Compressor Valve Simulation Using ANSYS and CFX

2008 International ANSYS Conference

Compressor Valve Simulation Using ANSYS and CFX Sachin Pagnis, CFD Engineer Emerson Design and Engineering Center, India Zhichao Wang, Manager Of Analytical Services Emerson 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 © 2008 ANSYS, Inc. All rights reserved.

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Objective

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

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Simulation Difficulties • Transient coupled Fluid Structure Interaction problem for transient flows are still challenging when large deflection is involved • Rather large deformation due to the fluctuation of fluid load causes significant fluid mesh distortion such that solver stability is impacted • This is particularly challenging as the time step size for stable computation could be impractically small • An alternate approach is adopted to effectively resolve the physics of flow Induced deformation in the thin valve assembly

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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 geometry

2. 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) © 2008 ANSYS, Inc. All rights reserved.

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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 © 2008 ANSYS, Inc. All rights reserved.

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Geometry and Boundary Conditions for CFD & Structural Models

Valve

(b) (a)

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

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Boundary Conditions : CFD Materials Property Name Settings Density 961 kg/m3 Viscosity 0.028 kg/ms

Boundary Conditions Name

Settings

Inlet

Flow Direction = Normal to Boundary Condition Flow Regime = Subsonic Mass And Momentum = Total Pressure Relative Pressure = 1 [psi] Turbulence = Low Intensity and Eddy Viscosity Ratio

Outlet

Flow Regime = Subsonic Mass And Momentum = Static Pressure Relative Pressure = 0 [psi]

INTF1 (thin valve Wall Influence On Flow = Free Slip Top surface) INTF2 (thin valve Wall Influence On Flow = Free Slip Bottom surface) © 2008 ANSYS, Inc. All rights reserved.

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•CFD Results Valve deflection 0.79mm Top surface Pmean = 1.42e-3 MPa

Bottom surface Pmean = -1.93e-5MPa

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Valve deflection 0.49mm

Pmean

Top surface = 2.35e-3MPa

Bottom surface Pmean = -1.26e-5MPa

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Valve deflection 0.32mm

Pmean

Top surface = 3.34e-3MPa

Bottom surface Pmean = -4.61e-5MPa

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Structural Results Deflection

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

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Thin valve thickness = Thk#2 thin valve deflection (assumed) = 0.49mm (equilibrium position)

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Thin valve thickness = Thk#3 thin valve deflection (assumed) = 0.32mm (equilibrium position)

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Structural Results Equivalent Stress

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

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Thin valve thickness = Thk#2 thin valve deflection (assumed) = 0.49mm (equilibrium position)

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Thin valve thickness = Thk#3 thin valve deflection (assumed) = 0.32mm (equilibrium position)

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Simulation Results Summary

Valve Deflection (mm)

Valve Deflection Vs Reed Thickness 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

Thk#1

Thk#2 Valve Thickness

Thk#3

Stress Vs Valve Thickness

220

Stress (MPa)

• Note the asymptotic behavior of the deflection response to valve plate thickness – These are consistent with the stress in the plate for different thicknesses – Mathematically, this method is acceptable. In reality, the accuracy of this type of predictions may require more study (full FSI in progress)

Thin valve Static deflection thickness (in) (mm/in) 0.002 0.800 / 0.031 0.003 0.475 / 0.019 0.004 0.325 / 0.013

200 180 160 140

Van Mises

120

Max. Principal

100 Thk#1

Thk#2

Thk#3

Valve Thickness (mm) © 2008 ANSYS, Inc. All rights reserved.

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Calculation of Valve Deflection 4 Differential Total Pressure (MPa)

Deflection (mm)

3.5 3

Deflection Thk#1 thk Deflection Thk#2 thk Deflection Thk#3 thk

2.5 2 1.5 1 0.5 0 0.0

0.001

0.002

0.003

0.004

0.005

0.006

Total Pressure (MPa)

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• Predicted excessive deformation due to pressure solution from CFD appears to create unmanageable mesh distortion • Successive mesh displacements cause pressure spike on the CFD solver

– Although, all these could be potentially resolved as apparent from multiple success stories available at ANSYS on the couple FSI applications

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Reed Deflection Vs Reed Thickness 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

Thk#1

Thk#2 Reed Thickness

Thk#3

Stress Vs Reed Thickness

220

Stress (MPa)

• As next step, more investigation will be needed to use the manual procedure of getting these curves for any valve using ANSYS MFX solver – Current convergence difficulties are under investigation

Reed Deflection (mm)

Implicit Approach

200 180 160 140

Van Mises

120

Max. Principal

100 Thk#1

Thk#2

Thk#3

Reed Thickness (mm)

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Coupled FSI Applications with ANSYS MFX Solver

Courtesy: ANSYS Inc.

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Summary • Using sequential one-way coupled FSI procedure, an explicit procedure is developed to design thin valve assemblies • The current procedure is under validation studies. Strain gauge testing is on going. • Knowledge gain can now be used to improve performance of the coupled FSI problem with ANSYS MFX solver

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