaic system for environmental conditions of the Atacama Desert. However, the calculated levelized cost of energy (LCOE) of the HPV-TEG system is 0.071 USD/kWh which is relatively close to current LCOEs for PV systems in Chilean energy market. Further, LCOE analysis economically quantifies the advantages of HPV-TEG system over PV system and opens the possibility for HPV-TEG systems to be competitive in desert locations.

Chile presents a combination of favorable climatic conditions which result in the highest levels of solar irradiation in the world. In this paper, the performance of a hybrid CSP + PV plant at utility-scale integrated with a large-scale Battery Energy Storage System (BESS) located in northern Chile was studied. The model considered a solar power tower integrated with a two-tank molten salt direct TES and a fixed-angle PV plant coupled to a BESS to deliver a baseload of 100 MWe. A parametric analysis was carried out in terms of the PV size, solar multiple, TES hours capacity, and BESS size. The hybrid plant performance was studied in terms of the LCOE and the capacity factor. The minimum LCOE configurations were identified and a sensibility analysis was carried out to evaluate the effect of a cost reduction of the BESS in the hybrid plant configuration for different BESS sizes. Results showed that current investment costs of the storage section of the BESS make unprofitable its integration to the hybrid plant, so, it is required a reduction of approximately 60–90% of the storage cost (based on a reference value of 300 USD/kWh) to achieve competitive LCOEs in comparison to those obtained for a hybrid plant without BESS. Under this BESS cost reduction scenario, it was found a solutions domain with different hybrid plant configurations, which allow to integrate and complement the production of both CSP and PV plants with both storage types in a synergetic operation. Also, a comparative study evaluating the differences between implementing a fixed-tilt PV configuration or a one-axis tracking system in the hybrid plant scheme was carried out, obtaining that the annual production of the hybrid plant increases by no more than 5% with the tracking system, while the LCOE is reduced up to -1.98% due to a decrease of the BESS participation in the total output. These results indicate that the minimum LCOE configurations of a hybrid CSP + PV plant with tracked PV arrays (given a BESS size) could result in smaller PV plants, since the PV production increases in comparison to that with a fixed-tilt PV configuration.

Numerical simulations were performed to investigate a microchannel heat sink device as cooling option for a high concentration photovoltaic system. COMSOL Multiphysics 5.1 software is used to solve three-dimensional equations which consider conjugate heat transfer, viscous dissipations, and temperature-dependent-properties. This study investigates the integration of microchannels with complex geometric features on its inner walls into the solar cell structure, to enhance the heat transfer performance of a microchannel heat sink-based active cooling system. Inner sidewall mounted forward triangular ribs are considered in aligned and offset distributions along the microchannel walls. In addition, numerical analysis is developed for a conventional flat plate heat sink integrated to a high concentration photovoltaic system to stablish a baseline solar cell temperature. The numerical results show that a micro-channel heat sink device can control and keep in very low range the solar cell temperature ( 200. At Re = 400, the pumping power reaches 41% and 23% of the total power generated by a multi-junction solar cell in the aligned and offset rib distribution, respectively. The pumping power is greatly reduced while using smooth microchannel, because the maximum pumping power is only 9.5% of the solar cell power at Re = 400, however, the resulting solar cell temperature is slightly higher compared to microchannels with aligned and offset rib configurations. A microchannel heat sink provides a more effective cooling solution compared to a passive flat plate heat sink for a high concentration photovoltaic system. In addition, the possibility of direct integration of a microchannel heat sink into a solar cell structure as proposed in this study, represents an interesting option to feasibly increase thermal performance to a considerable level by maintaining the solar cell temperature in a very low range.

A solar thermoelectric generator (STEG) system composed of an optical concentrator system (OPS), a Bismuth Telluride thermoelectric module (TEG), and a cold plate-based cooling system (CPCS) was numerically simulated, to measure the efficiency of electric generation of a commercial thermoelectric module under controlled temperatures. The OPS is composed by a Fresnel lens that allows a temperature of around 200 °C, the OPS works with a solar irradiance of 1000 W/m2 (AM 1.5 Reference) and an optical concentration of 60. The OPS is coupled to the hot side of the TEG that consists of a commercially available thermoelectric module. The CPCS maintains a temperature of around 50 °C on the cold side of the TEG. To evaluate the configuration, a computational fluid dynamic (CFD) analysis was carried out to evaluated the thermal performance of the CPCS and the temperature achieved on the upper surface of the cooling device. Based on the numerical results generated by the CFD analysis, an analytical TEG efficiency of around 5% was achieved when a temperature difference, between the hot and cold sides of the commercial TE module, of 150 °C was maintained. We perform an analysis using the Hogan and Shih model that uses the thermoelectric material properties exposed by Chen et al.

The development of renewable energy technologies to take advantage of clean energy sources, such as solar, is crucial for sustainability. Here, we show a hybrid solar thermoelectric (HSTE) concept that simultaneously recovers heat from the sun with a thermosyphon and generates electricity with a thermoelectric generator (TEG) via the Seebeck effect. In this work, we experimentally demonstrated a relatively low-temperature HSTE prototype that combines a cartridge heater (to emulate the sun), a TEG to generate electricity, and a thermosyphon. This thermosyphon consists of three zones: an evaporator, an adiabatic region, and a condenser. The TEG was located between the heater and the evaporator. The thermosyphon was used to maintain constant the low-temperature face of the TEG and to recover heat in the condenser zone. A thermal resistance model was developed to understand the HSTE. Our model shows good agreement with the experimental data and guides the further development of the system. To evaluate the technical feasibility and performance of the HSTE, we characterized the HSTE by measuring the temperatures, condenser flow rate, voltage, and current under 72 different experimental conditions. We reported an electrical power of up to 96 mW and a recovered heat up to 68.18 W, when 118 W were supplied through the cartridge heater. Deionized water was the working fluid showing a higher electrical power generation than a 40% ethylene glycol solution.

