Heat transfer, friction factor and exergy efficiency analysis of nanodiamond-Fe3O4/water hybrid nanofluids in a tube with twisted tape inserts

In the present study, the Nusselt number, exergy efficiency, entropy generation and themal performance factor of nanodiamond-Fe3O4/water hybrid nanofluids in a tube with twisted tape inserts have been conducted experimentally. The experiments have been performed under turbulent flow regime in different Reynolds number ranging from 2000 to 22000, volume loadings from 0 to 0.2 %, and twisted tape inserts, H/D of 5, 10, and 15, respectively. It is found that, compared to water plain tube, the Nusselt number and exergy efficiency of 0.2 % vol. of nanofluid in a tube is raised by 29.55 % and 18.13 % with twisted tape insert, H/D of 5, the Nusselt number and exergy efficiency is further raised by 73.04 % and 161.31 % at a Reynolds number of 20,095 with a maximum friction factor penalty of 61.93 %. The thermal entropy generation is dropped by 30.72 % for 0.2 % vol. of nanofluid in a tube and with twisted tape insert, H/D of 5, the thermal entropy generation is further dropped by 48.58 % compared to water. Maximum increase of frictional entropy generation is about 74.81 % at 0.2 % vol. of nanofluid with twisted tape insert of H/D = 5 over base fluid. New Nusselt number and friction factor correlations were developed.