twinmesh for positive displacement machines: structured ... · twinmesh for positive displacement...
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Dipl.-Ing. Jan Hesse, Dr. Andreas Spille-Kohoff
CFX Berlin Software GmbH
Karl-Marx-Allee 90 A
10243 Berlin
TwinMesh for Positive Displacement
Machines: Structured Meshes and reliable
CFD Simulations
05.06.2014
Contents
• Introduction
• Methods for chamber modelling
• TwinMesh
• Simulation results
• Summary
ANSYS Conference & 32nd CADFEM Users' Meeting 2014, June 4 – 6, 2014, NCC Ost, Messe Nürnberg 2
Introduction
• Positive displacement machines (examples)
ANSYS Conference & 32nd CADFEM Users' Meeting 2014, June 4 – 6, 2014, NCC Ost, Messe Nürnberg 3
Screw compressor Lobe pumps
Gear pumps
GHH Boerger
Introduction
• Characteristics of the geometries
– Two rotating rotors (often screwed)
– Size-changing chambers with very small clearances between the lobes
and between rotors and casing
• Characteristics of the flow
– Cavitation (Multiphase)
– Non-newtonian fluid
– Compressibility
– Real-gas properties
– Turbulence
– Viscous heating, etc.
ANSYS Conference & 32nd CADFEM Users' Meeting 2014, June 4 – 6, 2014, NCC Ost, Messe Nürnberg 4
Lobe pump
Methods for chamber modelling
ANSYS Conference & 32nd CADFEM Users' Meeting 2014, June 4 – 6, 2014, NCC Ost, Messe Nürnberg 5
How can I model this
complex behavior in my
CFD-simulation?
Methods for chamber modelling Immersed-Solid
• Advantages
– Fast – only one mesh for fluid and solid volumes
– Variable time step control
• Disadvantages
– Insufficient wall treatment
– Multiphase is not available
(e. g. Cavitation)
– Only incompressible fluids
– Possible numerical instabilities depending
on local pressure gradients (e. g. gap flow),
small time steps necessary
– Very large number of elements depending on
geometry especially for unstructured meshes
ANSYS Conference & 32nd CADFEM Users' Meeting 2014, June 4 – 6, 2014, NCC Ost, Messe Nürnberg 6
Schwotzer, T.: „Simulation einer Drehkolbenpumpe
mit der Immersed-Solid-Methode“, Bachelorarbeit,
Technische Universität Berlin, 2009
Methods for chamber modelling Mesh Deformation and Remeshing
• Advantages
– Automatic mesh generation (less manpower required)
– Fluid volume is represented by the mesh
– Full model support
(e.g. Multiphase with Cavitation,
turbulence model)
• Disadvantages
– Mesh-generation for almost each iteration
(increasing computation time)
– Mesh generation leads to very high
element numbers in gaps (increasing computation time)
– Mesh quality issues due to mesh deformation and
element topology when using remeshing
– Numerical errors due to frequent interpolation of calculation results
between different meshes
ANSYS Conference & 32nd CADFEM Users' Meeting 2014, June 4 – 6, 2014, NCC Ost, Messe Nürnberg 7
Methods for chamber modelling Manual Generation of Structured Hexahedral Meshes
• Idea
– Manual grid generation in ANSYS ICEM CFD Hexa
for many rotor positions per rotation
• Advantages
– Best mesh and numerical quality
– High resolution of gaps is possible
– Element topology allows manageable model size
– No interpolation errors since the grid topology
remains the same
• Disadvantages
– Extremely high manual effort: grid
generation for 2D-models would
need 4 weeks
block structured grid
ANSYS Conference & 32nd CADFEM Users' Meeting 2014, June 4 – 6, 2014, NCC Ost, Messe Nürnberg 8
Fuchs, M.: „Numerische Simulation der instationären
Strömung in einer Drehkolbenpumpe“, Bachelorarbeit,
Technische Universität Berlin, 2010
TwinMesh Seven steps from CAD to Mesh
• TwinMesh is a novel software, developed by CFX Berlin Software
GmbH which generates high-quality hexahedral meshes for the
rotating parts of axis parallel rotary positive displacement machines.