The present work in nanofluids is focusing into using the electro-kinetic phenomenal occurring around nanoparticles immersed in a base fluid as a method to stabilize a nanofluid and enhance its thermal conductivity. The electro-kinetic physic establishes, that when an electrolyte solution is in contact with a solid, an electric double layer (EDL) is produced on the solid surface. Due to the high concentration of ions with the same charge around of the particle surface, “it is possible to stabilize a nanofluid by the action of an electro repulsive force caused by ions over the nanoparticle surface and enhance its thermal conductivity as the concentration of the solutions increases”. The nanofluid samples were prepared by the two-step method and a continuous ultrasonication. 1wt% and 3wt% concentration (mass fraction) of Titanium oxide, Anatase (TiO2) nanoparticles, is added in an electrolyte solution (base fluid) made of different concentration of Potassium Chloride (KCl), and deionized water. The pH of the base fluid is maintained constant adding HEPES as a buffering agent. To measure the different level of stability for the nanofluid we used the thermal conductivity enhancement of the base fluid by nanoparticles. The experimental results under controlled temperature condition show that an electrolyte solution with nanoparticles after 20 days of preparation, presents a higher thermal conductivity with respect to the base fluid with an improvement rate ranging from 0.43±0.12% to 0.72±0.12% for 1wt%, and 2.15±0.17% to 3.03±0.21% for 3wt% of nanoparticles added respectively. The higher improvement shows sign of a major level of homogeneity of the nanofluid, and this behavior seems to be directly proportional to the KCl concentration.

The development of long-time stable nanofluids for practical use in heat transfer processes is a tremendous scientific challenge because nanoparticles tend to precipitate and agglomerate when in a solution, affecting both their thermophysical properties and their stability. This work experimentally investigates the role of the electro-repulse force by electric charges around the nanoparticle, as a way of improving the stability of an electrolyte-based nanofluid. Nanofluid samples were prepared in a two-step method, with 1 wt% and 3 wt% concentrations (mass fraction) of titanium oxide (TiO2) nanoparticles added to a base fluid consisting of an electrolyte solution with a different concentration of potassium chloride (KCl) and deionized water. The pH of the base fluid was maintained constant, adding HEPES as a buffering agent. The stable condition of the nanofluid was established when the temporal variation of the thermal conductivity was negligible. When stability was established, the dynamic viscosity, zeta potential and the enhancement of the thermal conductivity were measured under controlled temperatures. Experimental results showed that the stable behavior of the nanofluid was directly influenced by the electric charge around the nanoparticles and the electro-repulse force between the nanoparticles (represented by the zeta potential), producing a consistent and homogenous stable condition for an extended 30-day period. Due to the greater number of nanoparticles in the 3 wt% solution, the dynamic viscosity of the nanofluid at 3 wt% was higher than at 1 wt%. It was noted that the addition of the nanoparticles did not affect the Newtonian nature of the fluid (except that it was slightly for higher KCl concentrations) and it produced an increase of a 41.75 ± 2.4% for 1 wt% and 59.32 ± 2.1% for 3 wt% of the nanofluid dynamic viscosity, with respect to that of the pure water. Significant enhancement of thermal conductivity enhancement was also obtained, ranging from 0.46 ± 0.11% to 1.47 ± 0.12% for the 1 wt%; and, 2.15 ± 0.11% to 4.7 ± 0.13% for the 3 wt% of nanoparticles added. This noteworthy improvement was attributed to the higher level of homogeneity of the nanofluid, caused by the high electro-repulse force between nanoparticles. Stable electrolyte-based nanofluids, such as KCl, which increase the electro-repulse forces between nanoparticles, can bolster the application of this type of nanofluid in energy conversion and electronic cooling. Enhanced stability properties (particularly in microchannel heat sinks) give these nanofluids the ability to use electric fields for the fluid motion, rather than traditional pumping devices.

The flow and heat transfer characteristics are investigated in micro and mini communicating channel pressure driven flows by 2D numerical simulations using a computational method. The continuum based Navier–Stokes and continuity equations are solved by the Spectral Element Method (SEM). Flow and heat transfer characteristics are determined for 10 2 L ˆ and an aspect ratio of r = a ˆ / ( 2 L ˆ ) is used, where a ˆ is the height of block within the channel and L ˆ is the periodic length. For low Reynolds number, viscous forces dominate and two stationary symmetric vortices are generated between blocks with very laminar parallel viscous flow in the upper and lower communicating channel. For moderate Reynolds numbers, numerical results show a transition scenario with two Hopf flow bifurcations, as the flow evolves from a laminar to a time-dependent flow regime. The first Hopf bifurcation B1 occurs at a critical Reynolds number (Rec1) leading to a periodic flow characterized by a frequency ω1. A quasi periodic flow sets in for higher Reynolds numbers through a second Hopf flow bifurcation B2 occurring at a critical Reynolds number (Rec2

