Effect of the In-Hole Vortical Structures on the Cylindrical-Hole Film-Cooling Effectiveness

Film cooling is one of the most important technique when it comes to the cooling of the turbine blades in gas turbines. Understanding of the flow from an individual cooling hole is thus important in the design and analysis of the film cooling system in turbines. Efforts have been done to carefully study the vortex features associated with such a flow, which also replicate to some extent to those in a jet in cross flow. Recently, it has been shown using large eddy simulations, that the hairpin vortices are the basic structures found in such a flow situation. It is these vortices that are responsible for the formation of counter rotating vortex pairs (popularly known as CRVP) and other vortex structures as well.

The current study uses large eddy simulations to analyze the flow inside a standard cylindrical film cooling hole as well as outside in the mainstream flow. It is seen that the hairpin vortices located outside in the mainstream flow are found inside the cooling hole as well. It is also observed that these vortices originate at the strong shear layer formed due the flow separation near the hole entry irrespective of the hole length. One such structure is captured and compared to a standard hairpin vortex outside the cooling hole.

Other important contributions of this study include the comparative study of these in-hole hairpin vortices in two cases with different metering length. The change in film cooling performance due to this change in the in-hole hairpin vortices is also investigated. For this purpose, two different hole geometries with standard cylindrical hole shape but with different metering lengths are simulated with LES. Although various reasons are given to attribute for the change in film cooling effectiveness with the change in hole length, it is shown here that the effect due to these in-hole vortices is one of the fundamental reasons for a change in film cooling performance. The hole length plays a major role in the sense that, longer the hole the greater are the chances that these distinct vortex features will dissipate within the hole itself. In such a case there will be a uniform flow at the hole exit. However, if the hole length is decreased, it is seen that these vortices rather exit the hole and are responsible for the reduction in the film cooling effectiveness by affecting the vortex structures outside the hole.

The current study thus also implies that the proper study and analysis of the in-hole flow features is an important aspect to be kept in mind when it comes to the design of the cooling holes for optimum cooling performance.