Flow Details

In contrast to a less labour – and CPU-intensive set-up with source terms (Kluge et. al. (2003)) the meshing of every single cooling hole, including the plenum provides a much higher level of detailed information. Since the local

Steady, Mixing Plane

o-o Unsteady, Time Averaged

— Unsteady, Time Dependent

Normalized Axia Distance

Figure 9. Heat Transfer Coefficient in NGV

fbw conditions at the cooling hole exits are not longer a fixed boundary con­dition as in the source term approach, the local blowing rate has the freedom to adapt itself according to the local few conditions. Consequently, the local blowing rate varies from hole to hole, obvious in the distribution of the heat transfer coefficient in Figure 9.

The cooling flow enters the blade passage in a typical flow pattern (Fig. 10). In dependence on the inclination angle, the local blowing rate and the shape of the cooling hole, the emerging jet acts much like a solid obstacle. The incoming boundary layer of the main flow rolls up into a horseshoe vortex, causing a counter-rotating kidney vortex behind the jet. (Hildebrandt et. al. (2002), Wilfert (1994)). This vortex configuration is responsible for the hot gas entrainment beneath the cool air, a distinct and undesired feature of cylindrical cooling holes.


Unsteady calculations of a transonic film cooled turbine stage where the cooling holes and the cold air plenum is discretized represent a high level

of very detailed information from the fbw. Clearly, on the downside of this approach are the high CPU requirements and the quite labour intense pre­processing. Both limitations prohibit the use of such a method in the frame of the daily design work in industry, which is characterized by short turn around times. The source term approach, presented in Kluge et. al. (2003) is more suitable in such an environment, but suffers not only from a lack of detailed fbw information, but more important from an uncertainty in the specification of the correct boundary condition for the source terms. Here, a full discretiza­tion offers the advantage that no boundary conditions are necessary on the exit surface of the cooling holes as long as the plenum is taken into account. How­ever, the boundary conditions for the plenum are relatively straightforward to obtain. An option is proposed to combine these two approaches. First, a set of fully discretized simulations are conducted for typical configurations and operating conditions. From these results, boundary conditions for the source term approach can be derived in order to calibrate the source term boundary conditions. But even then, the immediate vicinity of the cooling holes will be better captured using a full discretization of holes and plenum.


The reported work was carried out under the contract of the European Com­mission as part of the BRITE EURAM project contract number BRPR-CT-97- 0519, Project number BE97-4440 (TATEF).The authors wish to acknowledge the financial support as well as the contributions from ALSTOM POWER,



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