Natural Convection Heat Transfer in a Horizontal Thermal Insulation Layer Underlying an Air Layer

Doktorsavhandling, 1995

In the thesis a study is made of the influence of natural convection on heat transfer in a composite system comprising a porous material heated from below and an air space situated above this. The measuring apparatus, a Wind Box which has been designed and developed at the Department of Building Physics is described. This apparatus is a prototype; its main difference from a traditional guarded hot plate apparatus is its large measuring area (0.6 x 1.2 m) over which the material can be installed up to a thickness of 0.5 m and its thermal resistance when exposed to an upwards heat flux determined. In designing the measuring process, calculation of the maximum systematic error has been a control parameter; this is less than 6%. The study includes a review of the literature, measurements in a horizontal guarded hot plate apparatus, measurements in the Wind Box and calculations with a recently developed simulation program (CONVBOX) for three-dimensional convection problems. The combination of simulation and experiments performed in the Wind Box has been found to be an important instrument in understanding the complex coupled convection processes which occur in the insulation material. Good agreement between modelling and experiment has been obtained for the studied case of natural convection. Two types of experimental investigations have been performed. The first is made in a horizontal guarded hot plate apparatus in order to find whether small scale apparatus can be used to study the influence of natural convection on heat transfer through a porous material. The second is made in the Wind Box in order to study the influence of the air space on the convective component of heat transfer in the underlying material. The investigations focus on a model material consisting of polystyrene pellets of cylindrical shape.