The turbine department was able to gain design and optimization capability during the last years. This enables to fully design and aerodynamically validate a turbine (simplified process shown in Fig. 1) even if only few initial requirement data like RPM or power are known.
The one-dimensional turbine predesign is a DLR in-house tool, which was developed at turbine department to be able to design complete, if needed multi-stage and multi-spool, turbines. In the following process, the initial geometry is supposed to be defined in more and more detail and then even two-dimensional calculation codes can be used for a fast design evaluation and further optimization. For this, a commercial blade-to-blade solver (S1 plane) and two different in-house through flow codes for annulus flow (S2m plane) are available at the turbine department. The resulting three-dimensional geometry can be calculated via DLR’s CFD code TRACE and then further optimized (Detailed design).
Predesign
With the year 2008 and start of DLR internal project EVITA (led by AT-TWK) the turbine department began to develop a concept of a general design process for axial turbines with intended use in research, aviation, power generation or traffic. First there has been defined a process separation in two dominant parts, namely predesign and detailed design. Following this concept, predesign should include all necessary actions (and tools) which are reasonably carried out or used before a 3D Navier-Stokes CFD calculation is set up for further enhancing the design. At this point, detailed design begins per definition. In order to realize the predesign part of the concept, the following processes were implemented and tools were developed:
PrEDiCT: 1D (quasi-2D) turbine predesign program aiming to yield annulus and initial blade geometries from basic thermodynamic input data. It provides different means of design parameter variation and accounts for turbine cooling flows. It enables the designer to consider an arbitrary number of stages at different spools => universality
GP³S: Graphical Predesign Program Processing Suite => framework computer program which enables configuring, executing and result post processing in predesign => user interface with input aids. Currently configured to work with PrEDiCT, upgrade to OS-independent compatibility with all predesign tools is ongoing. Fig. 2 shows the vision behind and intention of the predesign framework GP³S, which can potentially process many turbines at once, a virtually 3D designed blade of a certain stage and finally the real manufactured part after being finalized in 3D detailed design (here: low pressure turbine blade for project TATT ).
BLADEGENERATOR: Parameterization of turbomachinery blades by spline curves in two or three dimensions, possibility of specific, local form variation => is currently being used in turbine predesign and represents interface to 3D detailed design process.
SLC4T: 2D streamline curvature program to simulate annulus flow with correlation-based representation of blade geometry. Possible usage directly after 1D design to get a better thermodynamic state estimation and simulate off-design behavior. In the following predesign it can also be used to optimize 2D annulus contour and further on, after calibration against 3D CFD, to get more profound operating point calculation results.
FEMT: 2D finite element solver with generally identical application as SLC4T. In contrast however, this program has partly complementary attributes compared to SLC4T => FE discretization enables slow flow and backflow capability and a true 2D high resolution calculation.
MISES: 2D blade-to-blade finite volume solver to simulate turbine cascade (profile) flow in consideration of boundary layer effects => used to detail the initial blade profile geometries from PrEDiCT generated by BLADEGENERATOR and to guarantee for desired deflection and acceleration over the cascade. Coupling to multi objective optimizer during design is possible and common.
AutoOpti: Independent optimization program code employing a genetic algorithm and with the possibility to enhance optimization speed and/or result quality by using mathematical surrogate models. Gets used in turbine predesign for 2D profile optimization and variation of annulus geometry.
Figure 3: Turbine predesign process
An instantly obvious advantage from establishing of a design process chain by dedicated persons is the capability to design turbines by contract for research purposes or for industrial, third-party customers. This was shown during the DLR project EVITA and is currently being advanced in the scope of the project PEGASUS. During the TPS project with DNW and NLR, the turbine department’s new design capability could be used in its full scope to finally yield a real manufactured and functional part at DNW, which at the same time helped to prove the universality and flexibility of the design process (because of very special constraints especially compared to standard aviation or power turbines).
Detailed design
The main tools for the aerodynamic simulation of turbomachines are DLR’s CFD code TRACE and the automatic optimizer AutoOpti. The tools for the generation of parametrized 3D blades already existing in the Institute of Propulsion Technology were adapted to turbines. This was necessary as turbine blades exhibit a far larger turning as compressor blades.
Together with the departments Fan and Compressor and Numerical Methods a program for the generation of non-axisymmetric endwalls was written and implemented into the grid generator.
To aim at the complete aerothermodynamic and mechanical simulation of turbines it is necessary to include cooling in more detail. Especially the internal cooling channels have to be incorporated into the geometrical description of the turbine blade. A tool generating parametrized cooling channels is presently built up.