Simultaneous determination of overall thermal diffusivity and conductivity of composite building elements in situ

Abstract

A method has been developed for the simultaneous determination of thermal diffusivity and conductivity of enclosing fabrics of an unconditioned enclosure exposed to outdoor periodic excitations. These fabrics may be homogeneous or multilayered. It is only necessary to make measurements over a complete period of 24 h of the external and internal surface temperatures and of the heat flow rates at the internal surface. The observed surface temperatures and heat flow rates are represented by series of complex harmonic terms of which these were the real part. For any particular harmonic, the argument of the resulting complex expression representing the heat flux in terms of the two surface temperatures and the material properties could be equated with the argument of the complex expression, representing the observed heat flux, giving the value of thermal diffusivity. Having determined the thermal diffusivity, the thermal conductivity could be obtained by equating their modulii. The thermophysical coefficients, thus obtained, were more realistic in connection with the precise prediction of indoor climate of an unconditioned enclosure exposed outdoors and this method has been found to be sufficiently accurate for similar applications.