الفهرس 
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Abstract
The objectives of the investigation of this study are concentrated on the effect of surface roughness on the heat transfer coefficient during nucleate pool boiling using refrigerant R-134a as a working fluid.An experimental apparatus has been designed and constructed here to be suitable for measuring and reading the experiment data. The experimental apparatus consists of evaporator, cooling system, insulated room, control panel, and cooling circuit to collect the liquid refrigerant after each run.The evaporator was installed in an insulated room. The system was provided by a cooling water circuit to cool the refrigerant vapor. Test surfaces used are horizontal steel heating plates of different surface roughness, the test surface plates was machined by spark cutting machine to make different surface roughness according to the machine standard.The surface roughness’s grades are choice from the machine standard ruler typical as the machine roughness production standards. This surface roughness’s ranged from 0.5μm to 45μm.The test condition was performed on refrigerant R-134a at saturation pressure ranging from 9 bars to 17 bars (which corresponds to 0.22 to 0.41 of the critical pressure). A heat flux 50kW/m2 up to 150kW/m2 was applied using the five different surface roughness’s. The results were presented for different values of saturation pressure.The results showed that:- The average heat transfer coefficient increases with the increase of heat flux for all test plate surfaces.2. The average heat transfer coefficient increases with the increase in pressure for all test plates surface and the rate of increase of the average heat transfer coefficient with the normalized pressure is almost constant.3. The average heat transfer coefficient increases with the surface roughness.4. The rate of increase in the normalized heat transfer coefficient (α/αmir) increases at high heat flux for all surface roughness.5. At the low applied heat flux (60kW/m2) the minimum rate of increasing of heat transfer coefficient was 167% while at the high heat flux (100kW/m2) the maximum rate of increasing of heat transfer coefficient was 193%, this means that the rate of increasing the average heat transfer coefficient has increased with the increasing of average heat flux A general correlation equation is deduced here to describe the variation of the normalized heat transfer coefficient with normalized heat flux, normalized pressure and normalized surface roughness7 This correlation equation is valid for the range: The deviation of the calculated normalized heat transfer coefficient from that obtained from the experimental measurement was found to be ±10%.