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ABSTRACT
The main purpose of this work is to conduct coupled CFD analysis of the steam
flow in the secondary part of turbine and study thermal rotor/stator steady state.
We consider a 11 stage high-pressure cylinder of a power steam turbine with extended terminal labyrinths. Domain of interest is only the secondary flow path of the cylinder, therefore the shroud and blades are beyond the scope of this work. All blades are virtually removed and on the imaginary cylindrical surface the effective boundary conditions are assigned. This takes into account heat fluxes that came to disks/diaphragms from blades in reality.
Practical engineering method of obtaining axi-symmetric solution has been developed and results are presented and discussed. Thermal state analysis shows a significant influence of secondary steam bleeding on temperature distribution of rotor and stator.
INTRODUCTION
Prediction of steam turbine rotor and stator thermal state is a complex problem that usually is divided into a set of interdependent problems related to steam flow, heat transfer and thermo-mechanical modeling. Some of most important modeling issues can be identified as follows:
- Turbine flow path heat transfer boundary conditions;
- Heat transfer boundary conditions related to the secondary part;
- Secondary steam flow including the flow in disk/diaphragm cavities and seals;
- Windage losses;
- Turbine rotor and stator thermal steady analysis;
- Seals radial/axial clearance optimization.
The most reasonable and correct way to solve the problem is a coupled solution when all boundary conditions and thermal state of turbine parts are interactively evaluated. Usually, all mentioned tasks are addressed separately due to obvious difficulties of computation. This typical approach
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lacks accuracy since temperature of the secondary steam with relatively small mass flow
rate is changing significantly through the secondary part channels. Meanwhile, the steam
temperature computation can be easily made more precise and close to real conditions by
successive iterative calculations of both solid thermal state and steam-solid heat exchange.
Thus, the key to the problem is the secondary steam flow prediction, which, as a rule, is
obtained with the help of network model that includes labyrinth seals, disks balance holes,
cavities between disk and shroud, etc. This approach not always adequately reflects
the process because the network model is based on a simplified concept of flow behavior
in disk cavities and in some cases usage of such model could lead to significant errors.
Therefore, a CFD analysis is more preferable way for obtaining the solution. Numerous
studies of rotated disk-stator system have been carried out using CFD approach (see, for
example, Wilson, Pilbrow, Owen, 1997-1999 and Kapinos, Matveev, Pustovalov, 1983).
All these studies contain theoretical and experimental analyses that show very intricate
phenomena of flow and heat transfer in such kind of systems. Vortexes in the terminal parts
of cavities, pumping effect, mixing in balance holes are only a few phenomena that cant be
accurately predicted by a simple model based on idea of boundary layers on disk and stator
surfaces. Unfortunately, the majority of published CFD works is restricted to the gas
turbines. The present work is an attempt to fill the void in secondary flow analysis for
steam turbines using the new method developed by authors.
NOMENCLATURE
| p
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pressure
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| v
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specific volume
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| T
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temperature
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| G
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mass flow rate
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| ΅, φ
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flow rate factor
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| D
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diameter of labyrinth seal
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| d
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diameter of balance holes
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| δ
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labyrinth seal clearance
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| z
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number of knifes in labyrinth seal or balance holes
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| uθw
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shaft rotational velocity
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