The enhancement characteristics of heat transfer, through a transition scenario of flow bifurcations, in asymmetric wavy wall channels, are investigated by direct numerical simulations of the mass, momentum and energy equations, using the spectral element method. The heat transfer characteristics, flow bifurcation and transition scenarios are determined by increasing the Reynolds numbers for three geometrical aspect ratios r = 0.25, 0.375, and 0.5, and Prandtl numbers 1.0 and 9.4. The transition scenarios to transitional flow regimes depend on the aspect ratio. For the aspect ratios r = 0.25 and 0.5, the transition scenario is characterized by one Hopf flow bifurcation. For the aspect ratio r = 0.375, the transition scenario is characterized by a first Hopf flow bifurcation from a laminar to a periodic flow, and a second Hopf flow bifurcation from a periodic to quasi-periodic flow. The periodic and quasi-periodic flows are characterized by fundamental frequencies ω 1 , and ω 1 and ω 2 , respectively. For all the aspect ratios and Prandtl numbers, the time-average mean Nusselt number and heat transfer enhancement increases with the Reynolds number as the flow evolves from a laminar to a transitional regime. For both Prandtl numbers, the highest increase in the Nusselt number occurs for the aspect ratio r = 0.5; whereas, the lowest increases happen to r = 0.25. The increase of the Nusselt number occurs at the expense of a higher pumping power, which, for both Prandtl numbers, grows as the aspect ratio increases from r = 0.25 to r = 0.5 for reaching a specific Nusselt number. This enhancement is obtained without the necessity of high volumetric flow rates associated with turbulent flow regimes, which demand much higher pumping powers. Significant heat transfer enhancements are obtained when the asymmetric wavy channel is operated in the appropriate transitional Reynolds number range.

Blood flow dynamics under physiologically realistic pulsatile conditions plays an important role in the growth, rupture, and surgical treatment of intracranial aneurysms. The temporal and spatial variations of wall pressure and wall shear stress in the aneurysm are hypothesized to be correlated with its continuous expansion and eventual rupture. In addition, the assessment of the velocity field in the aneurysm dome and neck is important for the correct placement of endovascular coils. This paper describes the flow dynamics in two representative models of a terminal aneurysm of the basilar artery under Newtonian and non-Newtonian fluid assumptions, and compares their hemodynamics with that of a healthy basilar artery. Virtual aneurysm models are investigated numerically, with geometric features defined by beta = 0 deg and beta = 23.2 deg, where beta is the tilt angle of the aneurysm dome with respect to the basilar artery. The intra-aneurysmal pulsatile flow shows complex ring vortex structures for beta = 0 deg and single recirculation regions for beta = 23.2 deg during both systole and diastole. The pressure and shear stress on the aneurysm wall exhibit large temporal and spatial variations for both models. When compared to a non-Newtonian fluid, the symmetric aneurysm model (beta = 0 deg) exhibits a more unstable Newtonian flow dynamics, although with a lower peak wall shear stress than the asymmetric model (beta = 23.2 deg). The non-Newtonian fluid assumption yields more stable flows than a Newtonian fluid, for the same inlet flow rate. Both fluid modeling assumptions, however, lead to asymmetric oscillatory flows inside the aneurysm dome.

Biomedical engineers have the tools to address some of the world’s most challenging nondisease–focused problems.

The enhancement of the optical efficiency in both, organic and non-organics photovoltaic cells, with inclusion of metallic nanoparticles that induces surface plasmon resonant effects, is determined and studied by computational simulations. The Maxwell equations are solved in the frequency domain using a Finite Element Methods (FEM) based computational program. The absorption of the active layer is directly obtained and weighted by the corresponding solar spectrum. Then, the photovoltaic cell optical efficiency is ultimately determined. This investigation demonstrated that for photovoltaic cells without nanoparticles, there exist three optimal configurations: an organic glass/PEDOT:PSS/CuPc:PTCBI/Ag cell; and non-organic glass/ ITO/CuInSe2/Ag and glass//ITO/CdTe/Ag cells. The numerical simulations show that optimal efficiency depends on the cell material and positioning of the nanoparticle within the cell. For an organic cell, the optimal efficiency was obtained with silver nanoparticles positioned at the bottom of the active layer (position 3); whereas, for non-organic cells, the optical efficiency was obtained with aluminum nanoparticles positioned between the glass and TCO layers (position 1). From the three dimensional simulations, it was determined that silver nanoparticles with a diameter of 80nm within a cubic cell of period 230nm positioned in position 3 of the active layer of CuPc:PTCBI of an organic photovoltaic cell allow the augmentation of the efficiency such that a similar efficiency can be obtained with a cell of the same material but without nanoparticles and an active layer thickness 94% higher than with nanoparticles. For aluminum nanoparticles with a diameter of 30 nm in a cubic cell of period 40nm positioned in position 1 of the active layer de CuInSe2 of a non-organic photovoltaic cell, the efficiency is augmented to such a value that this value can be obtained with a non-organic photovoltaic cell with no nanoparticles and a an active layer thickness 137% higher than with nanoparticles.

