Effect of morphology of carbon nanotubes on thermal conductivity enhancement of nanofluids

An experimental study was conducted to investigate the effect of the morphology of carbon nanotubes on the thermal conductivity of suspensions. Three different types of carbon nanotubes, classified by aspect ratio as well as their aqueous suspensions, were prepared and characterized through the analyses of Raman spectroscopy: thermogravimetric analyzer, scanning electron microscopy, and transmission electron microscopy. The effective thermal conductivities of the samples were measured using a steady-state cut bar apparatus method. Enhancements based on the thermal conductivity of the base fluid are presented as functions of both the volume fraction and the temperature. Although functionalized single-walled nanotubes; produced a more stable and homogeneous suspension, the addition of small amounts of surfactant to suspensions of "as produced" single-walled nanotubes demonstrated a greater increase in effective thermal conductivity than functionalized single-walled nanotubes alone. The effective thermal conductivity enhancement corresponding to 1.0% by volume approached approximate to 10%, which was observed to be lower than expected but more than twice the value, 3.5%, obtained for similar tests conducted using aluminum oxide suspensions. However, for suspensions of multiwalled nanotubes, the degree of enhancement was measured to be approximately 37%. Comparison of the measured data and that predicted by a previously developed theoretical model indicated good agreement for a corresponding shape factor, n, of 12 for the single-walled nanotubes, whereas multiwalled nanotube suspensions were well matched when the shape factor was 36, indicating that extension of the theoretical model may allow it to be applied to carbon nanotube suspensions if the effect of clustering can be accurately predicted.