Graduation Semester and Year

2018

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering

Department

Mechanical and Aerospace Engineering

First Advisor

Donghyun Shin

Abstract

For many years, scientists enhanced the specific heat capacity of molten salt nanofluids by dispersing only minute concentrations of different nanoparticles (e.g., 1 wt.%). While the specific heat capacity for molten salts was enhanced in many types of research, other engineering fluids (i.e., water, ethylene glycol, oil) showed decreased specific heat capacity with doping the nanoparticles. Recently, researchers discovered that a unique nanostructure formed by molten salt molecules, which were doped and interacted with nanoparticles, were the reason for enhanced specific heat capacity. Therefore, the enhanced specific heat capacity only happens for molten salts rather than other fluids that may not naturally form these nanostructures. In the first study, the specific heat capacity of molten salt nano-eutectic (����₂����₃ − ��₂����₃ doped with ������₂ nanoparticles) was theoretically investigated. Corresponding to the proposed theory in the literature [1], the specific heat capacity of a nano-eutectic can be significantly increased by the formation of needle-like nanostructures by salt eutectic. To investigate the effect of the formed nanostructure, the model presented by Wang [2] for nano-sized particles was used and expanded to theoretically calculate the specific heat capacity of the nanostructure of molten salt nano-eutectic. The mass fraction of the formed nanostructure was estimated by MATLAB using the reported material characterization study [3]. The theoretical predictions were in a good agreement with the measured specific heat capacity values from the literature, with the error of 3 - 4%. An additional verification of the proposed model was done by a different lab partner using a molecular dynamics study. The error between the theoretical prediction and the simulation is only 3.4%, and the value was in a good agreement with the experiment (1.9% max. error) [66]. The result confirms the enhanced specific heat capacity of a nano-eutectic can be described by the contribution of the formed nanostructure. Hence, we artificially fabricated similar nanostructures and dispersed them in a non-salt medium to see whether it enhances specific heat capacity or not. In the second study, we theorized such nanostructures could be mimicked through the in-situ formation of fabricated nano-additives, which are the acknowledged nanoparticles coated by organic materials (e.g., polar-group-ended organic molecules) resulting to these structures called superstructures. We first portrayed this approach by studying the polyalphaolefin (PAO) oil as the base fluid and coated the ������₂ nanoparticles with Polyethylene-block-poly (ethyleneglycol) (PBP). A differential scanning calorimeter (DSC) device, a discovery hybrid rheometer (HR-2), and an in-lab built thermal conductivity apparatus were used to conduct measurements for the specific heat capacity, thermal conductivity, and viscosity of pure PAO and PAO with fabricated nano-additives. Results showed 44.5% enhancement for specific heat capacity and 19.8% and 22.98% enhancement for thermal conductivity and viscosity, respectively, by adding fabricated superstructures compared to pure PAO. Furthermore, a peak representing the partial melting of the PBP was detected in the first thermal cycle, which disappeared in the following cycles; so, this specifies that the in-situ formation of fabricated nano-additives spontaneously happens in the thermal cycle to form such superstructures. At last, we analyzed the figure of merit for PAO-superstructure to evaluate the value of its performance for heat transfer and storage media.

Keywords

Polyalphaolefin (PAO), Nanostructure, Molten salt, Nanofluids, Nano-eutectic, Specific heat capacity, Thermal conductivity, Viscosity, Ionic liquid, Polyethylene-block-poly (ethylene glycol)

Disciplines

Aerospace Engineering | Engineering | Mechanical Engineering

Comments

Degree granted by The University of Texas at Arlington

Download

Share

COinS