Time filter

Source Type

Piraeus, Greece

The Hellenic Naval Academy is a military academy with university status and has the responsibility to educate and suitably train competent Naval Officers for the Hellenic Navy. Its full name is Hellenic Naval Cadets Academy and was founded in 1845. The academy is one of the oldest educational institutions in Greece.The academy educates Deck and Engineering Naval cadets. It may also educate Supply Officer cadets as well as Coast Guard Officer cadets. Foreign nationals are also accepted to study as naval cadets in the academy. Wikipedia.

Kosmadakis G.M.,National Technical University of Athens | Pariotis E.G.,Hellenic Naval Academy | Rakopoulos C.D.,National Technical University of Athens
International Journal of Hydrogen Energy | Year: 2013

The present work investigates the effect of heat and mass transfer on the combustion process of a hydrogen-fueled spark-ignition engine, using an in-house CFD code. The main scope is to compare the calculated local heat fluxes with the available measured ones, using three heat transfer models of increasing complexity (two existing and one developed by the authors). Moreover, the effect of mass transfer through the crevice regions is also investigated using a phenomenological crevice model. The calculated results (cylinder pressure traces, local heat fluxes and NO exhaust emissions) are compared with the corresponding measured data, at various operating conditions, maintaining constant engine speed and altering the compression ratio and the equivalence ratio. It is revealed, that the proposed heat transfer model is more accurate than the standard wall-function formulation, while with the use of the crevice model a more reliable prediction of engine performance is achieved. © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights. Source

Rakopoulos C.D.,National Technical University of Athens | Kosmadakis G.M.,National Technical University of Athens | Pariotis E.G.,Hellenic Naval Academy
International Journal of Hydrogen Energy | Year: 2010

The present work deals with the evaluation of a combustion model that has been developed, in order to simulate the power cycle of hydrogen spark-ignition engines. The motivation for the development of such a model is to obtain a simple combustion model with few calibration constants, applicable to a wide range of engine configurations, incorporated in an in-house CFD code using the RNG k-ε turbulence model. The calculated cylinder pressure traces, gross heat release rate diagrams and exhaust nitric oxide (NO) emissions are compared with the corresponding measured ones at various engine loads. The engine used is a Cooperative Fuel Research (CFR) engine fueled with hydrogen, operating at a constant engine speed of 600 rpm. This model is composed of various sub-models used for the simulation of combustion of conventional fuels in SI engines; it has been adjusted in the current study specifically for hydrogen combustion. The basic sub-model incorporated for the calculation of the reaction rates is the characteristic conversion time-scale method, meaning that a time-scale is used depending on the laminar conversion time and the turbulent mixing time, which dictates to what extent the combustible gas has reached its chemical equilibrium during a predefined time step. Also, the laminar and turbulent combustion velocity is used to track the flame development within the combustion chamber, using two correlations for the laminar flame speed and the Zimont/Lipatnikov approach for the modeling of the turbulent flame speed, whereas the (NO) emissions are calculated according to the Zeldovich mechanism. From the evaluation conducted, it is revealed that by using the developed hydrogen combustion model and after adjustment of the unique model calibration constant, there is an adequate agreement with measured data (regarding performance and emissions) for the investigated conditions. However, there are a few more issues to be resolved dealing mainly with the ignition process and the applicability of a reliable set of constants for the emission calculations. © 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. Source

Rakopoulos C.D.,National Technical University of Athens | Kosmadakis G.M.,National Technical University of Athens | Dimaratos A.M.,National Technical University of Athens | Pariotis E.G.,Hellenic Naval Academy
Applied Energy | Year: 2011

A theoretical investigation is conducted to examine the way the crevice regions affect the mean cylinder pressure, the in-cylinder temperature, and the velocity field of internal combustion engines running at motoring conditions. For the calculation of the wall heat flux, a wall heat transfer formulation developed by the authors is used, while for the simulation of the crevices and the blow-by a newly developed simplified simulation model is presented herein. These sub-models are incorporated into an in-house Computational Fluid Dynamics (CFD) code. The main advantage of the new crevice model is that it can be applied in cases where no detailed information of the ring-pack configuration is available, which is important as this information is rarely known or may have been altered during the engine's life. Thus, an adequate estimation of the blow-by effect on the cylinder pressure can be drawn. To validate the new model, the measured in-cylinder pressure traces of a diesel engine, located at the authors' laboratory, running under motoring conditions at four engine speeds were used as reference, together with measured velocity profiles and turbulence data of a motored spark-ignition engine. Comparing the predicted and measured cylinder pressure traces of the diesel engine for all cases examined, it is observed that by incorporating the new crevice sub-model into the in-house CFD code, significant improvements on the predictive accuracy of the model is obtained. The calculated cylinder pressure traces almost coincide with the measured ones, thus avoiding the use of any calibration constants as would have been the case with the crevice effect omitted. Concerning the radial and swirl velocity profiles and the turbulent kinetic energy measured in the spark-ignition engine, the validation process revealed that the developed crevice model has a minor influence on the aforementioned parameters. The theoretical study has been extended by investigating in the same spark-ignition engine, during the induction and compression strokes, the way crevice flow affects the thermodynamic properties of the air trapped in the cylinder. © 2010 Elsevier Ltd. Source

Karagianni E.A.,Hellenic Naval Academy
Progress In Electromagnetics Research M | Year: 2015

In this paper the propagation of electromagnetic waves in a medium with non zero conductivity is discussed, analyzing the dielectric properties of the sea water, in order to accurately characterize a wireless communication channel. Mathematical models for sea water dielectric constant, wavelength, propagation speed and path loss when an electromagnetic wave at 2.4GHz propagates through sea water are presented. A Bow-Tie microstrip antenna that is required to overcome the high path loss and bandwidth requirements in sea water is studied. A dual-band antenna, with arcshaped circular slots, operating for IEEE802.11 b/g/n standards, at 2.4GHz and 5.1 GHz for WLAN communications, with dimensions 1.4 cm2 is implemented. Return loss, input impedance and gain have been extracted in order to characterize antennas’ performance in a conductive medium. © 2015, Electromagnetics Academy. All rights reserved. Source

Papagiannakis R.G.,Hellenic Air Force Academy | Zannis T.C.,Hellenic Naval Academy
Journal of Energy Engineering | Year: 2014

The gasification of wood allows the production of wood gas, which can be used as an energy source in large spark-ignition (SI) piston engines located in agricultural areas for generating electric power. The composition of wood gas depends on the fuel source and the processing technique. The primary objective of this paper is to investigate the main performance and emission characteristics of a multicylinder, four-stroke, turbocharged, spark-ignited engine fueled with three different types of wood gas at various air to fuel excess ratios. This engine is used for electricity production especially in small generator sets. In order to examine the effect of wood-gas composition on performance and exhaust emissions, a theoretical investigation is conducted by using a comprehensive two-zone phenomenological model. The results concern some of the main engine performance characteristics, i.e., brake specific fuel consumption and maximum cylinder pressure, and specific NO and CO emissions. The predictive ability of the model has been tested against experimental measurements. The results of simulation are found to be in good agreement with the variation trends of the experimental data with engine load. The conclusions from this investigation are valuable for the use of wood gas as a full supplement energy source in a heavy-duty, spark-ignited engine used for electric power generation in agriculture areas, where the composition of the produced wood gas is not fixed but depends on the fuel feedstock source and the type of gasification. © 2013 American Society of Civil Engineers. Source

Discover hidden collaborations