A numerical study has been undertaken to explore the details of forced convection heat transfer in finned aluminum foam heat sinks. Calculations are made using a finite-volume computational fluid dynamics (CFD) code that solves for the flow and heat transfer in conjugate fluid/porous/solid domains. The results indicate that using unfinned blocks of porous aluminum results in low convective heat transfer due to the relatively low effective thermal conductivity of the porous aluminum. The addition of aluminum fins to the heat sink significantly enhances the heat transfer with only a moderate pressure drop penalty. The convective enhancement is maximized when thermal boundary layers between adjacent fins merge together and become nearly developed for much of the length of the heat sink. It is found that the heat transfer enhancement is due to increased heat entrainment into the aluminum foam by conduction. A model for the equivalent conductivity of the finned/foam heat sinks is developed using extended surface theory. This model is used to explain the heat transfer enhancement as an increase in equivalent conductivity of the device. The model is also shown to predict the heat transfer for various heat sink geometries based on a single CFD calculation to find the equivalent conductivity of the device. This model will find utility in characterizing heat sinks and in allowing for quick assessments of the effect of varying heat sink properties.

A linear stability analysis is undertaken for plane pulsatile Poiseuille flow. The stability picture is described in terms of two distinct ranges of the Womersley number, Wo, which is the ratio of pulsatile to viscous forces. The low-frequency range, termed precritical, has dominant viscous effects, while the high-frequency range, termed postcritical, has dominant inertial effects. A critical Womersley number, Wocr=1.107, is identified, where viscous and inertial forces are in balance. In the precritical range, viscous forces become rapidly dominated by impulse forces, resulting in a sharp drop of the critical Reynolds number with increasing Wo. In the postcritical range, inertial forces become increasingly dominant over impulse forces, resulting in flow stabilization. In the limits Wo→0 and Wo→∞, the flow recovers the stability characteristics of the steady base flow, independently of the pressure amplitude. In other words, pulsating the flow is always destabilizing. The results reported in the postcritical range are in agreement with existing theoretical and experimental works.