Πλοήγηση ανά Συγγραφέα "Vlahostergios, Z."
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Τεκμήριο Computational modeling of vortex breakdown control on a delta wing(2013) Vlahostergios, Z.; Missirlis, D.; Yakinthos, K.; Goulas, A.We present an effort to model the development and the control of the vortex breakdown phenomenon on a delta wing. The pair of the vortices formed on the suction side of a delta wing is the major contributor to the lift generation. As the angle of attack increases, these vortices become more robust, having high vorticity values. The critical point of a delta wing operation is the moment when these vortices, after a certain angle of attack, are detached from the wing surface and wing stall occurs. In order to delay or control the vortex breakdown mechanism, various techniques have been developed. In the present work, the technique based on the use of jet-flaps is numerically investigated with computational fluid dynamics by adopting two eddy-viscosity turbulence models. The computational results are compared with the experimental data of Shih and Ding (1996). It is shown that between the two turbulence models, the more advanced one, which adopts a non-linear constitutive expression for the Reynolds-stresses, is capable to capture the vortex breakdown location for a variety of jet exit angles. The performance assessment of the models is followed by the investigation of the effect of the jet-flap on the lift and drag coefficients.Τεκμήριο Development of a ball valve with PVD-coated Metal-to-Metal sealing mechanism(2012) Bouzakis, K. D.; Klocke, F.; Tsouknidas, A.; Kombogiannis, S.; Missirlis, D.; Vlahostergios, Z.; Sideridis, A.; Yakinthos, K.Conventional ball valves are based on elastomeric sealing technologies which exhibit restricted wear resistance in the presence of network impurities while requiring high operating torques. In the described investigations, the elastomeric sealing of a 2-inch floating ball valve was replaced by a metal-to-metal seating for attaining specific application requirements, without major dimensional changes. In order to avoid the use of elastomerics and achieve a high performance rating of the valve (Class VI shut-off), computational fluid dynamics (CFD) simulations were employed to predict the optimum contact geometry of ball/seating.A high accuracy contact was obtained through precision turning of the hardened metal shell (ball seating) and by grinding of its counter ball body. Both components, ball and seating, were appropriately heat treated and subsequently coated with a nanostructured diamond like carbon (DLC) coating, applied by physical vapour deposition (PVD) techniques, providing superior surface strength characteristics and nearly frictionless operation.Nanoindentations provided information regarding the coatings properties, while nanoimpacts were employed to determine the impact abrasive wear of the developed components.Τεκμήριο Effect of turbulence intensity on the pressure drop and heat transfer in a staggered tube bundle heat exchanger(2015-01) Vlahostergios, Z.; Missirlis, D.; Flouros, M.; Albanakis, C.; Yakinthos, K.This paper investigates experimentally the correlation and the effect of the turbulence intensity on the pressure drop and the heat transfer mechanism of a heat exchanger with elliptic tubes in a staggered arrangement. The heat exchanger studied in this work is an air–water cross flow heat exchanger with air being the external fluid and water the internal one. The heat exchanger consists of 144 elliptic tubes placed in a staggered arrangement. The experiments were carried out for two setups. The first setup was referring to isothermal conditions for which only air was used, flowing around the tubes. The second setup was referring to non-isothermal conditions with air as the external fluid and water as the internal working fluid, flowing inside the tubes. The Reynolds number for the external air for the isothermal experiments ranged between 3100 and 5200 based on the maximum velocity between the elliptic tubes. The turbulence intensity values varied between 0.9% and 3%. For the non-isothermal measurements the Reynolds number took values from 3100 to 7700 and the turbulent intensity ranged from 0.9% to 3%. The measurements showed that the increase of turbulent intensity led to a decrease in the total pressure drop of the external flow of the heat exchanger together with an enhancement of the heat transfer mechanism.