Design Optimization of a Refrigeration Compressor

2/282 November 2021, CFturbo, Inc.

• Conceptual design of a centrifugal compressor R134a

• Software: CFturbo, integrated into Ansys Workbench

• One initial design + one manual design variation, “Design2”

• Mathematical optimization using CFturbo/Ansys Workbench/Design Explorer o 8 geometry parameters, 141 different designs in totalo All design points meet the performance requirements at selected rpm

• Substantial stage efficiency improvement at design speedo Initial design: η=56% o Manual “Design2”: η= 59% o Optimized “Design107”: η= 73%

• Total project time: 10 days• Next steps

o Adjustments and fine-tuning, for example blade loading variation and volute parameters o 360° impeller model and transient simulations for final check of steady-state results

Summary


1 Design target points

Speed Pt Tt Mass flow rate

Pressure ratio Πts

rpm psi F lb/s -

44000 50 60 4.1 3.9

Design target 36000 50 60 2.7 2.8

32000 50 60 1.7 2.4

30000 50 60 0.8 2.0

1

1.5

2

2.5

3

3.5

4

4.5

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5Pr

essu

re ra

tio [-

]Mass flow rate [lb/s]

Inlet Total pressure p = 50 psiOutlet Mass flow or static pressure

Fluid R134a (Real Gas)

Rotational speed numbers for off-design points were determined during the project


Mesh statistics Nodes Elements

Impeller (Segment) ≈ 750k ≈ 700k

Volute ≈ 875k ≈ 2.02m

Overall (with Pipe,in + Pipe,out) ≈ 1.76m ≈ 3.10m

2 Computational Mesh


Inlet

Outlet

Rotor-Stator-Interface (RSI) RSI1

Periodic boundaries

2 CFD Setup, Boundaries conditions and control sections

ANSYS CFX 2020 R2Steady state, Mixing Plane, Total Energy,SST-Turbulenzmodel, High-Resolution-Differencing-Scheme, viscose work term

Inlet Total pressure p = 50 psiOutlet Mass flow rate or static pressure

Rotor-Stator-Interface (RSI) RSI2

Discharge Diffuser Outlet (DDO)

Pressure ratio Πstage = P DDO / P INLET


3 Initial Design

Impeller Main DimensionsDS=62 mm,D2=122 mmB2= 4mm

Impeller Blade Anglesβ,B1= 43 ... 16 β,B2= 60

7/282 November 2021, CFturbo, Inc. Design & Optimization HVAC Compressor

1

1.5

2

2.5

3

3.5

1.5 2 2.5 3 3.5 4 4.5 5

Pres

sure

ratio

Sta

ge [-

]

Mass flow [lb/s]

Design point

Initial Design Stage (t-t)

Initial Design Stage (t-s)

3 Initial Design, Pressure ratio (Stage)@36000rpm


0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

1.5 2 2.5 3 3.5 4 4.5 5

η ise

ntro

pic

(tota

l -st

atic

) [-]

Mass flow rate [lb/s]

Initial Design Stage

Design & Optimization HVAC Compressor

Initial design meets the pressure ratio however the peak efficiency occurs at a higher mass flow rate.

Targets -> Increase efficiency and shift peak efficiency to lower mass flow rates.

3 Initial Design, Efficiency ηisentropic (Stage)@36000rpm


4mm

3mm

4 Design2 – Manual modification, reduced outlet width b2Outlet width B2= 3 mm (previoulsy 4mm)


1

1.5

2

2.5

3

3.5

1.5 2 2.5 3 3.5 4 4.5 5

Pres

sure

ratio

Sta

ge [-

]

Mass flow [lb/s]

Initial Design Stage (t-t)

Initial Design Stage (t-s)

Design2 Stage (t-t)

Design2 Stage (t-s)

4 Design2 – Pressure ratio (Stage)@36000rpm


4 Design2 – ηisentropic@36000rpm

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.5 2 2.5 3 3.5 4 4.5 5

η ise

ntro

pic

(tota

l -to

tal)

[-]

Mass flow [lb/s]

Initial Design Impeller


Design2 Impeller

Design2 Stage


0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

1.5 2 2.5 3 3.5 4 4.5 5

η ise

ntro

pic

(tota

l -st

atic

) [-]



Design2 Stage

Design2 meets required the pressure ratio at design point; theisentropic stage efficiency has been increased by 7 points at the design flow rate (at 2.7 lb/s).

