The Queensland Geothermal Energy Centre of Excellence (QGECE) is investigating advanced cooling methods for a proposed geothermal power plant. Thermodynamic constraints placed upon the system by climatic conditions in the proposed location of the project (the Cooper Basin); mean that cooling and heating loads are limited, thus effecting thermal efficiency.
This thesis investigates a proposed solution to the aforementioned problem with an experimental investigation into the effectiveness of nanofluids in improving heat transfer rates. Nanofluids use nanoparticles dispersed in a base fluid, in order to enhance both conductive and convective heat transfer, in an attempt to increase efficiency or cooling and heating loads required by certain processes. Whilst results that have surpassed any current theoretical predictions have been well documented for thermal conductivity enhancement (as high as 150% when compared to base fluid properties (Das, et al., 2008)), data pertaining to convective heat transfer improvements have been relatively scarce and not as promising.
In this case, carbon nano-tubes and water have been used as the nanoparticle and base fluid respectively, with an investigation being carried out to examine possible improvements in the overall convective heat transfer coefficient of nanofluids when compared to the base fluid alone. Experiments were performed on a section of a car radiator, modelled as a micro-channel heat exchanger, placed inside a low speed wind tunnel facility at the University of Queensland. High quality water and the nanofluid were tested separately so that their heat transfer capabilities could be contrasted. The results were mostly positive, particularly at low wind velocities, with increases of up to 20% observed in the overall heat transfer coefficient. At the highest wind velocities tested, 10 and 12 m/s, results were less favourable and showed a slight decrease of up to 5% in the overall heat transfer coefficient, when compared to the test performed on water.
The results were cross correlated with previous work in literature reaching a positive degree of agreement, and in some cases showing results to be more positive than have been previously published, such as the work performed by (Leong, et al., 2010).
Additionally, the increased pumping power requirements due to increases in viscosity and density, which are inherent in nanofluids when compared to their base fluid, were calculated. The results of this analysis showed that there was only a 1.64% increase in the pumping power required for the nanofluids. It has been recommended that this be checked however, as empirical relations used to calculate increases in viscosity and density did not seem to correlate well with physical observations made for the nanofluids at the time they were used.
Future research focuses have been identified, with the main recommendation suggesting that more testing and research needs to be carried out before a definitive decision on the use of nanofluids in geothermal applications can be implemented.