Several well-known correlations to determine the heat transfer coefficients of quasi-turbulent and turbulent flow
in smooth tubes are available in literature. However, when the results of these correlations are compared with each other,
the results vary over a considerable range. Therefore, the purpose of this study was to conduct heat transfer and pressure
drop experiments in the quasi-turbulent and turbulent flow regimes and to develop an accurate heat transfer correlation. A
total of 1 180 experimental data points were collected from careful experiments that were conducted ourselves using two
different test section configurations. The first test section configuration consisted of a tube-in-tube test section on which
the wall temperatures were obtained either indirectly with the Wilson plot method or by direct surface temperature
measurements. The second test section configuration consisted of single tubes being electrically heated at a constant heat
flux. Different test sections covering a range of tube diameters from 4 mm to 19 mm and a range of tube lengths from 1 m
to 9.5 m, were used. Experiments were conducted from a Reynolds number of 2 445, which corresponded to the start of
the quasi-turbulent flow regime, up to 220 800, which was well into the turbulent flow regime. Water, as well as different
concentrations of multi-walled carbon nanotubes, were used as the test fluid, which gave a Prandtl number range of 3-10.
A new correlation was developed that could estimate 95% of all the experimental data points within 10% and an average
deviation of less than 5%.