Iman, Behroyan (2016) Computational modeling on convective flow of nanofluids through a heated pipe / Iman Behroyan. PhD thesis, University of Malaya.
Abstract
This study is focused on the CFD modeling of nanofluid heat transfer during convective flows. Depending on the measure of the wall heat flux, the convective flows are categorized in two general regimes of forced convection and convective flow boiling. The both regimes are numerically investigated in this study. The commercial Ansys-Fluent CFD codes are employed for this purpose. In case of the forced convection heat transfer, the research is limited to laminar and turbulent internal flows. Depending on whether the nanofluid is assumed as a homogeneous single-phase liquid or a colloidal mixture of nanoparticles and the base liquid, the nanofluid flows are simulated by either single-phase or two-phase approaches. The different single-phase models (i.e. Newtonian and non-Newtonian) and two-phase models (i.e. Eulerian-Eulerian, mixture and Eulerian-Lagrangian) are used in this study to simulate nanofluid forced convection through a heated pipe. Different fluid rheology, effective conductivity models and effective viscosity models are used in the single-phase approach to achieve the most accurate prediction of nanofluid heat transfer. Interphase interactions such as interphase heat transfer, Brownian motion, drag force, lift force, virtual mass force, thermophoretic force and nanoparticle migration, which exist between the nanoparticles and the base fluid, are considered in the different two-phase models to achieve the most accurate prediction of nanofluid heat transfer. In case of the convective flow boiling, the research is focused on subcooled flow boiling. The Eulerian-Eulerian two-phase model is used to simulate the nanofluids heat transfer during subcooled flow boiling through a vertical heated tube. The effects of the nucleate boiling parameters (i.e. nucleate site density, bubble frequency, and bubble departure diameter) and the bubble dynamics (i.e. interfacial area concentration of bubbles, non-drag forces and turbulence interaction resource) on the CFD model prediction of the boiling heat transfer coefficient (BHTC) are investigated. The effect of iv interphase interactions (i.e. interactions of the nanoparticles and the base liquid) and nonhomogeneous nanoparticles distribution on heat transfer predictions are also investigated. For this purpose, the Eulerian-Lagrangian CFD model is incorporated with the Eulerian-Eulerian model to track the thermal and hydrodynamic effects of the nanoparticles. The surface wettability improvement induced by the nanoparticles deposition is considered in the CFD model to find out how the heat transfer predictions are affected by such wettability improvement. Several User Define Function (UDF) programming codes are created and incorporated to Ansys-Fluent CFD software to define the thermal conductivity, the dynamic viscosity, the thermal dispersion models, the non-Newtonian rheology, the nucleate site density and the bubble departure diameter for the nanofluids. The UDF codes are incorporated with the commercial CFD codes of Ansys-Fluent. All the simulation results are benchmarked against the experimental ones from the literature. The single phase model and the Eulerian-Lagrangian two-phase model, overall, are the recommended models. The single-phase CFD model can predict the nanofluid heat transfer, if the nanofluid rheology and thermo-physical properties are determined accurately. The Eulerian-Lagrangian two-phase model no needs to determine nanofluid rheology and thermo-physical properties but it needs more computational effort than the single-phase model.
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