▎ 摘　　要

Carbon nanomaterials are of great interest for next generation nanofluids. Carbon nanotubes and graphene are, in particular, highly thermally conductive and their use as thermal fluid is gaining momentum among the research community. This work investigates the thermal performance of graphene nanoplatelet (GnP)-nanofluid and multi-walled carbon nanotube (MWCNT)-nanofluid by using a concentric pipe heat exchanger. The prepared nanofluid flowed in the inner pipe as hot fluid, which was in the opposite direction of a colder water flow in the annulus pipe. The investigation was conducted by varying the volumetric flow (from 1.5 to 2.5 L/min) and nanofluid concentration (from 0.01 to 0.35 vol%). The heat transfer coefficient (HTC) of GnP-nanofluid was observed to be superior to that of MWCNT-nanofluid. The maximum increase in HTC relative to the pure base fluid is approximately 16.8, 24.2 and 26.1% for GnP-nanofluid; and 8.8, 13.8 and 14.4% for MWCNT-nanofluid flowing at 1.5, 2.0 and 2.5 L/min, respectively. This positive impact on heat transfer performance enhancement reached the maximum when an optimum concentration is reached in both nanofluids. Compared to the maximum HTC ratio brought by MWCNT-nanofluid at 0.15 vol% and 2.5 L/min, GnP-nanofluid produced a greater HTC ratio, even at relatively low concentration (0.05 vol%) and volumetric flow (1.5 L/min) which is rather desirable for practical applications.