Dynamics of acoustically forced non-premixed flames close to blow-off

The effect of forced oscillations on the behaviour of non-premixed swirling methane flames close to the lean blow out limits was investigated using experiments in a lab-scale burner. Two different fuel injection geometries, non-premixed with radial -NPR- and non-premixed with axial -NPA- fuel injection, are considered. The flame behaviour is studied using 5 kHz OH* chemiluminescence and OH Planar Laser Induced Fluorescence (OH PLIF) imaging. In both systems, acoustic forcing reduces the stability of the flame, and in particular, the stability was found to decrease with the increase in forcing amplitude. Flame lift-off was observed in both configurations, with the magnitude of the effect of forcing depending on the fuel injection configuration. The results provide insight on the effect of superimposed flow field fluctuations in systems operating close to the lean blow out limits and offer useful data for the development and validation of numerical models for the prediction of the dynamic behaviour of flames of industrial interest

First application of hemi-body electron beam irradiation for Kaposi sarcoma at the lower extremities

Kaposi’s sarcoma (KS) is a systemic neoplastic disease that can present cutaneous symptoms and is usually treated with a systematic approach due to its extent. Due to its radiosensitivity, radiotherapy is considered one of its main treatments, for palliation and local control of the skin and mucosal lesions. The aim of this paper was to report the first case of KS treated by hemi-body electron irradiation protocol in Greece. A fractionated 40 Gy hemi-body electron irradiation was prescribed to a 60-year-old male patient with KS at his legs. Dose uniformity was verified on a daily basis by thermo luminescence dosimetry (TLD). The treatment resulted to complete clinical response. Limited irradiation-derived side effects appeared. This is the first case ever to be treated with hemi-body electron irradiation protocol in Greece. To the best of our knowledge, this is also the first time that a single field hemi-body electron beam irradiation at a total skin electron beam (TSEB)-like configuration is reported to be used for KS

EXPERIMENTAL INVESTIGATION OF THE RESPONSE OF FLAMES WITH DIFFERENT DEGREES OF PREMIXEDNESS TO ACOUSTIC OSCILLATIONS

This thesis describes an experimental investigation of the response of lean turbulent swirling flames with different degrees of premixedness (i.e. different mixture patterns) to acoustic forcing using the same burner configuration and varying only the fuel injection strategy. Special emphasis was placed on the amplitude dependence of their response. Also, the behaviour of self-excited fully premixed flames was examined. kHz OH* chemiluminescence was used to study qualitatively the heat release response of the flames, while kHz OH Planar Laser Induced Fluorescence (PLIF) was employed to understand the response of the flame structure and the behaviour of the various parts of the flame. The Proper Orthogonal Decomposition (POD) method was used to extract the dominant structures of the flame and their periodicity. In the first part of the thesis, self-excited oscillations were induced by extending the length of the duct downstream of the bluff body. It was found that the longer the duct length and the higher the equivalence ratio, the stronger the self-excited oscillations were, with the effect of duct length being much stronger. The dominant frequencies of the system were found to increase with equivalence ratio and bulk velocity and decrease with duct length. For some conditions, three simultaneous periodic motions were observed, where the third motion oscillated at a frequency equal to the difference of the other two frequencies. A novel application of the POD method was proposed to estimate the convection velocity from the most dominant reaction zone structures detected by OH* chemiluminescence imaging. For a range of conditions, the convection velocity was found to be in the range of 1.4-1.7 bulk flow velocities at the inlet of the combustor. In the second part, the response of fully premixed, non-premixed with radial fuel injection (NPR) and axial fuel injection (NPA) flames was investigated and compared. All systems exhibited a nonlinear response to acoustic forcing. The highest response was observed by the NPR flame, followed by the fully premixed and the non-premixed with axial fuel injection flame. The proximity of forced flames to blow-off was found to be critical in their heat release response, as close to blow-off the flame response was significantly lower than that farther from blow-off. In the NPR and NPA systems, it was shown that the acoustic forcing reduced the stability of the flame and the stability decreased with the increase in forcing amplitude. In the fully premixed system, the flame area modulations constituted an important mechanism of the system, while in the NPR system both flame area and equivalence ratio modulations were important mechanisms of the heat release modulations. The quantification of the local response of the various parts of the flame at the forcing frequency showed that the ratio (OH fluctuation at 160 Hz to the total variance of OH) was greater in the inner shear layer region than in the other parts in the case of NPR and NPA flames. In fully premixed flames, greater values were observed in large regions on the downstream side of the flame than those in the ISL region close to the bluff body. The ratio of the convection velocity to the bulk velocity was estimated to be 0.54 for the NPR flame, while it was found to be unity for the respective fully premixed flame. In the last part of the thesis, the response of ethanol spray flames to acoustic oscillations was investigated. The nonlinear response was very low, which was reduced closer to blow-off. The ratio was the highest in the spray outer cone region, downstream of the annular air passage, while values were very low in the inner cone region, downstream of the bluff body. Unlike NPR and fully premixed flames, in case of spray and NPA systems, it was found that forcing did not affect greatly the flame structure. The understanding of the nonlinear response of flames with different degrees of premixedness in a configuration relevant to industrial systems contributes to the development of reliable flame response models and lean-burn devices, because the degree of premixedness affects greatly the flame response. Also, the understanding of the behaviour of forced spray flames is of great interest for industrial applications, contributing to the development of thermoacoustic models for liquid fuelled combustors. Finally, the estimation of the convection velocity is of importance in the modelling of self-excited flames and flame response models, since the convection velocity affects the flame response significantly

