Convective heat transfer characteristics of CNT nanofluids in a tubular heat exchanger of various lengths for energy efficient cooling/heating system

This paper presents the heat transfer characteristics of CNT based nanofluids in a tubular heat exchanger of various lengths for cooling/heating applications. The nanofluid is prepared by dispersing multi-wall carbon nanotubes (MWCNT) in water–ethylene glycol mixture (70:30 by volume), and the value of the Prandtl number increases by 115.8% at 0 °C, and 180.2% at 40 °C for the nanofluids with 0.45 vol.% MWCNT, compared to the base fluid. In contrast to the conventional heat transfer concept, the value of the Nusselt number for the nanofluids increases with decrease in the Reynolds number as the MWCNT concentration increases. The results showed the significant enhancement in the convective heat transfer coefficient in the entrance region. The possible reason for the abnormal enhancement in the heat transfer coefficient for the shorter length of the test section is due to the migration of the carbon nanotubes. This migration of carbon nanotubes does not allow the thermal boundary layer to develop at the faster rate.