Σχεδιασμός & πειραματική μελέτη διάταξης «αεροκουρτίνας» για προστασία αμπερομετρικού αισθητήρα ΝΟΧ από επικαθίσεις αιθάλης

104 σ.Η παρούσα διπλωματική εργασία αναφέρεται στο σχεδιασμό και την πειραματική μελέτη μιας διάταξης κουρτίνας αέρα που θα προστατεύει έναν αμπερομετρικό αισθητήρα NOx, αρχικά σχεδιασμένα για εφαρμογές στην αυτοκινητοβιομηχανία, από επικαθίσεις αιθάλης. Ο αισθητήρας θα προστατεύεται μέσω της ροής πεπιεσμένου αέρα όταν δεν καταγράφει μετρήσεις ώστε να είναι μόνιμα εγκατεστημένος στον αγωγό καυσαερίων του κινητήρα. Δοκιμάστηκαν δύο τύποι αυτής της διάταξης που αποτελούνταν από ένα ομοίωμα αισθητήρα και ένα ακροφύσιο αέρα , για διαφορετικές τιμές της πίεσης του αέρα. Ο ρυθμός επικάθισης της αιθάλης αξιολογήθηκε μέσω τεχνικών επεξεργασίας των εικόνων των αισθητήρων, προκειμένου να καθοριστεί η πλέον αποτελεσματική διάταξη. Στη συνέχεια δοκιμάστηκε η τελική διάταξη που αποτελείται από τον πραγματικό αισθητήρα NOx και το επιλεγμένο ακροφύσιο αέρα. Οι μετρήσεις του αισθητήρων συγκρίθηκαν με ταυτόχρονες μετρήσεις από αναλυτή τύπου χημικής φωταύγειας, που πιστοποιείται από το Διεθνή Ναυτιλιακό Οργανισμό. Επιπλέον, οι μετρήσεις NOx από τον αισθητήρα καταγράφηκαν με τη συσκευή κουρτίνας αέρα σε λειτουργία, προκειμένου να διερευνηθούν οι επιπτώσεις της λειτουργίας της διάταξης στην ακρίβεια των μετρήσεων του αισθητήρα.This thesis refers to the design and experimental study of an air curtain arrangement that would protect an amperometric NOX sensor, initially designed for automotive applications, from excessive soot loading. The sensor is purposed to be permanently installed in the engine exhaust duct and it would be protected by the air curtain flow when it does not record measurements. Two types of this arrangement which consisted of a dummy sensor and an air nozzle were tested, for different values of air pressure. The amount of soot deposition was evaluated through processing techniques of dummy sensors’ images, in order to define the most effective arrangement. The final structure which consists of the actual NOX sensor and the selected air nozzle was tested. The sensor readings were compared to simultaneous measurements of a CLD type analyzer, specified by IMO. Moreover, NOX measurements by the sensor were recorded with the air curtain device switched on, in order to investigate the impact of the arrangement’s operation on the sensor’s output.Λάμπρος Δ. Ρίζο

Expected spectral characteristics of (101955) Bennu and (162173) Ryugu, targets of the OSIRIS-REx and Hayabusa2 missions

NASA's OSIRIS-REx and JAXA's Hayabusa2 sample-return missions are currently on their way to encounter primitive near-Earth asteroids (101955) Bennu and (162173) Ryugu, respectively. Spectral and dynamical evidence indicates that these near-Earth asteroids originated in the inner part of the main belt. There are several primitive collisional families in this region, and both these asteroids are most likely to have originated in the Polana-Eulalia family complex. We present the expected spectral characteristics of both targets based on our studies of our primitive collisional families in the inner belt: Polana-Eulalia, Erigone, Sulamitis, and Clarissa. Observations were obtained in the framework of our PRIMitive Asteroids Spectroscopic Survey (PRIMASS). Our results are especially relevant to the planning and interpretation of in-situ images and spectra to be obtained by the two spacecraft during the encounters with their targets.Comment: 22 pages, 11 figures. Accepted for publication in Icarus on May 11, 201

Gauge Unification and Quark Masses in a Pati-Salam Model from Branes

We investigate the phase space of parameters in the Pati-Salam model derived in the context of D-branes scenarios, requiring low energy string scale. We find that a non-supersymmetric version complies with a string scale as low as 10 TeV, while in the supersymmetric version the string scale raises up to ~2 x 10^7 TeV. The limited energy region for RGE running demands a large tan(beta) in order to have experimentally acceptable masses for the top and bottom quarks.Comment: 11 pages, LaTeX, 7 figures include

Conserved cosmological structures in the one-loop superstring effective action

A generic form of low-energy effective action of superstring theories with one-loop quantum correction is well known. Based on this action we derive the complete perturbation equations and general analytic solutions in the cosmological spacetime. Using the solutions we identify conserved quantities characterizing the perturbations: the amplitude of gravitational wave and the perturbed three-space curvature in the uniform-field gauge both in the large-scale limit, and the angular-momentum of rotational perturbation are conserved independently of changing gravity sector. Implications for calculating perturbation spectra generated in the inflation era based on the string action are presented.Comment: 5 pages, no figure, To appear in Phys. Rev.

