Forced Convection Heat Transfer and Hydraulic Losses in Graphitic Foam

Abstract

Experiments and computations are presented to quantify the convective heat transfer and the hydraulic loss that is obtained by forcing water through blocks of graphitic foam (GF) heated from one side. Experiments have been conducted in a small-scale water tunnel instrumented to measure the pressure drop and the temperature rise of water passing through the foam and the base temperature and heat flux into the foam block. The experimental data were then used to calibrate a thermal non-equilibrium finite-volume model to facilitate comparisons between GF and aluminum foam. Comparisons of the pressure drop indicate that both normal and compressed aluminum foams are significantly more permeable than GF. Results of the heat transfer indicate that the maximum possible heat dissipation from a given surface is reached using very thin layers of aluminum foam due to the inability of the foam to entrain heat into its internal structure. In contrast, graphitic foam is able to entrain heat deep into the foam structure due to its high extended surface efficiency and thus much more heat can be transferred from a given surface area. The higher extended surface efficiency is mainly due to the combination of moderate porosity and higher solid-phase conductivity.

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“…Such features will undoubtedly have an impact on the flow field, thereby influencing the observed permeability. This may serve to explain why the results for 88% reported by Straatman et al [26] are of the same magnitude as those predicted using the present model for 80%. Also included for comparison in Fig.…”

“…Klett et al [25] ran experiments on POCO™ foam of 75% porosity, while the experimental results shown from Straatman et al [26] correspond to a porosity of 88%. Though the results are in reasonable correspondence with the current predictions, physical SVP porous media is known to have non-spherical pores, jagged pore windows and blocked regions, all of which will disrupt the flow.…”

“…While the unit cube model greatly underpredicts the pressure drop, the results of Anghelescu [3] computed on a more realistic geometry are in closer correspondence to the present modeled results. The extracted permeability and Forchheimer coefficients are given in Table 2 where they are compared with the values obtained by DeGroot and Straatman [5], and Straatman et al [26].…”

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“…The plots in The results show that, for an air velocity of 1 m/s, the thin layer of foam can increase the heat transfer by a factor of three even if the foam ligaments have a conductivity of only 10 W/mK. The reason is that the ligaments are working as highly efficient fins [26]. The heat transfer in the porous channel is enhanced by the higher velocity due to restrictions in flow area and the thermal conductivity of the foam ligaments; but in this case the primary cause for the enhancement is the extended surface area available for convection.…”

“…At high velocities, the quadratic term in the Darcy-Forchheimer equation cannot be neglected and the flow regime is "non-Darcy". Equation (10) has been used to determine K and c f from experimental measurements of pressure drop and uniform filter velocity for a large variety of porous materials: aluminum, nickel and carbon foams [21], compressed aluminum foam [25] and carbon foam [26]. Experimental testing on aluminum foams showed that permeability of a porous material is a strong function of porosity and pore size and inertial coefficient is influenced by the solid phase shape and pore structure [23].…”