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Michailidis N.,Aristotle University of Thessaloniki | Stergioudi F.,Aristotle University of Thessaloniki | Omar H.,Aristotle University of Thessaloniki | Pavlidou E.,Aristotle University of Thessaloniki | And 4 more authors.
Journal of Alloys and Compounds | Year: 2010

The microstructural characterization of oxide surface morphologies formed on both high purity Ni and Inconel foams exposed to concentrated solar radiation is investigated by the use of scanning electron microscopy (SEM) and EDXS. Oxidation was achieved by concentrated solar radiation at temperatures ranging between 200 and 1100 °C for 30 min under conditions of dynamic air flow. SEM observations revealed a rapid homogeneous oxidation in the Ni-foam with three different surface oxide structures formed in relation with the process temperature. An important effect of substrate morphological and compositional features was only observed in Inconel foams, independent of the test temperature, with the existence of coarse and fine oxide regions denoting the non-uniformity of the oxidation process. The above-presented results give evidence for a potential use of the tested foam materials as volumetric solar receivers. © 2010 Elsevier B.V. All rights reserved.


Flouros M.,MTU Aerospace Engines | Kanarachos A.,Frederick Institute of Technology | Yakinthos K.,Laboratory of Fluid Mechanics and Turbomachinery | Salpingidou C.,Laboratory of Fluid Mechanics and Turbomachinery | Cottier F.,MTU Aerospace Engines
Journal of Engineering for Gas Turbines and Power | Year: 2016

In modern aero-engines, the lubrication system holds a key role due to the demand for high reliability standards. An aero-engine bearing chamber contains components like bearings and gears. Oil is used for lubrication and for heat removal. In order to retain the oil in a bearing chamber, pressurized seals are used. These are pressurized using air from the compressor. In order to avoid overpressurization of the bearing chamber, air/oil passages are provided in the bearing chamber. At the top, a vent pipe discharges most of the sealing air and at the bottom, a scavenge pipe is used for discharging the oil by means of a pump (scavenge pump). The scavenge pipe is setup in most cases by tubes of circular or noncircular cross sections. When the scavenge pipe has to be routed in a way that sharp bends or elbows are unavoidable, flexible (corrugated) pipes can be used. Because of the corrugation, considerable flow resistance with high-pressure drop can result. This may cause overpressurization of the bearing compartment with oil loss into the turbomachinery with possibility of ignition, coking (carbon formation), or contamination of the aircraft's air conditioning system. It is therefore important for the designer to be capable to predict the system's pressure balance behavior. A real engine bearing chamber sealed by brush seals was used for generating different air/oil mixtures thus corresponding to different engine operating conditions. The mixtures were discharged through a scavenge pipe which was partly setup by corrugated tubes. Instead of a mechanical pump, an ejector was used for evacuating the bearing chamber. An extensive survey covering the existing technical literature on corrugated tube pressure drop was performed and is presented in this paper. The survey has covered both single-phase and multiphase flows. Existing methods were checked against the test results. The method which was most accurately predicting lean air test results from the rig was benchmarked and was used as the basis for extending into a two-phase flow pressure drop correlation by applying two-phase flow multiplier techniques similar to Lockhart and Martinelli. Comparisons of the new two-phase flow pressure drop correlation with an existing correlation by Shannak are presented for mixtures like air/oil, air/water, air/diesel, and air/kerosene. Finally, numerical analysis results using ANSYS CFX VERSION 15 are presented. © 2016 by ASME.


