Τμήμα Χημικών Μηχανικών (ΜΔΕ)

Permanent URI for this collection

https://hdl.handle.net/10889/75

Browse

Now showing 1 - 20 of 144

Open Access
A constitutive rheological model for agglomerating blood derived from non-equilibrium thermodynamics
Τσιμούρη, Ιωάννα; Μαυραντζάς, Βλάσιος; Τσαμόπουλος, Ιωάννης; Δημακόπουλος, Γιάννης; Tsimouri, Ioanna
Τμήμα Χημικών Μηχανικών (ΜΔΕ)
Many deaths are the result of cardiovascular diseases associated with unusual blood rheological properties in the circulatory system [Yilmaz and Gundogdu (2008)]. Therefore, understanding the rheological behavior of blood is paramount in providing insights on the causes of various diseases and the tailor-design of the transport of drug directly to the infected area [Yilmaz and Gundogdu (2008)]. Blood is mainly a suspension of elastic particulate cells, among which red blood cells (RBCs) dominate, in plasma, usually considered as a Newtonian fluid. Red blood cells tend to aggregate in the presence of plasma proteins, forming structures known as rouleaux which can be decomposed upon the applied tension induced by the flow. In this master thesis, we derive a constitutive rheological model for human blood which accounts for the formation and dissociation of rouleaux using the generalized bracket formulation of non-equilibrium thermodynamics [Beris and Edwards (1994)]. Similar to the model derived by Owens and coworkers [Owens (2006); Fang and Owens (2006); Moyers-Gonzalez et al. (2008)] through polymer network theory, each rouleau in our model is represented as a dumbbell; the corresponding structural variable is the conformation tensor of the dumbbell. The kinetics of rouleau formation and dissociation is treated as in German et al. (2013) by assuming a set of reversible reactions, each characterized by a forward and a reverse rate constant. The final set of evolution equations for the microstructure of each rouleau and the expression for the stress tensor turn out to be very similar to those of Owens and co-workers. However, by explicitly considering a mechanism for the formation and breakage of rouleaux, our model further provides expressions for the aggregation and disaggregation rates appearing in the final transport equations, which in the kinetic theory-based network model of Owens were absent and had to be specified separately. Despite this, the two models are found to provide similar descriptions of the experimental data collected by Mehri et al. (2013) on the size distribution of rouleaux.
Open Access
A method for the in-situ measurement of the water content of atmospheric particles
Τσιλιγιάννης, Επαμεινώνδας; Πανδής, Σπύρος; Κουτσούκος, Πέτρος; Κορνάρος, Μιχαήλ; Tsiligiannis, Epameinondas
Τμήμα Χημικών Μηχανικών (ΜΔΕ)
The hygroscopic behavior of atmospheric aerosols influences their size, composition, lifetime, chemical reactivity, and light scattering. Hygroscopic growth plays an important role in a number of air pollution problems including visibility impairment, climate change, acid deposition, longrange transport and the ability of particles to penetrate the human respiratory system. The absorption of water by aerosol particles often exhibits a hysteresis. Thus, the physical state (liquid or solid) of particles and the amount of aerosol water at a specific relative humidity (RH) are uncertain, as they depend on the history of these particles. In this work, the reduced version of the Dry Ambient Aerosol Size Spectrometer (DAASS) that measures the water content of atmospheric aerosols has been redesigned and optimized. The DAASS measures the number distribution of the aerosols at ambient conditions and at low RH thus drying the particles. A comparison of these distributions allows the determination of the physical state of the particles and their water content. The new version of the DAASS is capable of operating at higher RH values than its predecessor. The instrument has been characterized regarding particle wall losses, in a set of smog chamber experiments using (NH4)2SO4 particles. An algorithm checking the consistency of the measurements and the applicability of the assumptions used in the data analysis was developed. The water concentrations observations were compared to the predictions of the aerosol thermodynamics model EAIM. Ambient measurements, using the original and the improved version of DAASS, were conducted during two different time periods in a suburban area in Patras. These tests allowed the testing and assessment of the operation of the DAASS and also the examination of the hygroscopic behavior of particulate matter. The original version was used during a moderate RH period, in which the water content of the aerosol represented 0-50% of the fine aerosol mass. The improved DAASS operated during high RH conditions. The particles retained water throughout the duration of the measurements. The measured volume growth factors were quite higher than the measured ones during the moderate RH period. The water concentrations observations were compared to the predictions of the aerosol thermodynamics model EAIM.
Open Access
Application of a non-linear thermodynamic master equation to three-level quantum systems
(2014-05-16) Αλατάς, Παναγιώτης; Μαυραντζάς, Βλάσης; Μαυραντζάς, Βλάσης; Τερζής, Ανδρέας; Πασπαλάκης, Εμμανουήλ; Alatas, Panagiotis
Τμήμα Χημικών Μηχανικών (ΜΔΕ)
In this Master’s thesis, we have focused on the description of three-level quantum systems through master equations for their density matrix, involving a recently proposed non-linear thermodynamic one. The first part is focused on a three-level system interacting with two heat baths, a hot and a cold one. We investigated the rate of heat flow from the hot to the cold bath through the quantum system, and how the steady-state is approached. Additional calculations here refer to the rate of entropy production and the evolution of all elements of the density matrix of the system from an arbitrary initial state to their equilibrium or steady-state value. The results are compared against those of a linear, Lindblad-type master equation designed so that for a quantum system interacting with only one heat bath, the same final Gibbs steady state is attained. In the second part of this thesis, we focus on the electromagnetically induced transparency (EIT), a phenomenon typically achievable only in atoms with specific energy structures. For a three level system (to which the present study has focused), for example, EIT requires two dipole allowed transitions (the 1-3 and the 2-3) and one forbidden (the 1-2). The phenomenon is observed when a strong laser (termed the control laser) is tuned to the resonant frequency of the upper two levels. Then, as a weak probe laser is scanned in frequency across the other transition, the medium is observed to exhibit both: a) transparency at what was the maximal absorption in the absence of the coupling field, and b) large dispersion effects at the atomic resonance. We discuss the Hamiltonian describing the phenomenon and we present results from two types of master equations: a) an empirically modified Von-Neumann one allowing for decays from each energy state, and b) a typical Lindblad one, with time-dependent operators. In the first case, an analytical solution is possible, which has been confirmed through a direct solution of the full master equation. In the second case, only numerical results can be obtained. We present and compare results from the two master equations for the susceptibility of the system with respect to the probe field, and we discuss them in light also of available experimental data for this very important phenomenon.
