Author: Ng, Wing-kin
Title: Determination of thermal conductivity of engineering materials using flash radiometry method
Degree: M.Sc.
Year: 1997
Subject: Materials -- Thermal properties
Heat -- Conduction
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Pages: v, 88 leaves : ill. ; 30 cm
Language: English
Abstract: Different types of commonly used porous polymeric insulants for chilled and hot water pipeworks inside buildings, such as polystyrene foam, opened-cell phenolic foam, and closed-cell phenolic foam, have been selected to verify the accuracy, validity, and applicability of the newly developed flash radiometry technique. The concept is to capture the thermal emission from the polymeric sample under measurement, which is irradiated by a high-power flash lamp, by using an infra-red detector. The obtained signal is then analyzed and mathematically fitted to the relevant theoretical relation, and the thermal diffusivity of the concerned sample can be estimated. This is the primary basis for determining the thermal conductivities of the polymeric samples. To evaluate the accuracy of the flash radiometry method, and the validity and applicability of the Effective Thermal Conductivity Method[6] (ETCM), the measured thermal conductivities were then compared with both the nominal values from manufacturers and the theoretical range demarcated by the longitudinal effective thermal conduction coefficient and the transverse effective thermal conduction coefficient. The results showed that the flash radiometry method is generally accurate, valid, and applicable for opened-cell and closed-cell phenolic foams. However, significant deviation was resulted for polystyrene foams. This can be explained by the influence from optical pulse and the existence of a virtual gap in the radiometry signal profile, due to the semi-transparent nature of the polystyrene samples. In addition, closed-cell phenolic foams with different porosities have been adopted to demonstrate the relationship between the thermal diffusivity, the thermal diffusion time constant, the thermal conductivity, and the porosity of polymeric foam. The results indicated that the thermal diffusivity increases steadily with the porosity of polymeric foam, while the thermal diffusion time constant, on the contrary, decreases steadily with the porosity of polymeric foam. The former can be interpreted by the significantly higher thermal diffusivity of air as compared with that of the polymeric matrix, which results in an increase of the overall thermal diffusivity of the composite as the porosity of polymeric foam becomes higher. This decreasing trend of the thermal diffusion time constant with the porosity of the polymeric foam is reasonable since it is inversely proportional to the thermal diffusivity. The thermal conductivity was found decreasing almost linearly with the porosity of polymeric foam. The result is obvious as air has a much lower thermal conductivity than that of the polymeric matrix. This is also in compliance with the arbitrary relationship predicted by Ziebland[18] for composite polymers.
Rights: All rights reserved
Access: restricted access

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