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Natural Convection in Solar Houses: A Study by Holographic Interferometry

Christian Durou
Laboratoire de Physique Electronique et Energie Solaire, Université Paul Sabatier, 118, route de Narbonne, 31062 Toulouse Cedex, France

Alain Cordier
Laboratoire de Physique Electronique et Energie Solaire, Université Paul Sabatier, 118, route de Narbonne, 31062 Toulouse Cedex, France


Natural convection plays a fundamental role in heat transfer processes in solar houses : convection in storage devices in active systems, air movements in passive systems. Analytical or numerical methods are practically useless for tridimensional problems encountered in solar building. Even in bidimensional situations, numerical methods are very time-consuming and expensive. On the contrary, experimental methods are usefull to investigate how heat transfer processes take place in a solar system. Among them, optical interferometry is a very interesting one which gives a direct picture of the whole map of temperature within the transparent space where heat transfer takes place and which allows precise measurements of heat transfer coefficients h and Nusselt numbers along the interfaces wall-transparent medium. /1/
Conventional interferometers, such as the Mach-Zehnder set-up, have been used for half a century to visualize thermal fields and to measure heat transfer coefficients. However, these methods very used in aeronautical research are very expensive because the model must be equipped with optically perfect windows. On the contrary, holographic interferometry allows the use of inexpensive materials such as Plexiglass to make the models, and then, it is very attractive for studies on natural convection in reduced scale models of active and passive solar buildings.
Interferometry is sensitive to refractive index variations due to pression, composition of the fluid or temperature variations. In our studies, the only parameter that varies is temperature. Then, isotherms are identical with equal index lines. The knowledge of n = f(T) for the fluid that fills the models permits to determine isotherms from the fringe patterns of the interferograms. /2/
Holographic interferometry, due to the optical memory property of holograms, allows us to compare two successive states of the same object. For example, if we consider a model in which heat transfer takes place, we can compare the model with non-uniform wall temperatures, with the same model whose walls are isotherm. In this case, each fringe in the interferogram represents an isothermal line which appears on the picture of the model. /3/
We must remember, however, that interferometry suffers two main limitations :
1 - Only two-dimensional heat transfer processes can be easily studied, because of the integration of the information along the light path in the third dimension ;
2 - Similitude probems arise when working on reduced scale models for the coupling of convective and radiative heat transfers.
Moreover, refractive effects must be made as low as possible in order to have negligible influence on the interferogram, even in the high temperature gradient parts of the model, i.e. near the hot and cold walls.

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