title: Nanofluid heat transfer under mixed convection flow in a tube for solar thermal energy applications.
abstract: The solar flat plate collector operating under different convective modes has low efficiency for energy conversion. The energy absorbed by the working fluid in the collector system and its heat transfer characteristics vary with solar insolation and mass flow rate. The performance of the system is improved by reducing the losses from the collector. Various passive methods have been devised to aid energy absorption by the working fluid. Also, working fluids are modified using nanoparticles to improve the thermal properties of the fluid. In the present work, simulation and experimental studies are undertaken for pipe flow at constant heat flux boundary condition in the mixed convection mode. The working fluid at low Reynolds number in the mixed laminar flow range is undertaken with water in thermosyphon mode for different inclination angles of the tube. Local and average coefficients are determined experimentally and compared with theoretical values for water-based Al2O3 nanofluids. The results show an enhancement in heat transfer in the experimental range with Rayleigh number at higher inclinations of the collector tube for water and nanofluids.
authors.affiliation: Energy Division, School of Mechanical and Building Sciences, VIT University, Vellore, 632014, Tamil Nadu, India. sekhar.rajay@gmail.com.
authors.forename: Y Raja
authors.initials: YR
authors.lastname: Sekhar
authors.affiliation: Department of Mechanical Engineering, Universiti Teknologi Petronas, Bandar Seri Iskandar, 32610, Tronoh, Perak, Malaysia.
authors.forename: K V
authors.initials: KV
authors.lastname: Sharma
authors.affiliation: Department of Petroleum Engineering, Universiti Teknologi Petronas, Bandar Seri Iskandar, 32610, Tronoh, Perak, Malaysia.
authors.forename: Subhash
authors.initials: S
authors.lastname: Kamal
mesh_terms: D018481:Convection; D006358:Hot Temperature; D053758:Nanoparticles; D012993:Solar Energy; D014867:Water
chemical_list: D014867:Water
keywords: Al2O3 nanofluid; Average heat transfer coefficient; Laminar mixed convection; Tube inclination