Heat transfer through cavity walls

Abstract

The aims of this study were: (a) to develop a numerical method and (b) to develop an experimental method for the prediction of heat transfer in a cavity in which radiative transfer, gaseous and solid conduction are occurring. In the experiments, the thermal conductivities of various materials (perspex, durotherm and particle board) were measured to within 1%. The rates of heat transfer across the materials (specimens) were measured for different values of: (a) temperature difference across the specimens - dT varied between 1°C and 51°C (b) material (specimen) thickness - 5.6mm, 9.7mm, 11mm, 12mm and 19mm thick specimens were used (c) cavity size - 30.5mm, 34.925mm, 35.5mm and 40.5mm diameter holes were used. The results are presented in graphical and tabular forms (rate of heat transfer versus temperature drop). As expected, the rates of total heat transfer decreased with increases in the hole size and specimen thickness. The effects of radiation transfer were assessed by blocking the holes with aluminium foil. The results showed that radiation transfer was small (about 5% of the total heat transferred). For four different values of the emissivity of aluminium (0.04, 0.11, 0.2 and 0.5), the method computed: (a) the heat transferred by solid conduction - this was found to be constant for all values of emissivity (b) the heat transferred by gaseous (air) conduction - this too did not vary with emissivity (c) the radiation transferred between the surfaces in the hole - this varied significantly with emissivity (d) the total heat transferred. The results obtained from the numerical simulations are presented in graphical and tabular forms. Depending on the emissivity value used, the percentage of radiation transfer varied between 2 and 15% of the total heat transferred. Correspondingly, the air conduction varied between 9 and 17% of the total heat transferred. Hence solid conduction was the dominant mode of heat transfer (68 to 89% of the total heat transferred). When using the material durotherm, the experimental and the theoretical results were in agreement within the limits of experimental error. When using perspex, only half of the results were within the limits of experimental error. The discrepancies for the perspex runs varied between 2 and 26%. Reasons are advanced to explain these discrepancies.