During the last years the aerodynamics characteristics of airfoils have been studied solving numerically the Navier-Stokes (NS) equations. These calculations require a significant computational cost due to both the second order and the nonlinear characteristics of the NS partial differential equations. Therefore, efforts have been devoted to reduce this cost and increase the accuracy of the numerical methods. The Lattice-Boltzmann Method (LBM) has become a great alternative to simulate this problem and a variety of fluid flows. In this method, the convective operator is linear and the pressure is calculated directly by the equation of state without implementing iterative methods. This work represents a preliminary investigation of a laminar flow over airfoils under low Reynolds number conditions (Re = 500). Solutions are obtained using a Multi-Block mesh refinement method. In order to validate the computational code, calculations are performed on a SD7003 airfoil at an angle of attack of 4° and 30°, which corresponds to the available numerical and experimental results. The results of this study agree well with previous experimental and numerical studies demonstrating the capabilities of the LBM to simulate accurately laminar flows over airfoils as well as capturing and predicting the laminar separation bubbles.Copyright © 2013 by ASME

Numerical investigation of laminar, transitional and chaotic flows in converging–diverging channels are performed by direct numerical simulations in the Reynolds number range 10 < Re < 850. The temporal flow evolution and the onset of turbulence are investigated by combining classical fluid dynamics representations with dynamical system flow characterizations. Modern dynamical system techniques such as timedelay reconstructions of pseudophase spaces, autocorrelation functions, fractal dimensions and Eulerian Lyapunov exponents are used for the dynamical flow characterization of laminar, transitional and chaotic flow regimes. As a consequence of these flow characterizations, it is verified that the transitional flow evolves through intermediate states of periodicity, two-frequency quasi-periodicity, frequency-locking periodicity, and multiple-frequency quasi-periodicity before reaching a non-periodic unpredictable behaviour corresponding to low-dimensional deterministic chaos.Qualitative and quantitative differences in Eulerian dynamical flow parameters are identified to determine the predictability of transitional flows and to characterize chaotic, weak turbulent flows in converging–diverging channels. Autocorrelation functions, pseudophase space representations and Poincaré maps are used for the qualitative identification of chaotic flows, assertion of their unpredictable nature, and recognition of the topological structure of the attractors for different flow regimes. The predictability of transitional flows is determined by analysing the autocorrelation functions and by representing their attractors in the reconstructed pseudophase spaces. The transitional flow behaviour is examined by the geometric visualization of the evolution of the attractors and Poincaré maps until the appearance of a strange attractor at the onset of chaos. Eulerian Lyapunov exponents and fractal dimensions are quantitative parameters to establish the onset of chaos, the persistence of chaotic flow behaviour, and the long-term persistent unpredictability of chaotic Eulerian flow regimes. Lastly, three-dimensional simulations for converging–diverging channel flow are performed to determine the effect of the spanwise direction on the route of transition to chaos.

A study of Lagrangian chaos, Eulerian chaos, and mixing enhancement in converging–diverging channel flows, using spectral element direct numerical simulations, is presented. The time‐dependent, incompressible Navier–Stokes and continuity equations are solved for laminar, transitional, and chaotic flow regimes for 100≤Re≤850. Classical fluid dynamics representations and dynamical system techniques characterize Eulerian flows, whereas Lagrangian trajectories and finite‐time Lagrangian Lyapunov exponents identify Lagrangian chaotic flow regimes and quantify mixing enhancement. Classical representations demonstrate that the flow evolution to an aperiodic chaotic regime occurs through a sequence of instabilities, leading to three successive supercritical Hopf bifurcations. Poincare sections and Eulerian Lyapunov exponent evaluations verify the first Hopf bifurcation at 125

The present numerical work describes the simulation and analysis of the absorptance and absorption efficiency of a solar cell, where the effect of utilizing photonic crystals as an active material of the cell was studied. The study was performed by numerical simulations using a computational code based on the Finite Element Method [1]. The results were obtained for photonic crystals with periodicity in both one and two dimensions [2]. In the first one, periodicity, thickness of the active material, and distance with respect to the electrode for hole collection were varied, and two organic materials for the active zone were tested, P3HT:PCBM and TDPT:PCBM. In the case of crystals with periodicity in two dimensions, only the period in one of the two dimensions was varied, based on the cell with the highest efficiency of absorption proposed for cells with periodic photonic crystals in one dimension. All simulations were obtained for waves with TM polarization, zero angle of incidence and wavelengths between 400 and 700 nm.Copyright © 2012 by ASME

Direct numerical simulations of the transition process from laminar to chaotic flow in converging–diverging channels are presented. The chaotic flow regime is reached after a sequence of successive supercritical Hopf bifurcations to periodic, quasiperiodic, and chaotic self‐sustained flow regimes. The numerical experiments reveal three distinct bifurcations as the Reynolds number is increased, each adding a new fundamental frequency to the velocity spectrum. In addition, frequency‐locked periodic solutions with independent but synchronized periodic functions are obtained. A scenario similar to the Ruelle–Takens–Newhouse scenario of the onset of chaos is verified in this forced convective open system flow. The results are illustrated for different Reynolds numbers using time‐velocity histories, Fourier power spectra, and phase space trajectories. The global structure of the self‐sustained oscillatory flow for a periodic regime is also discussed.

