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This work presents the synthetical mathematical analysis of available knowledge regarding physical phenomena of research interest. The work is not focusing on providing the phenomena according to the physical laws but rather because of them, hence, it is grounded on the philosophically defined concept of "similarity", and progresses to the mathematical treatments of those factors and parameters that are involved into the similarity validation among physical phenomena.A critical validation regarding the effectiveness of such an approach was also performed, in order to conceptualize the relevance of the factors and parameters interactions as a potential control tool against engineering-based hypothesis. Such factors and parameters are generated through the description and delimitation of the system of interest. A "matrix" is used for the classification of the existing knowledge regarding this system. It is consisted of the categorical descriptors of the system in question and the levels of these descriptors.A mathematical analysis of this "matrix" supports that all the existent perceptions of a physical phenomenon constitute a four-dimensional vector space. Within this space, the concept of similarity allows for the definition on which of a specific non-linear mapping that might be applied to strictly classify the existing knowledge about the phenomenon in question.Similarity is used here to define the conditions and the constrains that this mapping must satisfy. In conclusion, the applicability of the suggested approach on an engineering approach regarding a physical problem, was also demonstrated in this study.

Aim: The present work aimed to estimate the probability of bottled white wine exhibiting acceptable colour change during a certain predefined time using a predictive model based on a descriptive mathematically expressed theory.Materials and methods: Nine mono-varietal Greek white wines (produced by three varieties) were bottled and stored at two different temperatures (20 and 30 °C). Their absorbance at 420 nm (A420) was recorded every three months for 12 months. Results were processed by applying the root cause analysis versus storage conditions. Furthermore, a predictive mathematical model was adapted to the experimental data based on the probability of the product showing browning-index values higher than a specific threshold during its shelf life.Results: As expected, the absorbance at 420 nm increased with time, while the elevated storage temperature had a significant effect on colour alteration. The browning rate was dependent on grape variety. When storage temperature was 20 °C, a lower A420 increase was observed for Sauvignon blanc wine, while at 30 °C, browning was higher. Assyrtiko wines were more resistant to colour change at both temperatures. The probability of a bottled product maintaining an acceptable colour change by the end of one year, was used as a measure of its quality. A very successful fit was achieved between mathematical and experimentally obtained data for shelf life predictions.Significance and impact of the study: The modelling of the experimental results, as well as the exploitation of this fitting to make predictions independently of the specific results, indicate that storage temperature will potentially impact the colour of wines over time, depending on the evolution of the absorbance values at 420 nm. For the wine sector, the selection of the appropriate storage conditions for a given grape variety is an important factor to take into consideration.

A novel methodology for analyzing hybrid wind turbine-photovoltaic off-grid systems with battery storage is presented in this work. To support off-grid operation, small settlements were chosen as case studies for these autonomous hybrid systems. Μeteorological, demographic, geographical, geospatial, land use and load data with the design and implementation of off-grid power plants were used. The study aims at further supporting and improving off-grid hybrid energy systems in terms of design, analysis and integration, while a life-cycle analysis for the proposed systems was performed to quantify the relative environmental impact, as well. The findings show that the implementation of autonomous energy production by renewable energy systems for off-grid small settlements, based on the presented methodology, is feasible.

The present study concerns with an optimization method of Solid Oxide Fuel Cell (SOFC) systems for electricity generation, directly fed by CH4 (methane) or C2H5OH (ethanol). By considering a detailed thermodynamic analysis, an innovative simulation model, namely THERMAS (THERmodynamic MAthematical Simulation), was designed and implemented. A specific SOFC-based system was selected to be the reference simulation, which is assumed to be equipped with heat exchangers (preheaters), a reformer, a SOFC-stack system and an afterburner. THERMAS allows for an extended parametric analysis in terms of energy and exergy and offers the opportunity to investigate all the operational characteristics that affect system's efficiency. The optimization process relies on the difference between the energy and exergy efficiency, where an OPF (OPtimization Factor) has been introduced and particularly estimated for each simulated scenario, based on several operational parameters, such as fuel composition, extension of chemical reactions and temperatures. The introduction of this OPF is actually an innovative improvement towards an easy-to-use optimization of SOFC systems. It is found that the design of a SOFC-based power plant fueled by pure hydrocarbons, has to be carefully simulated before its, otherwise the purification of fuels sounds meaningless.

