Thermal characteristics of helical coiled heat exchanger with graphene-deionized water on waste heat recovery of combustion stack gas

Overall heat transfer coefficient of helical coiled heat exchanger with graphene- deionized water (DI-water) for waste heat recovery of combustion stack gas including heat exchanger effectiveness, ε, were experimentally carried out. Various sizes of tube diameter (1.2-1.5 cm), coil diameter (19-24.5 cm), and coil pitch (2.8-6 cm) of helical coils 1-4 including different heat transfer fluids, DI-water and nanofluid (graphene nanoparticles suspended in DI-water) at different particle fractions (0.05, 0.08 wt.%) had been experimentally studied. The results based on the LMTD-method of countercurrent flows showed that higher graphene fraction resulted in higher values of heat transfer rate, overall heat transfer coefficient and heat exchanger effectiveness. The ε*-NTU*min performance curves of the helical coils in series connection for DI-water and nanofluid with 0.05 wt.% graphene were created from Shah and Sekulić model. The calculated results showed that the helical coil 1 gave the best performance. With 5 numbers of this coil and the flow rate of 2 L/min, the outlet temperature of the nanofluid at 0.05 wt.% particle fraction was 1.3ºC over than that of the DI-water. When the coil number was over 7, the nanofluid gave advantage on the outlet temperature compared with the base fluid only slightly.