Enhancement of Film Cooling Effectiveness Using Dean Vortices

Abstract

Abstract Film cooling technology is widely used in gas turbines. Improvement of gas turbine thermal efficiency, specific power and specific thrust can be achieved by reducing the use of cooling air by improvements on film cooling technology. Experimental and numerical efforts have demonstrated that cooling effectiveness is reduced when kidney vortices created as the emerging film cooling jet flow interacts with the passage flow resulting in coolant lift-off and mixing. With higher blowing ratios, M, these kidney vortices become stronger and effectiveness worsens. Different technologies have been developed to enhance film cooling effectiveness by manipulating the kidney vortices. Some reduce local blowing ratios and local injection angles with expanded hole exits, called shaped holes. Others are employing hole geometries in an attempt to establish anti-kidney vortices in the flow field to weaken the effects of kidney vortices. Most of these film cooling technologies focus on methods that are within the present limits of manufacturing technology. However, with the additive manufacturing anticipated in the future, there will be more freedom in film cooling hole design. Exploiting this freedom, the present authors tried using curved holes to generate Dean vortices within the delivery line. These vortices have opposite direction of rotation to the vorticity of the kidney vortices and, thus, there is interaction between these vortices in the mixing region. It is shown that as a result of the inclusion of Dean vortices, the curved hole delivery leads to enhanced film cooling effectiveness. Numerical results, including film cooling effectiveness values, tracking of vortices in the flow field, heat transfer coefficients, and net heat flux reduction are compared between the curved hole (CH), round hole (RH) and a laidback, fan-shaped hole (SH) with blowing ratios, M, of 0.5, 1.0, 1.5, 2.0 and 2.5. The laidback, fan-shaped hole represents the state of the art in film cooling hole design. Another curved hole (RCH) with the opposite (to the CH hole) curvature of delivery line is checked for comparison, with M = 1.5. The comparison shows that film cooling effectiveness values with the CH curved hole are higher than those with cylindrical film cooling holes at every blowing ratio studied. The curved hole has lower film cooling effectiveness values than the laidback, fan-shaped holes when M = 0.5 and 1.0, but shows advantages when the blowing ratio is higher than 1.0. With the interaction between Dean vortices and kidney vortices when using curved holes, a large amount of coolant re-attaches to the wall after moving streamwise for some distance, providing improved downstream film cooling performance. There is heat transfer enhancement for the curved hole case due to a higher kinetic energy transferred to the near-wall region, however. Nevertheless, the curved hole still displays a higher net heat flux reduction (NHFR) when the blowing ratio is high.