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    Improvement of Performance by Using MgO+CuO/Water and MgOAl2O3/Water Nanofluids in Sequential Heat Pipe Heat Exchangers: A Comparative Experimental Study
    (Gazi Univ, 2021) Filiz, Cagdas; Yetisken, Yasar
    Systems used in pre-heating the required clean air; waste heat recovery units used in industry and many waste heat plants. In this study, it is aimed to improve the thermal performance of the heat pipe heat recovery system by using pure water and nanofluids in air to air heat recovery systems. The fact that nanofluids, which have recently found a lot of application area, have been used in the experimental setup has added originality value to the study. As working fluid, MgO+CuO/Water hybrid nanofluid and MgOAl2O3/Water nanofluid were used in the heat pipe heat exchanger. With the use of these nanofluids, the improvement rates of thermal performance of the heat recovery system relative to pure water have been determined and attempted to comment. In the condenser region, which is the lower part of the heat pipes, when the cold air velocity was 0.751 m/s and Re=12300, it was measured that there was 77 % improvement for MgO+CuO/Water hybrid nanofluid and 91 % for MgOAl2O3/Water nanofluid.
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    Upgrading of the Performance of an Air-to-Air Heat Exchanger Using Graphene/Water Nanofluid
    (Springer/Plenum Publishers, 2021) Soezen, Adnan; Filiz, Cagdas; Aytac, Ipek; Martin, Kerim; Ali, Hafiz Muhammad; Boran, Kurtulus; Yetisken, Yasar
    The aim of this study is to improve the thermal performance of air-to-air heat recovery units, containing heat pipes by using graphene/water nanofluid as a working fluid. The experimental set up of this work consists of two air ducts. To study the effect of the airflow rate and the temperature on the performance of the heat recovery unit, different values of airflow rates and temperatures are used. The values of Re numbers are calculated for each air duct. These Re numbers referred to the turbulent flow type in all cases. To compare the results of the graphene/water nanofluid and the pure water working fluid, thermal efficiency and thermal resistance values are calculated for both of them. The results showed that the graphene/water nanofluid was more efficient than pure water in all different conditions. Re number in the cold air duct was 6800, and the Re number in the hot air duct was 9000. The maximum thermal efficiency values were 34.1 % and 20.1 % for graphene/water nanofluid and pure water, respectively. The maximum improvement rate in thermal efficiency was 87.7 % when the average Re number in cold and hot air ducts was equal to 11,150 and 11,650, respectively. By comparing the results of graphene/water nanofluid with those of the pure water, it can be seen that using graphene/water nanofluid decreased the thermal resistance of the heat pipes. Therefore, the heat transfer increased. The maximum decreasing value of the thermal resistance was 52.3 % when cold and hot air duct Re numbers were 11,700 and 11,000, respectively.