Convective heat transfer of nanofluids in a concentric annulus

• We modeled convection of nanofluids with density variation included.
• Solutions for nanofluid convective flows in concentric annuli are obtained.
• Two distinctive thermal boundary conditions are examined in detail.
• Anomalous heat transfer exits for the case of the heated outer wall.
• The anomaly level is found higher for the smaller inner to outer diameter ratio.

Forced convective heat transfer of nanofluids in a concentric annulus is investigated theoretically to seek possible anomalous heat transfer enhancement associated with nanofluids convection, in which the heat transfer rate exceeds the rate expected from the increase in thermal conductivity of nanofluids. The Buongiorno model for convective heat transfer in nanofluids was modified to fully account for the effects of nanoparticle volume fraction distribution on the continuity, momentum and energy equations. The effects of the inner to outer diameter ratio, thermal boundary conditions on the fully developed Nusselt number have been investigated. Anomalous heat transfer enhancement has been captured for the case of the heated outer wall with the inner wall insulated. This anomaly level is found higher when the inner to outer diameter ratio is smaller. The effects of Brownian and thermophoretic diffusivities ratio, bulk mean nanoparticle volume fraction and nanoparticle type on pressure gradient and Nusselt number are discussed in depth for the case of the heated outer wall with the inner wall insulated and fixed inner to outer diameter ratio ζ = 0.5. It has been found that Nusselt number has optimal bulk mean nanoparticle volume fraction value for alumina–water nanofluids, whereas it only increases monotonously with bulk mean nanoparticle volume fraction for titania–water nanofluids.