The thermal properties of a variable aspect ratio cavity wall

Aviram, David (2000) The thermal properties of a variable aspect ratio cavity wall. (PhD thesis), Kingston University,


An experimental investigation of the thermal properties of a variable cavity wall has been conducted with the aid of a Guarded Hot Box (GHB). The main objective of the investigation was to determine the thermal trends of that exist in such a wall at different aspect ratios and internal configurations. In the course of this research effort, attention had initially focused on the suitability of the GHB as a tool for measuring building components of low thermal transmittance. Following the initial evaluation that included computational modelling of the GHB, a series of modifications were employed, which included a series of baffie plates in the Guard box area. Experimental trials have shown that these modifications culminated in a reduced thermal gradient distribution within the box and along the test wall. As a result, the test wall was subjected to a more uniform heat flux and lower peripheral heat loss. A variable cavity wall measuring 1.2m by 1.2m and 0.41m deep was the main 'focus of this study. By means of a moveable brick leaf, the aspect ratio of the cavity wall could be remotely altered during the experiment, thereby allowing immediate comparison of thermal trends without the errors that are associated with building and testing a series of individual walls of different geometric proportions. In particular, this set-up enabled an accurate comparative analysis of cavity aspect ratios over a range of 15 to 30. Lazer Doppler Anemometry (LDA) and thermal measurement on the four surfaces of the cavity wall leafs were the prime means for collating experimental data. Extensive computational modelling complemented the research, which provided important insights both prior to and following the experimental stage. The use of Computational Fluid Dynamics (CFD), while not intended for precise solutions to models of the GHB and cavity wall, was never the less instrumental in establishing trends and expanding the experimental range, once corroboration of experimental results had been achieved. The experimental and computational results show that with successive cavity closure an optimum aspect ratio is reached, where thermal resistance peaks and velocity of the convective flow is minimal. At this aspect ratio, the flow regime was found to be conductive. The main implication of this result is that decreasing aspect ratio beyond this aspect ratio, by widening the cavity, will result in increasing heat losses due to the circulation of convective currents in the cavity. Thus, it was concluded that when convection diminishes, the thermal resistance of the air cavity would rise. Further computational and experimental work on the same wall with an internal partition, corroborated the trends found during the clear cavity experiment. It was found that a centrally placed vertical partition will double the thermal resistance of the wall. Furthermore, the thermal resistance of the partition was found to equal that of one partitioned cavity, raising the possibility of eliminating cavities from wall construction The effect of mortar joints upon cavity walls, at various aspect ratios, was also investigated. Results show that a vertically sinusoidal flow pattern exists in such cavities due to the thermal bridging effect of the mortar joints. The results of this study were used for several recommendations, which deal both with design of cavity walls and Guarded Hot Box design and operation.

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