We report a method to prepare surfactant-free water based nanofluids containing multi-walled carbon nanotubes (CNTs). The as prepared CNTs with hard dispersibility, after being cut by mechanical ball-milling approach following strong acid treatment, can be directly dispersed into water. The thermal conductivity of the nanofluids is optimized by controlling the CNT length and straightness. It is realized by changed the ball-milling times. The thermal conductivity enhancement of water based CNT nanofluids with volume fraction of 1% attains 29.5% by controlling the CNT length and straightness when the temperature is 63.9°C.

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