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CFD modelling of radiators in buildings with user defined wall functions
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.ORCID iD: 0000-0003-0225-711X
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
2016 (English)In: Applied Thermal Engineering, ISSN 1359-4311, E-ISSN 1873-5606, Vol. 64, p. 266-273Article in journal (Refereed) Published
Abstract [en]

The most widely used turbulence model for indoor CFD simulations, the k-ε model, has exhibited problems with treating natural convective heat transfer, while other turbulence models have shown to be too computationally demanding. This paper studies how to deal with natural convective heat transfer for a radiator in order to simplify the simulations, reduce the numbers of cells and the simulation time. By adding user-defined wall functions the number of cells can be reduced considerably compared with the k-ω SST turbulence model. The user-defined wall function proposed can also be used with a correction factor for different radiator types without the need to resolve the radiator surface in detail. Compared to manufacturer data the error is less than 0.2% for the investigated radiator height and temperature.

Place, publisher, year, edition, pages
2016. Vol. 64, p. 266-273
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-9742DOI: 10.1016/j.applthermaleng.2015.10.134ISI: 000370770300028Scopus ID: 2-s2.0-84947125521Local ID: 868ebecd-4ce7-46ad-9b29-ded1785ffaebOAI: oai:DiVA.org:ltu-9742DiVA, id: diva2:982680
Note
Validerad; 2015; Nivå 2; 20151105 (andbra)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-07-10Bibliographically approved
In thesis
1. Analysis of the Thermal Indoor Climate with Computational Fluid Dynamics for Buildings in Sub-arctic Regions
Open this publication in new window or tab >>Analysis of the Thermal Indoor Climate with Computational Fluid Dynamics for Buildings in Sub-arctic Regions
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Analys av termiska inomhusklimatet med CFD för byggnader in subarktiskt klimat
Abstract [en]

This thesis aims to increase the knowledge of simulation of thermal indoor climate for nearly zero energy buildings in a sub-arctic climate. Air heating systems in cold climate generate temperature gradients, which negatively affects the thermal indoor climate. Stand-ard multi-zone modeling has problemswithpredicting these gradients.

In this work, Computerized Fluid Dynamics (CFD) simulations are used to model the tem-perature gradients. The consequences of reducing the cell sizes for the simulation volume are estimated and case studies of different building and heating systems are presented. The CFD method is validated for a traditional underfloor heating system and also for an air heating system.

Furthermore, the effects of snow on heat losses for common building foundations are in-vestigated, and snow is shown to be an important boundary for CFD simulations of a build-ing. The snow and ground freezing areshown to reduce the annual heat losses between 7-10%.

The CFD method is shown to be a suitable method for predicting thermal indoor climate. The method can determine the temperature variations inside a building, for different rooms, floors and heating systems. The CFD method is most appropriate for local distributed sys-tems. For traditional hydronic systems the method may be overambitious,since a good indoor climate is usually achieved anyway.

Heat transfer coefficients are inaccurate when calculated using standard wall functions used in many turbulence models (like the k-ε model) for surfaces with a high heat transfer rate and natural convection. Automatic wall functions have shown better accuracy for this type of problem, but they require more cells. In order to still use the k-εmodel, a user defined wall function is investigated. This method gave good results and a significant re-duction in the number of necessary cells in the simulation volume. The validation of the indoor climate shows that the wall boundary conditions are important for predicting the indoor temperaturevariations for steady state simulations.

New buildings have a higher thermal inertia, which affects the heat losses. It is important to include this effect in the boundary condition calculations for a CFD model.

The CFD simulations show that air heatingand local distributed heating systems have dif-ficulties infulfillinga good thermal comfort inside all rooms. This is especially true for rooms with exhaustair and closed doors and multi-storybuildings. Results from a CFD simulation can be used to improve the thermal comfort in these cases.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2018
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-68067 (URN)978-91-7790-086-3 (ISBN)978-91-7790-087-0 (ISBN)
Public defence
2018-05-24, E632, 971 87, Luleå Tekniska Universitet, Luleå, 09:00 (English)
Opponent
Supervisors
Available from: 2018-04-04 Created: 2018-03-27 Last updated: 2018-05-17Bibliographically approved

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Risberg, DanielRisberg, MikaelWesterlund, Lars

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