Effect of ultrasonication on colloidal thermophysical and rheological properties of alumina-water nanofluid / Mohammed Mahbubul Islam

Mohammed, Mahbubul Islam (2015) Effect of ultrasonication on colloidal thermophysical and rheological properties of alumina-water nanofluid / Mohammed Mahbubul Islam. PhD thesis, University of Malaya.

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Abstract

Nanofluids are promising fluids for heat transfer applications. Low stability and high viscosity are two important drawbacks for practical applications of nanofluids. The aggregation and sedimentation of nanoparticles are related to the colloidal dispersion characteristics, which directly affect the stability and thermophysical properties. An ultrasonic homogenizer can break the aggregation of particles and disperse them into a fluid to improve the stability of the suspension. Therefore, sound energy is needed to improve thermal energy. However, the research question is whether the improvement achieved in thermal application is feasible for the amount of used ultrasound energy. The aim of this research was to study the effect of the ultrasonic treatment on colloidal dispersion characteristics, thermophysical and rheological properties, and thermal performance analysis for a nanofluid. Specifically, a 0.5 vol.% of Al2O3–water nanofluid was prepared using a horn or probe (tip) ultrasonic dismembrator and 0 to 5 h of durations were applied. The microstructure, particle size distribution, and zeta potential were analyzed as the colloidal dispersion characteristics at 25% and 50% amplitude of the sonicator power. The thermophysical (thermal conductivity, viscosity, and density) and rheological properties of the nanofluids subjected to ultrasonic treatment for different durations were measured at different temperatures from 10 to 50 ºC. Thermal performance characteristics as: thermal resistance, heat transfer coefficient, pumping power, and figures of merit were also analyzed for a mini channel heat sink at different flow rates. It was found that higher sonicator amplitude took fewer periods to disperse the particles. An optimum dispersion of particles with high stability was observed at ~5 and ~3 h of ultrasonication duration with 25% and 50% power amplitudes, respectively. Thermal conductivity and density ratio were found to be increased, but viscosity ratio was decreased with increasing sonication time and temperature. At lower temperature, nanofluid showed Newtonian behavior at lower iv shear rate, but it showed non-Newtonian at higher shear rates. Nevertheless, at higher temperature, nanofluids were found to be almost non-Newtonian with shear thickening behavior. Moreover, a slight decrease in yield stress with increasing sonication time was also observed and it was found to be lower at a higher temperature. Higher heat transfer coefficient was observed for 4 h of ultrasonication duration, which was more effective at high-flow rates. However, pumping power was increased with the increase of sonication time and with low flow rates. Figure of merit analysis showed that a 4 h of ultrasonication could give optimum thermal performance. Nevertheless, the longer duration of ultrasonication is not fruitful in terms of productivity, considering the usage of sound energy and the gain in thermal engineering.

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