Metal assemblages in Late Bronze Age Greek mainland funerary contexts: an investigation for their identification as functional metal hoards

Bronze Age metal hoards in Europe have been extensively studied with a focus on their intended purpose and the reason/s behind their survival in the archaeological record, but definite answers cannot be given. A similar issue is also encountered in the Late Bronze Age Aegean more specifically and debate exists whether the increase in metal hoarding in the final centuries of the Late Bronze Age in the Aegean is the result of a copper/bronze shortage or abundance. Both in the Aegean and wider Europe, most scholars studying metal hoards are focusing on those from non-funerary contexts, dismissing the idea that metal assemblages in tombs interpreted as grave goods may also be hoards and which may have even been intended to be retrieved in the face of poverty and/or a metal shortage. This possibility has only been briefly looked at by Greek scholars and archaeologists and so the purpose of this thesis is to study further this possibility using the case studies of the Late Bronze Age tholos tombs at Kokla and Nichoria and the chamber tombs 2, 7 and 10 at Dendra in the Peloponnese. The research questions that are answered in this project are: can the content and context of selected non-funerary metal hoards provide any clues for their identification? Are there content similarities between funerary metal assemblages and non-funerary metal hoards? Could the funerary metal assemblages be retrieved from the tomb and put back into circulation? Can a metal shortage be responsible for the increase in metal hoarding on the late LBA Greek mainland? Through the study of metal assemblages from these tombs that are not clearly associated with any burials and their comparison to selected non-funerary hoards, also from the Late Bronze Age Greek mainland, this thesis shows that there are significant similarities between the two in the content and to some extent even the context. It concludes that these funerary metal assemblages can reasonably be considered as retrievable hoards to be used in times of need. The availability of scrap metal, the good-quality bronzework and the Linear B tablets from Pylos are argued to indicate that there was not a copper/bronze shortage on the mainland in the final centuries of the Late Bronze Age. Therefore, this research also concludes that the rise in metal hoarding during that time probably does not relate to an actual copper/bronze shortage

Proper Orthogonal Decomposition Analysis of a Turbulent Swirling Self-Excited Premixed Flame

Thermoacoustic oscillations constitute a serious threat to the integrity of combustion systems. The goal of the present work is to determine the effect of the equivalence ratio (φ), inlet flow velocity (U), and burner geometry on the characteristics of the self-excited oscillations and to reveal the dominant mechanisms. It also focuses on the data post-processing aiming at extracting information about the dynamics that are not captured through classical ensemble-averaging, and hence the Proper Orthogonal Decomposition technique is used. Experiments were conducted with a fully-premixed air/methane flame stabilized on a conical bluff body. Self-excited acoustic instabilities were induced by extending the length of the combustion chamber downstream of the bluff body. The flame was visualised using OH* chemiluminescence and OH PLIF at 5 kHz. Proper Orthogonal Decomposition (POD) and Fast Fourier Transform analysis were conducted on the imaging data. A strong effect of the chamber length was found, which primarily drove the generation of acoustic oscillation and flame-vortex interaction. Significant differences in the flame roll-up were found when either the burner geometry or the equivalence ratio was altered. Changes were detected in the frequency of oscillations, which showed a general trend to increase with φ and U and decrease with the length of the duct. Analysis of the POD modes allowed an estimate of the convection speed of the flame structures associated with the dominant frequency and it was found that this convection speed was about 1.5 U for most conditions studied

