Author

Osama Ahmed

Date of Award

11-2010

Degree Type

Thesis

Degree Name

Master of Applied Science (MASc)

Department

Mechanical Engineering

Supervisor

Mohamed S. Hamed

Language

English

Abstract

Pool boiling of nanofluids has been studied in the past decade and contradicting results have been found. Several parameters have been investigated, the most popular being the nanofluid concentration. The investigations in the literature have been carried out under different conditions. The effects of method of nanofluid preparation, pH value and boiling duration on the Heat Transfer Coefficient (HTC) and nanoparticle deposition have not been thoroughly investigated.
An experimental investigation has been carried out to investigate such effects and the effect of nanofluids concentration on the HTC and nanoparticle deposition. A flat copper surface with a mirror-finish (Ra = 50 - 150 nm) has bee n used as the boiling surface. Using a nanofluid prepared from a ready-made suspension showed to have a similar HTC to a nanofluid prepared from dry nanoparticles at a neutral pH value. Reducing the pH value of the nanofluids prepared from dry particles gave different responses in HTC at different concentrations. At 0.01 vol. % concentration the HTC decreased due to change in base fluid properties, at 0.1 vol. % the HTC increased due to enhanced electrostatic stabilisation, and no change took place at 0.5 vol. % due to a high intensity of nanoparticle deposition. Increasing the concentration of the nanofluids resulted in a reduction in HTC, higher nanoparticle deposition as well as a faster rate of deposition on the surface.
The effect of concentration on nanoparticle deposition has been studied by boiling water on nanoparticle deposited surfaces after boiling nanofluids. This approach has shown that nanoparticle deposition from boiling a higher concentration nanofluid gives a higher HTC, which is opposite to expectations. This approach has also shown that the surface condition depends on the intensity of the deposition as well as its uniformity.
An analysis has been carried out using the Rohsenow correlation to quantify the effect of nanoparticle deposition on the heat transfer. Prediction of the boiling heat transfer of nanofluids may be used by adopting a transient surface factor in the Rohsenow correlation.

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