الفهرس 
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Abstract
The aim of this study is to investigate experimentally the effect of the concentration of nanoparticles on the evaporating heat transfer coefficient during evaporation process of refrigerant R-134a in vapor compression system.The experimental runs include the variation of the heat flux, mass flux, nanoparticles concentration, nanoparticles sizes and type of nanoparticles materials and indicating their effect on the evaporating heat transfer coefficient. The refrigeration loop used in the present investigation consists of compressor, condenser, expansion valve, and hot water as a heat source to give the thermal load to the evaporator (the test section). The test section is a coiled tube made of copper immersed in heat exchanger made of galvanized steel, which is filled with hot water. The test section is provided with thirty six thermocouples distributed along the test section to measure the refrigerant temperatures. Two thermocouples are used to measure the inlet and outlet temperatures of the condenser in addition to two thermocouples used to measure the inlet and outlet temperatures of the hot water to and from the test section respectively.
Experiments were carried out using heat flux ranged from 2 to 70 kW/m2, mass flux ranged from 100 to 400 kg/m2s and nanoparticles concentrations ranged from 0.1 to 1%. Nanoparticles additives used in the experiments are CuO (sizes: 15, 25, 50 and 70 nm), TiO2 (size: 15 nm), and Al2O3 (size: 15 nm).
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The results indicated that for a certain concentration of the nanoparticles, the evaporating heat transfer coefficient increases with the increase in both heat flux and mass flux. In other words the average Nusselt number increases with the increase in both heat and mass fluxes in the ranges used. It is worth to mention that evaporating hat transfer coefficient shows its highest value in case of using CuO nanoparticles followed by Al2O3 nanoparticles, while TiO2 gives the lower heat transfer coefficient.
Two empirical correlation equations are deduced to estimate the average Nusselt number based on the present experimental measurements. These correlation equations fit the present experimental measurements with maximum deviations of ± 11.7% for equation (4.1) and ± 10.8% for equation (4.2) respectively. The present correlation equations seem simple and easy to use in calculating the average Nusselt number for a wide range of Reynolds number, nanoparticles concentrations, type of nanoparticles materials and nanoparticles sizes. The evaporating heat transfer coefficient of the present results are compared with available published data. The comparison shows fair agreement between the present results and available published data.