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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.,Purdue University | Filios A.E.,Technological Educational Institute of Piraeus | Margaris D.P.,University of Patras
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.

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.

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.

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