This work simulates electricity production in a Solid Oxide Fuel Cell (SOFC)-based power plant, fed by biogas of various compositions. Steam reforming of the gas feed stream is used to produce the required supply for the SOFC. Given the constraints of the feed stream compositions, resulting from the origin of biogas, i.e., by the biomass from which the biogas has been produced as well as by the operating conditions selected for its production, the overall plant performance is modelled in terms of energy and exergy. The model provides results on the efficiency, power output and thermal behavior of the system, thus presenting the potential to offer great advantages in generating electricity from biogas and reducing the environmental impact. This research study presents the efficiency of such a system in terms of energy and exergy, by considering several values of the operational parameters (extensions of reactions that take place in the apparatus, temperatures, feed stream compositions, etc.). It is found that moving towards a methane richer fuel, the energy and exergy efficiency can remain almost constant at high levels (around 70%), while in absolute value the electric energy can increase up to 35% according to the system’s needs. Therefore, under this prospect, the present research study reveals the usefulness of low content methane fuels, which through the optimization process can succeed identical energy management compared to high content methane fuels.

A statistical analysis on the wildfire events, that have taken place in Greece during the period 1985-2007, for the assessment of the extremes has been performed. The total burned area of each fire was considered here as a key variable to express the significance of a given event. The data have been analyzed through the extreme value theory, which has been in general proved a powerful tool for the accurate assessment of the return period of extreme events. Both frequentist and Bayesian approaches have been used for comparison and evaluation purposes. Precisely, the Generalized Extreme Value (GEV) distribution along with Peaks over Threshold (POT) have been compared with the Bayesian Extreme Value modelling. Furthermore, the correlation of the burned area with the potential extreme values for other key parameters (e.g. wind, temperature, humidity, etc.) has been also investigated.

Greece has to recycle 70% of Construction and Demolition Waste (CDW) by 2020, in the context of the enforcement of Waste Framework Directive (WFD) 2008/98/EC. CDW recycling has started only recently to a small extent and is restricted to road works. The reuse of CDW in new concrete production could contribute to the construction sector sustainability through minimizing landfill and the need for new raw materials. Literature data are found regarding the performance of recycled concrete, but limited knowledge is available regarding the chemical and mineralogical properties of recycled aggregates (RA) and how these affect concrete. Moreover, Greek aggregates are not siliceous aggregates as are most of the aggregates investigated in literature. Against this background, this study aims to evaluate for the first time mechanical and durability performance of concrete made with RA from different Greek recycling plants. All recycled materials used were characterized in terms of chemical and mineralogical composition and certain chemical and physical properties have been determined. It was found that all RA are enriched in Si, Al, and alkali oxides, which was mineralogically verified by the identification of quartz and minor quantities of mica and feldspars. They proved though innocuous to alkali aggregate reaction. Higher water absorption of coarse RA is their most negative physical characteristic. Fine fractions of RA have low Sand Equivalent and significantly high water absorption values, which make them unsuitable to replace natural sand. Water soluble ions are at the same level to that of natural aggregates. Furthermore, concrete mixtures were prepared using an RA percentage ranging from 0% to 75%. Results indicate that the compressive strength of recycled concrete ranges from significantly lower (37% reduction) to equal, compared to conventional concrete, depending on the composition of RA. The majority of literature data relate the inferior quality of RA to their high water absorption, the findings of the present research though indicate that the presence of clay minerals has a more pronounced adverse effect to concrete's strength. As for the durability properties examined, i.e. water absorption, sorptivity, frost and carbonation resistance, the recycled concrete mixtures exhibited somewhat lower performance than reference concrete.

The present research study presents the optimization of Solid Oxide Fuel Cell (SOFC) power plants directly fed by biogas. By considering energy and exergy balances for such a system, a detailed thermodynamic model (THERMAS) was designed and implemented. A specific SOFC-based system was selected as case study, equipped with three heat exchangers (preheaters), a reformer, a SOFC-stack system and an afterburner. The use of the simulation tool THERMAS give us the opportunity to investigate all the appropriate parameters that affect system’s efficiency based on exergy analysis while incorporating a detailed parametric analysis regarding the whole system. The optimization process relies on the difference between the energy and exergy efficiency by considering an innovative Optimization Factor (OPF) for each simulated system, which is dynamically affected by operational parameters, such as fuel composition, extension of chemical reactions and temperatures. It is found that the use of a pure fuels seems to be meaningless without optimization.

