compressor valve simulation using ansys and cfxvalve
TRANSCRIPT
© 2008 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary
2008 International ANSYS Conference
Compressor Valve Simulation Using ANSYS and CFX
Sachin Pagnis, CFD EngineerEmerson Design and Engineering Center, IndiaZhichao Wang, Manager Of Analytical ServicesEmerson Climate Technology, USA
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Outline
• Background & Objective• Simulation Procedure• Results & Discussions• Conclusions
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Background
• Background– A compressor valve is used in a bi-flow fluid path – The valve carries dynamic load and stresses due
to the fluctuation of pump pressure – The thin valve deflection (opening) is a critical
factor that affects the pump performance – High stress and strain may lead to thin valve
fatigue and compressor failure
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Thin Plate Valve for Flow Control
• The structural response of this thin plate controls the flow rate
• High pressure drop across the valve increase flow by forcing the valve plate to deflect more – Large deflection creates
high stress• In reverse flow condition, the
valve will stop the flow• Large structural deflection
application like this is one of the most challenging Fluid Structure Interaction problem
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Objective
Evaluate the deflection and stresses of thevalve due to fluid flow using ANSYS andCFX fluid and structure interaction feature(FSI)
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Simulation Difficulties
• Transient coupled Fluid Structure Interaction problem fortransient flows are still challenging when large deflectionis involved
• Rather large deformation due to the fluctuation of fluidload causes significant fluid mesh distortion such thatsolver stability is impacted
• This is particularly challenging as the time step size forstable computation could be impractically small
• An alternate approach is adopted to effectively resolvethe physics of flow Induced deformation in the thin valveassembly
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Simulation Procedure
• For a given reed valve thickness 1. Start with ANSYS
• Apply a pressure on it and obtain deformed reed geometry2. Take the deformed reed geometry to CFX
• Mesh the fluid volume; Set up the CFX case 3. Calculate steady state flow at this initial opening
• Note the pressure on the reed valve from CFX will be different from that used in ANSYS
4. Transfer fluid load from CFX to ANSYS • Calculate corresponding reed deflection
• Repeat steps 1 thru’ 4 for a number of pressure values – Step-3 will generate a curve of fluid load as a function
of reed openings– Step-4 will generate a curve of reed deflection
(opening) as a function of fluid load (pressure)
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Simulation Procedure
• Mathematically, we have two equations from the 2 curves– The solution of these equations can be easily obtained
if the two curves are not parallel– The cross point of these curves will be the actual
solution of the reed under steady state fluid flow• We expect relatively smaller changes in deflections for
different thicknesses of the reed valve– Flow will not be impacted significantly– ANSYS-only simulation (no CFX) will suffice to obtain
fluid pressure vs. reed deflection curves• This make this method more efficient and effective in
design optimization
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Geometry and Boundary Conditions for CFD & Structural Models
Valve(b)
(a)
Outlet
Figure 1 (a) CFD mesh & BC; (b) Structural geometry & BC
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Name Settings
Inlet
Flow Direction = Normal to Boundary ConditionFlow Regime = SubsonicMass And Momentum = Total PressureRelative Pressure = 1 [psi]Turbulence = Low Intensity and Eddy Viscosity Ratio
OutletFlow Regime = SubsonicMass And Momentum = Static PressureRelative Pressure = 0 [psi]
INTF1 (thin valve Top surface) Wall Influence On Flow = Free Slip
INTF2 (thin valve Bottom surface) Wall Influence On Flow = Free Slip
Name SettingsDensity 961 kg/m3
Viscosity 0.028 kg/ms
Boundary Conditions : CFD
Materials Property
Boundary Conditions
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•CFD Results
Top surfacePmean = 3.34e-3MPa
Bottom surfacePmean = -4.61e-5MPa
Top surfacePmean = 2.35e-3MPa
Bottom surfacePmean = -1.26e-5MPa
Valve deflection 0.79mm
Top surfacePmean = 1.42e-3 MPa
Bottom surfacePmean = -1.93e-5MPa
Valve deflection 0.49mm Valve deflection 0.32mm
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Structural ResultsDeflection
Thin valve thickness = Thk#1thin valve deflection (assumed) = 0.79mm (equilibrium position)
Thin valve thickness = Thk#2thin valve deflection (assumed) = 0.49mm (equilibrium position)
Thin valve thickness = Thk#3thin valve deflection (assumed) = 0.32mm (equilibrium position)
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Thin valve thickness = Thk#1thin valve deflection (assumed) = 0.79mm (equilibrium position)
Thin valve thickness = Thk#2thin valve deflection (assumed) = 0.49mm (equilibrium position)
Thin valve thickness = Thk#3thin valve deflection (assumed) = 0.32mm (equilibrium position)
Structural Results Equivalent Stress
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Simulation Results Summary
• Note the asymptotic behavior ofthe deflection response to valveplate thickness– These are consistent with the
stress in the plate for differentthicknesses
– Mathematically, this methodis acceptable. In reality, theaccuracy of this type ofpredictions may require morestudy (full FSI in progress)
Thin valve thickness (in)
Static deflection (mm/in)
0.002 0.800 / 0.0310.003 0.475 / 0.0190.004 0.325 / 0.013
Valve Deflection Vs Reed Thickness
00.10.20.30.40.50.60.70.80.9
Thk#1 Thk#2 Thk#3Valve Thickness
Valv
e D
efle
ctio
n (m
m)
Stress Vs Valve Thickness
100120
140160
180200
220
Thk#1 Thk#2 Thk#3Valve Thickness (mm)
Stre
ss (M
Pa)
Van Mises
Max. Principal
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Calculation of Valve Deflection
Total Pressure (MPa)
Def
lect
ion
(mm
)
0
0.5
1
1.5
2
2.5
3
3.5
4
0.0 0.001 0.002 0.003 0.004 0.005 0.006
Differential Total Pressure (MPa) Deflection Thk#1 thkDeflection Thk#2 thkDeflection Thk#3 thk
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Implicit Approach
• As next step, more investigation will beneeded to use the manual procedure ofgetting these curves for any valve usingANSYS MFX solver– Current convergence difficulties are
under investigation• Predicted excessive deformation due to
pressure solution from CFD appears tocreate unmanageable mesh distortion
• Successive mesh displacements causepressure spike on the CFD solver
– Although, all these could be potentiallyresolved as apparent from multiplesuccess stories available at ANSYS onthe couple FSI applications
Reed Deflection Vs Reed Thickness
00.10.20.30.40.50.60.70.80.9
Thk#1 Thk#2 Thk#3Reed Thickness
Ree
d D
efle
ctio
n (m
m)
Stress Vs Reed Thickness
100
120
140
160
180
200
220
Thk#1 Thk#2 Thk#3Reed Thickness (mm)
Stre
ss (M
Pa)
Van Mises
Max. Principal
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Coupled FSI Applications with ANSYS MFX Solver
Courtesy: ANSYS Inc.
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• Using sequential one-way coupled FSI procedure,an explicit procedure is developed to design thinvalve assemblies
• The current procedure is under validation studies.Strain gauge testing is on going.
• Knowledge gain can now be used to improveperformance of the coupled FSI problem withANSYS MFX solver
Summary