The flow and heat transfer characteristics of an impinging jet on a perpendicular flat surface are obtained by two dimensional numerical simulations of laminar and transitional flow regimes for the Reynolds number of Re = 300, 350, and 400 for a Prandtl number of Pr = 0.7. A fixed jet to plate spacing of H/W = 5 and a given heat flux on the plate surface are considered. Temporal evolution of velocity and temperature fields, Fourier spectra of the velocity temporal evolution and time average local and global Nusselt numbers are obtained for increasing Reynolds numbers for determining the time depending behavior and its effect on the heat transfer characteristics. Numerical simulation results demonstrate that self-sustained transitional periodic flow regimes arise from a laminar regime, when the Reynolds number is further increased to Re = 400 and that these regimes spread out to the whole domain with similar time dependent characteristics due to the flow incompressibility. Evaluations of time average local and global Nusselt numbers demonstrate the asymmetric Gaussian-type spatial distribution and the increase of both parameters when the flow evolves through the transitional periodic regime, with reasonable increases on the pumping power requirements.Copyright © 2010 by ASME

Heat transfer enhancement characteristics, through a transition scenario of flow bifurcations in symmetric wavy wall channels, are investigated by direct numerical simulations of the mass, momentum, and energy equations using spectral element methods. Flow bifurcations, transition scenarios, and heat transfer characteristics are determined by increasing the Reynolds numbers from a laminar to a transitional flow for the geometrical aspect ratios r =0.125 and r = 0.375. The numerical results demonstrate that the transition scenario to transitional flow regimes depends on the aspect ratio. For r =0.375, the transition scenario is characterized by one Hopf flow bifurcation in a frequency-doubling transition scenario, where further increases in the Reynolds number always lead to periodic flows; whereas, for r = 0.125, the transition scenario is characterized by a first Hopf flow bifurcation from a laminar to a time-dependent periodic flow and a second Hopf flow bifurcation from a periodic to a quasiperiodic flow. For r =0.125, the flow bifurcation scenario is similar to the Ruelle―Takens―Newhouse (RTN) transition scenario to Eulerian chaos observed in asymmetric wavy and grooved channels. The periodic and quasiperiodic flows are characterized by fundamental frequencies ω 1 , and ω 1 and ω 2 , respectively. For the aspect ratio r = 0.375, the Nusselt number increases slightly as the Reynolds number increases in the laminar regime until it reaches a critical Reynolds number of Re c ≈ 126. As the flow becomes periodic, and then quasiperiodic, the Nusselt number continuously increases with respect to the laminar regime, up to a factor of 4, which represents a significant heat transfer enhancement due to a better flow mixing.

There are several biomechanical factors involved in the formation, growth, remodeling, and eventual rupture of intracranial aneurysms. In particular, hemodynamic forces have a decisive role in the biomechanical environment of the aneurysmal cerebral vasculature. Most of the previous studies on vascular mechanics assessment of intracranial aneurysms are based on idealized geometries, where it has been suggested [1] that it is highly unlikely that saccular aneurysms expand due to a limit point instability. In addition, it has been reported [2] that some saccular aneurysms with non-spherical initial shape tend to become spherical when subjected to uniform pressure, because a spherical geometry is optimal to resist the pressure load, yielding a homogenous wall stress. In the present work, we present a comparison between anisotropic and isotropic constitutive models, which allows us to analyze the biomechanics of patient-specific cerebral aneurysmal arteries subjected to flow-induced pulsatile pressure. The results describe the effects of material anisotropy in the resulting wall mechanics of the intracranial vasculature geometries.

The Lattice-Boltzmann Method (LBM) has been used for investigating flow behavior and characteristics in mini, micro and nano channels with the objective of describing the transition among different length scales. In particular, we have used the LBM to describe the air bearing lubrication problem at very small scales. For doing this, first we simulate and characterize the Poiseuille flow through different length scale and compare the LBM numerical results to existing experimental and numerical results. We put special attention on the application of the slip boundary condition on the channel wall for very small length scales. Our numerical results for the Poiseuille flow show an acceptable agreement with the Fukui & Kaneko numerical solution for continuous and slip-velocity regimes. For both, the rarified flow regime and the free molecular flow regime our solutions do not show an acceptable agreement with the Fukui & Kaneko Model. Then, we focus on the Couette flow characterization at very small length scales. The pressure distribution on both walls for different Knudsen numbers is obtained and compared to existing numerical results. Last, we concentrate in the air bearing problem. We have looked at the best simulation parameters for successfully describing this device flow dynamics, and particularly, for determining the pressure distribution and the net force with a good accuracy.

The stretching and flow mixing enhancement characteristics in mini and micro wavy channels have been investigated by solving the Boltzmann Transport Equation (BTE) with the Lattice-Boltzmann method (LBM). The Eulerian and Lagrangian flow characteristics are obtained for a 2D symmetric wavy channel with the geometrical aspect ratios r = 0.375 and 0.1875, by performing numerical simulations of a Newtonian incompressible flow and a Newtonian compressible flow. Extended and reduced domain are used for determining the existence of spatial periodicity the Eulerian and Lagrangian characteristics for increasing Reynolds numbers. The Eulerian flow characteristics for micro channels configuration are determined for different Knudsen numbers, pressure ratio and accommodation coefficients with the objective of obtaining reliable velocity and flow patterns. The Lagrangian characteristics are obtained by integrating the Eulerian velocity field. Thousands of massless fluid particles are used to determine fluid particle Lagrangian trajectories, future and past stretching fields, and Lagrangian Lyapunov exponents, which are used for determining the channel regions with high stretching and flow mixing enhancement. The numerical results demonstrate that mini and micro wavy channels flows develop different Lagrangian characteristics. In mini wavy channels, flow mixing enhancement develops for time dependent 2D flows; whereas, in micro wavy channels, future and past stretching fields describe existence of flow mixing enhancement for very low, stable and time independent Reynolds number flow regime.Copyright © 2008 by ASME

