Thermal performance enhancement in a parabolic trough receiver tube with internal toroidal rings: A numerical investigation

The present work investigates the incorporation of internal toroidal rings in an absorber tube of a solar parabolic trough collector (SPTC) with an objective to enhance the thermal performance. To produce substantial effect on the heat transfer rate, nine different cases of absorbers have been considered. The fully developed turbulent heat transfer characteristics of the absorber tube have been numerically studied and validated under varying inlet temperatures and flow rate of the heat transfer fluid (HTF). Realizable k-epsilon two-equation turbulence model with enhanced wall treatment has been used with ANSYS FLUENT 15 commercial codes. The influence of toroidal rings on heat transfer and fluid flow are evaluated and presented. The absorber with toroidal ring having a diameter ratio (H) of 0.88 and pitch size (p) of 2d is the thermally efficient optimal case. While the absorber with H = 0.92 and p = 2d is the energy efficient optimal case, with H being the ratio of inner to outer diameter of the toroidal ring and 2d being the distance between the adjacent rings, which is equal to two times the inner diameter of the absorber tube. When the inlet temperature of HTF is 600 K, the increase in efficiency for the thermally efficient and energy efficient optimal case are found to be 3.74% and 1.88%, while the increase in Nusselt number is found to be 2.33 and 1.49 times higher than the smooth absorber tube (SAT), respectively.