Open Access
Atomistic modeling and simulation of the mechanical properties of sPMMA - graphene nanocomposites
(2014-08-26) Σκούντζος, Εμμανουήλ - Θεόδωρος; Μαυραντζάς, Βλάσης; Μαυραντζάς, Βλάσης; Τσαμόπουλος, Ιωάννης; Γαλιώτης, Κωνσταντίνος; Skountzos, Emmanouil - Theodoros
Τμήμα Χημικών Μηχανικών (ΜΔΕ)
Small concentrations of graphene can significantly alter the phase behavior and the mechanical and electrical characteristics of polymeric materials. In this Masters thesis, we present results from a hierarchical simulation methodology that leads to the prediction of the thermodynamic, conformational, structural, dynamic and mechanical properties of polymer nanocomposites. As a model system, we have chosen syndiotactic poly(methyl methacrylate) or sPMMA reinforced with uniformly dispersed graphene sheets. How graphene functionalization affects the elastic constants of the resulting nanocomposite is also examined. The simulation strategy entails three steps: 1) Generation of an initial structure which is subjected to potential energy minimization and detailed molecular dynamics (MD) simulations at T=500K and P=1atm, to obtain well relaxed melt configurations of the nanocomposite and to extract any interested properties. Furthermore, for the sPMMA/graphene nanocomposite: 2) Gradual cooling of selected configurations down to room temperature to obtain a good number of structures representative of its glassy phase, and 3) Molecular mechanics (MM) calculations of its mechanical properties following the method originally proposed by Theodorou and Suter. The MD simulations have been executed with the LAMMPS code using the all-atom DREIDING force-field. By analyzing MD trajectories under constant temperature and pressure, all nanocomposite systems were found to exhibit slower terminal and segmental dynamics than the unfilled ones. The addition of a small fraction of graphene sheets in the polymer matrix led to the enhancement of its elastic constants especially when functionalized graphene sheets were used.
Open Access
Computations of two-phase fluid flows with phase-field models
Βασιλόπουλος, Γιάννης; Τσαμόπουλος, Ιωάννης; Τσαμόπουλος, Ιωάννης; Δημακόπουλος, Ιωάννης; Pieter (Ko) van der Weele, Jacobus; Vasilopoulos, Yannis
Τμήμα Χημικών Μηχανικών (ΜΔΕ)
Two-phase fluid flows arise in a wide variety of industrial and scientific applications. Inherent feature of the two-phase flows is the topological evolution of the interface between the two fluids, which leads to the formation of various flow patterns or regimes that depend strongly on the properties of both fluids, the properties of the interface and the flow rate. Τhe detailed description of a moving interface remains challenging from physical and computational points of view due to the strong impact of surface tension and the discontinuity that arises in the stress and pressure field across the interface. This Master thesis deals with the simulation of prototype two-phase flows via the phase-field model. Throughout the present work, the two fluids are supposed to be Newtonian, incompressible, immiscible and of matched densities. Thus, the Navier-Stokes equations along with the continuity equation, govern the flow of the two fluids, while the Cahn-Hilliard equation is used to describe the evolution of the interface as well as to produce an additional term in the Navier-Stokes equations representing the surface tension force acting only on the interfacial region. Two novel numerical algorithms (NSCH) have been developed for accurate time integration of the governing equations; the first one is used for the simulation of moderate and high Reynolds number flows and the second one for the simulation of low Reynolds number flows. In both cases, the Cahn-Hilliard equation is always treated implicitly, while the modified NS equations are integrated explicitly in the first case and semi-implicitly in the second case. The efficiency of the algorithm is verified on a set of problems. In particular, phase separation processes have been studied, to ensure the validity of the Cahn-Hilliard solver excluding flow effects. Then, the efficiency of the NSCH model is tested by tracking a fluid/fluid interface when it experiences large deformations due to an imposed vortex flow. We have validated the entire algorithm through simulations of two-phase flows and comparison to results found in the literature. Specifically, we studied the interfacial instabilities due to viscosity stratification in a planar Couette flow and the effects of inertia and capillarity on the deformation of liquid drops in simple shear flow.
Open Access
Continuous real-time measurement of the chemical composition of atmospheric particles in Greece using aerosol mass spectrometry
(2014-11-04) Φλώρου, Καλλιόπη; Πανδής, Σπύρος; Κουτσούκος, Πέτρος; Μιχαλόπουλος, Νικόλαος; Florou, Kalliope
Τμήμα Χημικών Μηχανικών (ΜΔΕ)
Atmospheric aerosol is an important component of our atmosphere influencing human health, regional and global atmospheric chemistry and climate. The organic component of submicron aerosol contributes around 50% of its mass and is a complex mixture of tens of thousands of compounds. Real-time aerosol mass spectrometry was the major measurement tool used in this work. The Aerodyne High Resolution Time of Flight Aerosol Mass Spectrometer (HR-ToF-AMS) can quantitatively measure the chemical composition and size distribution of non-refractory submicron aerosol (NR-PM1). The mass spectra provided by the instrument every few minutes contain information about aerosol sources and processes. This thesis uses the HR-ToF-AMS measurements in two areas of Greece to quantify the contributions of organic aerosol sources to the corresponding organic aerosol levels. Local and regional air pollution sources were monitored and characterized in two sites during intensive campaigns. The first campaign took place during the fall of 2011 (September 24 to October 23) in Finokalia, Crete, a remote-background coastal site without any major human activity. The aim of the study was to quantify the extent of oxidation of the organic aerosol (OA) during autumn, a season neither too hot nor cold, with reduced solar radiation in comparison to summer. The second one took place during the winter of 2012 (February 26 to March 5), in the third major city of Greece, Patras. The measurements were conducted in the campus of the Technological Educational Institute of Patras (TEI), in order to quantify the severity of the wintertime air pollution problem in the area and its sources. The contributions of traffic and residential wood burning were the foci of that study. The Finokalia site is isolated and far away from anthropogenic sources of pollution, making it ideal for the study of organic aerosol coming from different directions, usually exposed to high levels of atmospheric oxidants. The fine PM measured during the Finokalia Atmospheric Measurement Experiment (FAME-11) by the AMS and a Multi Angle Absorption Photometer (MAAP) was mostly ammonium sulfate and bisulfate (60%), organic compounds (34%), and BC (5%). The aerosol sampled originated mainly from Turkey during the first days of the study, but also from Athens and Northern Greece during the last days of the campaign. By performing Positive Matrix Factorization (PMF) analysis on the AMS organic spectra for the whole dataset the organic aerosol (OA) composition could be explained by two components: a low volatility factor (LV-OOA) and a semi-volatile one (SV-OOA). Hydrocarbon-like organic aerosol (HOA) was not present, consistent with the lack of strong local sources. The second field campaign took place in the suburbs of the city of Patras, 4 km away from the city center during the winter of 2012. During this 10-day campaign, organics were responsible for 70% during the day and 80% during the evening of the total PM1. The OA mean concentration during that period was approximately 20 μg m-3 and reaching hourly maximum values as high as 85 μg m-3. Sulfate ions and black carbon followed with 10% and 7% of the PM1. PMF analysis of the organic mass spectra of PM1 explained the OA observations with four sources: cooking (COA), traffic (HOA), biomass burning (BBOA), and oxygenated aerosol (OOA), related to secondary formation and long range transport. On average, BBOA represented 58% of the total OM, followed by OOA with 18%, COA and HOA, with the last two contributing of the same percentage (12%).
