The conventional thermal design method of cooled turbine blade consists of independent decoupled analysis of the blade external flow, the blade internal flow and the analysis of heat conduction in the blade itself. To make such a calculation, proper interface conditions need to be applied.
In the absence of film cooling, a proper treatment would require computation of heat transfer coefficient by computing heat flux for an assumed wall temperature. This yields an invariant h to the wall and free stream temperature that is consistent with the coupled calculation. When film-cooling is involved the decoupled method does not yield a heat transfer coefficient that is invariant and a coupled analysis becomes necessary. Fast computers have made possible this analysis of coupling the conduction to both the internal and external flow. Such a single coupled numerical simulation is known as conjugate heat transfer simulation.
The conjugate heat transfer methodology has been employed to predict the flow and thermal properties, including the metal temperature, of a particular NASA turbine vane. The three-dimensional turbine vane is subjected to hot mainstream flow and is cooled with air flowing radially through ten cooling channels. The passage flow and heat transfer, internal coolant flow and heat transfer, and conduction within the metal are solved simultaneously using conjugate heat transfer methodology. Unlike the decoupled approach, the conjugate simulation does not require thermal boundary
conditions on the turbine walls.
It is believed that turbulence has a large influence on heat transfer and hence in the present study, a modified k-omega model by Wilcox was used for the external flow and
it was compared to Wilcox's standard k-omega model to see if the new model provided any improvement in predicting the heat transfer on the blade surface. Results here have shown that, for the present geometry, the standard model is still much better in predicting temperature and hence the heat transfer over the blade surface than the modified model.
A thermal barrier coating (TBC) “system” is used to thermally protect turbine engine blades and vanes from the hot gases in gas turbine engines. The overall results from this study, including turbulence modeling and the use of TBC, are encouraging and indicate that conjugate heat transfer simulation with proper turbulence closure is a viable tool in turbine heat transfer analysis and cooling design.