Property of water-nanoparticle flow and heat transfer in the double-tube heat exchanger with nanofluid

The three-dimensional numerical simulation is conducted to study the impact of Al2O3/water nanofluid on the flow property and heat transfer of a counter flow double-tube heat exchanger where a laminar flow of cold water is in the outer tube, and in the inner tube there is a turbulent flow of hot nanofluid with particle volume concentrations ranging from 0.05% to 3%. The effects of temperature, flow rate, and particle volume concentration on the property of nanofluid flow and heat transfer are explored. The results show that the friction factor decreases with the increase of temperature and flow rate, and the pressure drop increases with increasing particle volume concentration. The temperature of the nanofluid at the outlet first decreases rapidly with time and then increases gradually to a stable value. Along the flow direction, the temperature of nanofluid decreases slowly at x/D < 10, and the axial temperature gradient increases at the full development stage. At a constant Reynolds number, the lower the temperature or the larger the particle volume concentration, the stronger the heat transfer. The local heat transfer coefficient on the wall of the inner tube decreases rapidly after reaching the peak x/D = 2.3, and then increases slowly until the outlet. The particle number density decreases sharply at x/D < 5, and then slowly decreases, while the change of the geometric mean diameter of particles is just the opposite. Increasing the flow rate or the temperature will strengthen particle coagulation, resulting in the decrease of particle number density and the increase of geometric mean diameter of particles.