Heat transfer enhancement in a parabolic trough receiver using wall detached twisted tape inserts

In this paper, heat transfer and fluid friction performance of a parabolic trough receiver with twisted tape inserts detached from the absorber tube's wall is numerically studied. The numerical investigations were conducted for twist ratios in the range 0.30 ≤ ỹ ≤ 2.40 , width ratios in the range 0.53 ≤ w̃ ≤ 0.91 and Reynolds numbers in the range 10,260 ≤ Re ≤ 320,000 . The numerical simulations were performed using a finite volume method with the realisable k-ε turbulence model and Syltherm 800 as the heat transfer fluid. The use of twisted tape inserts shows a significant increase in the heat transfer and fluid friction performance of the receiver. The study also reveals significant reduction in absorber tube's circumferential temperature difference due to the improved heat transfer performance. For the range of parameters considered, the Nusselt number, fluid friction and thermal enhancement factor are 1.01-3.36, 1.32-21.8, and 0.74-1.25 times those in a receiver with a plain absorber tube respectively. The absorber tube's circumferential temperature difference reduces between 4-76% compared with a plain absorber tube. Correlations for Nusselt number and fluid friction are also reported for the range of parameters considered.