Untersuchung des Wärmeübergangs bei erzwungener Konvektion und gleichzeitiger Erstarrung eines Phasenwechselmaterials an einer rotierenden Trommel

Kurzfassung

This scientific work investigates theoretically and experimentally the heat transport processes on the outer surface of a rotating drum partially immersed in a liquid PCM that solidifies on the surface of the drum. For this purpose, the external heat transfer coefficients for the cylinder with and without solidification on the surface are determined experimentally for the first time. Furthermore, an existing numeric simulation tool is adapted to allow the simulation of the maximum thickness of the solid layer. Within the scope of a measuring campaign on a test stand of the rotating drum, heat transfer coefficients between 49 W/(m2 K) and 2106 W/(m2 K) are determined for tests without solidification and between 64 W/(m2 K) and 1282 W/(m2 K) for tests with solidification. The experiments are performed for speeds from 1 min-1 to 35 min-1 with a cylinder of 0,184 m diameter, corresponding to Reynolds numbers from 24 to 11470. For all measurements the temperature differences between the liquid PCM and the heat transfer fluid are between 5 °C and 30 °C and the temperature differences between the heat transfer fluid and the melting temperature of the PCM is 2 °C for measurements without solidification and -5,5 °C for measurements with solidification. The PCM is capric acid with a melting temperature of 32 °C. For the measurements without solidification the heat transfer coefficients are calculated from the power transmitted at the drum surface and for measurements with solidification from the maximum thickness of the PCM layer. Both calculations are carried out based on a quasistationary approach, which neglects the heat storage capacity of the solids. It is found that the Nusselt correlation for a plane, moving surface in a resting liquid from the VDI Heat Atlas is best suited to describe the external heat transfer coefficients. When using this correlation, the factors for adapting the heat transfer coefficients to the measured values - averaged over speed and temperature - are reduced from 5,3 to 1,7 for the case without solidification and with an immersion depth of 25 mm, from 4,4 to 1,4 for the case without solidification and an immersion depth of 45 mm and from 4,0 to 1,2 for the case with solidification and an immersion depth of 35 mm in comparison to the previously used correlation of the the flat, laminar overflowed plate. The adaptation of the simulation tool allows the qualitative simulation of the maximum solidified PCM-layer thickness that builds up over several rotations, but it cannot be validated.