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Modelling leakage flows in gas turbines

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Old   March 4, 2018, 21:22
Default Modelling leakage flows in gas turbines
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I'm modelling leakage flows within the turbine stage of a gas turbine engine. Essentially leakage flows can occur through the stator disk. As the leakage flow increases, this would disrupt the conventional rotor-stator flow structure and vortices starts forming as can be seen from Figure 1.

I'm basically trying to model this using CFD. I also have to acquire some values such as the pressure coefficient distribution and swirl ratio as a function of the vertical distance within the wheel space; I have experimental results to compare these values to. I have created a basic geometry to model the fluid in the wheel space between stator and rotor disks. I have attached a pdf drawing to show what it looks like (excuse the lack of professionalism). The left one is the stator disk and the right one is the rotor disk. I'm only modelling an 11.25 degree sector with one leakage flow hole to utilise axisymmetry; I'm using the periodic boundary pair on either side of the wheel space cross-section to do this. I have applied a total of 5 boundary conditions to the problem. I have attached the CFD file along with the support file so it is clear what I have done.

I have a few questions.

1. I have made the inlet (bottom) sufficiently long for the flow to develop, moreover Autodesk didn't let me apply a mass flow rate boundary condition on a non-planar surface which meant that I had to add an extension to the front end of the inlet (the bottom). Is there a way to add the mass flow rate condition on a non-planar surface on Autodesk?

2. Would you recommend doing a rotating region analysis? If so how can I go about doing this as I'm not sure how I would model the rotating region into this. Right now I'm just doing a steady state analysis, applying the boundary condition of rotational velocity to the rotor disk surface. The rotor disk is rotating at a velocity of 2500 rpm.

3. When I run the model, it seems to give similar flow pattern as Figure 1 by looking at the velocity field. However, the values I get is not correct for pressure coefficient for example. I get negative pressure coefficients from the built in pressure coefficient result parameter in Autodesk, which is not what I should be getting. In some cases these values are extremely large, much greater than 1. How does Autodesk calculate this parameter? Figure 2 below shows the formula I need to calculate this result, where b = 0.185 (wheel space radius) and omega = 2500rpm and P is just the normal static pressure at each point. Pbar is the mean static pressure within the wheel space. How can I obtain the mean static pressure from Autodesk?

4. With regards to calculating the swirl ratio (defined as the ratio between tangential velocity of the air to the rotational velocity of the disk), I'm not able to find how to identify the tangential velocity of the air in Autodesk.

5. When I use the xy plot to plot some results, how can I see the origin of the co-ordinate system it is obtaining the x and y co-ordinates from?

6. Also, with regards to meshing, I have tried to set wall layer parameters to essentially decrease the y+ value. This causes the model to be stuck at volume meshing when I run it. How could I resolve this? When I run it with the normal default wall layer parameters, it gives me y+ values of zero when I run the model. I don't know why this is the case.

7. Is there a way to do a structured mesh in Autodesk? If so how?

Just to make things clear, I have attached the drawing as an image and I have attached both Figure 1 and Figure 2 images. I can also find a way to attach the Autodesk CFD file if anyone wants to see what I have done.

Hope I'm not bombarding you with questions. I will really appreciate any help with any or all of the questions.

Attached Images
File Type: jpg Figure 1.jpg (102.7 KB, 11 views)
File Type: png Figure 2.PNG (9.7 KB, 8 views)
File Type: png Geometry Drawing.PNG (29.3 KB, 7 views)
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Old   March 7, 2018, 04:42
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Jon Wilde
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I see that you posted this over on the Autodesk forum also. I will monitor the thread there (linked above for others reference) and jump in if needed
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