| Popis: |
This study numerically explores the thermo-hydraulic performance of a twisted annular circular tube. The annular configurations are investigated at two twisting pitch ratios (y = H/Do=1 and 2) which represents the ratio of the twisting step to the outer tube diameter. This results in nine cases including the straight annulus. The considered cases are AT (typical annulus), TAT (twisted annular tube with both inner and outer surfaces are twisted in the same direction), TOSI (twisted outer surface with a straight inner one), SOTI (straight outer surface with a twisted inner surface), and CW-CCW (both inner and outer surfaces are twisted in opposite directions – outer tube twisted clockwise, inner tube twisted counter-clockwise). The 3D computational domains of the considered cases were modelled and solved using ANSYS 17, SIMPLE algorithm and utilizing the standard k-ε turbulent model, where 10,000 ≤ Re ≤ 30,000 was the range of turbulent flow of air. The important parameters, Nusselt number (Nu) and friction factor (f), as well as thermal performance factor (TPF) are examined. The results indicated that the smallest (y) values yield higher TPF, while larger values result in the opposite trend for all annular cases. TPF for all investigated twisted annuli surpasses unity. The most significant increase is observed in the CW-CCW case (TPF=2.039 at y = 1 and Re = 10,000), showing a 103.9 % rise over the corresponding straight annulus. Conversely, the trivial increase is noted in TPF=1.588 at y = 2 and Re = 30,000, exhibiting an increase of 58.8 %. The obtained results are compared with available data for a plain tube, computed using the well-known Gnielinski and Dittus-Boeltor equations for the Nusselt number, yielding relative errors of 5.3 % and 4 %, respectively. Additionally, the friction factor is determined through Petukhov and Blasius correlations, showing relative errors of 5 % and 3 %, respectively. |