Open Access
Deformation study and analysis of static and rotating aluminium blades by using magnetoelastic sensors
Σαμουργκανίδης, Γεώργιος; Κουζούδης, Δημήτριος; Μπογοσιάν, Σογομών; Παρασκευά, Χριστάκης; Samourgkanidis, Georgios
Τμήμα Χημικών Μηχανικών (ΜΔΕ)
A key part of this work is the in situ study and analysis of the mechanical deformation of rotating metallic aluminum plates, import disaster on them, using magnetoelastic sensors. The purpose of this study is to detect in real time the deformation through the magnetoelastic sensor, caused by the introduction of disasters and then analyze these to derive the health of the blades as they rotate. The magnetoelastic sensor playing the role of the transmitter which receives the information from the blades and sends it to a coil (magnetic speaker) which having the role of the receiver. The information the magnetoelastic sensor receives, contains the response of the resonance frequencies of the blades when introduced into these mechanical impulse excitation. The significance to this information is that as damages inserted into the blade, the natural frequencies of it are shifted relative to the original blade, in which the material is not deformed.
Open Access
Denitrification of potable water with electrochemical and biological resources
Ζιουβέλου, Αθηνά-Μαρία; Βαγενάς, Δημήτριος; Βαγενάς, Δημήτριος; Μαντζαβίνος, Διονύσιος; Κατσαούνης, Αλέξανδρος; Ziouvelou, Athina-Maria
Τμήμα Χημικών Μηχανικών (ΜΔΕ)
Urbanization, population growth and production needs of food and every day goods, but above all the excessive use of fertilizers in crops and the increasing production of new chemical compounds, result in a significant reduction in available potable water reserves and in their quali-tative degradation. One of the most important problems of surface and groundwater is the pres-ence of nitrates and other nitrogen compounds. The presence of these compounds in potable wa-ter causes undesirable effects on natural systems (such as eutrophication) and in humans (such as methemoglobinemia, blue-baby syndrome, and various forms of cancer) making it essential to develop technologies to effectively remove them. Various treatment methods have been studied until this day, however in recent years research-ers have focused on the electrochemical treatment of potable water and in biological denitrifica-tion systems as they achieve high yields with small operating costs and are environmentally friend-ly. Electrochemical denitrification is a treatment method based on the movement and separation of ions in the solution under the influence of an external electric field. Its arrangement consists of cathode and anode electrodes where anion and cation exchange occur. The process takes place on a small surface of electrodes and there is no use of chemicals before and after the water treat-ment. Depending on the material selected for the anode and cathode electrodes, the process that is performed is either electrochemical reduction or electrocoagulation. Biological denitrification is a treatment method carried out by appropriate cultures of microor-ganisms using NO3- ions as the ultimate electron donor under anaerobic conditions. Hydrogen-otrophic denitrification is a promising method of nitrate removal. H2 is an excellent choice of elec-tron donor due to its pure nature, ease of production by electrolysis in the laboratory, low bio-mass production, as well as the removal of it or its agents is not required from the treated water. In the present study, the electrochemical denitrification of drinking water was studied in a la-boratory scale (electrolytic cell of 0.150 L volume) to remove nitrates. Experiments were conduct-ed under various operating conditions, aiming at maximum nitrate removal performance, and avoiding operational problems. Specifically, the effect of the initial concentration of nitrates (10-100NO3--Nmg/L), current intensity (10-40mA/cm2), electrolyte concentration (0-1gNaCl/L), and the electrodes’ construction material (Al, Cu, S-S, Fe, Brass, Sn, Ti/IrO2, Ti/IrO2-Pt, Ti/RuO2-Pt) were studied. Also, autotrophic hydrogenotrophic denitrification using mixed hydrogenotrophic cultivation was studied in laboratory scale reactors of suspended (1L) and attached (0.25L) biomass growth in batch as well as in continuous operation mode. Various operational parameters such as initial nitrate concentration (10-100NO3--Nmg/L), nutrient addition (Na2PO4, KH2PO4), residence time, and appropriate filling material (zeolite) for the fixed bed reactors have been studied. Also, to reduce the operating costs of the processes, renewable sources of energy from α pho-tovoltaic and a wind turbine were used for the power supply needed.
