Upgrading of the Performance of an Air-to-Air Heat Exchanger Using Graphene/Water Nanofluid

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.