Effect of employing a new biological nanofluid containing functionalized graphene nanoplatelets on thermal and hydraulic characteristics of a spiral heat exchanger

In this study, thermal and hydraulic attributes of an ecofriendly graphene nanofluid flowing within a countercurrent spiral heat exchanger are evaluated. The cold water flows in one side while the hot nanofluid or hot base fluid moves in the other side of heat exchanger. The heat transfer rate and overall heat transfer coefficient enhance with increasing either Reynolds number or concentration. The effect of adding nanoparticles becomes more important at higher Reynolds numbers. At great Reynolds number, the higher heat transfer occurs in the end sections of the heat exchanger whereas at low Reynolds number, the chief heat exchange happens at the initial sections. The results show that the value of effectiveness is much great (higher than 0.85) in all cases under investigation. Moreover, the effectiveness and number of transfer units decrease by increasing the Reynolds number. The pressure drop intensifies with the Reynolds number increment, and the nanofluid demonstrates a greater pressure drop than the base fluid especially at higher Reynolds numbers. Meanwhile, the cold fluid demonstrates a higher pressure drop compared with the hot fluid due to the greater viscosity. The performance index, i.e. the ratio of heat transfer rate to pressure drop, enhances with increase in either Reynolds number or concentration, which is a promising result. Thereby, the performance index for the nanofluid increases almost 142% by increasing the Reynolds number from 1000 to 3000.