Large Eddy Simulation of Flow and Heat Transfer in a Channel Roughened by Square or Semicircle Ribs

The internal cooling passage of a gas turbine blade can be modeled as a ribbed channel. So far, most studies have considered square ribs. However, the ribs can be rounded due to improper manufacturing or wear during the operation. Round ribs have also been tested expecting that they may enhance the thermal and aerodynamic performance. Hence, we have studied two different rib geometries in this study, i.e. square and semicircle ribs. Large eddy simulations (LES) of turbulent flow in a ribbed channel with a dynamic subgrid-scale model are performed. In our simulation, the no-slip and no-jump conditions on the rib surface are satisfied in Cartesian coordinates using an immersed boundary method. We have also conducted an experimental study to validate the simulation. The velocity and temperature fields are measured using hot wire and thermocouple, respectively. The surface heat transfer is measured using the thermochromic liquid crystal with a high spatial resolution. LES predicts the detailed flow and thermal features such as the turbulence intensity around the ribs and the local heat transfer distribution between the ribs, which have not been captured by simulations using turbulence models. By investigating the instantaneous flow and thermal fields, we propose the mechanisms responsible for the local heat transfer distributions between the ribs; i.e. the entrainment of the cold fluid by the vortical motions and the impingement of the entrained cold fluid on the ribs. We also discuss the local heat transfer variation of the ribs in connection with flow separation and turbulent kinetic energy. The total drag and heat transfer are calculated and compared between the square and semicircle ribs, showing that two ribs produce nearly the same heat transfer, but the semicircle one yields lower drag than the square one.