Hot-wire method for measuring effective thermal conductivity of porous media

The effective thermal conductivity lambda(e) in porous media is often measured by the transient hot-wire method. This technique uses an identification method based on the Blackwell solution that makes some valuable assumptions (heat transfers are one-dimensional and by conduction only, and the sample is considered as an infinite medium), which are the origin of vagueness. A two-dimensional and dynamic model of heat transfer describing the functioning of the hot-wire method is developed. The numerical solution of the equations allows the tracing of the time-space evolution of the state variables (temperature, pressure, velocity) inside the porous medium and the determination of the validity domain of the Blackwell identification method. The results show that the Blackwell technique is accurate (better than 1%) for building materials where the wire is thin compared to the sample (gamma < 5 x 10(-3)), the aspect ratio of the sample is :>= 1.5, and the Rayleigh number is <= 100.