Tzempelikos D.A.,University of Patras | Vouros A.P.,Laboratory of Fluid Mechanics and Turbomachinery | Bardakas A.V.,Laboratory of Fluid Mechanics and Turbomachinery | Filios A.E.,Technological Educational Institute of Piraeus | Margaris D.P.,University of Patras
Engineering in Agriculture, Environment and Food | Year: 2015

The objective of the present work is to study the drying kinetics of quince slices during convective drying. The details of the laboratory-scaled convective dryer are presented. Experiments were carried out at air temperatures 40°C, 50°C and 60°C and 2ms-1 bulk velocity. The whole process occurred within the falling rate period. Results showed that the temperature of the air stream has a significant effect on the drying curves. In particular, a temperature increase from 40 to 60°C produced a decrease of the total time of drying of about 54%. Eight thin-layer drying models were used to fit the temporal distributions of the moisture data using non-linear regression analysis. Among the various models, the Weibull formula was best fitted to measurements. For the range of the drying temperatures examined, Fick's law of diffusion was used to determine the effective moisture diffusivity, which varied between 3.23×10-10m2s-1 and 7.82×10-10m2s-1. Assuming an Arrhenius type model for the drying process, a value of 38.291kJmol-1 was estimated for the activation energy. © 2015 Asian Agricultural and Biological Engineering Association.


Tzempelikos D.A.,University of Patras | Vouros A.P.,Laboratory of Fluid Mechanics and Turbomachinery | Bardakas A.V.,Laboratory of Fluid Mechanics and Turbomachinery | Filios A.E.,Laboratory of Fluid Mechanics and Turbomachinery | Margaris D.P.,University of Patras
International Journal of Mechanics | Year: 2013

In order to overcome the lack of experimental data in the open literature and the necessity to validate numerical models, as well as increase the efficiency of the drying process, a new laboratory convective (LC) dryer has been designed, constructed and equipped with an integrated measurement and automated control instrumentation. The main sections of the LC dryer, which can be arranged for operation in a closed or open circuit mode through manually controlled dumpers, are the vertical flow drying chamber, the tube heat exchanger, the thermal boiler and finally the fan-motor with a smooth speed control unit. The experimental facility tested and monitored the moisture content removal of horticultural and agricultural products. The current paper outlines the methodology applied for the design and optimization of the LC dryer, which has been achieved through the analysis of the flow field by means of computational fluid dynamics (CFD). The prediction of the 3d flow problem was accomplished through the solution of the steady-state incompressible, Reynolds-Averaged Navier-Stokes (RANS) equations with the incorporation of the standard k-ε turbulence model. The measurement and control instrumentation with the inclusion of the innovative, pc-controlled, 3d traverse system that serves detailed surveys of the temperature and velocity inside the drying chamber, are also discussed. The performance test and evaluation of the LC dryer was conducted using quince slices as a test material at an average temperature of 60°C and air at 2 m/s into the drying chamber.


Vouros A.P.,Laboratory of Fluid Mechanics and Turbomachinery | Mentzos M.D.,University of Patras | Xanthopoulos E.I.,Laboratory of Fluid Mechanics and Turbomachinery | Filios A.E.,Laboratory of Fluid Mechanics and Turbomachinery | Margaris D.P.,University of Patras
Journal of Applied Fluid Mechanics | Year: 2013

The flow of the axisymmetric jet constitutes a subject of research from the origins of fluid dynamics; however it remains a subject of interest due to the recent findings that denote the influence of flow and geometry conditions in configurations that diverge from the theoretical "free-jet" case. In the present study, the effect of a wall boundary produced by a circular disk of twice the jet diameter, which is imposed on the exit of the jet is investigated experimentally and numerically. Computational simulations are performed to predict the flow characteristics incorporating different turbulence models (k-ε and Reynolds Stress) and solvers (Simplec and Coupled). Supportive pressure measurements are used to evaluate the predictions within the initial region of a circular jet at two Reynolds numbers (50,000 and 65,000). The results indicate that the presence of the exit wall boundary results to the formation of recirculation zone behind the exit, which prevents the entrainment of ambient fluid. Comparing with the flow field of the free from confinement jet, it is shown that the imposition of the wall has a minor effect on the mean velocity field; it is however capable of altering the turbulent flow properties, including the normal and the Reynolds shear stresses, in the region before the establishment of the self-similarity zone.