The food preservation hypothesis as impacted by overall packaging applications is considered in this work. The objective was to devise a decision supportive method for the selection of “just-right” packaging materials, techniques and procedures. For that, food preservation was critically approached in order to identify the optimum outcome at experimental and packaging selection decision-making levels. A mathematically supported and proven knowledge classification, and the establishment of a straightforward coherence mode among the principles of the natural systemic phenomena, were used. The ultimate aim of this work was to justifiably surpass a simple description of packaging according to its measurable specifications, and instead, engage its inherent properties into a cyclic 8-steps-process for eventually understanding its potential to support any particular preservation hypothesis in question. The proposed methodology includes primarily, the consideration of the study hypothesis and, in parallel, the conclusive remarks and claims with respect to the experimental factors involved (properties, parameters, relations and conditions). Considering the experimentally controlled set-ups that a researcher has to expose the food system to and the role of packaging in obtaining its preservation potential, our method supports the experimenters in selecting the experimental conditions under which the preservation hypothesis can be disclaimed and furthermore, it could indicate the way to reduce experimentation research waste.

Aim: The aim of this project was to identify the characteristic flavor and off-flavor compounds that could be used as potential oxidation markers to establish the quality of the stored wines and to predict the wines’ shelf life employing a mathematical model.Materials and methods: Six mono-varietal Greek white wines (produced by three varieties) were bottled using two types of corks having different oxygen permeability properties. Volatile compounds, resistance to oxidative degradation and total and free-sulfur dioxide content were recorded in all samples. Results were processed using the root cause analysis versus packaging and storage conditions. Additionally, a predictive model has been constructed to estimate the shelf life of the bottled wine.Results: Physicochemical analysis indicated the absence of significant oxidative degradations during the first 7 months. Furthermore, 12-month stored samples showed a significant alteration in their flavor profile. Based on the concentration of the selected oxidation marker (isoamyl-alcohol), the possibility of the bottled product not reaching the end of its shelf life, has been used to quantify the product’s quality. A very successful fit was achieved between mathematical and experimentally obtained data concerning shelf life predictions.Significance and impact of the study: The modeling of the results indicated the importance of cork selection due to the Oxygen Transmission Rate (OTR) values that potentially impact the quality of the wines in time, according to the evolution of the isoamyl-alcohol concentration. For the wine industry, the selection of the appropriate cork according to the wine type is an important consideration.

A previously developed model which effectively predicts the probability of olive oil reaching the end of its shelf-life within a certain time frame was tested for its response when the convective diffusion of oxygen through packaging material is taken in account. Darcy’s Law was used to correlate the packaging permeability with the oxygen flow through the packaging materials. Mass transport within the food-packaging system was considered transient and the relative one-dimensional differential equations along with appropriate initial and boundary conditions were numerically solved. When the Peclet (Pe) number was used to validate the significance of the oxygen transport mechanism through packaging, the model results confirmed the Arrhenius type dependency of diffusion, where the slope of the line per material actually indicated their –Ea/R. Furthermore, Pe could not be correlated to the hexanal produced in samples stored under light. Photo-oxidation has a significant role in the oxidative degradation of olive oil confirmed by the shelf-assessing test. The validity of our model for the oxygen diffusion driven systems, was also confirmed, for that reason the predictive boundaries were set. Results safely indicated the significance of applying a self-assessing process to confirm the packaging selection process for oxygen sensitive food via this model.

