Numerical study of nanofluid-based direct absorber solar collector systems with metallic/carbon nanoparticles, multiple geometries and multi-mode heat transfer

Nanofluids are complex colloidal suspensions comprising nanoparticles (metallic or carbon based or both) suspended in a base fluid (e.g. water). The resulting suspension provides demonstrably greater thermal performance than base fluids on their own without the agglomeration or sedimentation effects associated with larger (micron-sized) particles. The substantial elevation in thermal conductivity achieved with nanoparticles has made nanofluids very attractive for numerous energy applications including solar collectors. Solar energy is a clean, renewable source available and is essential for all life to exist on earth. Current technology which harvests solar energy with heat transfer fluids (HTFs) e.g., Direct Absorber Solar Collectors (DASCs), Flat Plat Solar Collector (FPCs), Parabolic Trough Solar Collector (PTSCs) etc, still requires continuous improvement in achieving higher efficiencies and greater sustainability. Nanotechnology has emerged as a significant area in recent years and features the use of sophisticated “green” nanomaterials embedded in conventional engineering materials. In this PhD a range of different DASC geometries are explored (annular, trapezoidal, prismatic, quadrilateral, biomimetic channel etc) with a variety of real nanofluids (water-based with metallic nanoparticles such as silver, copper, gold, zinc, titanium etc or carbon based e.g. diamond, graphite etc). Viscous incompressible laminar flows using Newtonian fluid models (Navier-Stokes equations) with thermal convection and radiative heat transfer are considered both with and without thermal buoyancy. Several thermal radiative flux models are deployed to mimic solar radiation effects such as the Rosseland model, P1 Traugott model, Chandrasekhar discrete ordinates model (DOM). ANSYS FLUENT and MAPLE symbolic software are used as the numerical tools to solve the relevant boundary value problems. Generally, the Tiwari-Das nanoscale model is used although the Buongiorno two-component nanofluid model (with thermophoresis and Brownian motion) has also been deployed. Extensive visualizations of streamline and isotherms are computed. Validation with alternative numerical methods and experimental studies is also included. Comprehensive appraisal of the relative performance of different nanofluids is evaluated. Generally, non-magnetic nanoparticles are studied although for the biomimetic channel (solar pump) case magnetic nanoparticles are addressed. The simulations show the significant improvement in thermal conductivities (and thermal efficiency) achieved with different types of geometry and nanoparticle type. Aspect ratio and inclination effects are also considered for some DASC cases. Extensive physical interpretation of thermofluid characteristics is provided. Where possible key dimensionless scaling parameters (Rayleigh number, Nusselt number, Prandtl number, Rosseland number etc) are utilized. The analyses reported herein constitute significant novel developments in solar collector nanofluid dynamics and many chapters have been published in leading international journals and conferences. The results have furnished good guidance for solar designers to assist in the selection of different geometries, nanoparticle types and volume fraction (percentage doping) for larger scale deployment in the future. Furthermore, some pathways for extending the current simulations to e.g. non-Newtonian nanofluid physics, turbulence etc are also outlined

On the role of nanofluids in thermal-hydraulic performance of heat exchangers - a review

Heat exchangers are key components in many of the devices seen in our everyday life. They are employed in many applications such as land vehicles, power plants, marine gas turbines, oil refineries, air-conditioning, and domestic water heating. Their operating mechanism depends on providing a flow of thermal energy between two or more mediums of different temperatures. The thermo-economics considerations of such devices have set the need for developing this equipment further, which is very challenging when taking into account the complexity of the operational conditions and expansion limitation of the technology. For such reasons, this work provides a systematic review of the state-of-the-art heat exchanger technology and the progress towards using nanofluids for enhancing their thermal-hydraulic performance. Firstly, the general operational theory of heat exchangers is presented. Then, an in-depth focus on different types of heat exchangers, plate-frame and plate-fin heat exchangers, is presented. Moreover, an introduction to nanofluids developments, thermophysical properties, and their influence on the thermal-hydraulic performance of heat exchangers are also discussed. Thus, the primary purpose of this work is not only to describe the previously published literature, but also to emphasize the important role of nanofluids and how this category of advanced fluids can significantly increase the thermal efficiency of heat exchangers for possible future applications