Open Access
Determination of water effective diffusivity within CNT/PMMA nanocomposite membranes from kinetic Monte Carlo simulations
(2015-05-25) Μερμίγκης, Παναγιώτης; Μαυραντζάς, Βλάσης; Τσαμόπουλος, Ιωάννης; Βογιατζής, Γεώργιος; Mermigkis, Panagiotis
Τμήμα Χημικών Μηχανικών (ΜΔΕ)
Membranes find extensive applications today in numerous processes ranging from gas purification techniques to the treatment of industrial wastewater and the production of clean water because of their potential for better energy utilization and reduced production and equipment costs. A typical example is seawater desalination, where the use of advanced membrane technologies based on nanoporous, semipermeable materials with well controlled pore architectures would be favored over reverse osmosis due to lower operating cost and minimal environmental impact. But for membranes to achieve the desired levels of purification efficiency and effectiveness (they are also often susceptible to fouling and tend to exhibit low chemical resistance) they must possess an array of desired and novel properties such as high tensile strength and a well-defined nanoscale porous structure; the latter could allow the selective transport of (e.g.) water while simultaneously blocking undesired compounds (e.g., organic molecules). A typical such membrane operation is nanofiltration (NF), driven by applying a pressure difference between the two sides of the membrane. In the last decade, a large number of experimental studies have identified carbon nanotubes (CNTs) as a very attractive new class of nanoporous materials for designing nanostructured polymeric membranes characterized by exceptionally selective and permeable nanopores. Unfortunately, contradicting experimental results have often been reported as far as the magnitude of flow enhancement is concerned during water transport through nanometer-wide CNTs embedded in micrometer thick membranes. For example, Holt et al. [Nano Letters, 2004] reported an enhancement factor of 4 to 5 orders of magnitude higher while Majumder et al. [Nature, 2005] found water flows that are 2 to 4 orders of magnitude larger than the predicted ones by macroscopic continuum models. More recent experimental results [Qin et al., Nano Letters, 2011] on individual ultra-long (several micrometers) CNTs with diameter in the range 0.81-1.59 nm reported flow enhancement rates below 1000, thus contradicting for the same diameter the results of the two previous studies. A thorough review of the existing literature [Kannam et al., JCP, 2013] has shown that data for the slip length (which characterizes the flow rate of water in CNTs) are scattered over 5 orders of magnitude for nanotubes of diameter 0.81–10 nm. To help clarify some of the above observations, in this Master’s thesis, we have developed and implemented a coarse-grained method for simulating diffusion of a small molecule (water) within a glassy PMMA membrane containing CNTs which has allowed us to probe significantly longer times than what is possible today by atomistic molecular dynamics (MD) simulations. The method is known as kinetic Monte Carlo, is realized on a lattice, and uses as input data only the transition rates for a water molecule to hop from one lattice site to another. To take into account the nanostructure of the polymeric membrane and the fact that water diffuses much faster within a CNT than within a glassy polymer, lattice sites belonging to PMMA regions of the membrane have been assigned a different rate constant than lattice sites belonging to the interior of a CNT. The two constants have been computed by borrowing data for water diffusivity in the PMMA matrix and in a CNT either from experimental measurements or from independent simulation studies. At T=300K and for CNTs with a diameter D larger than about 2 nm, the rates are equal to 1.3x108 s-1 for PMMA and 2.3x1011 s-1 for CNT. That is, CNT sites correspond to “fast-diffusing” regions while PMMA ones to “slow-diffusing” regions, for a given water molecule. The simulations begin by distributing a large number of ghost water molecules on the sites of the lattice and letting them hop from site to site by using the above predetermined transition rates. In the simulations, hopping from a PMMA site towards a CNT interior site and backwards is forbidden; the only possible way for a walker to enter-exit a CNT is via the CNT entrance region. From the KMC method we compute the mean square displacement (msd) of all walkers as a function of time and then we apply Einstein’s equation to extract the corresponding effective diffusivity Deff quantifying water transport in the entire polymeric membrane given that the diffusive motion of the penetrants is Fickian. We conducted several such KMC runs both for randomly placed and perfectly aligned CNTs in the matrix, and we calculated the dependence of Deff on the size of CNTs (their diameter D and length L) and their concentration C (% vol.) in the PMMA matrix. Our simulation results indicate that CNT orientation does not significantly affect the water effective diffusivity. We also found that Deff varies practically linearly with both the CNT aspect ratio and CNT concentration. This allowed us to come up with a simple linear expression for Deff as a function of C and L/D describing the mobility of water molecules in the membrane. The predictions of this analytical equation are in excellent agreement with the simulation findings.
Open Access
Development and testing of a photochemical model based on the master chemical mechanism for the atmospheric simulation of chambers
Uruci, Petro; Πανδής, Σπυρίδων; Πανδής, Σπυρίδων; Κουτσούκος, Πέτρος; Κούκος, Ιωάννης; Ουρούτσης, Πέτρος
Τμήμα Χημικών Μηχανικών (ΜΔΕ)
This thesis is motivated by the need to develop a photochemical box model that would be a valuable tool for the analysis of the ICE-HT/FORTH smog chamber results but could be also applied to other chambers around the world. The Master Chemical Mechanism (MCM) is a detailed gas phase, near-explicit, mechanism for the photooxidation of 143 primary volatile organic compounds (VOCs) in the atmosphere. This work uses as a case study the toluene oxidation focusing on the oxygenated products of toluene degradation. The reactions of toluene are translated to differential equations (ODEs) and they are integrated with a Rosenbrock solver. This solver proved to be effective for stiff systems, which is a major challenge in the numerical simulation of atmospheric transport-chemistry processes. The toluene model was applied to many different initial conditions. The results show that most of the carbon ends up as CO and CO2 in a matter of hours in typical laboratory experiments. The rest is peroxy acetyl nitrates (PAN), maleic anhydride, glyoxal, and others. The model predictions were evaluated against experimental data found in the literature. Despite some discrepancies, the model seems to be promising if a suitable auxiliary mechanism for each chamber is included. Moreover, the evaluation results confirm that CO and CO2 are indeed major products in these experiments. Finally, a partitioning model combined with the volatility basis set (VBS) is incorporated in the main model in order to predict the particulate mass in the system. Presence of sufficient initial seeds is assumed in the system so homogeneous nucleation is not simulated. The results show that the current model tends to overestimate particulate mass, especially that of organonitrates. Improvements in the estimation of the vapor pressure of these complex organic molecules are needed. The addition of the partitioning model reduced the predicted production of CO and CO2.
