The current study explained the mechanism of heat transfer enhancement of nanofluids from the micro-flow aspect. The Eulerian-Eulerian and Euler-Lagrange multiphase models are implemented respectively to explore the flow field of nanofluids inside a horizontal circular tube under turbulent state.The pressure loss is used to validate the accuracy of predictions for each model

which is the most important flow parameter in turbulent flow but also is frequently ignored in previous numerical work.The results indicated that there is obvious velocity slip phenomenon between water and nanoparticles.Compared with the pure water

the development of nanofluids boundary layer is inevitably disturbed

which consequently decreases its thickness and thermal resistance.The distribution of nanoparticles in the entire flow field is not uniform

which improves overall heat transfer capacity inside the boundary layer. The present authors suggest the enhanced heat transfer capability of nanofluids is the result of altered flow characteristics when nanoparticles are present and the flow conditions are most important drivers of heat transfer enhancement in nanofluids. The thorough understanding on flow characteristics of nanofluids is essential for nanofluids flow

which is the basis of further utilization of nanofluids in engineering applications.