4 Design2, Efficiency ηisentropic (Stage)@36000rpm


1

1.5

2

2.5

3

3.5

4

4.5

5

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Pres

sure

ratio

Sta

ge [-

]


Design2 Stage (t-t)

Design2 Stage (t-s)

CFD n44000rpm

CFD n33000rpm

CFD n30000rpmDP0

DP1

DP2

DP3

Design2, select rpm, all other operating points meet or exceed the target pressure ratios.

4 Design2, Other operating points


Bi-directional Integration for Automated Simulation, Design Space Exploration, Optimization

5 CFturbo in Ansys Workbench

Design Parameter Exchange

Connected parameter sets in CFturbo and Workbench!


5 Mathematical optimizationStart optimization from „Design2“8 geometry parameters were selected for optimization

Optimization methodMISQP (ANSYS Design Explorer, default setting)

Objectives and Constraints


5 Mathematical optimization, design parameters D2, b2 , β2

tm

Mean lines

Leading edge

βB2

βB1


5 Mathematical optimization, design parameter „Wrap Angle φ“

tm

Mean lines

tLE,S tTE,H

tTE,S

Leading edge

φ


5 Mathematical optimization, design parameter „Radial Offset“

Offset

The radial offset between impeller outlet and volute inlet is used to add a non-rotating component between impeller and volute. Technically it workd as a short lenght radial diffuser.


5 Mathematical optimization „Volute Radius and Diffusor Height“


5 Mathematical optimization, Results

Computational effort:Approx. 1 hour per design on a 12 core AMD Ryzen Threadripper PRO 3945WX 4.00 GHz workstation

• Optimization process 141 designs• Three ecxellent design proposals

Selected best fit „Design107“ • Peak efficiency η(t-s) > 73% • Π (t-s) ≈ 3.0


5 Mathematical optimization – Comparision

Design2

Design107

Impeller Main Dimensions

DS [mm] 62.0 62.0D2 [mm] 122.0 115.0B2 [mm] 3.0 3.0

Mean Blade AnglesL.E. β,B1 [°] 43/16 43/16T.E. β,B2 [°] 60 55

Blade Wrap Angle φ [°] 99/70 108/79

Number Blades Z [-] 14 (7/7) 12 (6/6)

Radial Diffuser L [mm] 2.0 6.6

Discharge Diffuser H [mm] 120.0 116.6

Design2 Design107


5 Mathematical optimization, Results Design 107

n=36000 rpmṁ=2.7 lb/s



n=36000 rpmṁ=2.7 lb/s



Span = 50%

n=36000 rpmṁ=2.7 lb/s


5 Mathematical optimization – Pressure ratio (Stage)@36000rpm

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Pres

sure

ratio

Sta

ge [-

]


DP 0 DP 1

DP 2 DP 3

n30000rpm (t-t) n30000rpm (t-s)

n32000rpm (t-t) n32000rpm (t-s)

n36000rpm (t-t) n36000rpm (t-s)

n44000rpm (t-t) n44000rpm (t-s)

„Design 107“


5 Mathematical optimization – ηisentropic (Stage)@36000rpm

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

η ise

ntro

pic

(tota

l -st

atic

) [-]


Stage@n30000rpm

Stage@n32000rpm

Stage@n36000rpm

Stage@n44000rpm

„Design 107“

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Design Optimization of a Refrigeration Compressor

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