Abstract:
The use of nanofluids in buoyancy-driven heat transfer can be very useful in enhancing the
performance of various heat transfer applications. In this thesis, natural convection by multiwalled-
carbon nanotubes (MWCNT) was studied in a square enclosure with differential
heating by two opposite walls. Low particle concentrations of 0 1% based on volume were
considered at Rayleigh numbers of 104 108. Thermal conductivities and viscosities of the
nanofluids were experimentally determined. It was found that thermal conductivity and
viscosity increased with increasing concentration by 6% and 58%, respectively. Models based
on these experimental results were obtained and subsequently used in a numerical study of a
two-dimensional simulation of natural convection in a square cavity using a commercial
code. Results revealed an initial enhancement in the Nusselt numbers to a maximum of 22%
which occurs at 0.14 % particle concentration and a Rayleigh number of 108. Beyond the
maximum, the Nusselt number deteriorated. This was true for the different Rayleigh numbers
studied with percentage enhancement in the Nusselt number increasing with increasing
Rayleigh numbers. Further analysis was done to predict heat transfer performance of higher
particle concentrations up to 8% which showed a general decline in the Nusselt numbers by
increasing particle concentration. An experimental setup was subsequently used to study
natural convection in an insulated square cavity with different temperature differences
between the two opposite sides for particle concentrations of 0 1% at Rayleigh numbers
between 2.1 ?? 10?? and 6 ?? 10??. Results from the experimental and numerical studies were
subsequently compared and the validity of projected results for higher particle concentration
was therefore assessed. The experimental results supported the overall behaviours of the
nanofluids obtained from the numerical analysis. However, the experimental results of
maximum enhancement in the Nusselt number was 42% at particle concentration 0.1% and a
Rayleigh number of 6 ?? 10??. Nevertheless, both results indicated the existence of an optimum particle concentration at which heat transfer in MWCNT nanofluids is maximised. The
variation in the performance nanofluid was attributed to the counteracting, non-linear effects
of thermal conductivity and viscosity both of which increases by increasing particle
concentration. The thermal conductivity effect which improves heat transfer performance was
observed to be more dominant for a very narrow range of low particle concentration up to 0.1
% while the viscous effect which diminishes heat transfer performance was found to be more
dominant at higher particle concentration.
