Experiments were performed to investigate the pressure drop
and heat transfer performances of Al2O3/water and
MWCNT/water nanofluids flowing in a PHE. Results were compared
with those of water. In order to get the HTC for nanofluids,
experiments with water used as both hot and cold fluids were carried
out first to obtain the heat transfer correlation of cold water.
The new heat transfer correlation also agrees with the experimental
data of nanofluids flowing inside the PHE very well. Comparable
results showed that the heat transfer performance of nanofluids
was better than that of water at a constant Reynolds number.
The heat transfer enhancement may be caused by the increase in
thermal conductivity. Besides, the increase in viscosity makes the
flow rate of nanofluids larger than that of water, which also benefits
the heat transfer of nanofluids. However, heat transfer deterioration
was observed based on a constant flow velocity. The heat
transfer deterioration may be caused by the suppression of turbulence
intensity by adding nanoparticles and the increase in viscosity.
The heat transfer deterioration of MWCNT/water nanofluids
was more intensive than that of Al2O3/water nanofluids due to
the relatively large increase in viscosity for MWCNT/water
nanofluids. For MWCNT/water nanofluids, HTC also decreased with
increasing particle volume concentrations. This might be due to
the counteracting effects of the increased thermal conductivity
and the increased viscosity. As the effect of viscosity outweighed
the effect of thermal conductivity, the HTC decreased with increasing
volume concentrations. The pressure drop of nanofluids was
somewhat higher than that of water and increased with increasing
particle volume concentrations due to the increase in viscosity. A
correlation for predicting the friction factor was obtained and fitted
the experimental data very well.