Date of Award

8-9-2014

Document Type

Open Access Dissertation

First Advisor

Jamil A. Khan

Abstract

Concentrated Solar Power (CSP) is a prominent alternative energy technology, where mirrors or lenses are used to concentrate sunlight from a large area and stored in a collector filled with heat transfer fluid (HTF). The energy from this HTF is used to produce steam for power generation. CSP system requires high heat storage capacity and thermally stable HTF to reduce its operating cost. Having suitable thermophysical properties, ionic liquids (ILs) is considered as a potential HTF for the CSP applications; however thermophysical properties of ILs can be further enhanced by dispersing small volume percentages of nanoparticles. This liquid is called Nanoparticle Enhanced Ionic Liquids (NEILs). The present research focuses on the experimental and numerical evaluation of the NEILs as a potential working fluid for the CSP applications.

The experimental assessment includes thermophysical property measurements, and convective heat transfer (forced and natural convection) performance evaluation. For this research, four representative ILs ([C4mpyrr][NTf2], [C4mim][NTf2]. [C4mmim][NTf2], [N4111][NTf2]) are selected. The thermophysical properties of Al2O3 NEILs demonstrate enhanced density, thermal conductivity, viscosity, and heat capacity compared to the base ILs. Plausible reasons of enhanced properties are discussed and compared with the existing model.

To evaluate the forced convection performance of ILs and NEILs experiments are conducted in a circular tube with constant heat flux condition. The experimental results obtained for ILs correlate well with the Shah’s equation in laminar flow condition and Gnielinski’s equation for turbulent flow condition. Whereas, results obtained for NEILs show higher forced convection heat transfer coefficient than the base ILs. This is due to enhanced thermal conductivity and particle migration behavior in the boundary layer. The numerical simulation by FLUENT also shows the enhancement of heat transfer coefficient of NEILs compared to base ILs.

Natural convection experiments were performed in rectangular cavity with different aspect ratios (1 and 1.5) heated from below. New correlations for Nusselt Number as a function of Rayleigh number is proposed for ILs. It is noted that the natural convection behavior of NEILs demonstrates much lower heat transfer coefficient compared to the base ILs. The relative change of effective thermophysical properties are not fully responsible for the degradation of the natural convection of NEILs which also confirms the numerical simulation of natural convection of ILs and NEILs. In addition to thermophysical properties, particle-fluid interaction and clustering of nanoparticles also plays a role in degrading the natural convection heat transfer.

Share

COinS