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

Abstract

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.