experimental investigation on heat transfer of silver-oil nanofluid in concentric annular tube

in order to examine the laminar convective heat transfer of nanofluid, experiments carried out using silver-oil nanofluid in a concentric annulus with outer constant heat flux as boundary condition. silver-oil nanofluid prepared by electrical explosion of wire technique and observed no nanoparticles agglomeration during nanofluid preparation process and carried out experiments. the average size of particles established to 20 nm. nanofluids with various particle weight fractions of 0.12%wt., 0.36%wt. and 0.72%wt. were employed. the nanofluid flowing between the tubes is heated by an electrical heating coil wrapped around it. the effects of different parameters such as flow reynolds number, diameter ratio and nanofluid particle concentration on heat transfer coefficient are studied. results show that, heat transfer coefficient  and nusselt number increased by using naanofluid instead of pure oil. maximum enhancement of heat transfer coefficient occurs in 0.72% wt. also results indicate that heat transfer coefficient  increase slightly by using low wt. concentration of nanofluids.