Performance evaluation of nanofluid flow in conical and helical coiled tubes Milad Zare 1 • Mohammad Mahdi Heyhat 1 Received: 15 May 2018 / Accepted: 28 June 2018 Ó Akade ´miai Kiado ´, Budapest, Hungary 2018 Abstract Nanofluid and coiled tubes have been employed as two passive methods for enhancing the heat transfer. In the present study, the turbulent flow of CuO–water nanofluid in helical and conical coiled tubes was numerically investigated with constant wall temperature through mixture model. The thermophysical properties of base fluid (water) were considered as temperature-dependent functions, while Brownian effects were adopted in thermal conductivity and dynamic viscosity of nanofluid. Simulation results were validated using experimental data for heat transfer coefficient and pressure drop in helical coiled tube for different Reynolds numbers. Four different geometries were simulated and compared. The first one was a conical coiled tube; the others were helical coiled tubes whose coil diameters were minimum, maximum, and median of the conical coiled tube pitch coil diameter. The velocity profiles indicated stronger secondary flow in conical coiled tube at a specified Dean number. The obtained results also showed higher heat transfer enhancement in the conical coiled tube in comparison with helical coiled tube with the same average pitch coil diameter. Moreover, the nanoparticle-induced heat transfer enhancement was more effective in conical coiled tube. Keywords Conical and helical coiled tubes  Thermal performance  Nanofluid  Mixture model  Heat transfer enhancement  k–e model List of Symbols C p Specific heat capacity (J kg -1 ) D Pitch circle diameter (m) d Pipe diameter (m) De Dean number h Heat transfer coefficient (W m -2 K -1 ) f Friction factor k Turbulence kinetic energy (J kg -1 ) Nu Nusselt number p Pressure (Pa) P Helical pitch (m) T Temperature (K) y ? Dimensionless wall distance y node First near-wall node (m) Greek letters q Density (kg m -3 ) l Dynamic viscosity (Pa s) u Volume fraction d Curvature ratio s Torsion ratio h Cone angle (radian) g Performance index Subscripts b Bulk dr Drift f Fluid eff Effective m Mixture p Particle T Turbulence w Wall Introduction Due to the presence of coil curvature-induced secondary flow, coiled tubes have higher heat transfer rates as com- pared with straight tubes. Hence, the coiled tube heat exchangers have been extensively applied in industrial & Mohammad Mahdi Heyhat mmheyhat@modares.ac.ir 1 Faculty of Mechanical Engineering, Tarbiat Modares University, Tehran, Iran 123 Journal of Thermal Analysis and Calorimetry https://doi.org/10.1007/s10973-018-7516-0