Hellenic Air Force Academy

Athens, Greece

Hellenic Air Force Academy

Athens, Greece

Time filter

Source Type

Rakopoulos D.C.,National Technical University of Athens | Rakopoulos C.D.,National Technical University of Athens | Papagiannakis R.G.,Hellenic Air Force Academy | Kyritsis D.C.,University of Illinois at Urbana - Champaign
Fuel | Year: 2011

An experimental study is conducted to evaluate the effects of using blends of diesel fuel with either ethanol in proportions of 5% and 10% or n-butanol in 8% and 16% (by vol.), on the combustion behavior of a fully-instrumented, six-cylinder, turbocharged and after-cooled, heavy duty, direct injection (DI), 'Mercedes-Benz' engine installed at the authors' laboratory. Combustion chamber and fuel injection pressure diagrams are obtained at two speeds and three loads using a developed, high-speed, data acquisition and processing system. A heat release analysis of the experimentally obtained cylinder pressure diagrams is developed and used. Plots of histories in the combustion chamber of the heat release rate and temperatures reveal some interesting features, which shed light into the combustion mechanism when using these promising bio-fuels that can be derived from biomass (bio-ethanol and bio-butanol). The key results are that with the use of these bio-fuels blends, fuel injection pressure diagrams are very slightly displaced (delayed), ignition delay is increased, maximum cylinder pressures are slightly reduced and cylinder temperatures are reduced during the first part of combustion. These results, combined with the differing physical and chemical properties of the ethanol and n-butanol against those for the diesel fuel, which constitutes the baseline fuel, aid the correct interpretation of the observed engine behavior performance- and emissions-wise. © 2010 Elsevier Ltd. All rights reserved.


The effects of uniaxial tensile strain on the ultimate performance of a dual-gated graphene nanoribbon field-effect transistor (GNR-FET) are studied using a fully analytical model based on effective mass approximation and semiclassical ballistic transport. The model incorporates the effects of edge bond relaxation and third nearest neighbor (3NN) interaction. To calculate the performance metrics of GNR-FETs, analytical expressions are used for the charge density, quantum capacitance, and drain current as functions of both gate and drain voltages. It is found that the current under a fixed bias can change several times with applied uniaxial strain and these changes are strongly related to strain-induced changes in both band gap and effective mass of the GNR. Intrinsic switching delay time, cutoff frequency, and Ion/Ioff ratio are also calculated for various uniaxial strain values. The results indicate that the variation in both cutoff frequency and Ion/Ioff ratio versus applied tensile strain inversely corresponds to that of the band gap and effective mass. Although a significant high frequency and switching performance can be achieved by uniaxial strain engineering, tradeoff issues should be carefully considered. © 2014 Kliros; licensee Springer.


Kliros G.S.,Hellenic Air Force Academy
Proceedings of the International Semiconductor Conference, CAS | Year: 2012

Scaling effects on the gate capacitance of graphene nanoribbon field-effect transistors (GNRFETs) are studied by means of a semi-analytical model. The influence of nanoribbon width, gateinsulator thickness and dielectric constant scaling on the capacitance - voltage characteristics is explored. Gate capacitance has non-monotonic behavior with ripples for thin and high-k gate-insulators. However, beyond the quantum capacitance limit, the ripples are suppressed and smooth monotonic characteristics are obtained. © 2012 IEEE.


Kliros G.S.,Hellenic Air Force Academy
Proceedings of the International Semiconductor Conference, CAS | Year: 2010

We present a simple phenomenological model for the quantum capacitance of bilayer graphene. Quantum capacitance is calculated from the broadened density of states taking into account electron-hole puddles and possible finite lifetime of electronic states through a Gaussian broadening distribution. The obtained results are in agreement with many features recently observed in quantum capacitance measurements on gated bilayer graphene. The temperature dependence of quantum capacitance is also investigated. © 2010 IEEE.


Kliros G.S.,Hellenic Air Force Academy
Proceedings of the International Conference on Microelectronics, ICM | Year: 2010

Gate voltage control of carrier density and quantum capacitance is an important step for understanding the device physics and assessing the performance of nanoscale transistors. In this paper, we present a simple phenomenological model for the carrier density and quantum capacitance of graphene nanoribbon field-effect transistors as functions of gate voltage, Fermi level position and temperature. Quantum capacitance is calculated from the broadened density of states incorporating the presence of electron-hole puddles and possible finite lifetime of electronic states through a Gaussian broadening distribution. Thin gate-insulators of high-κ dielectric constant are used in our calculations in order to approach the quantum capacitance limit. © 2009 IEEE.