We study here the urban use of electric vehicles (EVs), focusing on the electricity production for charging purposes. This work proposes an innovative charging scheme for EVs, by introducing a home-applied power station, consisting of a fuel cell combined with an internal reforming unit, which is fed by natural gas and can thus be directly connected to the already-established natural gas grid. We therefore overcome the barriers posed by hydrogen use (establishment of storage equipment, energy consumption for keeping high pressure, development of supply grid, etc.) while we eliminate the environmental impact, since no fossil fuels are required for electricity production. Furthermore, comparisons against EVs charged by grid and vehicles fueled by petrol, both subjected to urban everyday use, have been carried out here. Precisely, we financially compare the use of an EV charged directly through the national electric grid against our innovative power station. Both options are also compared with the same vehicle, powered by an internal combustion engine fueled by petrol. This study also implements a detailed thermodynamic analysis for this state-of-the-art power station and an additional financial analysis for the everyday use of these vehicles under the three different scenarios. For the sake of equivalence, the new Peugeot 208 was selected as the reference-vehicle, as it is equipped either with a petrol engine or an electric motor, under a roughly identical performance profile. This work also introduces the use of the existing grid of natural gas to produce the necessary electricity for charging EVs, rather than using other renewables (solar, wind, etc.), and could further strengthen the worldwide acceptance of the EVs as a viable and a financially feasible solution for everyday urban transportation.

This chapter presents a developmental approach through a well-defined 12-fold classification matrix scheme, recognized via a synthetical mathematical analysis.

The main objective of this study is to investigate the adsorption process from both experimental and modeling point of view of phosphate onto Phoslock™, an increasingly used worldwide restoration tool to control phosphorus from natural water ecosystems. Bench-scale batch experiments were performed to examine its efficiency as a phosphate adsorbent and detailed simulations were carried out, allowing for a deep insight of the removal process. The adsorption efficiency calculated by the simulations is 87.41%, being in excellent agreement with that experimentally measured (approx. 87%). However, it is important to notice the discrepancy between experimental measures and simulation estimates. Although the agreement is perfect for T = 25 °C, it becomes more and more poor as temperature decreases, approaching approx. 10% difference for the low temperature of 10 °C. This inconsistency between experiments and model can be attributed to the internal inefficiencies of unit cell approach, which in particular overestimates the adsorption efficiency.

Recent advances in upscaling techniques have investigated the importance of upscaling, regarding mass transport phenomena in porous media. In order to obtain a better understanding of the significance of pore scale, new analytical approaches as well as numerical simulations must be carried out. Toward this aim, three numerical simulations of porous media for three different scales are developed and validated in this study, and also analyzes the mass transport phenomena for each scale. This technique has been successfully applied to three-dimensional porous systems. More precisely, the first system is a sphere-in-cell model, consisted of two concentric spheres, whose volume ratio is supposed to represent the porosity of the medium. The second one is an assemblage of spheres, being a more accurate model of the pore structure. Finally, the third system is a porous 3-D box in the macroscopic scale. This study also emphasizes on the simulation of flow as well as mass, and heat transfer phenomena taking place in complex porous structure, providing details for velocity profiles, species distribution, and temperature levels. It is shown here that the details in the porous structure at the mesoscopic scale—shape, size, and positioning—have significant effects on the results since they lead to errors, up to 10%. It is also concluded that a microscopic description at the pore scale is insufficient for studying mass transport phenomena in porous media due to discontinuities of the structure and very high local velocity. Therefore, a methodology including globally valid rules has been presented here, to allow for an accurate description of the parameters and variables used in different scales of porous structures description.

The scale-up and scale-down process is of great importance regarding modeling of mass transport phenomena in porous media. Three numerical simulations of porous media for three different scales were developed and validated in this study, analyzing the mass transport phenomena for each scale. More precisely, the first system is a spheres assemblage of various spatial distributions and sizes (mesoscopic), the second is a porous box (macroscopic), and the third is a sphere-in-cell model (microscopic). It was found that the porous structure—shape, size, and positioning—of the spheres at the mesoscopic scale have significant effects on the adsorption efficiency (up to 10%) of highly convective regimes. It was also concluded that a microscopic description at the pore scale is insufficient to adequately describe mass transport phenomena in porous media due to discontinuities of the structure and high local velocity. By comparing the results from the three scales, we obtained a method of matching all geometrical,...

The objective of this work is to investigate, from both experimental and simulation points of view, the feasibility of a flywheel energy storage system (FESS) for buffering energy when implemented in off-grid (autonomous) electricity production. Toward this aim, a prototype FESS was built and measured to identify its optimal design and operational characteristics as they have been obtained theoretically. After simulating two different materials and shapes for the rotating mass, the hollow aluminum cylindrical design found to provide excellent energy storage results. By identifying the storage capacity of a prototype laboratory-scale FESS, it is also found that such a device can be promoted as a uninterruptible power supply system to cover peak loads during limited time periods. Finally, the scale of the project found to be a crucial parameter for its feasibility.

