Buoyancy driven flow and heat transfer of nanofluids past a square cylinder in vertically upward flow

The present work simulates the buoyancy driven mixed convective flow and heat transfer characteristics of water-based nanofluid past a square cylinder in vertically upward flow using a SUPG (Streamline Upwind Petrov-Galerkin) based finite element method. Nano sized copper (Cu) and alumina (Al2O3) particles suspended in water are used with Prandtl number (Pr) = 6.9. The range of nanoparticle volume fractions (phi) considered is 0 <= phi <= 20%. Computations are carried out at a representative Reynolds number (Re) of 100. Effect of aiding and opposing buoyancy is brought about by considering the Richardson number (Ri) range -0.5 <= Ri <= 0.5. Al2O3-water and Cu-water nanofluids show suppression of vortex shedding at Ri >= 0.15. Vortex shedding process is initiated and a completely new phenomenon is discovered when the nanofluid solid volume fraction,phi, is increased. For Al2O3-water nanofluids, at Ri = 0.15, completely periodic vortex shedding is found for phi >= 10%. For Cu-water nanofluid, shedding is observed for both Ri = 0.15 and Ri = 0.5. At Ri = 0.15 shedding is found at phi >= 5%, whereas at Ri = 0.5 it is at phi >= 15%. A new expression for convective instability of nanofluids is derived to calculate critical nanofluid solid volume fraction. The local Nusselt number increases with increasing phi. At a fixed phi, the time averaged local Nusselt number is higher for Cu-water nanofluids, as compared to Al2O3-water nanofluids. The average Nusselt number (Nu(avg)) increases with the concentration of phi. Cu-water based nanofluids show higher magnitudes of Nu(avg) compared to Al2O3-water nanofluids. (C) 2012 Elsevier Ltd. All rights reserved.