Experimental Tests of Conduction/Convection Heat Transfer in Very High Porosity Foams with Lattice Structures, Immersed in Different Fluids

This experimental work presents the results of measurements of thermal conductivity λ and convection heat transfer coefficient h on regular structure PLA and aluminium foams with low density ratio (~0.15), carried out with a TCP (thermal conductivity probe), built by the authors’ laboratory. Measurements were performed with two fluids, water and air: pure fluids, and samples with the PLA and aluminium foams immersed in both fluids have been tested. Four temperatures (10, 20, 30, 40 °C) and various temperature differences during the tests ΔT (between 0.35 and 9 °C) were applied. Also, tests in water mixed with 0.5% of a gel (agar agar) have been run in order to increase the water viscosity and to avoid convection starting. For these tests, at the end of the heating, the temperature of the probe reaches steady-state values, when all the thermal power supplied by the probe is transferred to the cooled cell wall; thermal conductivity was also evaluated through the guarded hot ring (GHR) method. A difference was found between the results of λ in steady-state and transient regimes, likely due to the difference of the sample volume interested by heating during the tests. Also, the effect of the temperature difference ΔT on the behaviour of the pure fluid and foams was outlined. The mutual effect of thermal conductivity and free convection heat transfer results in being extremely important to describe the behaviour of such kinds of composites when they are used to increase or to reduce the heat transfer, as heat conductors or insulators. Very few works are present in the literature about this subject, above all, ones regarding low-density regular structures.