Open Access
Dielectric barrier discharge plasma reactor construction and characterization
(2022-11-24) Σταυρογιαννόπουλος, Σταύρος; Stavrogiannopoulos, Stauros
Τμήμα Χημικών Μηχανικών (ΜΔΕ)
The scope of this thesis was the detection, recognition and recording of electrical characteristics of the dielectric barrier discharge in order to define the effect of external parameters on plasma efficiency. The procedure that was used for the definition of these parameters, is synopsized in the construction of a special arrangement that allows the generation of dielectric barrier discharge as well as the recording of data in a computer environment, which will result in their subsequent easy and efficient analysis. In particular, the dielectric barrier discharge was studied at the frequency of 60Hz, in or-der to evaluate and classify the importance of external parameters in plasma production. A complete new method of analysis of the electrical data was developed, the types of interference were presented and eliminated, electrical signals were simulated, characteristic curves of voltage and current electric pulses were derived, the effect of electrodes’ distance was approached, the number and the density of streamers were calculated in relation to the ap-plied power, and the effect of external parameters were studied with respect to the electrical characteristic of the electric pulse. The developed methodology can be a guide for similar measurements in any frequency range, with simple changes to the configuration of the experiment. The generality of the methodology lies in the analytical recording of all individual steps, from the protection of personnel and equipment to all the stages of conducting measurements and analyzing the results.
Open Access
Estimation of dust emissions and their effects on atmospheric pollution over Europe
Κακαβάς, Στυλιανός; Πανδής, Σπυρίδων; Παρασκευά, Χριστάκης; Κορνάρος, Μιχαήλ; Kakavas, Stylianos
Τμήμα Χημικών Μηχανικών (ΜΔΕ)
Simulation of the concentration of dust and its interactions with other inorganic components of particulate matter (nitrate, ammonium, sulfate) remains a major modeling challenge due to the lack of accurate dust emission inventories. In this study the chemical transport model PMCAMx is applied over Europe during the period of May 2008 (EUCAARI campaign). First, the existing EUCAARI emission inventory is used and the model predictions are compared with available measurements of PM10 from 43 stations in Europe. Periods of Saharan dust transport are excluded. The predicted dust levels are too low compared to measurements especially in the urban areas (fractional bias for PM10 equal to − 67%). We test the hypothesis that the error is related to underestimation of road transport dust emissions. The fractional bias decreases after the increase of dust emissions related with on-road transportation by a factor of ten. There is significant improvement of the model performance for PM10 in both southern and northern Europe, but also in urban and rural and remote areas. In the second part of the work the hybrid version of PMCAMx is used with the improved dust emissions, in order to study mineral dust interactions with the rest of the pollutants. The average predicted PM1-10 (particles with diameter between 1 and 10 μm) calcium, magnesium, and potassium concentrations are 0.02, 0.006, and 0.01 μg m-3 respectively over Europe. Average PM1 (particles with diameters less than 1 μm) concentrations of nitrate, ammonium, and sulfate decrease 0.04, 0.06, 0.16 μg m-3, while PM1-10 concentrations increase 0.06, 0.03, 0.03 μg m-3 respectively over Europe due to mineral dust. These results are sensitive to the assumed composition of dust.
Open Access
Experimental study and characterization of magnetoelastic ribbons as vibration sensors and their application for the identification of cracks in cantilever beams through the dynamic behavior of the beam
Σαμουργκανίδης, Γεώργιος; Samourgkanidis, Georgios
Τμήμα Χημικών Μηχανικών (ΜΔΕ)
In the current thesis thin magnetoelastic ribbons of metallic glass alloy known as Metglas 2826MB were investigated, characterized and applied as vibration-based structural health monitoring sensors. Such materials have the property of changing their magnetic state (magnetization) when they are stressed mechanically (Villary effect), and vice versa they are stressed mechanically when they are magnetized by an external magnetic eld (magnetostriction effect). These materials were used in the form of thin ribbons in contact with a mechanical structure, such as a cantilever beam, as a vibration sensor, in order to monitor the structure's mechanically health state. The monitoring was established through the detection of the natural frequencies of the mechanical structure. The study of the thesis is divided into three main parts which are, the "proof of concept" of the work, the characterization procedure and the application process. As far as the first part is concerned, the ability of the ribbons in sensing and transmitting the vibrational state of a cantilever beam was investigated, as well as the accuracy of the recorded data in detecting the change of the vibrational state of the structure due to damages. To carry out this task, a number of different beam specimens, undamaged and damaged, of aluminum alloy 6063 material were used and the results were compared to computational ones using ANSYS modal analysis. The second part was the characterization of the ribbons as structural vibration sensors and the process involved seven different sensor parameters such as the frequency response, linearity, signal to noise ratio (SNR), quality factor, stability, repeatability and sensitivity. The experiment was accomplished using two different experimental setups, one to examine the frequency response parameter and one to examine the rest of the parameters. The last part included the application of the under consideration vibration sensors to detect and identify cracks in cantilever beams, through a proposed crack identi cation methodology. The methodology involved the use of a pattern matching process, through a minimization procedure, in order to identify the crack location and depth. Each one of the three parts was examined in detail and thoroughly, with the results of the experiments being properly presented and described.