Τεκμήριο Flow, thermal and structural application of Ni-foam as volumetric solar receiver(2013-02) Michailidis, N.; Stergioudi, F.; Omara, H.; Missirlis, D.; Vlahostergios, Z.; Tsipas, S.; Albanakis, C.; Granier, B.Open-cell nickel foams with 92% porosity and uniform pore size and distribution were used in this study. The main objective of this work was to evaluate the behaviour of Ni-foam, when treated as volumetric receivers under concentrated solar radiation while improving their oxidation resistance, in order to make them attractive for such applications. The experimental investigation showed that their efficiency was depending on both materials parameters and flow conditions, the latter affecting the pressure drop and the heat transfer behaviour. The microstructural characterisation of oxide surface morphologies formed on the open-cell Ni foams exposed to concentrated solar radiation is investigated by the use of SEM and EDXS. SEM observations revealed a rapid homogeneous oxidation in the Ni-foam with three different surface oxide structures formed in relation with the process temperature. A novel slurry-based process for aluminising nickel foams while retaining their geometrical properties is applied in order to develop an aluminide–nickel intermetallic coating on a Ni foam thus enhancing the oxidation resistance. Scanning electron microscopy and X-ray diffraction were applied to assess the effectiveness of the aluminising process and determine the optimum parameters of the procedure (slurry composition, holding temperature and time).Τεκμήριο Modelling Operation of System of Recuperative Heat Exchangers for Aero Engine with Combined Use of Porosity Model and Thermo-Mechanical Model(2012) Yakinthos, K.; Missirlis, D.; Sideridis, A.; Vlahostergios, Z.; Seite, O.; Goulas, A.The present work describes an effort to model the operation of a system of recuperative heat exchangers of an aero engine for real engine operating conditions. The modelling was performed with the combined use of a porous medium model and a thermo mechanical model. The porous medium model was taking into account the heat transfer and pressure loss behaviour of the heat exchangers while the thermo mechanical one was used for the calculation of the wall temperature distribution of the elliptic tubes of the heat exchangers. As it is presented, the combined use of these models can provide a useful tool which can help in the prediction of the macroscopic behaviour of the system of recuperative heat exchangers of the aero engine which can be used for optimization purposes and numerical studies.Τεκμήριο Thermodynamics Cycle Analysis, Pressure Loss, and Heat Transfer Assessment of a Recuperative System for Aero-Engines(2015-04-01) Goulas, A.; Donnerhack, S.; Flouros, M.; Misirlis, D.; Vlahostergios, Z.; Yakinthos, K.Aiming in the direction of designing more efficient aero-engines, various concepts have been developed in recent years, among which is the concept of an intercooled and recuperative aero-engine. Particularly, in the area of recuperation, MTU Aero Engines has been driving research activities in the last decade. This concept is based on the use of a system of heat exchangers (HEXs) mounted inside the hot-gas exhaust nozzle (recuperator). Through the operation of the system of HEXs, the heat from the exhaust gas downstream the LP turbine of the jet engine is driven back to the combustion chamber. Thus, the preheated air enters the engine combustion chamber with increased enthalpy, providing improved combustion and by consequence, increased fuel economy and low-level emissions. If additionally an intercooler is placed between the compressor stages of the aero-engine, the compressed air is then cooled by the intercooler; thus, less compression work is required to reach the compressor target pressure. In this paper, an overall assessment of the system is presented with particular focus on the recuperative system and the HEXs mounted into the aero-engine's exhaust nozzle. The herein presented results were based on the combined use of CFD computations, experimental measurements, and thermodynamic cycle analysis. They focus on the effects of total pressure losses and HEX efficiency on the aero-engine performance especially the engine's overall efficiency and the specific fuel consumption (SFC). More specifically, two different hot-gas exhaust nozzle configurations incorporating modifications in the system of HEXs are examined. The results show that significant improvements can be achieved in overall efficiency and SFC, hence contributing to the reduction of CO2 and NOx emissions. The design of a more sophisticated recuperation system can lead to further improvements in the aero-engine efficiency in the reduction of fuel consumption. This work is part of the European funded research program Low Emissions Core engine Technologies (LEMCOTEC).