Learning Forum

[Jpret99]

Hi all.

To give a run down, I have several models of a gas turbine blade with varied parameters to study their influence on the blade, which is subject to high temperatures and pressures. The parameters I have changed are the thermal barrier coating thickness (0.1 to 1mm), and the velocity (40 to 60 m/s) of internal cooling gas which simply flows from the base of the blade to the top without any complex flow channels. I obtained the influence of these through CFX. The general trend of increasing the TBC thickness is that the thermal gradients between the blade and coating decrease and the thermal gradient in the blade itself also decreases, which reduces thermally induced stress and strain (von Mises) in the blade, as shown by static structural analyses. (Note that i am only analysing the stresses on the external surface of the blade)

However, for increased cooling gas velocity, the temperature of the blade decreases (as expected) but the thermal gradients between the blade and coating, and in the blade itself increase. Due to this, the maximum stress increased for models with higher cooling gas velocity. However, the elastic strain decreases, which seems very unusual. From further investigation, the total deformation and thermal strain also decrease, meaning only the von Mises stress increases for increased cooling gas velocity.

The location of maximum stress and strain occurred at the same location for all models.

Does anyone have an idea or any experience with this sort of trend ?

[John Doyle]

Stress increasing while strain is decreasing intuitively sounds non-physical. However, the application you describe also sounds pretty complicated. Perhaps the combination of spin softening and thermal differential expansion together with temperature dependent material properties might help explain it. Could you try turning off spin softening (as a test), remove temperature dependent properties (as another test) to better understand the influences of these effects individually?