In order to permit high fuel efficiency and power output, jet engines needs to deal with high rotor speeds and turbine inlet temperatures (TIT). For modern jet engines, this temperature is usually above 2000K for take-off conditions. The component that is faced to those temperatures, represents the high pressure turbine.
The task of this component is to efficiently extract the energy from the hot gas and transfer it into shaft power. In order to deal with the high temperatures, high pressure turbine blade consists of highly heat resisting materials. Moreover, extensive cooling is needed.
The basic cooling technique of high pressure turbines represents internal cooling. Here, a coolant passes through multiple channels and picks up heat from the surrounding walls—leading to a reduced blade temperature. Typically, this technique is coupled with impingement and film cooling.
All three techniques have in common, that air from the upstream located compressor is taken as coolant medium. As a result, this extracted air cannot be used for the subsequent combustion process. Thus, the usage of cooling air always provokes a decrease of the overall efficiency. Therefore, jet engine manufacturers try to keep the amount of cooling air as low as possible and use it as efficiently as possible.
We asked Jens Iseler, Senior Technical Specialist with SIMULIA who developed this workflow, to provide answers to some common questions.
Q. Why does this workflow need to be analyzed?
A. Since the internal cooling plays a key-role for cooling concepts, a significant effort was put into the design of efficient internal cooling systems through the last several decades. Today, those systems are realized by serpentine-shaped channels, including heat transfer enhancements like turbulators, which is acceptable for current temperatures and pressures.
However, the next generation of engines will have much higher temperatures and pressures. Current concepts will barely render a sufficient blade cooling. In order to meet future cooling requirements, individually shaped cooling channels are needed in order to deliver the cooling air as efficiently as possible to a location that needs it.
This means jet engine manufacturers will need to have a design optimization tool, which is able to propose in very short time efficient fully three dimensional cooling passages tailor-made for their application (click on the above image to view more details).
So far, jet engine manufacturers design cooling passages manually or use parametric optimization. Both methods lead to enormous development times. Moreover, channels designed in that manner will suffer from a small degree of individuality and will therefore not use the available potential of improvement. Topology optimization enables those customers to obtain individual & better performing design in a fraction of time. This workflow documents how innovative channel designs can be created and validated by applying our best-in-class simulation and optimization techniques.
Q. Describe the workflow.
A. This workflow deals with a design optimization of the internal cooling system of a high pressure turbine blade followed by an evaluation of flow losses, heat transfer and heat conduction, stresses and life. The design optimization was achieved by applying the topology optimizer, Tosca Fluid. The usage of Tosca Fluid allowed an automated creation of innovative flow ducts with minimized amount of re-circulation and large surface area. As a result, cooling air is transferred at low pressure loss through the passages, while the large surface area permits a high heat transfer.
The Tosca Fluid process encompasses setup, run and post-processing and export of the obtained design proposals. By applying Isight, those steps were conducted automatically. The next step was to redesign the obtained Tosca Fluid ducts via 3DEXPERIENCE. With that, inappropriate features of the design were eliminated and a smooth connection to non-optimized sections was realized. The flow behavior and the heat transfer were evaluated afterwards by running a loosely coupled conjugate heat transfer analysis with a third party CFD solver. With that, the pressure loss and heat absorption by the coolant was quantified.
Since a change of the flow channels leads to a modification of the blade solid, a subsequent mechanical analysis with Abaqus/Standard was required. The study was set up as a linear elastic analysis. As material, the single crystal nickel superalloy CMSX-4 was applied. By using the mapped temperature values from the CFD solution and prescribing a centrifugal body force for the considered rotational speed, the available blade and root stresses were predicted.
The final step of this workflow was the life prediction with fe-safe. This analysis was conducted based on a prescribed duty cycle including three operational points (idle->take-off->cruise->idle) with an overall mission time of 3 hours. The results of the analysis showed how many flights can be processed before an initial damage on the blade occurs.
(Click on both images above to view in greater detail)
Q. What are you trying to learn from the simulation?
A. The goal of this workflow is to use simulation in order to virtually design new innovate cooling passages with improved flow performance and increased cooling efficiency. Since a change of the cooling passage geometries leads to a modified blade structure, a subsequent evaluation of stress and life is considered, as well.
Q. Which SIMULIA solutions did you use?
A. This workflow leveraged these products:
Q. What were the benefits of using simulation?
A. The following benefits were realized:
- Lower development time
- Innovative design creation by coupling simulation technology with smart design optimization algorithm
- High degree of automation
- Reduce number of physical prototypes