Natural convection of multi-walled carbon nanotubes with water in a square enclosure

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.