Numerical investigations of transition scenarios due to flow bifurcations from laminar to time-dependent transitional flows in asymmetric wavy channels with different spatial periodicity on the sinusoidal wall are performed by direct numerical simulations of the mass and momentum conservation equations using a spectral element method computational program. Three aspect ratios r = a/(2L) are considered in this study: 0.125, 0.25, y 0.375. Computational meshes for both extended and periodic computational domains are used to determine first, the existence of spatial periodicity and second, the transitional flow behavior for increasing Reynolds numbers. Numerical results shows that the transition scenarios are highly dependent on the aspect ratio, r. The following scenarios develop: a) a first transition scenario with one flow bifurcation to a relatively low Reynolds numbers, Rec ; b) a second scenario, with also one flow bifurcation, to a relatively high Reynolds numbers, Rec* ; and, c) a third flow transition scenario with two Hopf bifurcations B1 and B2 , occurring at critical Reynolds numbers Rec1 y Rec2 , respectively. In this third scenario, fundamental frequencies ω1 and ω2 , and sub and super harmonic combinations of the fundamental frequencies develop as the Reynolds number increases from a laminar to higher transitional flow regime. This transition scenario from a laminar flow to high transitional flow regimes is very similar to the Ruelle-Takens-Newhouse (RTN) found in another confined flow channels such as symmetric wavy, grooved and communicating channels.Copyright © 2008 by ASME

The Eulerian and Lagrangian characteristics of a rarified gas flow have been investigated and related to flow mixing enhancement in micro grooved channels. The governing Boltzmann Transport Equation (BTE) is solved by the Lattice-Boltzmann method (LBM) for the Knudsen number range of 0.01−0.1. First, the Eulerian flow characteristics are determined by performing numerical simulations for different Knudsen numbers, pressure ratio and accommodation coefficients with the objective of obtaining reliable velocity characteristics and flow patterns, and determining the transition characteristics from the macro to microscale. The numerical predictions are compared to existing analytical and numerical results. Then, the Lagrangian characteristics are obtained by integrating the Eulerian velocity field by a 4t order Runge-Kutta scheme. Ten thousands (10,000) pairs of fluid particles are used to determine fluid particle Lagrangian trajectories, future stretching fields, and Lagrangian Lyapunov exponents, which are used for determining the grooved channel regions with high and low flow mixing enhancement. Our results demonstrate that rarified gas flows develop a future stretching field leading to the existence of Lagrangian chaos and flow mixing enhancement for very low, stable and time independent Reynolds number flow regime. This flow behavior departure from the well accepted concept that Lagrangian chaos and flow mixing enhancement need a time dependent 2D flow.Copyright © 2008 by ASME

Numerical simulations of time-dependent thermal energy transport in semiconductor thin films are performed using the Lattice Boltzmann Method applied to phonon transport. The discrete Lattice Boltzmann Method is derived from the continuous Boltzmann transport equation assuming nonlinear, frequency-dependent phonon dispersion for acoustic and optical phonons. Results indicate that the heat conduction in silicon thin films displays a transition from diffusive to ballistic energy transport as the characteristic length of the system becomes comparable to the phonon mean free path, and that the thermal energy transport process is characterized by the propagation of multiple, superimposed phonon waves. The methodology is used to characterize the time-dependent temperature profiles inside films of decreasing thickness. Thickness-dependent thermal conductivity values are computed based on steady-state temperature distributions obtained from the numerical models. It is found that reducing feature size into the subcontinuum regime decreases the thermal conductivity when compared to bulk values, at a higher rate than what was displayed by the Debye-based gray Lattice Boltzmann Method.Copyright © 2007 by ASME

We investigate the heat transfer enhancement due to flow mixing enhancement in a frequency-doubling transition scenario in symmetric wavy channels by direct numerical simulations of the mass, momentum and energy equations. The governing equations are solved for laminar and transitional flow regimes by the spectral element method, using a periodic computational domain, and with a high aspect ratio of r = a /(2L) = 0.375, where L is the periodic length, and a , the wavy wall amplitude. The frequency-doubling transition scenario is characterized by one flow bifurcation that develops to a critical Reynolds numbers Rec , leading to a periodic flow. Further increases in the Reynolds number leads to successive periodic flows where the fundamental frequency ω1 , increases continuously. This scenario is different to the Ruelle-Takens-Newhouse transition scenario obtained for a symmetric wavy channel with an aspect ratio of 0.125, where periodic and quasi periodic flow regimes develop as the Reynolds number increases. Heat Transfer simulations are carried out assuming a constant heat flux on one wall and an adiabatic condition on the other wall. Numerical results demonstrate that the time-average mean Nusselt number increases significantly as the flow passes from a laminar to a periodic flow regime. As the flow becomes periodic and for increasing Reynolds numbers, the Nusselt number increases with respect to the laminar flow Nusselt number, up to a factor of 4, depending on the Reynolds number, which represents a significant heat transfer enhancement due to a better flow mixing. This increase is accompanied by a reasonable increase in both the friction factor and the pumping power. The obtained qualitative and quantitative features are compared to other channel geometries, such as grooved and asymmetric wavy channels, which also develop different transition scenarios.Copyright © 2007 by ASME