Open Access
Hemodynamics in microcirculation : a novel method for the accurate calculation of WSS
Γιαννοκώστας, Κωνσταντίνος; Δημακόπουλος, Ιωάννης; Κουτσούκος, Πέτρος; Τσαμόπουλος, Ιωάννης; Giannokostas, Konstantinos
Τμήμα Χημικών Μηχανικών (ΜΔΕ)
Ο σχηματισμός ενός αμιγούς στρώματος πλάσματος γύρω από τα τοιχώματα του μικροαγγειακού συστήματος καθορίζει σε μεγάλο βαθμό την διατμητική τάση (WSS) που αναπτύσσεται πάνω σε αυτά και είναι ρυθμιστικός παράγοντας για την ανάπτυξη παθογόνων καταστάσεων [1]. Η παρούσα έρευνα εστιάζει στον ακριβή υπολογισμό της διατμητικής τάσης μέσω της μοντελοποίησης της ροής του αίματος σε αρτηρίες εύρους διατομής 20μm-400μm και αιματοκρίτη 0.25-0.55 (in vitro ροή). Για το σκοπό αυτό αναπτύσσεται ένα διφασικό μη-Νευτωνικό μοντέλο οι προβλέψεις του οποίου βρίσκονται σε άριστη συμφωνία με την αιμοδυναμική συμπεριφορά που παρατηρείται πειραματικά και υπεερούν των προβλέψεων με προηγούμενα μοντέλα προσομοίωσης της ροής του αίματος σε αρτηρίες [2]. Η ανάγκη της χρήσης ενός πιο ευπροσάρμοστου μοντέλου οδηγεί στην αντικατάσταση της διφασικής ροής από ένα μονοφασικό μοντέλο πλήρως κατειλημμένο από ερυθρά αιμοσφαίρια στο οποίο επιβάλλεται συνθήκη ολίσθησης στο τοίχωμα καθορισμένη από τα αποτελέσματα της διατμητικής τάσης του αρχικού μοντέλου. Η συνθήκη ολίσθησης που εξαρτάται τόσο από τις γεωμετρικές όσο και από τις εγγενείς αιμορεολογικές παραμέτρους της ροής, συμβάλλει ώστε η αρχικά η υπερεκτιμημένη διατμητική τάση από την πρόβλεψη του μονοφασικού προβλήματος να υποβιβαστεί κατάλληλα ώστε να επαληθεύει την πρόβλεψη της πραγματικής ροής του μικροαγγειακού συστήματος, που σε ορισμένες περιπτώσεις φτάνει μέχρι και 60%. Συμπεράσματα για την αποτελεσματικότητα της συνθήκης εξάγονται από την ροή αίματος σε πιο περίπλοκες γεωμετρίες και συγκεκριμένα σε κάθετες αρτηρίες με διατομή συγκρίσιμή των ερυθρών αιμοσφαιρίων. Η υπόθεση της συμπεριφοράς του αίματος σαν συνεχές μέσο αλλά και η χρήση του κανόνα ολίσθησης που εξήχθηκε από την μονοδιάστατη μελέτη επαληθεύουν πλήρως τα αποτελέσματα που προέρχονται από την ίδια ροή με σωματιδιακή προσέγγιση των ερυθρών αιμοσφαιρίων [3]. Πραγματοποιώντας παραμετρική ανάλυση των αιμορεολογικών ιδιοτήτων της ροής, βρέθηκε ότι η ανάπτυξη της διατμητικής τάσης εξαρτάται αισθητά από την ροή μάζας αλλά και από το ποσοστό της συνολικής παροχής σε κάθε κλάδο ιδιαίτερα για τα σημεία που βρίσκονται κοντά στην διακλάδωση. Η έρευνα επεκτείνεται και στην ροή του αίματος σε μικροαγγεία όπου η ύπαρξη ενός στρώματος γλυκοκάλυκα που καλύπτει τα ενδοθηλιακά κύτταρα επιφέρει αλλαγή στην αιμοδυναμική και την διαμόρφωση της διατμητικής τάσης στο εσωτερικό ενδοθηλιακό τοίχωμα (in vivo ροή). Η παραμετρική ανάλυση του προβλήματος αποκαλύπτει μια αυξημένη επιρροή του γλυκοκάλυκα στην μέιωση της διατμητικής τάσης καθώς επίσης και το όριο όπου οι ροές in vitro και in vivo παρουσιάζουν όμοια αιμοδυναμική συμπεριφορά.
Embargo
In-silico rheometry of endothelial cells under start-up shear experiments
Ψαράκη, Κωνσταντίνα; Psaraki, Konstantina
Τμήμα Χημικών Μηχανικών (ΜΔΕ)
The endothelium, a monolayer of endothelial cells (ECs), constitutes the inner cellular lining of the blood vessels (arteries, veins, and capillaries) and the lymphatic system, and therefore is in direct contact with the blood and the circulating cells. It is now recognized to be a main pillar in the control of blood fluidity, platelet aggregation and vascular tone, a predominant factor in the regulation of immunology, inflammation and angiogenesis, a metabolizing and endocrine organ. Therefore, the response of the endothelium in variations of the hemodynamic environment is of vital importance. This implies that we should quantify fundamental dynamic quantities such as the developing shear stresses, the effect of flowing conditions on Wall Shear Stress (WSS), in addition to recirculation zones, which are indicators of atherosclerosis. Because of the 𝑂(10 μ𝑚) length of the ECs and the invasive nature of hands-on techniques when referring to the human cardiovascular system, such an approach would be difficult to be pursued experimentally. Here, we propose an in-silico rheometric emulation based on start-up and pulsating shear experiments in a representative two-dimensional domain of endothelial monolayer that accounts for the interaction of the blood plasma and the deformable ECs. Moreover, we create a three-dimensional domain representing endothelial cells and nuclei and perform compressive tests in order to investigate the poroelastic nature of the EC’s cytoplasm and retrieve its elasticity depending on the volume fraction of the cytoskeletal network. We present quantitative predictions for the shear and normal stresses on each cell for blood flow under physiological conditions and conclude that the imposition of a uniform, mean stress value above the endothelium does not correspond to true conditions. Finally, we show that wall thinning is slightly more prominent at the locus of high WSS in the range of physiological velocities, but under extreme velocities wall thinning intensely prevails at the locus of flow separation.
