Characterisation and Optimisation of Heat Transfer Nanofluids – an Optical Approach

Efficient heat transfer is widely needed in various technical and engineering applications. For example, the development of integrated circuit chips is currently limited by a feasible and robust solution for efficient heat dissipation due to its significantly increased current density during the past decades. Intensification of heat transfer can be carried out in either a passive manner or an active manner. Regardless of which approach is adopted, the heat transfer coefficient is governed by the thermal conductivity of the fluid medium for the same design.
A nanofluid is a functional colloidal dispersion with nanoparticles that serves the purpose of intensifying the heat transfer process. Nanoparticles with high thermal conductivity, like metal and metal oxides, are dispersed in heat transfer fluids to improve the heat transfer performance. While the results seem promising for the potential of nanofluids, most experiments were conducted with fresh nanofluids with spherical nanoparticles. Aggregation caused by Brownian Motion over time would restrict the improvement of the thermal conductivity of nanofluids, which is even more complicated for anisotropic nanofluids. Developing a strategy for formulations which would prevent aggregation while limiting the impact on the heat transfer performance for nanofluids is thus vital to keep nanofluids as a practical solution for efficient heat transfer.
Quantum dots are nanocrystals that are photoluminescent upon being illuminated by UV light. Traditional quantum dots have their photoluminescence wavelength directly related to the size of the individual nanocrystal due to the quantum confinement effect. Perovskite quantum dots have their band gap depending on the bond length within the lattice, giving a much narrower distribution of the photoluminescence wavelength than traditional quantum dots. Halide perovskite quantum dots stand out in optics applications for their high photoluminescent quantum yield, narrow peak width, short response time, and high tolerance toward structural defects. The photoluminescent intensity of the quantum dots is directly linked to the temperature due to the thermal quenching effect, which gives a stable relationship between the photoluminescent intensity and temperature.

This thesis proposes a strategy for formulations that balances the anti-aggregation and the heat transfer performance of anisotropic nanofluids. The study is thus divided into two parts - formulation development for anisotropic nanofluids and development of a characterisation method to verify the heat transfer performance of the enhanced nanofluids with different formulations at their working conditions.
In the first part, a detailed study is performed on the dispersion and aggregation performance of γ - Al2O3 - DI and β - SiC - DI nanofluids with formulations with different additives. Following the DLVO theory, pH regulators, surfactants and polymer dispersants are added to the nanofluids to investigate their contribution to preventing the aggregation of the dispersed nanoparticles. The results have demonstrated that the contribution of each additive towards the decrease of the size distribution is slightly different when applied to γ - Al2O3 - DI nanofluid and cubic anisotropic β - SiC - DI nanofluid, implying that the curvature of the surfaces has a dominating effect on the aggregation kinetics for nanoparticles with a similar size and sphericity.
To develop a characterisation method that can measure the temperature in situ and in 2D, a new type of n-octylamine substituted CsPbBr3 halide perovskite quantum dots is synthesised, providing twice higher thermal sensitivity compared to CsPbBr3 QDs synthesised via conventional Ligand Assisted Reprecipitation (LARP) approach. Along with the red CsPbBr1.2I1.8 QDs, the synthesised QDs are cast into a thin film with PMMA, and the thermal sensitivity of the newly synthesised QDs is thoroughly examined, along with the thermal stability of the QD-PMMA film. Utilising the CsPbBr3 QD-PMMA film, a 2D in situ thermometer is developed for microchannels with excellent spatial and temporal
resolution. A case study on producing in situ 2D temperature maps with a certain combination of wall and fluid temperatures and under laminar flow conditions is generated to demonstrate the proposed approach's spatial and temporal resolution. A 2D in situ steady-state heat flux meter is built by adding the CsPbBr1.2I1.8 QD-PMMA film into the design. The heat flux meter is tested with PDMS and nanofluids developed in the first part with two separate approaches with one and two cameras.
At last, a 2D in situ heat flux meter is built for studying the heat flux of an individual sessile drop during the drying process using a combination of thermocouples and the CsPbBr3 QD-PMMA film. A selection of nanofluids and their corresponding nanofluid-PDMS emulsions have been studied using this developed in situ 2D heat flux meter. Different heat flux patterns during drying are observed for each type of working fluid due to their morphology during drying.