Some Methods to Estimate the Thermal Conductivity of Insulating Media - Application to the Characterization of Fluids in Laminar Couette Flow

In first part, the method described here is to measure thermal conductivity of super insulating materials. The principle is based on a simple transient experiment and a single temperature measurement. The main idea is to control the heat flux diffusion in the sample by adjunction of a semi-infinite highly conductive medium. In second part, we present a transient method to estimate thermophysical properties and viscosity of fluid in Couette flow. It is an extension of the previous method. INTRODUCTION Designing an experimental device to estimate thermophysical conductive properties of superinsulating materials is generally difficult. The use of the transient flash method (see Parker and al , 1961; Degiovanni, 1977) to measure thermal diffusivity is also difficult due to the influence of heat losses around the sample. Some authors (see Martin et al, 1994) have tried to improve the experiment by adding 2 metal plates on either side of the sample. However, the experiment becomes more complicated and the influence of the lateral heat losses is only attenuated. The popular hot wire method (see Carslaw and Jaeger, 1959) is easier to implement. However, even if the cylindrical semi-infinite medium assumption avoids the problem of considering heat losses and at medium faces, some loss effects are possible at the ends of the wire. Moreover, large temperature gradients around the wire, due to the geometry, can introduce some estimation errors in the case of non-linear heat transfer. The new device proposed here tries to combine all advantages of previous methods. The main idea is to control the heat flux diffusion inside the insulating sample by addition of a highly conductive metal support. No regulated heat sink and fluxmeter is then needed. A probe similar to the hot wire system is used to measure only one temperature evolution on a planar heating device. Therefore, the transfer becomes quite 1D and steady, even if a model considering 2D geometry and transient state is necessary. In second part, an extension of this method is proposed for thermal characterization of fluid in Couette flow. The main difficulty is to solve the transient heat transfer trough the multilayer system (see David and al, 1993; Soliman and al, 1967; Osizik). We present in this paper an extension of quadripole formalism. NOMENCLATURE A, B, C, D Quadripole elements Q Excitation heat flux Rc Contact resistance T Temperature a Thermal diffusivity x, y, z Spatial coordinate p Laplace parameter t Time E Thermal effusivity b, L Lateral dimensions