Open Access
Investigation of electrokinetic phenomena in polymeric flows
Μοσχόπουλος, Παντελής; Δημακόπουλος, Ιωάννης; Τσαμόπουλος, Ιωάννης; Μαυραντζάς, Βλάσης; Moschopoulos, Pantelis
Τμήμα Χημικών Μηχανικών (ΜΔΕ)
We develop a comprehensive model for the electro-osmotic flow of a polyethylene oxide (PEO) solution in a NaCl aquatic solvent under steady-state conditions in a microchannel. Based on the experimental data of Huang et al. (2016) for microchannels made of poly-dimethylsiloxane (PDMS) with bonded glass interior, we show that (a) the solvent contribution in the viscosity of the fluid, (b) the polymer migration and ionic steric effects and (c) the physicochemical description of the zeta potential are essential features of the model for capturing the complex rheological behavior adequately, whereas neglecting them leads to inaccurate predictions. The formation of a polymer depletion layer along the microchannel wall maintains the polymeric chains away from the area of the highest shear-rates, thus preserving their integrity. This makes the shear-stress at the solvent/solution interface a critical quantity for the characterization of the effect of the flow on the integrity of a polymeric chain and the design of microdevices for PEO transport. The values of this interfacial shear stress are found to be almost 30% lower than estimations of the wall shear stress based on a simplified single-phase formulation, which neglects the formation of the polymer depletion layer. However, the critical stress component is not the shear, but the elongational one at this interface. The latter is found to be an order of magnitude larger than the former (!) and almost 50% lower than the elongational stress at the wall that the single-phase model predicts for the experimental conditions of Huang et al. (2016). Sweeping a wide range of values of the applied potential (340 V/cm - 500 V/cm) and the ionic concentration (0.001 mM – 10 mM), the model shows that they affect the dynamics of the depletion layer in a non-monotonic way. Indicatively, the system can also achieve higher electroosmotic mobilities and lower shear and elongational stresses at the interface of the phases by increasing the bulk ionic concentration.
Open Access
Kinetic study of biomass growth and lipid synthesis of the microalga Chlorella vulgaris under heterotrophic conditions and energy valorization of the produced biomass
Σακαρίκα, Μυρσίνη; Κορνάρος, Μιχάλης; Κορνάρος, Μιχάλης; Μαντζαβίνος, Διονύσης; Παρασκευά, Χριστάκης; Sakarika, Myrsini
Τμήμα Χημικών Μηχανικών (ΜΔΕ)
The present study focuses on the kinetic study of biomass growth and lipid synthesis of the microalga C. vulgaris under heterotrophic conditions in addition to the energy valorization of the produced biomass. The main reason of this strain attracting so much attention is that it constitutes a versatile microorganism. It has the ability to grow fast, in various kinds of media and wastewaters, on extreme conditions such as extreme pH values and temperatures and tolerate the presence of toxic compounds. Moreover, it accumulates intracellular lipids resulting in high lipid content, the composition of which is suitable for biodiesel production. The goal of the first set of experiments was to investigate the pH range that can support the growth of C. vulgaris, and, more specifically, to identify the optimal pH for the microalga’s growth, under heterotrophic conditions. Furthermore, the effect of pH on accumulation of intracellular lipids was studied. A wide range of pH values was tested using buffer solutions. The optimal pH for biomass growth and lipid accumulation under sulfur starvation was found to be 7.5, resulting in the maximum specific growth rate of 0.541 days-1 and the maximum total lipid content of 53.43% g gDW-1. The aim of the second set of experiments was to determine the effect of starvation from different nutrients (S, P and N) on biomass growth and lipid accumulation of C. vulgaris. The potential differences in the effect of nutrient starvation could affect the selection of the most appropriate wastewater mixture for microalgal cultivation, as in many cases, in a realistic application of wastewater treatment using microalgae the mixture of various wastewaters is used as a substrate. The nutrient starvation that had the most significant effect on lipid accumulation was that of sulfur. Moreover, the use of Volatile Fatty Acids as potential carbon sources for C. vulgaris was investigated. VFAs are metabolic products, intermediate or final, of many processes, such as anaerobic digestion, dark fermentation and acidogenesis. The goal was to identify the VFAs that can be used as carbon sources by C. vulgaris, individually or in a combination with glucose, grown without the presence of light. The VFAs tested were acetic, propionic, butyric and isobutyric acid. C. vulgaris failed to grow on propionic, butyric and isobutyric acid as sole carbon sources. However it was able to grow on acetic acid, resulting in a maximum specific growth rate of 0.429 days-1. Moreover, the combination of the VFAs mentioned above in addition to glucose was used as carbon source, simulating the case in which the produced effluent streams of these processes contain VFAs in addition to other organic compounds. The microalga was again unable to assimilate propionic, butyric and isobutyric acid. The maximum specific growth rate obtained from the assimilation of glucose and acetic acid was 0.534 days-1. In the following part of the present study, phenolic compounds were tested as potential carbon sources for the microalga. Before carrying out the experiments with different phenols used individually as sole carbon source, an acclimation stage took place. The phenolic compounds tested as potential carbon sources for C. vulgaris were vanillic, ferulic, syringic, gallic, p-hydroxybenzoic acid and catechol, in a concentration of 0.5 g L-1. Catechol inhibited the growth of the microalga from the second stage of the acclimation process, in a concentration of 100 mg L-1, verifying the highly toxic effect of the specific phenolic compound. C. vulgaris failed to grow on vanillic, ferulic and p-hydroxybenzoic acid as sole carbon sources in a cultivation period of 15 days. However, gallic and syringic acid were degraded by C. vulgaris in the respective experiments, resulting in low μmax (0.063 and 0.103 days-1 respectively). The biotreatment of Olive Mill Wastewater using C. vulgaris was subsequently investigated. OMW is a wastewater with complex composition with the most distinctive features being the high concentration of phenolic compounds, and the low nitrogen content, all present in the form of organic nitrogen, which make OMW a non-easily degradable substrate for microalgae. The microalga was unable to grow on OMW, even after the regulation of pH at the optimal value and the addition of a sulfur and nitrogen source. However, the growth of the microalga in a medium containing 10% v v-1 OMW supplemented with glucose and BG-11 broth showed that it can tolerate the phenols present (in a concentration of approximately 0.5 g L-1), and that the inhibiting factor in OMW is the lack of nutrient bioavailability. The next part of the present study concerns the energy valorization of the produced C. vulgaris biomass. Biodiesel production from microalgal biomass apart from harvesting and drying also requires the additional steps of transesterification and purification. These steps augment the cost of biofuel production, thus making it an economically unviable solution. In an effort to explore the potentials of microalgal biomass exploitation, the energy valorization of C. vulgaris biomass through direct combustion and anaerobic digestion were also investigated. The specific calorific value of C. vulgaris dry biomass was found to be 24,525 ± 182 kJ kg-1 or 5,861 ± 44 kcal kg-1. The methane potential of C. vulgaris biomass was evaluated by conducting BMP assays in mesophilic and thermophilic conditions, with and without pretreatment (ultrasonication). The maximum yield was obtained in mesophilic conditions without pretreatment, presenting the value of 389.07 ml gVS-1added. Finally, by comparing the energy valorization methods, the highest energy productivity (20.331 kJ Lreactor-1 day-1) was obtained from the direct combustion of C. vulgaris biomass. Last but not least, the modeling of the behavior of C. vulgaris in pH 7.5 under sulfur stress took place, resulting in satisfying data fitting and parameter estimation.
