Catalytic synthesis of high aspect ratio Al₂ O₃ impregnated carbon nanotubes used to improve thermophysical properties of nanofluids with a case study on an industrial gasoline-water heat exchanger

Thermophysical properties of nanofluids containing multiwall carbon nanotubes (MWCNTs) are highly dependent on the surface morphology of the MWCNTs. In this study, a vertical-injection reactor was used to produce Al₂ O₃-impregnated MWCNTs by free substrate catalytic cracking in the presence of aluminum isopropoxide. The synthesized MWCNTs were characterized by BET SEM, TEM, XRD, and TGA. The Al₂ O₃ nanoparticles act as nano-spacers, reducing the entanglement of MWCNTs, thereby increasing the aspect ratio to 5500. These MWCNTs were used to prepare nanofluids with low concentrations (NF0.1, NF0.3, and NF0.5 wt%) in water. Thermophysical properties of the nanofluids were determined under static lab conditions. Values of the thermal conductivity, heat capacity, and viscosity increase with increasing concentration of MWCNTs. The thermophysical properties were further used to calculate the heat transfer rate (q) and the pressure drop (∆P) in a full-scale gasoline-water shell and tube heat exchanger used in the petrochemical industry. When NF0.1 is used in the heat exchanger, q increases by 34% with a ∆P penalty of 5%. A comparison of Nu, Pr, and Re of water with those of the nanofluids under given industrial flow conditions, indicates that the effects due to the viscosity of the nanofluids significantly influence the heat transfer mode distribution.