Effective Thermal Conductivity of Layered Porous Media

Sintered porous media has been employed on mini heat pipes (MHP) in the capillary transport of working fluid from the condenser to the evaporator section. High porosity media has high permeability represen ting low liquid pressure drop, while low porosity media increases the capillary pumping capacity of the hea t pipe. LABTUCAL (Heat Pipe Laboratory ‐ UFSC, Brazil) developed porous media made of layers of different porosities, which can provide high capillary pumpin g performance and keep low liquid pressure drops. T he combination of these two properties should enhance the heat transfer capacity of the mini heat pipe. T he effective thermal conductivity of such porous media is an important parameter employed in the design o f mini heat pipes. In this work, a set up developed f or measuring the effective thermal conductivity of porous media is described. Samples with porous media layers were constructed and tested in vacuum and filled with saturated distilled water. The tests were conducted in the temperature range of 20 °C to 80 °C. The experimental data were compared with literature model results. The samples were tested on a vacuum chamber with a guard heater as radiation insulator. The samples used had nominal dimensions of 200mm x 30mm x 10mm. The two porous media layers were fabricated from 95% of pure atomized copper powder, with different particle size (average size of 20 an d 50µm) and deposited as overlapping layers resulti ng in porosities of 42 and 52%, respectively. The interfa cial characteristic length was determined applying statistical image analyses using software IMAGO ® . Porosity and frequency correlations were used as evaluation parameters. The effect of porosity and o f the presence of the working fluid in the effectiv e thermal conductivity was analyzed and evaluated.