Open Access
Linear stability analysis of viscoelastic fluid extrusion through a planar die
Πέττας, Διονύσιος; Τσαμόπουλος, Ιωάννης; Τσαμόπουλος, Ιωάννης; Δημακόπουλος, Ιωάννης; Μαυραντζάς, Βλάσης; Pettas, Dionisios
Τμήμα Χημικών Μηχανικών (ΜΔΕ)
It is well-known that, increasing the flow rate in polymer extrusion, the flow becomes unstable and the smooth extrudate surface becomes wavy and disordered to an increasing degree. In order to investigate the mechanisms responsible for these instabilities we perform a linear stability analysis of the steady extrusion of a viscoelastic fluid flowing through a planar die under creeping flow conditions. We consider the Phan-Thien-Tanner (PTT) model to account for the viscoelasticity of the material. We employ the mixed finite element method combined with an elliptic grid generator to account for the deformable shape of the interface. The generalized eigenvalue problem is solved using Arnoldi’s algorithm. We perform a thorough parametric study in order to determine the effects of all material properties and rheological parameters. We investigate in detail the effect of interfacial tension and the presence of a deformable interface. It is found that the presence of a finite surface tension destabilizes the flow as compared to the case of the stick-slip flow. We recognize two modes which are found to become unstable beyond a critical value of the Weissenberg number and perform an energy analysis to examine the mechanisms responsible for the destabilization of the flow and compare against the mechanisms that have been suggested in the literature.
Open Access
Linking metabolic networks with integrated white biotechnology process design
Μιχαηλίδη, Αικατερίνη; Κούκος, Ιωάννης; Κούκος, Ιωάννης; Βαγενάς, Δημήτριος; Μαντζαβίνος, Διονύσιος; Michailidi, Katerina
Τμήμα Χημικών Μηχανικών (ΜΔΕ)
For the sake of sustainability and environmental considerations, the chemical industry has turned to biomass-based production of fuels and chemical commodities. In view of that, significant research studies have been conducted on microorganisms producing high yields of the desirable end or intermediate products. Succinic acid has been identified by the U.S. Department of Energy as a top 12 target molecule due to the reactivity of the two carboxylic groups leading to versatile end-products and the cost-competitiveness of biotechnological over petrochemical production. Over the past years, there is an increasingly number of studies concerning microorganisms producing succinic acid, some of which are Basfia succiniciproducens, Actinobacillus succinogenes and Mannheimia succiniciproducens. The aforementioned bacteria produce high yields of succinic acid since it is a major metabolic intermediate, as reflected by their names. In recent years, optimization has been employed to predict the distribution of carbon flux in the metabolism of a bacterium. In this thesis, the specifications of each microorganism, the set of reactions implied and the results regarding the polyhedra of the feasible regions provided by metabolic flux analysis are studied. The developed model is employed to support experimental data and its validity is assessed. A mathematical model for designing and optimizing the biotransformation (upstream) section of biotechnological processes is presented. The model has been augmented by equations for the estimation of the equipment cost derived from a recent publication by the US National Renewable Energy Laboratory. The processes of the downstream section for the recovery of succinic acid are then presented, along with a financial analysis of the case study for the biobased production of succinic acid. A key step in the decision making in the design of a fermentation process is to choose an appropriate strain and/or improve it, since the product of interest is primarily determined by the properties of the microorganism. Consequently, a quantitative approach in which basic principles of the biosciences is combined with core disciplines from the engineering fields will indicate the best design of any bioprocess. The novelty of this study is that it incorporates significant details of the bioprocess design and also use is made of a relatively accurate model that simulates the metabolic pathways of microorganisms. Therefore, the model provides a linkage between the metabolism of bacteria and the operation and design of white biotechnology processes.
Open Access
Low temperature plasma enchanced chemical vapor deposition of graphene layers
Σωτηρίου, Νίκη; Αμανατίδης, Λευτέρης; Ματαράς, Δημήτρης; Γαλιώτης, Κωνσταντίνος; Sotiriou, Niki
Τμήμα Χημικών Μηχανικών (ΜΔΕ)
The subject of this master thesis is the synthesis of graphene layers by Plasma Enhanced Chemical Deposition at low temperature, below 300 ˚C, on both nickel and copper foil. The experimental work is mainly focused on the adjustment of the experimental parameters like temperature, pressure, gas mixture ratio, deposition time and power in order to produce graphene layers with low defect density on nickel foil. Raman spectroscopy is used in order to characterize the deposited materials. In order to produce images of the deposited films, scanning electron microscope (SEM) measurements took place and for further investigation of the surface of the deposited graphene layers, Atomic Force Microscope (AFM) measurements where performed on nickel foil before and after deposition. Finally, to further verify the growth of 〖sp〗^2carbon and the amount of oxygen in our samples, XPS experiments were carried out. The experimental results suggest that the deposited films are graphene oxide layers, since C/O ratio is calculated 5,6 by XPS measurements. Moreover, the average thickness of the deposited graphene layers on nickel after the transfer was measured by profilometer to be 200nm for 10min deposition time, indicating the formation of multilayer graphene oxide film. The presence of oxygen in our experiments is attributed to the fact that our experiments were carried out in a low pressure reactor and the base pressure of the system is in the mtorr region. On the other hand, unlike the synthesis of graphene oxide layers on nickel foil, where large area films where synthesized, the density of nucleation on copper foil is quite low and the deposited films are of small lateral dimensions (nanosheets). We attribute the low uniformity of the deposited films on copper to the absence of thermal annealing of the substrate before deposition.