In this work, Al 2 O 3 Nanoparticles with mean diameter of 25nm, surface area 180m 2 /g dispersed in base fluid using two step method with the nanoparticles volume concentrations 0.1%, 0.2%, 0.3%, 0.4% and 0.5% using magnetic stirrer and sonicator. Two different base fluids were used, Distilled Water (DW) and Distilled Water:Ethylene Glycol (EG) mixture (70:30%, 80:20% & 90:10%). 40 samples were prepared with the four base fluids with and without capping agent (Cetyltrimethyl Ammonium Bromide-CTAB). First part of the work is Rheological investigation of all samples. Thermal conductivity, viscosity and density were measured. Obtained values were compared in three ways, Nanofluids with and without capping agent, Nanofluids with different percentages of ethylene glycol and without ethylene glycol. It was observed that capping agent plays a vital role in keeping good stability of nanofluids and in the case of Water:Ethylene glycol base fluid, capping agent is very useful to reduce the agglomeration of nanoparticles and in turn increases thermal conductivity. Decreasing ethylene glycol percentage in base fluid increases the thermal conductivity. Second part of the work is the experimental investigation of heat transfer performance of nanofluids on double pipe counter flow heat exchanger. Results show that with the increasing nanoparticle concentration in nanofluids heat transfer coefficient increases. Heat transfer augmentation at 0.5% volume concentration of Al 2 O 3 is higher than the other volume concentrations.

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