Effects of blade lean on internal swirl cooling at turbine blade leading edges

Abstract

The blade lean technique has been extensively employed in the design of modern gas turbine blades. Since swirl cooling is a promising alternative for internal cooling techniques at the blade leading edge due to its strongly-enhanced heat transfer, it is thereby vital to completely understand the fundamental flow and heat transfer behavior of swirling flows in a leaned tube to provide guidance for improved swirl cooling design in today's advanced turbine blades. In this study, to model a realistic internal swirl passage, the flow and heat transfer patterns of swirl cooling were numerically investigated within leaned, convergent tubes. To examine the effects of blade lean levels, two leaned tubes with moderate and extra angles were considered. The simulations were carried out using the unsteady Reynolds-averaged Navier-Stokes (URANS) method for a strong geometrical swirl number of 5.3. Furthermore, to demonstrate the necessity of using the unsteady simulation in modeling swirling flows, steady and unsteady RANS simulations were compared against experimental data for a straight tube with constant cross section. Results reveal that the steady RANS method fails to capture the reverse flow in the downstream section of the tube where vortex breakdown occurs, leading to over-predicted axial velocity and under-predicted circumferential velocity. However, the unsteady RANS method provides satisfactory results for both flow and heat transfer, achieving a good compromise between computational efforts and numerical accuracy. In comparison with the straight tube, Dean vortices in the leaned tube decelerate the decay of the swirl in the upstream section, and vice versa in the downstream part by changing the axial velocity profile. In addition to the double helical vortex typically observed in the swirling flow, the inner vortex induced by strong shear in a transition layer between core and ring zones also contributes to enhanced turbulence mixing and thus improves heat transfer. The stronger swirl in the inlet regions of the leaned tubes decreases the turbulence production in the core zone, resulting in a lower flow loss relative to the straight tube. Globally, the extra-leaned tube has an increase of heat transfer coefficients by 5.3% and a reduction of pressure loss by 24.90%, while the moderate-leaned tube has a decrease of heat transfer coefficients by 6.1% and a reduction of pressure loss by 12.89%, relative to the straight tube.