Heat transfer coefficients for quasi-turbulent and turbulent flow in solar receiver tubes

09 Jul 2020

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%.