CFD Modelling of Turbo-machinery

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 How to model Turbo-machinery using CFD simulations

   Correct prediction  of Rotating fluid flow  for Better performance 

By Dr. Sharad Pachpute

 

An Introduction to Turbo-machinery

• Turbo-machinery is defined as the machines (device) which transfer fluid energy between a rotor (a moving wheel with blades)  and a fluid.  It can be  turbines and compressors.
• As the turbine rotates and transfers energy from a fluid to a rotor, a compressor transfers energy from a rotor to a fluid

 
 
Fig.1 Flow pattern around the fan for clockwise and anti-clockwise rotation.
Fig.2 Steam-turbine in power plant for power generation
  • CFD users must be familiar with a basic understanding of turbo-machinery. Hence, the classification and representation of turbine and pump have been elucidated well with diagrams.
  • The CFD models are considered for turbo-machinery depending on assumptions, boundary conditions and selection of computational domain.
  • CFD users must-know advantages and disadvantages of each turbo -CFD model:  Best_practice_guidelines_for_turbomachinery_CFD

2. Classification of turbo-machinery
3. Major turbo-machinery in Thermal Power Plant
4. Governing equations for turbo-machinery
5. Modeling of turbo-machinery
6. ANSYS CFD Tools for turbo-machinery
7. Other CFD Tools for turbo-machinery
8. Resources and CFD Tools

Classification and Components of Turbo-machinery

Classification of Turbo-machinery

• Turbo-machines are classified into two types of power generation or power absorption 

 
 
 
Fig.3 Example of Turbo-machinery: steam turbine in  power generation
Fig.4 Example of Turbo-machinery for power absorption 

Turbo-machines  based on  Directions of fluid flow 

 

Parts of Axial Turbine Stage

  • Major components of axial flow turbine are given as below:
    • Stator comprises  a series of stationary blades that acts as guide vane for incoming flows
    •  The rotor comprises  a series of moving  blades and provides work/power to the shaft
  •   Application of Axial Flow Turbine: thermal  power plant and gas turbine of (aircraft engine)
  • Major parts of axial stage turbine: The following figure shows part of turbine and blades for a single-stage axial turbine

                 
 

 Representation of  A Single-stage Axial Turbine

  •  Cascade view of the axial turbine.
 
 
  •  Meridional  view of axial flow turbine

Multistage Axial Turbine

  •  A series of stages form a multistage turbine
  • The energy (work) transfer in a single stage is limited by the blade speed
  • If more energy transfer per unit mass is required, then the number of stages are added one after other
  •  Multi-stage steam turbine for thermal power plant
(a) Schematic of multi-stage steam turbine
                                                                            (b) turbine power-house
 
(c) Multi-stage Steam turbine without casing

Parts of Radial Turbine

  • Application of radial turbine: Gas turbine, turbocharger, and process industry   
 
 

                 

  • Components of radial turbine
 
Components of the radial turbine
 
                                     
 

 Impulse Turbine vs Reaction Turbine:

  • The difference between  impulse and reaction turbine is illustrated with schematics, axial variations of velocity, and pressure
  • In the impulse turbine, a high-velocity jet of fluid impinges in the moving bucked and provides the angular momentum to create the torque. The amount of power generated is the product of the torque and angular speed of the turbine.
  • In contrast, high-pressure fluid flow is used to create the reaction force of turbine and generation of torque
 

Power Absorbing Turbo-machinery

Compressor Types

 Axial Fans/blower 

 Fig.10 Different types of fans/blowers. 
  • Types of blowers that are popular in many industries and domestic applications like cleaning and cooling purposes

Water Pump

 
  • Parts of Water pump:

Representation of Flow through the Pump

  • The pump is easy to understand with two sectional view of pumps
 
  • The impeller is an essential part of the pump which rotates to create the desired pressure of water

Cascade view of Pump
 
 
  • Cascade view of a centrifugal pump

 

Governing Equations for Turbo-machinery

 Single Rotating Frame (SRF)

• Governing Equation for Single Rotating Frame (SRF) model in a rotating frame:

 
 
 
 

Multiple Reference Frame (MRF)

  • This part consists of stationary and rotating zones. Equations are solved in bot domains
 
           
where for each rotating zone: (1) the Coriolis force is added in the governing equations, (2) the flux is calculated from the relative velocity.


Euler’s Turbo-machine Equations

  •  Velocity diagram for the pump is given below
 

 CFD Modeling of Turbo-machinery

 Single Rotating Frame (SRF)

  • This model computes fluid flow in a rotating frame of reference that is adhere to a rotating machinery.
  • Flow is considered to be steady
           
        Refer for a detailed explanation: Rotating machinery training at OFW10_PDF
        Refer available OpenFOAM solver: OpenFOAM_SRF_Solvers
  •  Computational Domain

                            

 
  • Boundary conditions for SRF:
———————————————————————————————-

Multiple Reference Frame (MRF)

  • The Multiple Reference Frame (SRF) model computes fluid flow using both the rotating and stationary reference frames.
  • Multiple frames to be modeled : i)  Rotating zone is solved in the rotating frame, ii)Stationary zone is solved in the stationary frame:
a)  Computational Domain for MRF:

b) Computational Mesh for MRF:

b) Velocity Contours for MRF:
 

Mixing Plane Interface

      • This approach is used to model the axial flow turbines which comprises guide vanes, rotor and draft tubes


Periodic domain for CFD Analysis

 


Mixing Plane tutorial: Axial Turbine with Boundary conditions

                                       
 


Mixing Plane with a 3D view

 
 

Axial Turbine: Analysis of A single rotating blade

• Create the blade model using ANSYS BladeGen
• Import it into ANSYS ICEM CFD and mesh the model

Rotor CFD Domain:
Stator CFD Domain:

CFD Tools for Turbo-machinery

  • ANSYS modelers consist of three parts:
    • Geometry Creation based on blade profile and performance curve for a selected turbo-machinery
    • Create of the mesh using meshing platform
    • Simulation of turbo-machinery using suitable CFD models
 
  • ANSYS Tools : BladeGen,TurboGrid, CFX, FLUENTS
  • OpenFoam: SRFSimlpeFoam, MRFSimlpeFoam,
  • Turbo-machinery: WIKI_Turbomachinery
  • ANSYS for Turbo-machinery Solutions using ANSYS CFX, Mechanical and  DesignXplorer: watch the following video:

ANSYS Fluent for Turbo-machinery modeling

Conclusion

  • Turbo machinery are classified based on power absorption or generation, the direction of fluid flows
  • The moving part is a rotor which is a common part in all turbo-machinery
  • Different CFD models are developed for the analysis of turbo-machinery

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