Tzempelikos D.A.,University of Patras | Mitrakos D.,Greek Atomic Energy Commission | Vouros A.P.,Laboratory of Fluid Mechanics and Turbomachinery | Bardakas A.V.,Purdue University | And 2 more authors.
Journal of Food Engineering | Year: 2015

A numerical model for non-steady heat and mass transfer during convective drying of cylindrical quince slices, with axis parallel to the air flow, is developed. The model is based on the numerical solution of the coupled one-dimensional heat and mass transport equations, assuming moisture transport due to Fick's diffusion, with an effective moisture diffusion coefficient derived by fitting the analytical solution of the Fick's law to experimentally derived drying curves, on the basis of an Arrhenius-type temperature dependence. The necessary convective heat and mass transfer coefficients are obtained from CFD calculations of the turbulent flow field around the slices using a commercial CFD package. A new correlation of the Nusselt number, as a function of Prandtl and Reynolds numbers is proposed for the specific geometric flow configuration. The model is validated against experimental data for different air stream velocities (1 and 2 m/s) and temperatures (40, 50 and 60 °C). The model was found to be robust, computationally efficient and able to capture with sufficient accuracy the time evolution of the temperature and the moisture loss, with a minimum need for experimental adjustment, and hence, is considered suitable from an engineering point of view. © 2015 Elsevier Ltd. All rights reserved.


Tzempelikos D.A.,University of Patras | Vouros A.P.,Laboratory of Fluid Mechanics and Turbomachinery | Bardakas A.V.,Laboratory of Fluid Mechanics and Turbomachinery | Filios A.E.,Technological Educational Institute of Piraeus | Margaris D.P.,University of Patras
Case Studies in Thermal Engineering | Year: 2014

The objective of the current study is to examine experimentally the thin-layer drying behavior of quince slices as a function of drying conditions. In a laboratory thermal convective dryer, experiments were conducted at air temperatures of 40, 50 and 60 °C and average air velocities of 1, 2 and 3 ms-1. Increasing temperature and velocity resulted to a decrease of the total time of drying. The experimental data in terms of moisture ratio were fitted with three state-of-The-Art thin-layer drying models. In the ranges measured, the values of the effective moisture diffusivity (Deff were obtained between 2.67 x 10-10 and 8.17 x 10-10 m2 s -1. The activation energy (Eα) varied between 36.99 and 42.59 kJ mol-1.


Tzempelikos D.A.,University of Patras | Vouros A.P.,Laboratory of Fluid Mechanics and Turbomachinery | Bardakas A.V.,Laboratory of Fluid Mechanics and Turbomachinery | Filios A.E.,Laboratory of Fluid Mechanics and Turbomachinery | Margaris D.P.,University of Patras
International Journal of Mathematics and Computers in Simulation | Year: 2012

Batch dryers are some of the most widespread equipment used for fruit dehydration. Nevertheless, the optimization of the air distribution inside the drying chamber of a batch dryer remains a very important point, due to its strong effect on drying efficiency as well as the uniformity of the moisture content of the drying products. A new scale laboratory batch-type tray air (BTA) dryer was designed, constructed and evaluated for the drying of several horticultural and agricultural products. The airflow field inside the dryer was studied through a commercial computational fluid dynamics (CFD) package. A three-dimensional model for a laboratory BTA dryer was created and the steady-state incompressible, Reynolds-Averaged Navier-Stokes equations that formulate the flow problem were solved, incorporating standard and RNG k-ε turbulence models. In the simulation, the tray, used inside the BTA drying chamber, was modeled as a thin porous media of finite thickness. The simulations for testing the chamber were conducted at an average velocity of 2.9 m/s at ambient temperature. The CFD models were evaluated by comparing the airflow patterns and velocity distributions to the measured data. Numerical simulations and measurements showed that the new scale laboratory BTA dryer is able to produce a sufficiently uniform air distribution throughout the testing chamber of the dryer.


