Leonid Moroz
SoftInWay, Inc.
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n recent years, methods for turbo-machinery flowpath design have greatly improved
due to the intro-duction of three dimensional (3D)
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viscous flow concepts.
But one dimensional (1D) and two dimensional (2D) calculations are also beneficial,
especially at the early stages of the design.
This article describes the steps
in a turbomachinery conceptual design, which uses ID, 2D and 3D calculations.
Design of new turbomachinery is a long and complex process. It can be considered
as a sequence of several steps. They are:
- Preliminary design (sizing)
- Meanline and streamline flowpath analysis
- Profiling and 3D blade design
- Structural and modal analysis
- 3D flow analysis
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Preliminary design
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Design from scratch (preliminary design) is the first step of the conceptual workflow.
In this, the designer determines key parameters, such as the number of stages, diameters,
blade heights and cascade metal angles, reactions of the future flowpath and other
features that have a significant influence on the efficiency of the final design,
its manufacturing cost and reliability. Results of a preliminary design are in Figure 1.
The objective of the preliminary design is to determine optimal flowpath parameters and provide the best attainable performance while staying strictly within design limits. The preliminary design process typically uses the "inverse problem formulation" in which the flow-path dimensions are selected based on given geometrical and operational constraints.
The design is refined by pitchwise and spanwise optimization on S1-S2 surfaces,
taking into account general estimations of structural and modal constraints.
Optimization of the S1 surface in the pitchwise direction helps find the optimum
chord and pitch/chord ratio. The criterion used in this optimization is the minimization
of total losses. Optimization of the S2 surface
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Clockwise from top
Figure 1: Multistage turbine flowpath
after preliminary design
Figure 2: LP gas turbine
Figure 3: Contra-rotating turbine
Figure 4: Efficiency DoE study
Figure 5: Centrifugal compressor
in the
spanwise direction helps find optimal flowpath outline dimensions.
The next step is to validate the solution with the help of flowpath analysis through
the "direct formulation" process, in which flowpath performance is estimated with known
geometrical dimensions and fixed operating conditions. When the preliminary design is
finished, the designer uses flowpath meanline analysis (1D) in direct formulation
to validate the solution, and estimate more precisely the influence of flow leakages.
The next task in the design process is meanline and streamline flowpath analysis.
Meanline analysis uses data specified for the mean cross-sections to calculate
the solution. Axisymmetric analysis (2D) is used for further refinement, taking
into account the distribution of metal angles in spanwise direction. This type
of analysis uses data specified on several spanwise cross-sections along
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the blade
height to calculate a solution with the help of streamline solver methods.
Meanline and axisymmetric solvers are typically given the following tasks regarding boundary condition specifications:
- Find Mass Flow Rate (MFR) for a given outlet pressure
- Find outlet pressure for a given MFR
- Find inlet total pressure for a given MFR
- Find gauging angles for a given MFR and pressure
Gauging angles are used to predict the flow discharge in the
various stages of the flowpath. Outlet pressure can be taken as
static or total. In this step, it is possible to minimize incidence
(angles) by adjusting inlet blade angles to match flow angles.
The analysis can be carried out for multistage turbomachine stages
with full blade crowns, only rotor or stator crowns, stages with
contra-rotation, and mixed
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