The flow transition scenario in symmetric communicating channels has been investigated using direct numerical simulations of the mass and momentum conservation equations in the Reynolds numbers range of Re = [170–227]. The governing equations are solved for laminar and time-dependent transitional flow regimes by the spectral element method, using a periodic computational domain, for a periodic length of nL and an aspect ratio of r = aˆ / (2Lˆ) = 0.0405, where aˆ = 2a is the height of block within the channel, n an integer and Lˆ = L + 1 is the periodic length. Periodic computational domains with n = 1 and 2 are used in this investigation to determine the periodic length effect on the flow pattern characteristics. Numerical investigations with different domain meshes are carried out for determining the appropriate discretization for capturing transitional time-dependent flows. The numerical results show a transition scenario with two-flow Hopf bifurcations which develop as the pressure gradient is increased from a laminar to a time-dependent flow regime. The first Hopf bifurcation occurs to a critical Reynolds number of Rec1 and leads to a time-dependent periodic flow characterized by a fundamental frequency ω1. Further increases in the pressure gradient lead to successive quasi periodic flows after a second Hopf bifurcation B2 occurring to a critical Reynolds number Rec2 < Rec1, with two fundamental frequencies ω1 and ω2, and linear combinations of both frequencies—where the fundamental frequency ω1 increases continuously—and ω2 > ω1. This transition scenario is somewhat different from the Ruelle-Takens-Newhouse transition scenario obtained for symmetric wavy channels; in symmetric wavy channels, periodic and quasi periodic flow regimes develop as the Reynolds number increases. The friction factor for the symmetric communicating channel in the transitional regime is higher than the friction factor for the Poiseuille plane channel. The qualitative and quantitative behavior is compared to other channel geometries that also develop other transition scenarios.

The flow bifurcation scenario and heat transfer characteristics in grooved channels, are investigated by direct numerical simulations of the mass, momentum and energy equations, using the spectral element methods for increasing Reynolds numbers in the laminar and transitional regimes. The Eulerian flow characteristics show a transition scenario of two Hopf bifurcations when the flow evolves from a laminar to a time-dependent periodic and then to a quasi-periodic flow. The first Hopf bifurcation occurs to a critical Reynolds number Rec1 that is significantly lower than the critical Reynolds number for a plane-channel flow. The periodic and quasi-periodic flows are characterized by fundamental frequencies ω1 and m· ω1+n·ω2, respectively, with m and n integers. Friction factor and pumping power evaluations demonstrate that these parameters are much higher than the plane channel values. The time-average mean Nusselt number remains mostly constant in the laminar regime and continuously increases in the transitional regime. The rate of increase of this Nusselt number is higher for a quasi-periodic than for a periodic flow regime. This higher rate originates because better flow mixing develops in quasi-periodic flow regimes. The flow bifurcation scenario occurring in grooved channels is similar to the Ruelle-Takens-Newhouse transition scenario of Eulerian chaos, observed in symmetric and asymmetric wavy channels.

Computational investigations of flow mixing and oxygen transfer characteristics in an intravenous membrane oxygenator (IMO) are performed by direct numerical simulations of the conservation of mass, momentum, and species equations. Three-dimensional computational models are developed to investigate flow-mixing and oxygen-transfer characteristics for stationary and pulsating balloons, using the spectral element method. For a stationary balloon, the effect of the fiber placement within the fiber bundle and the number of fiber rings is investigated. In a pulsating balloon, the flow mixing characteristics are determined and the oxygen transfer rate is evaluated. For a stationary balloon, numerical simulations show two well-defined flow patterns that depend on the region of the IMO device. Successive increases of the Reynolds number raise the longitudinal velocity without creating secondary flow. This characteristic is not affected by staggered or non-staggered fiber placement within the fiber bundle. For a pulsating balloon, the flow mixing is enhanced by generating a three-dimensional time-dependent flow characterized by oscillatory radial, pulsatile longitudinal, and both oscillatory and random tangential velocities. This three-dimensional flow increases the flow mixing due to an active time-dependent secondary flow, particularly around the fibers. Analytical models show the fiber bundle placement effect on the pressure gradient and flow pattern. The oxygen transport from the fiber surface to the mean flow is due to a dominant radial diffusion mechanism, for the stationary balloon. The oxygen transfer rate reaches an asymptotic behavior at relatively low Reynolds numbers. For a pulsating balloon, the time-dependent oxygen-concentration field resembles the oscillatory and wavy nature of the time-dependent flow. Sherwood number evaluations demonstrate that balloon pulsations enhance the oxygen transfer rate, even for smaller flow rates.

This work investigates the transition scenario and heat transfer characteristics in a channel with a block tandem, as the flow evolves from a laminar to a transitional regime, by two-dimensional direct numerical simulations (DNS) of the time dependent, incompressible continuity, Navier-Stokes and energy equations. This investigation uses an extended computational domain with 10 blocks to determine the existence of a fully developed flow and self-similar temperature profiles, and a reduced computational domain to investigate the heat transfer enhancement for laminar and transitional flow regimes. This investigation demonstrates that significant heat transfer enhancements can be obtained at supercritical transitional flow Reynolds numbers with a minimum of dissipation due to viscous stresses. This enhancement is obtained without the necessity of operating this channel to high volumetric flow rates associated to turbulent flow regimes, which demand high pumping powers. In this channel, the transitional flow regime is more efficient than a laminar flow regime as a method of cooling electronics.

