الفهرس 
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Abstract
In this work, evacuated tube solar collector using air as a working fluid was investigated experimentally under the meteorological conditions of Suez, Egypt. A solar air heater containing six evacuated tubes is used for heating purpose. The length of the evacuated tube is 1800 mm long with inner and outer diameters of 42 mm and 58 mm respectively. A co-axial heat pipe was inserted at each evacuated tube to increase the thermal efficiency. Comparisons were done in the present work for two experiments. The first one was aimed to investigate the effect of using air as a working fluid, either than water, at different mass flow rates on the thermal efficiency and heat transfer. The other experiment was aimed to investigate the effect of inserting the co-axial heat pipe at same range of air mass flow rate on the thermal efficiency and heat transfer. The inlet mass flow rate was varying from 0.6 kg/min to 1.2 kg/min for the two cases. Results are showed that inserting co-axial heat pipe lead to increases the instantaneous thermal efficiency and the heat transfer rate of 92% greater than without heat pipe. In addition, the experimental results for both instantaneous and average Nusselt numbers are presented and discussed for various parametric conditions. Correlations for the average Nusselt numbers were presented for solar evacuated tube with and without heat pipe.