Suppressing dimension-5 operators in general SU(5) models

We discuss dimension-5 operators in supersymmetric models containing extra hypercharge 1/3 color-triplets. We derive a general formula relating dimension-5 operator to the color-triplet mass matrix. We show that certain zeros in the triplet mass-matrix together with some triplet coupling selection rules can lead to elimination of dimension-5 operators. In particular we focus on SU(5) models and show that (a) Dimension-5 operators can be eliminated in the standard SU(5) model by the introduction of an extra pair of 5+5b Higgses with specific couplings (b) Flipped SU(5) models with extra 10+10b Higgses are free of dimension-5 operators (c) Flipped SU(5) models with extra 5+5b and/or extra 10+10b Higgses can be made free of dimension-5 operators for a textured form of the triplet mass-matrix accompanied by constraints on the 5-plet couplings to matter. Our analysis is motivated by the recently put forward M-theory phenomenological framework that requires a strong string coupling and reintroduces the problem of eliminating dimension-5 operators.Comment: 10 pages, Latex2e, minor changes, references adde

Stringy Toda Cosmologies

We discuss a particular stringy modular cosmology with two axion fields in seven space-time dimensions, decomposable as a time and two flat three-spaces. The effective equations of motion for the problem are those of the $SU(3)$ Toda molecule, and hence are integrable. We write down the solutions, and show that all of them are singular. They can be thought of as a generalization of the Pre-Big-Bang cosmology with excited internal degrees of freedom, and still suffering from the graceful exit problem. Some of the solutions however show a rather unexpected property: some of their spatial sections shrink to a point in spite of winding modes wrapped around them. We also comment how more general, anisotropic, solutions, with fewer Killing symmetries can be obtained with the help of STU dualities.Comment: 22 pages, latex, no figures. Several minor typos correcte

Dilatonic Black Holes in Higher Curvature String Gravity

We give analytical arguments and demonstrate numerically the existence of black hole solutions of the $4D$ Effective Superstring Action in the presence of Gauss-Bonnet quadratic curvature terms. The solutions possess non-trivial dilaton hair. The hair, however, is of ``secondary" type", in the sense that the dilaton charge is expressed in terms of the black hole mass. Our solutions are not covered by the assumptions of existing proofs of the ``no-hair" theorem. We also find some alternative solutions with singular metric behaviour, but finite energy. The absence of naked singularities in this system is pointed out.Comment: 22 pages, Latex file, 7 Latex figures already include

Fully Anisotropic String Cosmologies, Maxwell Fields and Primordial Shear

We present a class of exact cosmological solutions of the low energy string effective action in the presence of a homogeneous magnetic fields. We discuss the physical properties of the obtained (fully anisotropic) cosmologies paying particular attention to their vacuum limit and to the possible isotropization mechanisms. We argue that quadratic curvature corrections are able to isotropize fully anisotropic solutions whose scale factors describe accelerated expansion. Moreover, the degree of isotropization grows with the duration of the string phase. We follow the fate of the shear parameter in a decelerated phase where, dilaton, magnetic fields and radiation fluid are simultaneously present. In the absence of any magnetic field a long string phase immediately followed by radiation is able to erase large anisotropies. Conversely, if a short string phase is followed by a long dilaton dominated phase the anisotropies can be present, in principle, also at later times. The presence of magnetic seeds after the end of the string phase can induce further anisotropies which can be studied within the formalism reported in this paper.Comment: 19 pages in Revtex style, 14 Encapsulated figure

Nature of singularities in anisotropic string cosmology

We study nature of singularities in anisotropic string-inspired cosmological models in the presence of a Gauss-Bonnet term. We analyze two string gravity models-- dilaton-driven and modulus-driven cases-- in the Bianchi type-I background without an axion field. In both scenarios singularities can be classified in two ways- the determinant singularity where the main determinant of the system vanishes and the ordinary singularity where at least one of the anisotropic expansion rates of the Universe diverges. In the dilaton case, either of these singularities inevitably appears during the evolution of the system. In the modulus case, nonsingular cosmological solutions exist both in asymptotic past and future with determinant $D=+\infty$ and D=2, respectively. In both scenarios nonsingular trajectories in either future or past typically meet the determinant singularity in past/future when the solutions are singular, apart from the exceptional case where the sign of the time-derivative of dilaton is negative. This implies that the determinant singularity may play a crucial role to lead to singular solutions in an anisotropic background.Comment: 21 pages, 8 figure