compressor valve simulation using ansys and cfxvalve

© 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


Outline

• Background & Objective• Simulation Procedure• Results & Discussions• Conclusions


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


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


Objective

Evaluate the deflection and stresses of thevalve due to fluid flow using ANSYS andCFX fluid and structure interaction feature(FSI)


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


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)


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


Geometry and Boundary Conditions for CFD & Structural Models

Valve(b)

(a)

Outlet

Figure 1 (a) CFD mesh & BC; (b) Structural geometry & BC


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


•CFD Results

Top surfacePmean = 3.34e-3MPa

Bottom surfacePmean = -4.61e-5MPa

Top surfacePmean = 2.35e-3MPa


Valve deflection 0.79mm

Top surfacePmean = 1.42e-3 MPa


Valve deflection 0.49mm Valve deflection 0.32mm


Structural ResultsDeflection

Thin valve thickness = Thk#1thin valve deflection (assumed) = 0.79mm (equilibrium position)







Structural Results Equivalent Stress


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


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


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


Coupled FSI Applications with ANSYS MFX Solver

Courtesy: ANSYS Inc.


• 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

compressor valve simulation using ansys and cfxvalve

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