Effect of Bend Geometry on Heat Transfer and Pressure Drop in a Two-Pass Coolant Square Channel for a Turbine

This paper presents a comparative numerical study of turbulent flow inside a two-pass internal cooling channel with different bend geometries. The goal is to find a geometry that reduces the bend related pressure loss and enhances overall heat transfer coefficient. A square channel with a round U-bend is taken as a baseline case and the heat transfer and pressure drop for nine different bend geometries are compared with the baseline. Modifications for the bend geometry are made along the channel divider wall and at the end wall of the 180 deg bend. The bend geometries studied include: (1) a turning vane geometry, (2) an asymmetrical bulb, (3) three different symmetrical bulbs, (4) two different bow shaped geometries at the end wall, (5) a bend with an array of dimples in the bend region, and (6) finally a combination of bow geometry and dimples. The solution procedure is based on a commercial finite volume solver using the Reynolds averaged Navier–Stokes (RANS) equation and a turbulence model. A two equation realizable k-ɛ model with enhanced wall treatment is used to model the turbulent flow. It was found that the bend geometry can have a significant effect on the overall performance of a two-pass channel. The modified bend geometries are compared with the baseline using Nusselt number ratios, friction factor ratios, and thermal performance factors (TPF) as the metrics. All the modified bend geometries show increase in the TPF with the symmetrical bulb configuration showing nearly a 40% reduction in friction factor ratio and a 30% increase in thermal performance. The highest TPF (41% increase over baseline) is observed for the symmetrical bulb combined with a bow along the outer walls and surface dimples.