Heat transfer mechanism in ribbed twisted-oval tubes

This paper presents the fluid flow and heat transfer mechanisms in ribbed twisted-oval tubes. Simulations were conducted for a uniform heat-flux wall condition in turbulent flow (5000 ≤ Re ≤ 20,000). Optimal conditions were evaluated from the following parameter ranges: rib arrangement (vertical and horizontal); number of ribs, N = 1, 2, 4; rib height ratios, RR = 0.025–0.1; spiral depth ratios, DR = 0.05–0.14 and twist pitch ratios, PR = 1.5–5.0. It was determined that the ribbed twisted-oval tubes generated stronger swirl flow than a conventional twisted-oval tube, indicated by their greater turbulent kinetic energy. With increasing RR, DR and N, the turbulent kinetic energy significantly increased while the opposite trend was observed as PR became larger. The greater turbulent kinetic energy led to better fluid mixing between the core and near wall region, which directly varied with the heat transfer between the two regions. The optimal parameters that provided a maximal thermal performance factor (TPF), 1.32, were PR = 3.5, DR = 0.1, RR = 0.075, N = 4 and Re = 5000. •A novel ribbed twisted-oval tube is proposed for improving thermal performance.•The ribbed twisted-oval tube produces flow separation and reattachment behind the ribs.•Thermal performance is increased, compared to both the conventional twisted-oval tube and straight-circular tube.•The optimal geometric parameters is the PR = 3.5, DR = 0.1, RR = 0.075 and N = 4.