Design, characterization and heat transfer performance evaluation of carbon-based nanofluids for renewable energy applications

• Autores: Javier Pérez Vallejo
• Directores de la Tesis: Luis Lugo Latas (dir. tes.), José Fernández Seara (dir. tes.)
• Lectura: En la Universidade de Vigo ( España ) en 2019
• Idioma: inglés
• Tribunal Calificador de la Tesis: Patrice Estellé (presid.), Leonor Hernández López (secret.), Francisco Javier Navas (voc.)
• Programa de doctorado: Programa de Doctorado en Investigación en Tecnologías y Procesos Avanzados en la Industria por la Universidad de Vigo
• Materias:
  • Física
    • Química física
      • Física del estado liquido
    • Termodinámica
      • Física de la transmisión del calor
  • Ciencias tecnológicas
    • Procesos tecnológicos
      • Transferencia de calor
• Enlaces
  • Tesis en acceso abierto en: Investigo
• Resumen

  • The low thermal conductivity of the fluids commonly employed in heat transfer processes of industry and renewable energy applications is the main obstacle to increase their energy efficiency. The dispersion of nanoadditives with high thermal conductivity, constituting nanofluids, provides a great opportunity for the improvement of their heat transfer performance. Furthermore, dispersions at even very low nanoadditive concentrations have been discovered to produce huge variations in the optical profiles of the base fluids, leading to the creation of alternative volumetric light absorbers.

    Fluids widely employed in heating and cooling applications [water (W), ethylene glycol (EG), propylene glycol (PG) or mixtures among them] have been used as base fluids through this work. Regarding nanoadditives, carbon-based nanomaterials (graphite, carbon black, nanodiamonds) focused the attention, particularly graphene due to its outstanding thermal properties. Furthermore, nitride nanoparticles were employed in particular studies.

    Contributions to the entire proposal process of new nanofluids were carried out: nanopowder characterization, nanofluid design, searching for long-term stability, determination of rheological, thermophysical or optical profiles and evaluation of convection heat transfer performances. This thesis consists of different studies distributed into three main groups. The first includes rheological studies of various dispersions of carbon-based nanomaterials in glycolated waters, the second involves diverse characterizations of physical properties of nanofluids focusing on different renewable energy applications, and the third comprises experimental heat transfer performance evaluations for different functionalized graphene nanoplatelet (fGnP) dispersions.

    The rheological studies aim to describe the dynamic viscosity behaviour of different nanofluids. Dispersions of fGnP, carbon black, different-phase nanodiamonds (Nd) and different-pure graphite/diamond mixtures at mass fractions ranging from 0.0025 to 0.02 in W and EG:W and PG:W mixtures were analysed by means of two rotational rheometers coupled with suitable geometries at temperatures ranging from 278.15 to 353.15 K.

    The determination of the physical properties of nanofluids is fundamental to describe their efficiency. Different characterizations were conceived for various particular applications, as the heat transfer enhancement in geothermal collection and wind turbines refrigeration or the sunlight absorption improvement in direct solar absorbers. Vibrating tube or picnometry methods for density, differential scanning calorimetry for heat capacity, guarded heat flow and transient hot wire methods for thermal conductivity or rotational rheometry for viscosity were some of the employed techniques. The thermophysical profiles of different-concentrated nD/EG, fGnP/PG:W 30:70 wt%, and fGnP/commercial coolant nanofluids with temperature ranging from 278.15 to 353.15 K were experimentally measured. Furthermore, a specific study focused on heat capacity and density of various nitride/EG nanofluids was carried out. The optical profiles of fGnP/commercial coolant nanofluids were described by means of spectrophotometry.

    The heat transfer performance was evaluated through an experimental setup based on a tube-in-tube heat exchanger for fGnP/W, fGnP/PG:W 30:70 wt% and fGnP/commercial coolant nanofluids at four nanoadditive mass fractions from 0.0025 to 0.01, three temperatures and various flow rates. These studies allow to determine the optimal concentration and the temperature and turbulence dependences of convection heat transfer coefficients and pressure drops.