1. Difference between using tabulated and exact values of thermal properties of materials in numerical simulations of heat transfer through a high-performance windowMiha Jukić, Sabina Jordan, 2019, published scientific conference contribution Abstract: The thermal properties of materials, primarily the thermal conductivity, are an essential input for numerical modelling of heat transfer in buildings and building components. When determining them according to relevant European standards, it is not uncommon to encounter materials for which the exact values are not appropriately specified and the tabulated values in standards are overly conservative. In such situations, the thermal conductivity of the material can be determined by measurement. However, this approach may prove inconvenient and too expensive, especially if the material in question turns out to have little influence on the overall thermal performance of the product. It is, therefore, of great interest to know how the thermal performance is affected by choosing either the accurate (measured) or the conservative (tabulated) value of the thermal conductivity. In this work, the two approaches are compared in a practical example – a high-performance window, Jelovica Jelofuture – using numerical simulations. Our study shows that modifying the thermal properties of individual materials generally leaves the thermal transmittances of the frame (Uf) and the window (UW) almost unaffected. If all of the materials considered are modified simultaneously, Uf changes by 1–2% while the change in UW remains below 1%. However, due to their small values, the calculated changes of Uf and UW may be significantly affected (further increased or reduced) by the rounding of the results according to the relevant standards. In contrast, using the tabulated value of linear thermal transmittance (Ψg) of the junction with the glazing leads to an overestimation of UW by up to 15%.
Keywords: thermal transmittance, numerical simulation, conductivity, tabulated and exact values, practical example
Published in DiRROS: 08.03.2024; Views: 87; Downloads: 44
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2. Novel thermal insulation with gas-filled cavities - assessment of thermal performance of different designs based on numerical simulations of heat transferMiha Jukić, Sabina Jordan, Danijel Lisičić, 2019, original scientific article Abstract: Not only is the energy efficiency of buildings nowadays becoming more and more important; the legislative requirements, the people’s awareness of the environmental questions and their thermal comfort expectations are also on a much higher level. All of these issues can be addressed by making the building envelope more thermally resistant. However, with the traditional thermal insulation materials the thickness of thermal insulation layers is already at the viable limits. Therefore, the development of new, more efficient thermal insulation products with a higher thermal resistance is highly promoted. Preliminary research results can be applied to models to develop and confirm the conceptual designs of such new materials. In this paper, an analysis of thermal performance is presented for a novel thermal insulation consisting of graphite polystyrene (GPS) matrix with cavities filled with an insulative gas, and a protective sheath to prevent it from leaking. Bearing in mind the suitability for later production, different configurations of the assembly were considered, regarding the matrix geometry, the type of the gas filling, and the surface emissivity of the cavities. A range of numerical simulations of heat transfer was conducted to determine the efficiency of different designs in reducing the conductive, the convective, and the radiative heat transfer. Advantages, limitations and some detailed parameters of the proposed design concepts were determined, which were then used for optimisation. The analysis of the results indicates that the equivalent thermal conductance of a GPS panel can be significantly reduced by the introduction of gas-filled cavities. The reduction is highly dependent on the type of the gas filling (thermal conductivity, viscosity, specific heat, etc.), the size of the cavities, and the cavity surface emissivity.
Keywords: gas-filled cavities, graphite polystyrene, numerical simulation, thermal insulation
Published in DiRROS: 15.09.2023; Views: 234; Downloads: 114
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4. Residual stress modeling and analysis in AISI A2 steel processed by an electrical discharge machineDinesh Kumar, Krishan Kant Singh Mer, Hoshiyar Singh Payal, Kapil Kumar, 2022, original scientific article Keywords: electro discharge machining, numerical simulation, residual stress, dielectric fluid, AISI A2 die steel, Gaussian distributed heat flux
Published in DiRROS: 22.06.2022; Views: 526; Downloads: 123
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