This paper deals with the feasibility of a Renewable Energy Sources (RES)-based stand-alone system for electricity supply based on a Flywheel Energy Storage System (FESS) located on the Greek Island of Naxos. The innovative use of flywheels in parallel connection with electrochemical batteries, as an integrated storage device in the same power plant, was selected to be simulated as it is a necessary buffer covering the load of a typical house. The optimal configuration for the electromechanical connection between the electrochemical batteries and flywheels is also considered in this study. Operational characteristics of the new storage systems were estimated and used in the simulations, while the financial aspects of the projects finalized using hand-made calculations and the HOMER software was used only for the energy calculations. It was found that an off-grid project using advanced and totally “green” technologies is possible and comparable to more conventional RES-based systems, in terms of energy and economical feasibility. Finally, it can be concluded that systems with low price flywheels are equivalent to those with electrochemical batteries.

The scope of this work is to model the mass transport process from a moving Newtonian fluid to an assemblage of spherical solid absorbers. The flow field for laminar flow was obtained by numerically solving the Navier–Stokes equation in the pore space of a stochastically constructed three-dimensional assemblage of spherical particles, where the convective/diffusive transport is simulated by involving an adsorption/reaction/desorption mechanism as an appropriate boundary condition. The spatial/volume-averaging technique was used here for up-scaling, where the simplified boundary-value problems have been described and numerically solved for the velocity field and concentration. Macroscopic mass transfer coefficient was then calculated and compared with other approaches. The process was found to be controlled by the Peclet number as well as the porosity of the porous structure. Through model validation, the sorption mechanism considered here proved to provide a reasonable estimation of the mass transfer.

This paper deals with the feasibility of RES-based systems for electricity supply in four different Greek Islands. Three specific typical loads were selected to be covered, where the grid connection was considered as optional. An optimization procedure in terms of energy efficiency and economical feasibility has also been applied, using the HOMER software for the energy part and a hand-made calculation procedure for the financial aspects. It is found that an off-grid project is not economically feasible and the grid connection is necessary only for the sale of excess electricity (not as a backup source). Finally, the relationship between environmental potential and produced energy is found to be linear, with almost constant behaviour for solar systems.

Mathematical modeling of transport and electrochemical phenomena within SOFCs can lead to improved understanding of the involved physical, electrical, and chemical processes and represents a powerful tool for their development. In this context, the present work illustrates a three-dimensional CFD simulation of a planar SOFC unit cell fuelled by modeled biogas/steam mixtures. The simulations estimate the distribution of gas species, the current densities and the potentials, as well as the temperature gradients and confirm that equimolar CH4/CO2 biogas leads to improved performance, while minimal steam addition can prevent carbon deposition.

The scope of this work is to estimate the effective mass-transfer coefficient in a two-phase system of oil and water fluid droplets, both being in a porous medium. To this end, a tracer is advected from the flowing aqueous phase to the immobile non-aqueous one. Partitioning at the fluid-fluid interface and surface diffusion are also taken into account. By using spatial/volume-averaging techniques, the appropriately simplified boundary-value problems are described and numerically solved for the flow velocity field and for the transport problem. The problem was found to be controlled by the Peclet number of the flowing phase, the dimensionless parameter Λ, containing both diffusion and partition in the two phases, as well as the geometrical properties of the porous structure. It is also verified that the usually involved unit cell-configurations underestimate the mass transport to the immobile phase.

By using the optimization software tool HOMER, this project aims at the energetic and economical optimization of a RES-based stand-alone system, already installed at Leicestershire, UK. Based on local meteorological data, an optimization strategy has been developed to identify the most economical and efficient scenarios for the generation of electricity to cover the desirable load in annual basis. Furthermore, the environmental-friendly character of the system was highly concerned in terms of emissions reduction, therefore the capability of an off-grid system was also investigated. The simulations show that an off-grid project with zero emissions is feasible, presenting the additional advantage of minimal capital investment costs. Finally, it is found that grid connection corresponds to very high operational costs in a long term.

