Heat transfer and thermally induced stresses in window glass coated with optically active nano-particles
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Reflective or absorptive coatings for solar control on windows are popular in the architectural and automotive industries. In general, noble metal coatings have been used in reflective applications, and various heat-absorbing dielectric compounds in absorptive ones. The ultimate objective is to moderate incoming infra-red radiation while simultaneously preserving the desirable transparent nature of the window. In addition, one problem with merely absorbing infra-red radiation by the glazing system is that the coating and hence the surface of the special glass becomes very hot. This increased glass temperature will result in thermal stresses leading to an expansion of the glass, which, if not matched by an expansion of the window frame, can cause buckling and cracking. The objective of this project has been to study heat transfer from and through glass surfaces to which IR-screening surface coatings have been applied, and to model the distribution of the resulting thermally-induced stresses in the glass. The use of coatings of gold nano-particles in an absorptive role has hardly been considered previously. The present study explores the characteristics of such coatings by subjecting panes of various experimental and commercially available glasses to illumination by an array of incandescent lamps at 500 W/m2, which is a representative figure for a vertical east- or west-facing window in Sydney, Australia or Houston, USA, during March and September. The heat transfer through the samples was determined and used to guide the subsequent finite element analysis. This provided an indication of the thermally induced stresses developed on the glass surfaces due to heat released by the absorptive coatings.
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