Heat Transfer Enhancement and Performance of Solar Thermal Absorber Tubes with Circumferentially Non-uniform Heat Flux

A numerical study was undertaken for investigating the heat transfer enhancement in a molten salt solar receiver tube with the twisted tapes. Various parameters of twisted tapes are studied in this paper. Comparisons of the Nusselt number and friction factor of twisted tape with previous correlation are made to evaluate the turbulence models used. Effects of the clearance ratios (C=0 (tight fit), 0.2, 0.5, 0.7 and 1) and twist ratios (γ=2.5, 5.0, 12.5, 15.6, 25, 41.7) on heat transfer rate (Nu), friction factor (f) are examined under non-uniform heat flux using molten salt as the testing fluid. Furthermore, the influence of grid generation on prediction results is also reported. Numerical calculations were performed with FLUENT 6.3.2 code, in the range of Reynolds number 7485-30553. The results show that the insert twisted tape can significantly improve the uniformity of temperature distribution of tube wall and molten salt. The decreases of clearance rate C and twisted rate γ can enhance the heat transfer effectively. Especially when clearance rate C=0, that is the heat transfer enhancement effect with tight-fit twisted tape is the most significant. But at the same time, the decreases of clearance rate and twisted rate also lead to increase the friction factor. These methods and results can be extended to the heat transfer enhancement of all the solar concentrated receivers.