Kehayas N.,Hellenic Air Force Academy
Journal of Aircraft | Year: 2011

It has been suggested that the basic configuration of subsonic civil transport aircraft is nearing its full evolutionary potential and a departure in the form of a new configuration or technology is needed. To accomplish this objective, a number of alternative concepts have been proposed, one of which is based on the jet flap. In this paper, the propulsion system of a jet-flapped subsonic civil transport aircraft design is evaluated. The jet engines of this design are embedded in the wings and exhaust through fishtail diffuser ducts, from high-aspect-ratio nozzles located at a small control flap at the trailing edge. The aim is to match jet engine, fishtail duct geometry and jet momentum coefficient requirements. It is found that it is possible to achieve the exceptional lift-to-drag ratios of the jet flap using very high bypass ratio geared turbofans operating at a lower temperature. The resulting jet-flapped design exhibits lift-to-drag ratios of over 60 without any significant effects on specific fuel consumption or weight. The jet-flapped design is then compared with other advanced technology designs and comes first on fuel consumption per seat . km, as well as in other areas of interest such as safety, emissions, and noise. © Copyright 2010.


Kliros G.S.,Hellenic Air Force Academy
Microelectronic Engineering | Year: 2013

The width-dependent performance of armchair GNRs-FETs is investigated by developing a fully analytical gate capacitance model based on effective mass approximation and semiclassical ballistic transport. The model incorporates the effects of edge bond relaxation and third nearest neighbor interaction as well as thermal broadening. To calculate the performance metrics of GNR-FETs, analytical expressions are used for the charge density, quantum capacitance as well as drain current as functions of both gate and drain voltages. Intrinsic gate delay time, cutoff frequency and Ion/Ioff ratio are also calculated for different GNR widths. Numerical results for a double-gate AGNR-FET operating close to quantum capacitance limit show that nanoribbon widths of about 3-4 nm at most are required in order to obtain optimum on/off performance. © 2013 Elsevier B.V. All rights reserved.


Kliros G.S.,Hellenic Air Force Academy
Proceedings of the International Semiconductor Conference, CAS | Year: 2013

We present a simulation study on the current-voltage characteristics of a dual-gated Graphene Nanoribbon Field Effect Transistor (GNR-FET) when its channel is under uniaxial tensile strain. Our study uses a fully analytical model based on effective mass approximation and semiclassical ballistic transport. The model incorporates the effects of edge bond relaxation and third nearest neighbor (3NN) interaction. It is found that the current under a fixed bias can change several times with applied uniaxial strain and these changes are strongly related to strain induced changes in both band gap and effective mass of the GNR. Furthermore, other characteristics as transconductance, gate capacitance and cutoff frequency are also calculated for various strain values. © 2013 IEEE.


In this paper, we investigate the propagation characteristics of thermally diffused expanded core (TEC) fibers with complex refractive index using the numerical Galerkin's method. Complex modal profiles and propagation constants for TEC fibers of different expanded core radii are calculated. The imaginary part of the electric field results in wavefront distortion showing that the power flows out of or into the doped region according to the sign of the imaginary part of the refractive index. The gain or loss as a function of both mode field radius and operating wavelength is calculated numerically. Moreover, an approximate formula for the loss or gain coefficient of these fibers is given and the obtained approximate results agree well with those from Galerkin's method applied to the complex refractive index fiber. © 2010 IOP Publishing Ltd.


Kliros G.S.,Hellenic Air Force Academy
Superlattices and Microstructures | Year: 2012

A semi-analytical model for the capacitance-voltage characteristics of graphene nanoribbon field-effect transistors (GNR-FETs), in the quantum capacitance limit, is presented. The model incorporates the presence of electron-hole puddles induced by local potential fluctuations assuming a Gaussian distribution associated with these puddles. Our numerical results show that the multi-peaks in the non-monotonic quantum capacitance-voltage characteristics are broadened as the potential fluctuation strength increases and the broadening effect is much more pronounced in wide GNRs. The influence of both gate-insulator thickness and dielectric constant scaling on the total gate-capacitance characteristics is also explored. Gate capacitance has non-monotonic behavior with ripples for thin gate-insulators. However, as we go beyond the quantum capacitance limit by increasing insulator thickness or decreasing dielectric constant, the ripples are suppressed and smooth monotonic characteristics are obtained. © 2012 Elsevier Ltd. All rights reserved.

Loading Hellenic Air Force Academy collaborators
Loading Hellenic Air Force Academy collaborators