Here, the effects of carbon nanotubes structures and different functional groups were studied on the pool boiling heat transfer coefficients (HTC) and critical heat fluxes (CHF). In order to enhance the the dispersion and stability in an aqueous media, multi-walled carbon nanotubes (CNT) were functionalized using two methods. First, covalent functionalization with cysteine and silver nano-particles and non-covalent functionalization via Gum Arabic (GA) were employed. Then, threenanofluids including GA -, Cys teine-and Ag treated CNT were prepared in different weight concentrations and their thermal performance were investigated nearly up to the CHF. In agreement with the results, while the pool boiling HTC of the non-covalent nanofluid was lower than that of the deionized water, the covalent nanofluids demonstrated a significant increase. In addition, the CHF of the nanofluids was exhibited a significant increase in all concentrations. Meanwhile, the CHF and HTC was also measured in the presence of different diameters. As the size of CNT decreased, the specific surface area increased, implying the higher CHF and HTC.

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