Cooled Turbines

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March 2011
The Flow
CFD Insights for the Turbomachinery Designer

INSIGHT: Cooling Flow--To Mesh or Model?
Several numerical methods are available for accounting for cooling flow in film cooled turbine design, yet one must take care to understand each technique's intended use and limitations.  In this month's issue of The Flow, we sit down with Bob Ni to discuss these techniques and how they are best applied for analyzing turbine durability and performance.  Prior to founding ADS, Bob spent nearly 30 years at Pratt & Whitney leading turbomachinery CFD development in support of turbine and compressor design.

FLOW: Bob, accounting for cooling flow seems like a straightforward topic--what's all the confusion about?
BOB: While the techniques for accounting for cooling flow are well understood, the confusion centers around how best to apply thse techniques to address performance and durability during design.  Depending on the type of cooling flow present in your design and your analysis objectives, some techniques make sense while others can lead to erroneous results.
FLOW: Let's start by identifying each of these techniques.
BOB: Sure thing.  Generally speaking, there are three approaches: "Level 0", "Level 1" and "Level 2".  With Level 0, cooling flow is modeled in the solver using source terms, with mass addition and temperature dilution values defined near the leading edge plane and/or trailing edge plane. With Level 1, all cooling flow (e.g. from each cooling hole) is accounted for using source terms and mapped to a mesh.  With Level 2, all cooling holes are meshed and the cooling flow is initialized in conjunction with the core flow.   

FLOW: Let's discuss each of these in a bit more detail.  When does it make sense to use Level 0 cooling?
BOB: Level 0 is effective for tracking the global effect of cooling flow (e.g. meanline-level analysis).  It is the easiest technique to set up as it simply generates a step function change in mass flow and temperature at the prescribed leading and/or trailing edge planes.  Level 0 is best used early in a design cycle, prior to identifying hole placement, to determine overall turbine design configuration.

FLOW: How about Level 1?
BOB: The Level 1 cooling technique is very useful for predicting downstream spanwise flow distribution.  It requires more effort to set up than Level 0 because each cooling hole must be defined.  Though it is an effective way to produce a 2D temperature profile at the exit for durability and performance, because of the transpiration boundary condition it isn't intended for gathering detailed flow field insight near the holes.  Level 1 cooling works best once you have defined your cooling holes and are trying to assess downstream impact.  We recommend Level 1 for multi-stage steady or time accurate analysis because it provides higher fidelity than Level 0 with little to no impact on computation time. 

FLOW: Got it.  How about Level 2?
BOB: With Level 2 cooling, cooling holes and internal cavities are fully meshed.  In addition to the core flow, boundary conditions are provided for each coolant supply such as cooling inlet temperature, pressure and flow angle.  Because the cooling holes and internal cavities are fully meshed, this approach does a far better job of providing detailed 3D local flow field insight but requires far more computational resource to conduct the analysis.  Level 2 cooling is best used when detailed flow insights are required near the holes, such as when conducting durability studies on a film cooled turbine vane.
FLOW: Now that these techniques have been defined, what are your thoughts about where these techniques fit in the design/analysis process?
BOB: Each of these techniques definitely has a place in the design/analysis process.  To predict aero performance and interaction effects, we recommend employing a combination of Level 0 and Level 1 cooling.  Early in a design process, Level 0 should be used to account for cooling flow in stage design prior to defining hole placement.  Once hole placements have been defined, Level 1 cooling should be used to improve the fidelity of the multi-stage steady/unsteady runs and refine your understanding of aero performance and interaction effects.  
On the durability front, since since a detailed understanding of surface metal temperatures and local flow field distribution are required, neither Level 0 nor Level 1 should be employed.  Here, we believe the best bet is to employ a Level 2 approach in conjunction with conjugate heat transfer analysis. 
FLOW: Is it possible to use Level 1 cooling with a finer mesh insteady of Level 2 to predict airfoil surface metal temperatures?
BOB: Great question, it is certainly possible but with caveats.  The answer strongly depends on how the Level 1 capability is implemented: using source terms (Code Leo) or by imposing prescribed boundary conditions at the airfoil surface where the holes are located.  The prescribed boundary conditions apporach yields reasonable surface temperature prediction but requires an extremely dense mesh similar to that required by the Level 2 cooling method.  In contrast, the source terms approach is not geared for surface temperature prediction; rather, it is used to account for cooling flow effects on the downstream flow field and do so at much lower computational cost.  The limitation on the source term approach is that the mesh element size should not be much smaller than the size of the cooling hole.  If the mesh is too fine, the cooling flow will jump too much and lead to erroneous results.
For durability studies requiring airfoil surface metal temperature prediction, the choice comes down to using Level 1 with a dense mesh or Level 2 with conjugate heat transfer.  While it is certainly possible to use Level 1 with a finer mesh, we advocate employing a Level 2 approach in conjunction with conjugate heat transfer, as it is tightly coupled and likely to produce the highest fidelity results with little loss in turnaround time.
FLOW: Thanks, Bob.
BOB: My pleasure.
TECHTIPS: Creating an Iso-Surface Using ParaView
Three dimensional iso-surfaces are an effective way of visualizing flow features when conducting three dimensional simulations.  ParaView provides a utility to create these iso-surfaces for use with your CFD results.  <more>
TECHTIPS: Creating Contour Plots with lines for 3D Objects in ParaView
  When generating contour plots, it is often helpful to have lines outlining the transition between different contour levels.  ParaView does not offer this option directly, but this effect can be achieved using the Extract Surface and Contour tools.  <more>
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