Turbine Blade Cooling

In the context of gas and steam turbines, the design and opti­miza­tion of turbine blade cooling struc­tures is a critical task for tur­bo­ma­chin­ery engi­neers. The higher tem­per­a­tures the first turbine stages can endure, the higher thermal effi­ciency can be realized. There are lit­er­ally unlim­ited pos­si­bil­i­ties in terms of how the inner struc­tures are designed and fine-tuned, to prevent turbine damage at high tem­per­a­tures under high cen­trifu­gal stresses. Finally, the most effi­cient way of solving this design problem comes down to an auto­mated shape opti­miza­tion process, where the design para­me­ters of such a cooling struc­ture are varied auto­mat­i­cally. For each gen­er­ated design, a sim­u­la­tion run is con­ducted and some objec­tive func­tions related to the stress, tem­per­a­ture, pressure, etc., are eval­u­ated and a ranking is assigned to each design can­di­date. Opti­miza­tion strate­gies, such as genetic algo­rithms and response surface methods help to minimize objec­tive func­tions in an auto­mated and effi­cient way, to find the optimal design in the shortest time. 

Geometry for Turbine Blade Cooling Structures

In order to be able to run shape opti­miza­tion studies for turbine blade cooling struc­tures, you have to create a flexible para­met­ric model that can reliably generate a large variety of design can­di­dates. Ideally, without breaking the solid geometry during the vari­a­tion process. CAESES provides a com­pre­hen­sive toolkit for the creation of inner air channels and variable hole con­fig­u­ra­tions. In par­tic­u­lar, Boolean oper­a­tions are used for gen­er­at­ing the solid turbine blade by merging the inner para­met­ric channel model with the outer blade surface geometry.

Channel Design Parameters

The fol­low­ing ani­ma­tions show a few para­me­ter changes for an example model in CAESES. The number of the inner ribs is an essen­tial design para­me­ter and can be controlled:

Not only the number of these channels is impor­tant, but also the dif­fer­ent angles that you can apply to these struc­tures, for which another para­me­ter can be introduced:

For turbine cooling struc­tures, the number of holes and their location within the blade are standard controls that turbine engi­neers need:

The vari­a­tion of the holes’ diam­e­ters is shown in this last ani­ma­tion and can be also con­trolled by a single parameter:

These are only a few para­me­ters, but there can be a variety of shape controls, which are all simul­ta­ne­ously changed within opti­miza­tion loops. Of course, there are also some geo­met­ric con­straints that need to be con­sid­ered as well. This makes sure that you create only feasible design can­di­dates during your studies. Note that we do not cover the fillet geometry (between the blade and the hub) in this blog post, to keep things short. There is another article about a tur­bocharger turbine, which also involves blade-hub fillets for con­sid­er­a­tion of stress and fatigue issues. 

Squealer Tips and Film Cooling Holes

Apart from the internal cooling passages detailed above, there are other struc­tures that deter­mine the cooling effi­ciency of a turbine blade. The tips of turbine blades expe­ri­ence espe­cially large thermal loads due to hot gases flowing through the gap between blade tip and shroud at high velocity. Apart from the thermal effects, this flow across the tip increases the losses in the flow. A recessed tip — a so-called squealer tip — can be used to reduce these effects. The tip gap can become smaller without risk of mechan­i­cal failure, which reduces the flow rate through the clear­ance. Also, the recess in the tip is believed to increase the resis­tance to the flow and can include cooling holes.

Turbine squealer tip modeling in CAESES[/caption] Cooling holes are gen­er­ally dis­persed through­out the blade and connect the internal cooling passages with the outer surface of the blade. The steady stream of exiting air creates a film on the blade surface that addi­tion­ally protects it from exposure to the hot gas. The cooling effec­tive­ness and per­sis­tence of the cooling film can be improved by reducing the velocity at the exit of the holes, i.e., by creating holes with a diffuser shape exit portion. Dif­fer­ent shapes of this diffuser portion further influ­ence the flow and can be explored for an optimal result. You can find some examples for squealer tip and film cooling hole opti­miza­tion in this pre­sen­ta­tion given by Siemens at our last users’ meeting.

More Infor­ma­tion

The turbine blade cooling struc­tures of this blog post orig­i­nally came from our partner NJTF, and the full article with a few more screen­shots and ani­ma­tions can be found here (Chinese). CAESES provides tur­bo­ma­chin­ery capa­bil­i­ties for geometry modeling, process automa­tion and shape opti­miza­tion strate­gies. Feel free to contact us if you have any ques­tions about this post, or if you are inter­ested in creating and using para­met­ric cooling struc­tures for turbine blades.