Engineered nanofields for heat transfer process intensification

Heat transfer equipment is one of the main unit operations in many industrial processes such as heating, cooling, transportation and power generation. Thus, convective heat transfer plays a major role in the heat equipment. In the past years, liquids such as water, oil and ethylene glycol had been used as the heat transfer fluids. These fluids have a major drawback since they possess low thermal conductivity. Thus innovation in developing advanced heat transfer fluids is needed in many industrial applications so that more energy efficient and compact systems can be achieved. This is the main impetus of this work. A nanofluid is a liquid suspension that consists of nano-sized solid particles. In this work, carbon nanotubes (CNT) and titanium dioxide (Ti02 ) were utilized in
formulating nanofluids. The shape and morphology of these nanoparticles make it a challenge in producing long term stable nanofluids. CNT nanofluids were produced using sonication and higher shear mixing, while the Ti02 nanofluids were produced by using the beads mill. The CNT nanofluids dispersion stability was enhanced by the aid
of gum arabic surfactant and the Ti02 was stabilized by means of electrostatic stabilization mechanism at pH - 11.0.
The nanofluids were characterised using electron microscopy and size analyzer. The multi-wall CNTs have a diameter of < lOnm and length up to micron size, thus the
aspect ratio is huge. The primary particles of Ti02 have an average diameter of 30-40 nm.

The heat transfer study involves several measurements and analysis: i) the thermal conductivity measurements, ii) viscosity analysis and iii) convective heat transfer measurements. A significant enhancement was observed for thermal conductivity of CNTs nano fluids, where nanoparticles concentration of 0.25 wt %, 25% enhancement was observed. On the other hand, for concentration of 0.2 wt% of TiO2 nanofluids, a maximum of 3.2% enhancement was observed, both measurements were conducted at 25°C.
The viscosity of CNT and Ti02 nanofluids showed shear thinning behaviour. The viscosity decreases with increasing shear rate, and decreases with increasing temperature. The viscosity of CNTs nanofluids was much greater than that of Ti02 nanofluids. At shear rate greater than 150 s the Ti02 nanofluids behaved like Newtonian fluids and the viscosity approached the viscosity values of water. The heat transfer behaviour of nanofluids was investigated for various
experimental conditions such as flow conditions (Reynolds Number), nanoparticle concentration, pH, and particle size. For flow in 45 mm diameter pipe, the heat transfer
coefficient decreases with increasing axial direction from the entrance, and increasing Reynolds Number. A significant enhancement for heat transfer coefficient was observed for CNT nanofluids. At Re = 800, a maximum of 350% enhancement of heat transfer coefficient was observed for 0.5wt % of CNTs. As the concentration increases, the maximum enhancement occurred at increasing axial direction along the pipe. On the other hand, the maximum enhancement (-16%), was observed at x/D = 150 for the Ti02 nanofluids. Moreover, the heat transfer coefficient of Ti02 increases with decreasing particle size for Reynolds Number > 2000.
Apart from the thermal conductivity of nanoparticles, several other possible mechanisms are believed to be operating towards the enhancement of heat transfer
coefficient. These include changes in the boundary layer thickness, particle migration and re-arrangement, thermal conduction increase due to shear and aspect ratio of
nanoparticles.