Martinopoulos G.,Process Equipment Design Laboratory | Missirlis D.,Laboratory of Fluid Mechanics and Turbomachinery | Tsilingiridis G.,Process Equipment Design Laboratory | Yakinthos K.,Laboratory of Fluid Mechanics and Turbomachinery | Kyriakis N.,Process Equipment Design Laboratory
Renewable Energy | Year: 2010

A polymer solar collector is developed and its behavior is investigated both experimentally and with computational fluid dynamics (CFD). Solar irradiation as well as convection and heat transfer in the circulating fluid and between the parts of the collector is considered in the model. The temperature and velocity distribution over its area as well as the collector efficiency at nominal flow rate were used in order to validate the CFD model. Temperature distribution during operation and average collector efficiency were found to be in good agreement between the experimental data and the results of the CFD modeling. © 2010 Elsevier Ltd. All rights reserved.


Flouros M.,MTU AG | Iatrou G.,Laboratory of Fluid Mechanics and Turbomachinery | Yakinthos K.,Laboratory of Fluid Mechanics and Turbomachinery | Cottier F.,MTU AG | Hirschmann M.,MTU AG
Proceedings of the ASME Turbo Expo | Year: 2014

In modern aero engines the lubrication system plays a key role due to the demand for high reliability. Oil is used not only for the lubrication of bearings, gears or seals, but it also removes large amounts of the generated heat. Also, air from the compressor at elevated temperature is used for sealing the bearing chambers and additional heat is introduced into the oil through radiation, conduction and convection from the surroundings. The impact of excessive heat on the oil may lead to severe engine safety and reliability problems which can range from oil coking (carbon formation) to oil fires. Coking may lead to a gradual blockage of the oil tubes and subsequently increase the internal bearing chamber pressure. As a consequence, oil may migrate through the seals into the turbo machinery and cause contamination of the cabin air or ignite and cause failure of the engine. It is therefore very important for the oil system designer to be capable to predict the system's functionality. Coking or oil ignition may occur not only inside the bearing chamber but also in the oil pipes which carry away the air and oil mixture from the bearing chamber. Bearing chambers usually have one pipe (vent pipe) at the top of the chamber and also one pipe (scavenge pipe) at the bottom which is attached to a scavenge pump. The vent pipe enables most of the sealing air to escape thus avoid over-pressurization in the bearing compartment. In a bearing chamber sealing air is the dominant medium in terms of volume occupation and also the in terms of causing expansion phenomena. The scavenge pipe carries away most of the oil from the bearing chamber but some air is also carried away. The heat transfer in vent pipes was investigated by Busam [1], [2]. Busam has experimentally developed a Nusselt number correlation for an annular flow in a vent pipe. For the heat transfer predictions in scavenge pipes no particular Nusselt number correlation exist. This paper intends to close the gap in this area. As part of the European Union funded research programme ELUBSYS (Engine LUBrication System TechnologieS), an attempt was done to simplify the oil system's architecture. In order to better understand the flow in scavenge pipes, high speed video was taken in two sections of the pipe (vertical and horizontal). In the vertical section the flow was a wavy annular falling film whereas the flow in the horizontal section was a an unsteady wavy stratified/slug flow. Heat transfer has been investigated in the horizontal section of the scavenge pipe, leaving the investigation on the vertical section for later. Thanks to the provided extensive instrumentation, the thermal field in, on and around the pipe was recorded, evaluated and also numerically modeled using ANSYS CFX version 14 [23]. Brand new correlations for two-phase flow heat transfer (Nusselt number) and for pressure drop (friction coefficient) in horizontal scavenge pipes are the result of this work. The Nusselt number correlation has been developed in such a way that smooth transition (i.e. no discontinuity) from two-phase into single phase flow is observed. This work was funded and conducted within the 7th EU Frame Programme for Aeronautics and Transport (AAT.2008.4.2.3). Copyright © 2014 by ASME.

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