There are two main approaches to modeling blade film cooling in CFX:
Use a full resolution mesh
You can create a detailed mesh model that resolves the flow path from the plenum interior to the blade and out through individual holes in the blade surface. This approach is accurate, but can be computationally expensive in terms of setup and solution.
Use injection regions
You can create a coarser mesh that does not fully resolve the individual holes directly, then use one or more injection regions (see Injection Regions in the CFX-Pre User's Guide) to model the injection of cooling air from hole positions on the blade surfaces. This approach is computationally less expensive and allows the holes to be modified independently of the aerodynamic surfaces, facilitating rapid blade design optimization. As the optimization procedure continues, you can refine the meshes to better resolve each hole and its effects. Finally, a full resolution model can be created from the best design in order to help confirm its performance.
The following topics are discussed:
Injection region settings are described in Injection Regions in the CFX-Pre User's Guide.
For modeling arrangements of cooling holes it is recommended to use the Cylindrical Hole
injection model, together with the Selected 2D Regions option for Injection Location Type.
This will allow you to reproduce multiple circular cross-section holes on a blade surface.
When combined with profile field data, from the Injection Positions definition, it is possible
to specify hole-specific size and drilling direction, as well as flow properties.
If the cooling holes are not circular but take the form of slots, such as might be found at the
trailing edge of a blade, then it is recommended to use the User Defined Slot injection model.
This allows the definition of a series of slots, or slot segments, that can be linear or
curvilinear, as dictated by the locations and connectivity defined by the Injection Positions
definition. When combined with profile field data, it is possible to define a slot with varying
width and flow properties.
If you want to model platform leakage on the hub (or shroud), then it is recommended to use
the Cylindrical Slot injection model, particularly if the machine is axial.
This also allows the definition of slots that can be
linear or curvilinear, but also exploits the surface-of-revolution nature of the underlying
Selected 2D Regions to produce an accurate radial intersection and slot representation. It is
also less sensitive to the radial coordinate of the Injection Positions with respect to the 2D
regions. Again, it is possible to define the slot with varying width and flow properties.
If the machine is radial, consider using the Axisymmetric Slot option on hub or shroud
surfaces where the local normal is tending towards the axial direction.
The flow of a compressible fluid through a hole/slot can choke at a sufficiently high mass flow rate. Such choking results in a local pressure rise at the hole/slot and a restriction of the local Mach number to be no greater than approximately 1.0. If you are representing high-mass-flow holes/slots using injection regions, consider activating the Sonic Limiter option in order to model this choking effect. For details, see Flow Regime in the CFX-Pre User's Guide and Injection Regions in the CFX-Solver Theory Guide.
For good solution convergence, it is recommended that high-mass-flow holes/slots having near-sonic flow speeds have adequate mesh resolution.
Information about post-processing injection regions in CFD-Post is provided in Injection Regions in the CFD-Post User's Guide.