Enhancing and attenuating heat transfer characteristics for circulating flows of nanofluids within rectangular enclosures

Due to wide applications of nanofluids to designs of thermal engineering systems, the free convection participated by nanofluids inside rectangular enclosures has been intensively investigated. Generally, rightward-traveling heat transfer across a rectangular enclosure is enhanced when a pure base fluid is replaced with a nanofluid. Amid numerous dedicated literature studies of such a phenomenon, a somewhat anomalous behavior for at least four types of nanofluids has been unreported. Here we have numerically discovered this behavior in terms of averaged Nusselt number versus the nanoparticle volume fraction parameterized in enclosure-geometry Aspect ratio and Rayleigh number. At AR >= 2.5 and Ra = 10(4), a regime, in which the average Nusselt number may enhance or diminish as the nanoparticle volume fraction increases, has unexpectedly emerged. Furthermore, as the aspect ratio increases, the average Nusselt number features mildly zigzag performances at large Rayleigh numbers. Being validated with simulation results, mechanisms that govern the emergence of such phenomena are also identified. Our study may benefit future investigations that desire to understand heat-transfer mechanisms of nanofluids and to optimize heat-transfer characteristics for various thermal systems.