Abstract: Aiming at the problems of large volume,easy fouling and low heat transfer efficiency of underground air coolers
in high temperature metal mines,the synergistic heat transfer enhancement technology of nanofluids and rotating bonds is proposed,
and the effect of composite heat transfer enhancement mechanism on the performance of heat exchange tubes is systematically
explored. The heat transfer characteristics of eight different concentrations of ZnO nanofluids in finned tubes with built-in
spiral ties were studied experimentally. The effects of nanoparticle concentration on heat transfer coefficient,Nusselt number
(Nu),friction coefficient and comprehensive thermal performance factor (PEC) were analyzed. The results show that there is a
significant threshold relationship between the concentration of nanoparticles and the heat transfer performance. When the concentration
of ZnO nanofluids is φ≤0. 5%,nanoparticles can effectively enhance heat transfer through micro-convection effect
and thermal conductivity enhancement,and its optimal heat transfer coefficient is 19. 39% higher than that of pure water. After
exceeding the critical concentration,the sharp increase in viscosity caused by particle agglomeration becomes the dominant factor,
which weakens the heat transfer efficiency. The secondary flow generated by the spiral ties can effectively inhibit the deposition
of nanoparticles,and this synergistic effect enables the best comprehensive thermal performance at a concentration of φ =
0. 5%. The research results provide a reference for improving the heat transfer efficiency of deep well air coolers. Future research
can focus on the stability of nanofluids and achieve better heat transfer enhancement.

Key words: nanofluids, spiral ties, synergistically enhanced heat transfer, comprehensive thermal performance factor