Πλοήγηση ανά Συγγραφέα "Missirlis, D."
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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 Modeling pressure drop of inclined flow through a heat exchanger for aero-engine applications(2007-06) Missirlis, D.; Yakinthos, K.; Storm, P.; Goulas, A.In the present work further numerical predictions for the flow field through a specific type of a heat exchanger, which is planned to be used in the exhaust nozzle of aircraft engines. In order to model the flow field through the heat exchanger, a porous medium model is used based on a simple quadratic relation, which connects the pressure drop with the inlet air velocity in the external part of the heat exchanger. The aim of this work is to check the applicability of the quadratic law in a variety of velocity inlet conditions configured by different angles of attack. The check is performed with CFD and the results are compared with new available experimental data for these inlet conditions. A detailed qualitative analysis shows that although the quadratic law has been derived for a zero angle of attack, it performs very well for alternative non-zero angles. These observations are very helpful since this simple pressure drop law can be used for advanced computations where the whole system of the exhaust nozzle together with the heat exchangers can be modeled within a holistic approach.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.