Modeling and Simulation Techniques of Nanofluid-Based Systems for Applications in Solar Energy

  • Omar Sharaf

Student thesis: Doctoral Thesis

Abstract

Motivated by their unique and adaptable optical, thermal, rheological, catalytic, biological, magnetic, and electrical properties, nanofluids (engineering colloidal dispersions with nanosized particles) have found numerous applications in engineering. The objective of this work is to advance our understanding of the highly coupled optical, thermal, colloidal, and hydrodynamic interactions in nanofluid-based solar and thermal systems. This requires a multidisciplinary approach based on multiphase flow, colloidal science, and nanoscale energy transfer. As such, novel fully continuous (FC) and continuous-discrete (CD) modeling techniques and computational algorithms with different levels of coupling were developed using in-house codes for simulating nanofluid-based low-flux direct-absorption solar collectors (DASCs) and parallel-plate microchannels (PPM). The FC approach was based on a Rayleigh scattering approximation of the nanofluid optical properties and coupling the radiative transfer equation (RTE) and energy equation with a transient vorticity-streamfunction formulation. The CD approach was based on a hybrid, four-way-coupled Eulerian-Lagrangian formulation with a deterministic interparticle interactions detection algorithm and a transient fluid-particle coupling algorithm that tracked the simultaneous evolution of the carrier and particulate phases while considering timescale differences between the two phases. By capturing the discrete nature of nanoparticles as well as momentum and temperature slips between the carrier and dispersed phases, the CD approach overcomes limitations of FC approach by coupling the fully resolved local nanoparticle concentration with nanofluid optical and thermal properties and capturing the physics of particle-fluid, particle-wall, and colloidal particle-particle interactions. This allowed us to reveal new findings regarding the impacts of incident radiation spectrum, hydrodynamic flow profile, bottom surface optical boundary conditions, colloidal stability, and the interplay of power and temperature gains in DASCs and nanoparticle migration, convective heat transfer, and spatial particle distribution in PPMs. Moreover, the FC and CD approaches were compared in both DASC and PPM systems and multi-variable, multi-objective design optimization of DASCs was conducted
Date of AwardDec 2019
Original languageAmerican English

Keywords

  • nanofluids; volumetric absorption; direct absorption solar thermal collectors; microchannels; discrete modeling

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