It is a common point for the current fuel cell research to correlate the composition of the feedstream to the output of a Solid Oxide Fuel Cell (SOFC). In this direction, the detailed steady-state transport processes (i.e. the flow regime, the heat transfer, the mass and the charge transport) are mathematically described here. A new mesoscopic mathematical model has been developed through the relative differential equations along with the appropriate boundary conditions, which have been numerically integrated by using the commercially available software CFD-ACE+, in order to calculate the electricity produced by the fuel cell. It is found that the produced current density increases with CH4 percentage in the feeding mixture.

The steady state heat transfer that takes place in a hydrogen-fed tubular Solid Oxide Fuel Cell is considered here. The heat is produced due to the electrochemical reaction of the hydrogen that feds the cell with oxygen anions. An averaging technique is used to formulate a relatively simple one-dimensional heat transfer problem. The conduction-convection equation describing the heat transfer from the electrolyte’s surface to the moving gas that surrounds the cell’s cathode is solved analytically under the assumption of iso-thermal conditions. Three different cases are considered for the flow of the cathode gas: (a) plug flow, (b) fully developed incompressible laminar flow, and (c) compressible flow. Analytical expressions for the spatial distribution of gas and cell temperature along the cell's length are obtained. For constant mass flow rate, different flow regimes produce almost the similar spatial distributions for the gas temperature and, consequently, the consideration of the flow regime is of low importance in the design of fuel cell stacks.

This work presents a mathematical model to support the electrical energy production in a specific area by using Renewable Energy Sources (RES). More precisely, three RES types (namely, wind, solar and hydropower) are considered while biomass is used to satisfy thermal demands. The actual goal of the model is to generate one or more scenarios for the efficient production of electrical energy for a specific area by selecting the most suitable RES in terms of energy efficiency and cost effectiveness. More specifically the decision criteria are the minimization of installation and operational costs produced for each scenario and the maximization of the electrical energy produced by the specific power plant. Finally, feasibility analyses for selected case studies as well as an evaluation against similar best practices are also carried out for each scenario proposed.

This paper deals with the prediction of a spatially averaged droplet size during transport within homogeneous porous media. More precisely, this transport process occurs in a mixture of a continuous aqueous phase which includes a discontinuous one in the form of droplets. The mixture flows in a homogeneous porous medium under laminar flow conditions. The collection of γ-order moments, Sγ, is used here to describe the time evolution of the spatially averaged mean diameter of spherical droplets, mainly because Sγ satisfies the convective/diffusive transient transport equation. As it is well known, break-up and coalescence are the primary local phenomena controlling the size

The integration of solid oxide fuel cells (SOFCs) in biomass gasification–turbine processes was studied for the estimation of the overall electrical efficiency. Since both processes operate close to 1000 °C, heat integration is one of the benefits of the proposed scheme. Heat generated at the SOFC and the afterburner of the integrated process was found sufficient to cover the demands of gasification and reforming, in any examined case, while a significant heat excess was available to a bottoming thermal cycle for additional power generation. The electrical efficiency of the integrated process was found to overcome 60% of the low heating value of the biomass feed. SOFC's contribution to the overall electrical power output was of the order of 70%, and fuel utilization at the SOFC was recognized as the most crucial operational parameter.

In the present work, a three dimensional model examining the fluid flow along with the fundamental transport phenomena occurring in a typical polymer electrolyte fuel cell (PEMFC), i.e. heat transfer, mass transport and charge transfer, has been developed. The flow field was simulated according to the well known Navier-Stokes equations, while the heat transfer was described by the typical conduction/convection equation and the mass transport by the convection/diffusion one. Furthermore, reaction kinetics were studied by the Butler-Volmer equation for the heterogeneous reactions occurring at the porous electrodes. The developed model was numerically solved by using the commercially available CFD package CFD-RC©, which is based on the multi-step finite volume method. The fuel cell performance in terms of velocity, temperature, mass fractions of active compounds and electric field has been investigated as well.

