Investigation of the thermophysical properties of coconut fibre based green nanofluid for heat transfer applications.

Doctoral Degree. University of KwaZulu-Natal, Durban.Significant resources are being channelled toward research on carbon nanomaterials obtained from biomass precursors because of their overall environmental acceptability, stability, low toxicity and simplistic use. Due to their unique nature, they have excellent thermo-physical properties which include improved thermal conductivity, electrical conductivity and viscosity. In this study, carbon nanotubes and nanospheres were successfully synthesized from coconut fibre activated carbon. The biomass was first carbonized, then physically activated followed by treatment using ethanol vapor at 700 °C to 1100 °C at 100 °C intervals. The effect of synthesis temperature on the formation of the nanomaterials was studied using scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive X-ray powder diffraction (XRD), Fourier transform infrared microscopy (FTIR) and thermo-gravimetric analysis (TGA). SEM analysis revealed nanospheres were formed at higher temperatures of 1000 °C and 1100 °C, while lower temperatures of 800 °C and 900 °C favoured the growth of carbon nanotubes. At 700 °C however, no tubes or spheres were formed. TEM and FTIR were used to observe spectral features, such as the peak positions, intensity and bandwidth which are linked to some structural properties of the samples investigated. All these provided facts on the nanosphere and nanotube dimensions, vibrational modes and the degree of purity of the obtained samples. In general, the TEM results showed spheres of diameter in the range 30 nm to 250 nm while the tubes had diameters between 50 nm to 100 nm. XRD analysis revealed that the materials synthesized were amorphous in nature with a hexagonal graphite structure. Experimental measurements of the thermal conductivity, electrical conductivity and viscosity of the synthesized nanomaterials dispersed in 60%:40% ethylene glycol (EG) and water (W) nanofluids containing gum arabic (GA) were performed, considering the effects of temperature and mass fraction. Stability testing of the nanofluids were determined by zeta potential, viscosity and UV spectroscopy measurements of nanofluids for 720 minutes. The green nanofluids prepared were observed to very stable for more than 720 minutes. Also the results of experiments showed that the addition of nanomaterials to the base fluid increased the viscosity and that with the increase in temperature, the viscosity decreased while the electrical conductivity improved when compared to the base fluid. On the other hand, the thermal conductivity results were observed to decrease with the addition of nanoparticles. This decrease observed has been attributed to high thermal boundary resistance, ratio of surfactant and inconsistent size of the nanoparticles

Reducing noise from wind turbines using active noise control.

M. Sc. Eng. University of KwaZulu-Natal, Durban 2014.Wind turbines while operating produce noise from the rotating mechanical parts and from the interaction of the blades with surrounding airflow. The noise produced by the blades consists of low frequency noise, airfoil self-noise and inflow turbulence noise. Active Noise Control (ANC) however, is a technique known to produce high level of attenuation in the low frequency range. The question therefore arose whether ANC can be used to reduce noise on wind turbines. The MATLAB simulation investigated the primary objective which was to introduce an opposite phase that is generated and combined with the primary “anti-noise” wave through an appropriate array of secondary noise, developed using a set of adaptive algorithms which consequently results in cancellation of both noises. The MATLAB simulation also investigated three secondary objectives: (i) to use filtered-x least mean squared (FXLMS) feed-forward ANC; (ii) to use a Finite Impulse Response (FIR) adaptive filter structure; and (iii) to minimize residual noise which consequently leads to reduction in low frequency aerodynamic noise from wind turbines. Field measurement was carried out in order to achieve one secondary objective: (i) to measure noise emission from a test turbine facility. Noise emission measurements were carried out at periods with the highest wind speeds which were between 10:00 am and 5:00 pm. Results show a reduction in sound pressure with increase in distance, with 64dBA at the foot of the tower and a sound pressure level of 54dBA at 30m away from the foot of the turbine. One-third octave analysis results indicate that although sound is attenuated with increasing distance, low frequency noise has higher frequency components having a value of 257Hz and a band power of 46dBA. Active Noise Control Simulations using FXLMS algorithm was carried out using sampled noise at 22050Hz and for 2 seconds and combining the noise signal, the FXLMS filter and the primary path filter. The FIR filter was used for the primary propagation path and a reduction of noise by 29dB has been achieved