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Dunaujvaros, Hungary

Fekete B.,Budapest University of Technology and Economics | Fekete B.,College of Dunaujvaros
Materials and Design | Year: 2015

A new low cycle fatigue criterion is presented based on the stored energy, which accumulates in the material's microstructure during fatigue loading. The new damage parameters are based on the assumption that only the stored part of the introduced energy causes the changes in the microstructure, while the other part does not contribute to the fatigue process. Moreover the dissipated part may depends on the fatigue conditions, especially on the strain rate, which could affect the lifetime prediction accuracy of the applied models. To demonstrate the prediction capability of the proposed model a comprehensive experimental work were conducted on two types of reactor pressure vessel structural material. Investigation on the fraction of plastic work dissipated to heat was carried out to provide information on thermo-mechanical behavior on the tested materials, which can be used as input parameters of the new model. Furthermore isothermal and thermo-mechanical low cycle fatigue tests were performed with in-service loading conditions. The result is higher prediction accuracy than by the classical strain amplitude and strain energy based approaches. The developed model may provide a useful analytical tool for the low cycle fatigue evaluation of reactor components. © 2015 Elsevier Ltd. Source


Szlivka F.,College of Dunaujvaros
Acta Polytechnica Hungarica | Year: 2014

The mathematical description of gas-pressure oscillations excited by a piston at the end of a straight duct has been investigated in an analytical way and it has been compared to numerical solutions and measured results. Two different analytical solutions have been found in the literature in series form which have been transformed into closed form solutions showing their identity. This evaluation can significantly be simplified and the comparison of the results of the analytical and the numerical solutions are presented in this study accordingly. In the results are presented the frequency of piston is close to the resonant frequency. Source


Zachar A.,College of Dunaujvaros
International Journal of Heat and Mass Transfer | Year: 2012

Natural convection induced heat transfer has been studied over the outer surface of helically coiled-tube heat exchangers. Several different geometrical configurations (curvature ratio δ [0.035, 0.082]) and a wide range of flow parameters (60 <= T tank <= 90, T in = 19 and 60 <= T in <= 90, T tank = 20, 4000 <= Re <= 45000) have been examined to broaden the validity of the results gained from this research. A fluid-to-fluid boundary condition has been applied in the numerical calculations to create the most realistic flow configurations. Validity of the numerical calculations has been tested by experiments available in the open literature. Calculated results of the inner side heat transfer rate have also been compared to existing empirical formulas and experimental results to test the validity of the numerical computation in an independent way from the outer side validation of common helical tube heat exchangers. Water has been chosen to the working fluid inside and outside of the coiled tube (3 < Pr < 7). Outer side heat transfer rate along the helical tube axis has been investigated to get information about the performance of the heat transport process at different location of the helical tube. It was found that the outer side heat transfer rate is slightly dependent on the inner flow rate of any helical tube in case of increasing temperature differences between the tank working fluid temperature and the coil inlet temperature. A stable thermal boundary layer has been found along the axial direction of the tube. In addition to this the qualitative behavior of the peripherally averaged Nusselt number versus the axial location along the helical tube function is strongly dependent on the direction of the heat flow (from the tube to the storage tank and the reversed direction). Inner side heat transfer rate of helical coils have also been investigated in case of fluid-to-fluid boundary conditions and the calculation results have been compared with different prediction formulas published in the last couples of decades. © 2012 Elsevier Ltd. All rights reserved. Source


Zachar A.,College of Dunaujvaros
International Journal of Heat and Mass Transfer | Year: 2015

Unsteady heat transfer process and the developed flow and temperature field inside hot water storage tanks have been studied with a helical tube-in-tube flow distributor. The primary purpose of the study is to improve the temperature stratification in hot water storage tanks operated with coiled-tube heat exchangers. The investigated operating mode is the extraction of thermal energy from hot water storage tanks. The studied flow distributor consists of two helically coiled pipes. The inner pipe is a common helically coiled tube heat exchanger and the outer pipe is a helically coiled flow distributor which separates the colder water from the significantly hotter bulk fluid of the tank to decrease the entrainment effect induced by natural convection. The flow distributor drives the cooling tank fluid to a helical flow path along the tube axis which induces additional secondary flow pattern in planes normal to the main flow direction over the outer surface of the inner coiled tube. Different inlet temperatures have been investigated to describe the impact for the developed velocity and temperature field. Comparison of the flow and temperature fields of hot water storage tank with common helical tube and the tube-in-tube flow distributor configuration are discussed. Numerical calculations have been carried out to examine different flow and temperature fields in laminar flow regimes. The proposed helical flow distributor ensures a highly stratified temperature field of storage tanks operating with helically coiled heat exchangers. In addition to this the suggested flow distributor geometry makes it possible to apply different heat transfer enhancement tools like spiral tape inserts, different kind of corrugations further improve if it is possible the poor outer side heat transfer rate of common helically coiled tube heat exchangers. © 2014 Elsevier Ltd. Source


Steady and unsteady heat transfer process and the developed flow and temperature field inside hot water storage tanks have been studied with a helical tube-in-tube flow distributor design. The studied flow distributor consists of two helically coiled pipes. The inner pipe is a common helically coiled tube heat exchanger and the outer pipe is a helically coiled flow distributor which separates the hot water from the significantly colder bulk water of the tank to decrease the entrainment effect induced by natural convection. The flow distributor drives the heated water to a helical flow path along the tube axis which induces additional secondary flow in planes normal to the main flow direction over the outer surface of the inner coiled tube. Different heat exchanger side flow rates, inlet temperatures and two gap size between the outer wall of the helical tube and the flow distributor have been investigated to describe the impact of the modification of these parameters for the developed velocity and temperature field. Comparison of the flow and temperature fields of hot water storage tank with common helical tube and the tube-in-tube flow distributor configuration are discussed. Numerical calculations have been carried out to examine different flow and temperature fields in laminar flow regimes. Calculated results of the inner side heat transfer intensity (Nu) have been compared to existing empirical formulas to test the validity of the numerical results in case of common helical tube heat exchanger. The proposed helical flow distributor ensures a highly stratified temperature field of storage tanks operating with helically coiled heat exchangers. In addition to this the suggested flow distributor geometry makes it possible to apply different heat transfer enhancement tools like spiral tape inserts, different kind of corrugations further improve if it is possible the poor outer side heat transfer rate of common helically coiled tube heat exchangers. © 2013 Elsevier B.V.All rights reserved. Source

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