A mathematical model for the simulation of the transport phenomena occurred in the anode of a typical fuel cell is presented here. The model initially considers a simple onedimensional geometry where the mass transport equation is combined with a Tafel-type description for the current density. By assuming isothermal conditions, the numerical solution of the differential equations was achieved with the use of a non-linear shooting scheme in conjunction with the multidimensional Newton algorithm. The space was discretized through a constant-step mesh while the resulting nonlinear system of ordinary differential equations was solved by using the 4th order Runge-Kutta method. The whole algorithm was implemented by developing a new FORTRAN code. In addition, a planar two-dimensional geometry is also considered, where the mass transport is described by the convection-diffusion equation within the catalyst layer together with the Navier- Stokes equation for laminar flow conditions and the electrochemical effects, while the convective heat transfer within the developed diffusion layer is also taken into account. This approach has been numerically implemented and solved by using the finite volume method being applicable through the CFD-RC© commercial package. For the sake of simplicity, the feedstream of the fuel cell was assumed to be a hydrogen-rich mixture (H2 >90%) for all cases. Both SOFC and PEM type fuel cells were considered in this study, while the results are presented in terms of fuel concentration, produced current density and overpotential.

In this paper the integrated process of biomass gasification and a solid oxide fuel cell (SOFC) was studied in terms of thermodynamics. The study is based on an ongoing project intending to develop an innovative sustainable technology with high efficiency. According to some assumptions, the energy balance revealed that the process can be auto-thermal. Furthermore, and due to the utilization of the hydrogen content of steam utilized in the reforming stage, the overall efficiencies to electrical power could reach very high levels.

Olive oil has gained much appreciation among consumers worldwide leading to increased markets as well as greater consumer expectation and thus more challenges for the relevant food sector. By understanding the product, its interactions with the environment, and the protective role of the package, decisions can be made on the barrier properties required of the packaging materials to achieve the desired shelf life. To this end, the shelf life of packaged olive oil under various storage and distribution environments can be predicted by mathematical modeling. This review examines the basic factors affecting the shelf life of olive oil in different packaging systems and describes the main oxidative degradation mechanisms for them. Since an experimental investigation to correlate the basic quality factors and the shelf life of a product is time- and effort-consuming, the use of mathematical modeling for the prediction of packaged olive oil shelf life is also discussed. In the presented works, shelf life predict...

Droplets transport in homogeneous porous media has been found to be an attractive problem applicable in a lot of industrial and scientific sectors such as enhanced oil recovery, food production, plastics etc. As applications become wider, a predictive method for the process is warranted. To this end, it has been widely accepted that the collection of γ-order moments, Sγ, can describe the time evolution of any spatially averaged quantity like the mean diameter of spherical droplets, while it has been also found that Sγ satisfies the transport equations [1]. Here, the so-called “Sγ concept” is applied in a CFD module for the modeling of the transport processes occurring in a mixture of a continuous aqueous phase which includes a discontinuous one in the form of droplets. This mixture flows within a homogeneous porous medium under creeping or laminar flow conditions. The momentums of the particle size distribution are evaluated using the local flow conditions as obtained from CFD simulations for the processes considered. To solve the transport equations, the microstructure droplets formation/destruction has been also taken into account by using already known analytical expressions for the source terms representing the break up and coalescence of the droplets [2-4]. The proposed constitutive model adequately simulates the effect of porous geometry on the droplets size distribution and could be helpful in understanding the phenomena that take place in microscopic scale.

We derive the effective mass-transfer coefficient between two fluid phases in a porous medium, one of which is flowing and the other is immobile. A passive tracer is advected by the flowing phase, becomes partitioned at the fluid–fluid interface and diffuses in the immobile phase. We use traditional volume-averaging methods to obtain a unit-cell boundary-value problem for the calculation of the effective mass-transfer coefficient. The problem is controlled by the Peclet number of the flowing phase, by a second dimensionless parameter that captures diffusion and partition in the two phases and by the geometrical properties of the porous medium. We derive asymptotic results for the scaling of the mass-transfer coefficient under various limiting conditions. Then, we use numerical methods that solve for the flow velocity field under Stokes flow conditions, and for the transport problem. The numerical results verify the asymptotic scaling expressions and provide estimates of the coefficient for a number of special cases. In particular, we find that when the immobile phase is wetting the solid (in the form of films), the mass transfer coefficient is larger than in the non-wetting case (where the phase is distributed in the form of blobs). Shape factors for practical applications are also obtained.