What dominates heat transfer performance of a double-pipe heat exchanger

ZnO–water nanofluid in an inner pipe was experimentally investigated to improve the thermal efficiency of double-pipe heat exchangers. In this work, a 57.6% increase in the Nusselt number was obtained at Re = 14,340 when the ZnO–water nanofluid flowed through a helically corrugated tube. It was found that helical corrugation played a more important role in heat transfer enhancement than thermophysical properties of the nanofluid at the Reynolds number below 10,221. In addition, with the increase of the Reynolds number, the advantages of nanofluids in thermal performance become obvious, and Nusselt numbers increase by 28.5 and 30.6% with the effects of helical corrugation and ZnO–nanofluid, respectively, at Re = 14,349.