Gergely Kristóf
Budapest University of Technology and Economics
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Publication
Featured researches published by Gergely Kristóf.
Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy | 2014
Zoltan Hernadi; Gergely Kristóf
Geometry of internally grooved tubes plays a significant role in the performance of heat exchangers. Although there are extensive databases of measurement data for some grooved tubes, it is generally challenging to predict the performance of nonexisting groove shapes. A computational fluid dynamics model based on wall-resolved large eddy simulation of turbulent pipe flow with heat transfer is proposed by using OpenFOAM simulation environment. Fully developed turbulence was simulated by solving the 3D unsteady Navier–Stokes equation in streamwise periodic variables. Simulations for a smooth pipe and a helically grooved tube were carried out at Re = 10,000 to 20,000. Mesh dependency study was performed. Simulation methodology was validated by collating simulated results with literature data and own measurements. The proposed model proved to be accurately predictive: 3.2% to 4.2% difference was found between simulations and measurements. The major novelties of this paper are the introduction of large eddy simulation technique in a new application area, validation for single-phase flow in the given Reynolds number regime, and guidelines for future modellers. This methodology can be used for optimizing parameters of future heat exchanger systems leading to more compact size and better performance.
international workshop on thermal investigations of ics and systems | 2016
Norbert Peter; Péter Tóth; Boldizsár Kovács; Gergely Kristóf
Polymeric materials are often used in assembling and packaging MEMS devices. Polymers are prone to absorb moisture which can lead to reliability issues and different types of failures of the package. In contrast to the IC components cavities are usually essential part of the MEMS devices. The gas tightness of these cavities must be ensured for proper operation. This paper presents an extension of the moisture diffusion simulation methodology towards gas filled cavities embedded in multi material systems. The formulation involves the transformation of convection diffusion vapour transport equation into the form of a general transport equation which is solved by a commercially available simulation package. The implementation allows the coupling of additional physical models to the simulation such as condensation models.
The Scientific World Journal | 2013
Aniko Rakai; Gergely Kristóf
Modelling pollutant dispersion in cities is challenging for air quality models as the urban obstacles have an important effect on the flow field and thus the dispersion. Computational Fluid Dynamics (CFD) models with an additional scalar dispersion transport equation are a possible way to resolve the flowfield in the urban canopy and model dispersion taking into consideration the effect of the buildings explicitly. These models need detailed evaluation with the method of verification and validation to gain confidence in their reliability and use them as a regulatory purpose tool in complex urban geometries. This paper shows the performance of an open source general purpose CFD code, OpenFOAM for a complex urban geometry, Michelstadt, which has both flow field and dispersion measurement data. Continuous release dispersion results are discussed to show the strengths and weaknesses of the modelling approach, focusing on the value of the turbulent Schmidt number, which was found to give best statistical metric results with a value of 0.7.
Slovak Journal of Civil Engineering | 2014
Balázs Istók; Gergely Kristóf
Abstract Environmental impacts of oil spills affecting urban sewage networks can be eliminated if timely intervention is taken. The design of such actions requires knowledge of the transport of surface pollutants in open channels. In this study we investigated the travel time and dispersion of pollutants by means of tracer experiments in sewage networks and a creek. The travel time of surface tracers has been found to be significantly shorter than that of a bulk flow tracer. The ratio of the travel times of a bulk flow tracer and surface tracers agreed with the known correlations obtained for rivers. An increasing tendency in the ratio of travel times has been observed for increasing bulk flow velocity. A segment-wise dispersion model was implemented in the existing hydraulic model of a sewer system. The simulation results were compared with the experimental observations. The dispersion rate of the bulk flow tracer has been found to obey Taylor’s mixing theory for long channels and was more intensive than that of surface tracers in community sewage channels.
international spring seminar on electronics technology | 2010
Balázs Illés; Gergely Kristóf; Laszlo Jakab; Miklos Ruszinko
In this paper the technology of the “Linn” type Si fluxless solder bonding oven and gas flow simulations of the oven are discussed. This oven is used for fixing silicon chips on metal substrates with high temperature solder bonding process. The solder is applied in a foil form which is placed between the Si chip and the metal substrate and does not contain any flux. Therefore a reducing agent (Forming gas which is a mixture of 10 vol% H2 and 90 vol% N2) has to be applied to avoid the oxidation of the joints. The key factors of this soldering process has been studied which are the suitable temperature and the adequate H2 concentration. A 3D gas flow model of the oven has been prepared which is based on the FVM (Finite Volume Model) method. The thermal and gas flow circumstances – used the basic and new theoretical oven settings – have been compared by simulations applied the ANSYS – FLUENT system. The model was verified by the measurements of the H2 concentration, the temperature and the pressure inside the oven. The aim was to find new oven settings in order to improve the mechanical stability and decrease the void percent of the joints.
Atmospheric Environment | 2013
Imre Salma; K. Dosztály; T. Borsós; B. Söveges; Gergely Kristóf; N. Péter; Zs. Kertész
Atmospheric Chemistry and Physics | 2016
Imre Salma; Zoltán Németh; Boldizsár Kovács; Gergely Kristóf
Journal of Fluids Engineering-transactions of The Asme | 2017
András Tomor; Gergely Kristóf
Archive | 2014
Aniko Rakai; Gergely Kristóf; Jorg Franke
Archive | 2010
Miklós Balogh; Gergely Kristóf