Τμήμα Μηχανολόγων και Αεροναυπηγών Μηχαν. (Δημοσ. Π.Π. σε περιοδικά)

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    Thermal conductivity of carbon nanoreinforced epoxy composites
    Κωσταγιαννακοπούλου, Χριστίνα; Φιαμέγκου, Ελένη; Σωτηριάδης, Γιώργος; Κωστόπουλος, Βασίλης; Kostagiannakopoulou, Christina; Fiamegkou, Eleni; Sotiriadis, George; Kostopoulos, Vassilis
    The present study attempts to investigate the influence of multiwalled carbon nanotubes (MWCNTs) and graphite nanoplatelets (GNPs) on thermal conductivity (TC) of nanoreinforced polymers and nanomodified carbon fiber epoxy composites (CFRPs). Loading levels from 1 to 3%wt. of MWCNTs and from1 to 15%wt. of GNPs were used.The results indicate that TC of nanofilled epoxy composites increased with the increase of GNP content. Quantitatively, 176% and 48% increase of TC were achieved in nanoreinforced polymers and nanomodified CFRPs, respectively, with the addition of 15% wt. GNPs into the epoxy matrix. Finally, micromechanical models were applied in order to predict analytically the TC of polymers and CFRPs. Lewis Nielsen model with optimized parameters provides results very close to the experimental ones in the case of polymers. As far as the composites are concerned, the Hashin and Clayton models proved to be sufficiently accurate for the prediction at lower filler contents.
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    Open Access
    Assessing the compression after impact behaviour of innovative multifunctional composites
    Παντελάκης, Σπύρος; Κατσιρόπουλος, Χρήστος; Πολυδωροπούλου, Παναγιώτα; Pantelakis, Spyros; Katsiropoulos, Christos; Polydoropoulou, Panagiota
    In the present work, the synergistic effect of multiwalled carbon nanotubes and glycidyl polyhedral oligomeric silsesquioxanes on the compression after impact behaviour of multifunctional carbon fibre–reinforced epoxy composite plates is investigated. For the qualitative evaluation of the damage accumulation after impact, non-destructive ultrasonic C-scan tests were performed. The impact-induced material degradation was correlated with data obtained from C-scan graphs through applying the concept of the damage severity factor. The test results obtained from C-scan analysis have shown a significant increase of the damaged area after the impact tests, as compared to the unfilled material considered as ‘reference’. A reduced compression after impact strength is observed for the enhanced material as compared to the reference material. To identify the type of damage, optical microscope, scanning electron microscope and energy dispersive spectroscopy analyses were made after the tests. The optical microscope analysis has shown more extended cracking and delaminations for the enhanced material. Scanning electron microscopic analysis has revealed the presence of carbon nanotube agglomerates and possible glycidyl polyhedral oligomeric silsesquioxanes aggregates which might be the cause for a degraded compression after impact behaviour of the multifunctional composites.
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    Open Access
    Case studies on the effect of the air drying conditions on the convective drying of quinces
    (2014-07-17) Τζεμπελίκος, Δημήτριος; Βούρος, Αλέξανδρος; Μπαρδάκας, Αχιλλέας; Φιλιός, Ανδρόνικος; Μάργαρης, Διονύσιος; Tzempelikos, Dimitrios; Vouros, Alexandros; Bardakas, Achilleas; Filios, Andronikos; Margaris, Dionissios
    The objective of the current study is to examine experimentally the thin-layer drying behaviour of quince slices as a function of drying conditions. In a laboratory thermal convective dryer experiments were conducted at air temperatures of 40, 50 and 60 oC and average air velocities of 1, 2 and 3 ms-1. Increasing temperature and velocity resulted to a decrease of the total time of drying. The experimental data in terms of moisture ratio were fitted with three state-of-the-art thin-layer drying models. In the ranges measured, the values of the effective moisture diffusivity (Deff) were obtained between 2.67 x 10-10 and 8.17 x 10-10 m2s-1. The activation energy (Eα) varied between 36.99 and 42.59 kJmol-1.
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    Open Access
    Design and implementation of a three axis digitally controlled traverse system for flow surveys in a drying chamber
    (2014-07-17) Μπαρδάκας, Αχιλλέας; Χασιώτης, Βασίλειος; Τζεμπελίκος, Δημήτριος; Φίλιος, Ανδρόνικος; Bardakas, Achilleas; Chasiotis, Vasileios; Tzempelikos, Dimitrios; Filios, Andronikos
    The current paper describes the design and development of a pc-controlled three axis traverse system that aims to serve extensive velocity and temperature measurements into the drying chamber of a laboratory scale convective dryer. The traversing gear design has been conducted based on the convective dryer’s specific experimental requirements and limitations. The fabricated traversing gear uses a trapezoidal lead screw-nut assembly supported by linear ball bearings, coupled with bipolar stepper motors for positioning in three dimensional spaces. A custom drive system solution was selected for achieving accurate motor control, paired with computer software developed using the LabVIEW® graphical programming environment. The functionality of the traversing system was experimentally verified by testing mechanical and electronic assemblies, but also by determining the per axis positioning error.
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    Open Access
    Design, construction and evaluation of a new laboratory convective dryer using CFD
    (2013-12-10) Tzempelikos, Dimitrios; Vouros, Alexandros; Bardakas, Achilleas; Filios, Andronikos; Margaris, Dionissios; Τζεμπελίκος, Δημήτριος; Βούρος, Αλέξανδρος; Μπαρδάκας, Αχιλλέας; Φιλιός, Ανδρόνικος; Μάργαρης, Διονύσιος
    In order to overcome the lack of experimental data in the open literature and the necessity to validate numerical models, as well as increase the efficiency of the drying process, a new laboratory convective (LC) dryer has been designed, constructed and equipped with an integrated measurement and automated control instrumentation. The main sections of the LC dryer, which can be arranged for operation in a closed or open circuit mode through manually controlled dumpers, are the vertical flow drying chamber,the tube heat exchanger, the thermal boiler and finally the fan - motor with a smooth speed control unit. The experimental facility tested and monitored the moisture content removal of horticultural and agricultural products. The current paper outlines the methodology applied for the design and optimization of the LC dryer, which has been achieved through the analysis of the flow field by means of computational fluid dynamics (CFD). The prediction of the 3d flow problem was accomplished through the solution of the steady-state incompressible, Reynolds-Averaged Navier-Stokes (RANS) equations with the incorporation of the standard k-ε turbulence model. The measurement and control instrumentation with the inclusion of the innovative, pc-controlled, 3d traverse system that serves detailed surveys of the temperature and velocity inside the drying chamber, are also discussed. The performance test and evaluation of the LC dryer was conducted using quince slices as a test material at an average temperature of 60oC and air at 2 m/s into the drying chamber.