Browsing by Author "Marais, Hannelie"
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- ItemThermal performance of foam concrete structural façade elements with cavities(Stellenbosch : Stellenbosch University, 2020-12) Marais, Hannelie; Van Zijl, Gideon P. A. G.; De Villiers, Wibke Irmtraut; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: The housing backlog in South Africa is an ever growing problem. Efforts are put together to study this problem and to reacha sustainable economical solution. Increased interest in the use of foam concrete in the building environment leads to renewed research in its use in structural housing elements where outside temperatures are the main influencing factor.Foam concrete is a low density andlightweight material withexcellent thermal properties. The Centre for Development of Sustainable Infrastructure has worked on increasing durability and strength of foam concrete while still maintaining a low density and 3Dprintingofthis low density material. This study focusses on the detailed analysis of foam concrete cavity walls in terms of the cavity geometry and sizes for thermal comfort.Parameters that greatly influence the thermal performance of foam concrete include its material density and its thermal conductivity. Other parameters includeits porosity and effective pore size, specific heat, moisture content, core temperature rises and radiation inside the pores. These parameters also havean influence on each other.In some existing theoretical models pore volume fractions of the pore structure matrix are used to determine the effective thermal conductivity of foam concrete.Heat exchange between the environment and the concrete surface occurs through convectionand radiation and heat exchange that happens inside the concrete material occurs through conduction. In foam concrete, convection and radiation also takes place in the pores of the material. These heat transfer principles are analysed throughfinite element modelling of foam concrete wall sections.An existing 3D printed foam concrete wall with air cavities isanalysed usingfinite elements to determine the effect of conduction, convection and radiation on the thermal performance of the wall. A heat flux of 342 W/m2isapplied on one side(the outside)of the wall section in different heat transfer analysis cases. When convection and radiation isincluded on the outer sides of the wall sectionand conduction in the wall section, the outside temperature is41.798°C and the inside temperature is20.216°C. Heat istransferred through the foam concrete only and not throughthe air cavities which resultsin a non-uniformheat transfer. When the effects of cavity radiation isadded to the air cavities, the outside temperature is40.690°C and the inside temperature is21.407°C.A more uniform heat transfer occurs through the wall. When heat transfer through cavity convection is also added, the outside temperature is 40.540°C and the inside temperature is 21.573°C. The slight decrease in the outside temperature is due to the release of some heat in the first cavity and as heat gathers closerto the inside of the wall section, the inside temperature increases slightly compared to when only cavity radiation is added tothe wall section. When only heat transfer through conduction in the foam concrete and in the air cavities are modelled, the outside temperature is 42.022°C and the inside temperature is 20.407°C. Here it is evident that the effects of cavity radiation andcavity convection are neglected. The effect of different cavity sizesand geometriesof walls with equal total thicknessesisalso researched. For fixed distances between rectangular cavities, the inside temperature is reducedby increasing the number of cavities. More cavities resultin a reduced cavity thickness and thus a reduced void ratio which is the ratio between the total thickness of the air cavities and the total thickness of the wall. However, when a wall sectionwith one setthickness between cavitiesand another wall section with another set thickness between cavities, are compared, the different void ratios as a result of the different thicknesses between cavities, can result in similar inside temperatures. Thus, a trend of reduced efficiency is suggested.An analytical verification is done on the numerical results of some of the wall sections with rectangular cavities. A thermal resistance networkis usedwhichonly accounts for conduction of thewall and the air cavities. Results are 3% higher than the numerical results.It is thus concluded that the effect of cavity radiation and to a lesser extent cavity convection should not be neglected. Negligence ofthe effect of cavity radiation and cavity convection may lead tothe prediction of a betterthermal performance results than the reality. Itis also concluded that cavity walls with more cavities rather than less, and thinner cavities giverise to an increased thermal performance.