Split of the simulation
domains into steady
and rotating parts
Grid generation for the rotors
with TwinMesh and for the
Steady parts with ANSYS ICEM
CFD or ANSYS Meshing
Rotor 1 Rotor 2
Stator
Numerical calculation
with ANSYS CFX
Simulation Workflow
ANSYS Conference & 32nd CADFEM Users' Meeting 2014, June 4 – 6, 2014, NCC Ost, Messe Nürnberg 9
TwinMesh 1. Geometry import
• File format (2D cross section of the machine)
– IGES
– CSV-File with point coordinates
ANSYS Conference & 32nd CADFEM Users' Meeting 2014, June 4 – 6, 2014, NCC Ost, Messe Nürnberg 10
TwinMesh 2. Boundary definition
• Boundary types
– Rotor curvature
– Casing curvature
– Additional curvature for interface creation
ANSYS Conference & 32nd CADFEM Users' Meeting 2014, June 4 – 6, 2014, NCC Ost, Messe Nürnberg 11
TwinMesh 3. Geometric characteristics
• Options
– Number of lobes
– Rotorposition
– Rotor curvature modifications
– Screwing
– Angle per timestep
– …
ANSYS Conference & 32nd CADFEM Users' Meeting 2014, June 4 – 6, 2014, NCC Ost, Messe Nürnberg 12
TwinMesh 4. Interface generation
• Automatic contact definition for the rotor meshes
– Interfaces between the rotor meshes are
automatically generated depending on
rotor curvature
ANSYS Conference & 32nd CADFEM Users' Meeting 2014, June 4 – 6, 2014, NCC Ost, Messe Nürnberg 13
Rotor 1 Rotor 2
Stator
TwinMesh 5. Mesh properties
• Definition of node distribution
– On curves
– In radial and axial direction
– Inflation layer
ANSYS Conference & 32nd CADFEM Users' Meeting 2014, June 4 – 6, 2014, NCC Ost, Messe Nürnberg 14
TwinMesh 6. Mesh generation and quality check
• Automatic mesh generation for each rotation angle
– Smoothing algorithm depending on orthogonality and volume change
– Different methods of mesh connection available (non-conforming or 1to1)
– Visual and quantitative quality check tools available
ANSYS Conference & 32nd CADFEM Users' Meeting 2014, June 4 – 6, 2014, NCC Ost, Messe Nürnberg 15
Mesh quality
Mesh
TwinMesh 7. Mesh export
• Export
– Mesh export to ANSYS CFX (format .cfx5) for the first rotor position
– Export of mesh displacement coordinates for each rotor position
– Including ANSYS CFX Session File for easy setup in ANSYS CFX
ANSYS Conference & 32nd CADFEM Users' Meeting 2014, June 4 – 6, 2014, NCC Ost, Messe Nürnberg 16
Simulation results Lobe pumps
• Challenges
– Incompressible fluids with cavitation (multiphase)
– Non-newtonian fluids even with high viscosity
ANSYS Conference & 32nd CADFEM Users' Meeting 2014, June 4 – 6, 2014, NCC Ost, Messe Nürnberg 17
Simulation results Gear pumps
• Challenges
– Complex rotor geometry
– High pressure gradients
– Incompressible fluids with cavitation
– Non-newtonian fluids even with
high viscosity
ANSYS Conference & 32nd CADFEM Users' Meeting 2014, June 4 – 6, 2014, NCC Ost, Messe Nürnberg 18
Simulation results Screw compressor
• Challenges
– Highly screwed rotors
– Compressible fluids
– High flow velocities
– Real-gas properties could be
defined
• Simulation conditions
– Rotational speed: 13000 rpm (male)
– Air (Ideal Gas, constant properties)
– 2 bar pressure difference
– Axial gap not modeled
ANSYS Conference & 32nd CADFEM Users' Meeting 2014, June 4 – 6, 2014, NCC Ost, Messe Nürnberg 19
Simulation results Screw compressor
• General analyses (Torque, Power and massflow)
– Power: 41.2 kW
– Averaged volume flow: 890 m³/h
ANSYS Conference & 32nd CADFEM Users' Meeting 2014, June 4 – 6, 2014, NCC Ost, Messe Nürnberg 20
Torque Massflow
Rotation angle [°] Rotation angle [°]
Massfl
ow
[kg
/s]
To
rqu
e [
Nm
]
Outlet
Simulation results Screw compressor
• Detailed analyses (Pressure change)
– Pressure at different measurement
points
ANSYS Conference & 32nd CADFEM Users' Meeting 2014, June 4 – 6, 2014, NCC Ost, Messe Nürnberg 21
Rotation angle [°]
Ab
so
lute
Pre
ssu
re [
Pa]
Point 1
Point 2
Behind the chamber
Point 1
Point 2
Summary
ANSYS Conference & 32nd CADFEM Users' Meeting 2014, June 4 – 6, 2014, NCC Ost, Messe Nürnberg 22
• There is now an efficient workflow for the simulation of rotary positive
displacement machines with ANSYS CFD Software
• Key features of TwinMesh
– Generation of high quality structured meshes with smoothing algorithm
– Easy to use (comfortable GUI)
– Works with ANSYS CFD
– Works already for many different machine types
– Works with gap sizes down to 1 m
– Individual node distribution and rotation angle steps
CFX Berlin Software GmbH
Folie 23 ANSYS Conference & 32nd CADFEM Users' Meeting 2014, June 4 – 6, 2014, NCC Ost, Messe Nürnberg