Piraeus, Greece
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.


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Rakopoulos C.D.,Internal Combustion Engines Laboratory | Kosmadakis G.M.,Internal Combustion Engines Laboratory | Pariotis E.G.,Hellenic Naval Academy
Applied Energy | Year: 2010

The scope of the present study is to try to determine a comprehensive heat transfer formulation, which would be able to predict adequately the heat transfer mechanism on a wide range of different reciprocating engine configurations (spark-ignition and diesel engines) and operating conditions. To this aim, four of the most popular heat transfer formulations used in commercial and research CFD (computational fluid dynamics) codes are evaluated comparatively against available experimental data, using an in-house CFD model that has already been applied satisfactorily for the simulation of a spark-ignition and a diesel engine running under motoring conditions. The comparison reveals that most of the existing wall heat transfer formulations fail to predict adequately both the history and peak value of the heat flux. Nonetheless, the predicted trends of the heat flux during the entire closed part of the engine cycle are similar, with higher differences occurring during the expansion phase. To overcome this, the present authors proceeded to the development of a new wall heat transfer formulation based on the existing ones. This new formulation is used in the in-house CFD model for the simulation of the heat transfer through the cylinder walls for the same engines and operating conditions as those used for the comparative evaluation of the existing heat transfer models. Comparing the calculated heat flux using the five heat transfer models with the corresponding measured one, it is concluded that in most cases the new model predicts more accurately the heat transfer during the compression stroke for motored operation and at the same time the predicted peak heat flux is closer to the experimental one. Although a more fundamental formulation is used to describe the heat transfer process, the computational time required is not affected, which is a parameter crucial for multi-dimensional modeling. © 2009 Elsevier Ltd. All rights reserved.


Alexandre J.,King's College London | Farakos K.,National Technical University of Athens | Tsapalis A.,National Technical University of Athens | Tsapalis A.,Hellenic Naval Academy
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2010

We consider a four-dimensional theory in the z=3 Lifshitz context, with an exponential (Liouville) potential. We determine the exact renormalized potential of the theory and derive the nonperturbative relation between the renormalized and bare couplings. In addition, we show that Lorentz symmetry is naturally generated by quantum fluctuations in the infrared regime, and conclude that the model can be relevant to high energy physics. © 2010 The American Physical Society.


Alexandre J.,King's College London | Farakos K.,National Technical University of Athens | Pasipoularides P.,National Technical University of Athens | Tsapalis A.,National Technical University of Athens | Tsapalis A.,Hellenic Naval Academy
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2010

We consider a 3+1 dimensional field theory at a Lifshitz point for a dynamical critical exponent z=3, with a scalar and a fermion field coupled via a Yukawa interaction. Using the nonperturbative Schwinger-Dyson approach we calculate quantum corrections to the effective action. We demonstrate that a first order derivative kinetic term as well as a mass term for the fermion arise dynamically. This signals the restoration of Lorentz symmetry in the IR regime of the single fermion model, although for theories with more than one fermionic species such a conclusion will require fine-tuning of couplings. The limitations of the model and our approach are discussed. © 2010 The American Physical Society.


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.


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.


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.


Katsanos C.O.,National Technical University of Athens | Hountalas D.T.,National Technical University of Athens | Zannis T.C.,Hellenic Naval Academy
Energy Conversion and Management | Year: 2013

In diesel engines, approximately 30-40% of the energy supplied by the fuel is rejected to the ambience through exhaust gases. Therefore, there is a potentiality for further considerable increase of diesel engine efficiency with the utilization of exhaust gas heat and its conversion to mechanical or electrical energy. In the present study, the operational behavior of a heavy-duty (HD) diesel truck engine equipped with an electric turbocompounding system is examined on a theoretical basis. The electrical turbocompounding configuration comprised of a power turbine coupled to an electric generator, which is installed downstream to the turbocharger (T/C) turbine. A diesel engine simulation model has been developed using operating charts for both turbocharger and power turbine. A method for introducing the operating charts into the engine model is described thoroughly. A parametric analysis is conducted with the developed simulation tool, where the varying parameter is the rotational speed of power turbine shaft. In this study, the interaction between the power turbine and the turbocharged diesel engine is examined in detail. The effect of power turbine speed on T/C components efficiencies, power turbine efficiency, exhaust pressure and temperature, engine boost pressure and air to fuel ratio is evaluated. In addition, theoretical results for the potential impact of electrical turbocompounding on the generated electric power, net engine power and relative improvement of brake specific fuel consumption (bsfc) are provided. The critical evaluation of the theoretical findings led to the basic conclusion that there is a significant potential for bsfc improvement of HD diesel truck engines with the proposed electrical turbocompounding concept, which reaches up to 4% at full engine load. The improvement of bsfc with electric turbocompounding appears to be more attractive solution in terms of technical complexity and installation cost against other competitive heat recovery technologies such as Rankine cycle systems. © 2013 Elsevier Ltd. All rights reserved.


Aretakis N.,National Technical University of Athens | Roumeliotis I.,Hellenic Naval Academy | Doumouras G.,National Technical University of Athens | Mathioudakis K.,National Technical University of Athens
Applied Energy | Year: 2012

The deregulation of the energy market has created an additional incentive for gas turbine plants operators to minimize and control performance deterioration with respect to the economical aspects of the plant. The most prevalent deterioration problem is compressor fouling, which has a significant impact on the power plant profit. Off-line washing is able to recover the engine's performance losses due to fouling, but has a variety of associated costs. A method to predict the impact of the compressor washing process on the power plant revenue is presented herein, allowing for the optimization of the process with regards to power plant specific data. For this reason, a detailed cost analysis module is formed and coupled with an engine model allowing for the study of both economic parameters and engine operation parameters like the increase of maintenance cost due to start-ups and the variation of the engine degradation rate. The method is applied for the case of an aeroderivative gas turbine of 42. MW. The parameters associated with the off-line washing process and the engine performance that affects the plant's revenue are examined and discussed, while recommendations on the optimal washing schedule are made. © 2012 Elsevier Ltd.


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.


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.

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