Χρονικές συστάσεις με βραχυπρόθεσμες και μακροπρόθεσμες προτιμήσεις

Ο χρόνος είναι ένας σημαντικός παράγοντας όταν γίνονται προτάσεις / συστάσεις και η αποτύπωση με ακρίβεια των προτιμήσεων των χρηστών στην πάροδο του χρόνου είναι μια μεγάλη πρακτική πρόκληση για τα συστήματα συστάσεων (recommender systems). Οι αλγόριθμοι συνεργατικού φιλτραρίσματος (Collaborative Filtering algorithms), που χρησιμοποιούνται σε συστήματα προτάσεων στο διαδίκτυο, συχνά αξιολογούνται όσον αφορά την ακρίβεια των προβλέψεων για την βαθμολογία του χρήστη σε δεδομένη στιγμή και πολλές από τις σημερινές τεχνικές αξιολόγησης αγνοούν το γεγονός ότι οι χρήστες συνεχίζουν να αξιολογούν τα αντικείμενα με την πάροδο του χρόνου, και να αλλάζουν τις προτιμήσεις τους λόγω διαφορετικών εξωτερικών γεγονότων. Η συμπεριφορά των χρηστών μπορεί συχνά να προσδιοριστεί από τις μακροπρόθεσμες και βραχυπρόθεσμες προτιμήσεις. Για την αντιμετώπιση αυτών των προκλήσεων, η πρώτη μέθοδος που ακολουθήσαμε ήταν χρονικός γράφος με βάση χρονικές περιόδους (Session-based Temporal Graph - STG), που μοντελοποιεί ταυτόχρονα τις μακροπρόθεσμες και βραχυπρόθεσμες προτιμήσεις των χρηστών με την πάροδο του χρόνου. Με βάση αυτόν τον γράφο, χρησιμοποιήσαμε τον αλγόριθμο για προτάσεις / συστάσεις Injected Preference Fusion (IPF). Για το δεύτερο μέρος, ακολουθήσαμε μια διαφορετική προσέγγιση με το collaborative Filtering, χρησιμοποιώντας Ανάλυση Κύριων Συνιστωσών (Principal Component Analysis - PCA) και ιεραρχική ομαδοποίηση (hierarchical clustering), για ομαδοποίηση παρόμοιων χρηστών και την δημιουργία συστάσεων με βάση τον χρήστη. Τέλος, αξιολογούμε την αποτελεσματικότητα των μεθόδων χρησιμοποιώντας την βάση δεδομένων του Yelp. Βάση των κριτικών (reviews) φτιάχνονται συστάσεις (recommendations) και αποδεικνύεται ότι η μέθοδος STG εμφανίζει πιο ακριβή αποτελέσματα από την PCA.Time is an important factor when making recommendations and accurately capturing user preferences over time is a great practical challenge in recommender systems. Collaborative Filtering (CF) algorithms, used to build web-based recommender systems, are often evaluated in terms of how accurately they predict user ratings and many current evaluation techniques disregard the fact that users continue to rate items over time and change their preferences due to different external events. User behavior can often be determined by individual’s long-term and short-term preferences. To address these challenges, the first method we used was the session-based Temporal Graph (STG) which simultaneously models users’ long-term and short-term preferences over time. Based on the STG model framework, we used the recommendation algorithm Injected Preference Fusion (IPF). For the second part we tried a different approach with the collaborative filtering, by using Principal Component Analysis (PCA) and hierarchical clustering, to group similar users and making user-based recommendations. Finally, we evaluate the effectiveness of the methods using Yelp dataset. Based on business reviews and making recommendations, we prove that the STG method presents more accurate results than the PCA method

Response of flames with different degrees of premixedness to acoustic oscillations

The response of three flames with different degrees of premixedness (fully premixed, non-premixed with radial, and non-premixed with axial fuel injection) to acoustic oscillations is studied experimentally. The flames were imaged using OH* chemiluminescence and OH Planar Laser Induced Fluorescence (OH PLIF) at 5 kHz. In addition to a flame kinematics analysis, the amplitude dependence of the transfer function was calculated. The dominant spatial structures of the heat release and their periodicity were examined using the Proper Orthogonal Decomposition (POD) method. The non-premixed system with radial fuel injection showed the highest response to acoustic forcing, followed by the fully premixed and the non-premixed system with axial fuel injection. In addition, the response of the non-premixed system with radial fuel injection was greater than that of the fully premixed system for various bulk velocities U, global φ, forcing amplitudes, A and forcing frequencies, f. About 70% of the energy of the total fluctuations in the NPR case is contained in the first four POD modes, a percentage that decreases with overall equivalence ratio, but only this drops to about 40% for the NPA flame. The frequency spectra of the coefficients of the POD modes have peaks at the forcing frequency, with increasing broadband contributions in higher modes and for the NPA flame