Three-dimensional analysis of the thermal and hydraulic performance of finned and un-finned tubes in a staggered array

Abstract

• Novel type of leeward cut fin is compared with a conventional circular fin. • Fin spacing was varied to evaluate thermo-hydraulic behaviour. • Leeward cut fin gives lower heat transfer rate and pressure drop than circular fin. • Performance study suggests the potential of leeward cut fins for material saving. A novel type of finned tube obtained by removing the downstream section of the conventionally used complete circular finned tube (CCFT) is studied. The resulting novel type of finned tube obtained is termed leeward cut finned tube (LCFT). The thermal and hydraulic behaviour of these novel finned tubes is compared with CCFT and un-finned tubes. Leeward cut fins are specifically studied to check their capability to give equivalent heat transfer like that of complete circular fins. This study is carried out in the Reynolds number range 4330 ≤ Re ≤ 8790 and fin spacing range 2 ≤ s ≤ 4 mm . The transition SST model is implemented to solve the RANS equations. Heat transfer coefficient at the downstream section of fins is found to be less than the upstream section; hence, it can be advantageous to remove some portion of the downstream section of the fin. The leeward cut finned tubes give a lower value of heat transfer rate and pressure drop when compared with complete circular finned tubes. The heat transfer Colburn factor j for leeward cut fins is higher than CCFT and un-finned tubes. The friction factor is the lowest for CCFT and the highest for un-finned tubes. Analysis of the effect of fin spacing showed an increment in heat transfer rate and a decrease in pressure drop when fin spacing is increased from 2 to 4 mm. The studied geometries are evaluated based on their thermo-hydraulic performance parameters ( PEC and Z/E ). The performance evaluation criterion PEC for LCFT is higher than CCFT by 30–46 % at fin spacing of 2 mm, whereas for spacing of 4 mm, it is 21–23 % higher. The ratio of heat transfer rate per unit temperature difference to power provided to the heat exchanger Z/E is lower for LCFT than CCFT by 5–8 % for fin spacing of 2 mm, whereas for fin spacing of 4 mm, it is lower by 9–12 %. Based on the results, CCFT performs better than LCFT, but they have more fin material requirements (∼50 %). Hence, the LCFT can be seen as a potential novel type of fin to give equivalent heat transfer with the advantage of material savings.