Spectral element computational simulations of the conservation of mass, momentum and species equations are performed to investigate the flow and oxygen transfer characteristics of an Intravenous Membrane Oxygenator (IMO). The simulations consider a three-dimensional IMO computational model consisting of equally-spaced fibers, an elastic balloon with non-permeable walls positioned longitudinally within the vena cava, and a Newtonian and time-dependent incompressible flow. Flow characteristics and oxygen transfer parameters are determined for operating conditions of a stationary and a pulsating balloon. For the stationary balloon configuration the flow is two-dimensional, parallel, laminar and without secondary flows for the Reynolds number range of 5.7-455.2. Evaluations of the oxygen transfer characteristics for the stationary balloon indicate that the main transport mechanisms are diffusion and convection in the crosswise and streamwise directions, respectively. Additionally, evaluations of oxygen transfer rates and Sherwood numbers in this Reynolds number range indicate that the oxygen transfer rate reaches an asymptotic limit at relatively moderate Reynolds numbers. For the pulsating balloon, flow characteristic results demonstrate the existence of a strong secondary flow around the fiber, and between the balloon and the fiber. This secondary flow induces oscillatory crosswise and streamwise velocities and a seemingly random spanwise flow which enhances the flow mixing as well as the transport of oxygen from the fiber surface to the bulk flow.

The Intravenous Membrane Oxygentor (IMO) device is an artificial respirator being developed at the University of Pittsburgh Medical Center (UPMC) with the objective of providing up to 50% of the O2 and CO2 exchange requirements to patients with the Acute Respiratory Distress Syndrome (ARDS) [1-4]. In-vitro and in-vivo experiments and computational simulations at the UPMC and Carnegie Mellon University (CMU), respectively, have been carried out to get a better understanding of the flow and mass transfer characteristics of the IMO [3,4, 7-11]. Numerically obtained pressure drop, for stationary and pulsating balloon conditions, presents a good agreement with the in-vitro experiments [4, 11]. The effect of the balloon pulsation on the flow pattern and mass transfer has also been investigated. Numerical results with a stationary balloon demonstrate a laminar flow regime with no secondary flows [10-11]. These results have indicated that the pulsating balloon generates a 3D oscillatory flow with a strong secondary flow that could increase the flow mixing and the mass transfer rate near the fibers [9-10]. These simulations however, have not considered the curvature of both the fibers and balloon and their effect on the flow mixing and mass transfer characteristics. This article reports numerical investigations of the effect of the curved shape of the balloon on the flow and oxygen transfer pattern of the IMO device shown in Fig. 1(a), for stationary and pulsating balloon regimes.

The Intravenous Membrane Oxygentor (IMO) is a device that consists of hundreds of hollow fibers and an elastic, non-permeable, balloon that is positioned within the vena cava. The balloon inflates and deflates rhythmically to a given amplitude and frequency to generate cross flow, promote blood mixing, and enhance the gas exchange. The IMO has been designed to provide temporarily up to 50% of the O2 and CO2 exchange requirements to patients who suffer Acute Respiratory Distress Syndrome (ARDS) [1–4]. This illness affects approximately 150,000 people per year in the U.S. and is characterized by non-cardiogenic pulmonary edemas and a rapid and progressive malfunctioning of the lung [4]. The IMO device is an alternative therapy to conventional treatments such as mechanical ventilation and extracorporeal membrane oxygenation [5–7]. This paper reports the investigation of the flow and oxygen transfer characteristics of the IMO device shown in Fig. 1(a) and describes the numerically obtained flow patterns and the oxygen transfer characteristics for stationary and pulsating balloon regimes.

The mass transfer performance of an Intravenous Membrane Oxygenator is investigated by computational simulations of the conservation of mass, momentum and species equations. The Intravenous Membrane Oxygenator (IMO), is a device developed experimentally to provide consistent and reproducible oxygen and carbon dioxide exchange. The IMO is composed by an elastic and non-permeable pulsating balloon positioned within the vena cava, and micro-porous-membrane fibers that transport oxygen and carbon dioxide, located longitudinally between the balloon and vena cava. During the operation regime, the blood flow motion is originated by a blood pressure gradient and a pulsating balloon motion. A three-dimensional physical-computational model consisting of equally-spaced fibers and a Newtonian and time-dependent incompressible flow is used for the simulations. The numerical simulation results for the stationary balloon configuration, obtained using the spectral element method, demonstrate that the flow remains parallel, laminar and with absence of secondary flows in the whole domain. Evaluations of the mass transfer characteristics and parameters, such as the oxygen concentration profile around the fiber and the Sherwood number, for increasing Reynolds numbers, indicate that the parabolic flow regime increase the oxygen transfer rate until an asymptotic limit in the oxygen transfer capability is reached. A further increase in the Reynolds number beyond this asymptotic limit does not increase the oxygen transfer rate.

An aneurysm is a permanent balloon-like, blood-filled dilatation on an arterial vessel. In an abdominal aortic aneurysm (AAA), the growth and eventual rupture of aneurysmal lesions depends on the interaction between the aneurysm wall and the blood motion along it. Rupture of an aneurysm occurs when the shear stresses exceed the strength of the wall [1]. When an AAA ruptures, 50 percent of patients die before reaching the operating room and 54 percent of the remaining will die within 30 days [2]. Therefore, AAA represents a great danger to the patient [3]. The objective of this study is to investigate the shear stresses in an aneurysm composed by two deforned regions and to examine the effect of the blood non-Newtonian behavior on the flow dynamics.