الفهرس 
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Abstract
The present study investigates experimentally the pool boiling characteristics of nanofluids during nucleate stage over a copper and steel heating surface at different heat flux, pressure and surface roughness. The nanofluid had been prepared by the suspension of copper oxide and aluminum oxide with average diameter 40 nm (according to the manufacturer, M K Impex Corp. - CANADA) in a base fluid R-134a and R-141b. The refrigerant R-134a was mixed with cooper oxide at different concentrations of 0.005, 0.01, 0.03 and 0.05 % by volume. The other refrigerant R-141b was mixed with Aluminum oxide at concentrations of 0.001, 0.01, 0.03, 0.05 and 0.1 % by volume, an experimental test rig had been designed and manufactured for this purpose. The results showed that the nucleate pool boiling heat transfer coefficient increases with increasing both applied heat flux and normalized pressure. The heat transfer coefficient also increases with the increase of the nanoparticles concentration in the range from 0.005 to 0.03% by volume, but it decreases with more concentration for CuO - R134a. For the other nano refrigerant, the heat transfer coefficient also increases with the increase of the nanoparticles concentration in the range from 0.001 to 0.05% by volume, but it decreases with more concentration for Al2O3 and R-141b. The effect of heating surface roughness was studied and the results showed that, the heat transfer coefficient increases with the increase of surface roughness. In general, an enhancement up to 82 % is observed during the investigation of present study with using CuO - R-134a nanofluid at 0.52 normalized pressure, 150 kW/m2 heat fluxes, 2.87 surface roughness and 0.03% volume concentration. An empirical correlation was deduced to generalize the experimental data as dimensionless groups within uncertainty of ± 10 %. The present results based on the experimental measurements were compared with the corresponding available published data showing agreement between the present results and published ones.