Date on Master's Thesis/Doctoral Dissertation

12-2015

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemical Engineering

Degree Program

Chemical Engineering, PhD

Committee Chair

Willing, Gerold

Committee Co-Chair (if applicable)

Watters, James

Committee Member

Watters, James

Committee Member

Berson, Eric

Committee Member

DePuy, Gail

Author's Keywords

nanofluids; cuo nanofluid; heat transfer coefficient; heat transfer enhancement; CFD; simulation

Abstract

Nanofluids are a class of heat transport fluids created by suspending nano-scaled metallic or nonmetallic particles into a base fluid. Some experimental investigations have revealed that the nanofluids have remarkably higher thermal conductivities than those of conventional pure fluids and are more suited for practical application than the existing techniques of heat transfer enhancement using millimeter and/or micrometer-sized particles in fluids. Use of nanoparticles reduces pressure drop, system wear, and overall mass of the system leading to a reduction in costs over existing enhancement techniques. In this work, the heat transfer coefficient is determined experimentally using copper oxide (CuO) based nanofluids. CuO nanoparticles (40nm) with different particles loadings (0%, 0.25%, 1% and 2% by weight) were dispersed into water without use of an additional dispersant. The heat transfer coefficient for each nanofluid was measured at specific flow rates and initial fluid temperatures through a high thermal conductivity copper tube with a constant heat flux supplied at the wall. The experimental results revealed that the heat transfer coefficient for each nanofluid increased with increasing Reynolds Number (Re) which is not unexpected in cases of convective heat transfer. Under conditions of constant particle loading and Re, the heat transfer coefficient was generally observed to increase with decreasing inlet nanofluid temperature. Since the generally observed trends were similar for all nanofluids under all conditions, the effectiveness of CuO nanoparticles in improving heat transfer coefficient relative to the heat transfer coefficient of the base was determined. This so called heat transfer enhancement is defined as the ratio of the heat transfer coefficient of the nanofluid and that of the base fluid. The goal of this investigation was to specifically determine the role of nanoparticles in the enhancement of the overall heat transfer coefficient of a nanofluid. Unfortunately, the variation of the heat transfer coefficient enhancement with respect to changes in nanoparticle concentration and Re is not consistent across the initial temperatures investigated. This variation was especially clear under laminar flow conditions where at 40°C the 0.25%wt gives the highest heat transfer coefficient enhancement while at 70°C it is the 1.0%wt that gives the highest enhancement. In an effort to understand the nature of these seemingly unpredictable variations, we developed a Computational Fluid Dynamics (CFD) model using an Eulerian-Lagrangian approach to study the nature of both the laminar and turbulent flow fields of the fluid phase as well as the kinematic and dynamic motion of the dispersed nanoparticles. The goal was to provide additional information about dynamics of both the fluid and particles to explain the experimentally observed trends of the heat transfer coefficient enhancement of CuO nanofluids relative to both nanoparticle concentrations and fluid flow conditions. The simulation results indicate that heat transfer enhancement significantly depends on particle motion relative to the tube wall and the thermal boundary layer. This is especially true in the case of laminar flow where heat transfer enhancement can only occur in cases where the nanoparticles can move beyond the thermal boundary layer and into the bulk flow of the fluid.

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