Influence of approaches in CFD Solvers on Performance ...

22nd International Compressor Engineering Conference at Purdue,

July 14 – 17, 2014

1

15th July 2014

Influence of approaches in CFD Solvers on

Performance Prediction in Screw

Compressors

Paper: 1124

Ahmed Kovacevic,

Sham Rane,

Nikola Stosic,

Centre for Positive Displacement

Compressor Technology,

City University London, UK.

[email protected]

Yu Jiang,

Michal Furmanczyk,

Sam Lowry,

Simerics Inc., Huntsville, USA.

[email protected]

mailto:[email protected]

mailto:[email protected]

Contents

2

1. Introduction

2. Objective

3. CFD Solver influences

4. Case Study

5. Summary

22nd International Compressor Engineering Conference at Purdue

Introduction – Screw Compressor CFD

3

• Rotary Screw Compressors are positive displacement machines

widely used in refrigeration, oil and gas and other industries.

• 3D CFD Analysis of Screw Compressors faces challenges like:

– Deforming Grid Generation

– Transient compressible fluid flow and boundary conditions

– Oil injection and Multiphase flows

• Many factors like grid quality, density, type, curvature refinement influence the solution accuracy.

• Similarly numerical solver formulations also impact the accuracy

and speed of calculations.


Introduction – Screw Compressor CFD

4 22nd International Compressor Engineering Conference at Purdue

• Prediction parameters – Pressure field,

– Velocity field,

– Mass flow rates,

– Leakage volume and Efficiency,

– Power,

– Dynamic losses,

– Noise and Design Improvements.

Literature Review

5

• CFD predictions better compare to

measurements at higher speeds than

at lower speeds. Similarly higher

deviations have been reported at

higher pressure ratios. [15]


• Thermodynamic chamber models are commonly used in the design and

analysis of twin screw compressors. [1,2]

• CFD has been increasingly used as a tool for design improvements in the

screw compressors, particularly for compressor ports. [10,13,14]

Rotor Grid Generation

6

SCORG Grid Generator for

a Twin Screw Machine

• 1999; SCORG – An algebraic, adaptive, block –

structured grid for twin screw rotors by

Kovacevic A, Stosic N and Smith I K. [3,4,5,6,7,8]

• 2005; Differential method with solution of

Laplace equation for unstructured to block –

structured grid for twin screw rotors by Voorde J

et al. [10]

• 2014; CFX Berlin – ‘TwinMesh’ tool for ANSYS

CFX solver. [12]


Objective

7

Solver – 1 : ANSYS CFX.

Rotor Grids from SCORG – Hexahedral

Port Grids from ANSYS – Tetrahedral

Element based control volume formulation

Pressure – Coupled Solver

Evaluate the influence of CFD solver formulations

on screw compressor flow calculations.

Solvers Identified:


Solver – 2 : Pumplinx.

Rotor Grids from SCORG – Hexahedral

Port Grids from Pumplinx – Body fitted

binary tree type

Cell centred control volume formulation

Pressure – Segregated Solver

Interface between Solver and Rotor Grid Generator


Solver – 1

User Fortran Library

Interface between Solver and Rotor Grid Generator


Solver – 2

Solver Comparison

10

Solver – 1 Solver – 2


Solver Comparison

11

Cell centred approach

Element based approach

Solver – 1

Solver – 2


Rotor Grids

12

SCORG

• Hexahedral

• Block Structured

• Deforming


Port Grids

13

Solver – 1

• Pressure Coupled

• Element based

• Tetrahedral

Solver – 2

• Pressure Segregated

• Cell Centred

• Body fitted Binary Tree


Case Study

14 School of Engineering and Mathematical Sciences

XK18 3/5 ‘N’ Profile, CD 93, Oil free, synchronized Twin Screw

Compressor

– Centre Distance, 93.00mm

– Main Rotor OD, 127.446mm

– L/D Ratio, 1.6

– Wrap Angle, 280º

– Built in Vi, 1.8

– Clearances,

• Interlobe 170 µm

• Radial 160µm

• End Axial 160µm

XK18 Compressor Working

Chamber

Model Clearance – 60µm

Solver Parameters


Criteria Solver – 1 Solver – 2

Turbulence Model SST – k Omega k-epsilon RNG

Inlet Boundary Condition Opening Pressure Pressure Inlet or

Total Pressure

Outlet Boundary Condition Opening Pressure Pressure Outlet

Advection Scheme Upwind Upwind

Pressure-Velocity Coupling Co-located layout SIMPLE – S

Turbulence Scheme First Order Upwind First Order Upwind

Transient Scheme Second Order First Order

Transient Inner Loop Coefficients

Up to 20 iterations per time step

Up to 25 iterations per time step

Convergence Criteria 1e-03 1e-03

Relaxation Parameters Solver relaxation fluids ‘0.1’ Pressure ‘0.5’

Pressure Variation


Solver – 1

Solver – 2

Experimental Measurement


Results – Pressure Angle Variation

18

6000 rpm, 2.0 bar

• Internal pressure calculated by both solvers is agreeing well with the

measured pressure curve.

• Some differences are noticed near the peak pressure at the moment of

opening of the discharge port. Solver 2 showed slightly better agreement

with measured data


Results – Performance

19

Experimental Range

• Solver-2 is predicting higher mass flow rate as compared to Solver-1 and is closer to

the experimental results.

• Both solvers are predicting similar indicated power, close to the experimental results.



20

Full Range CFD Comparison


• Over the full range, Solver-2 is predicting higher mass flow rate as compared to

Solver-1 even at higher discharge pressure.

• Only at 14000rpm, 2.0bar pressure, both solvers predict nearly equal mass flow.

• Both solvers are predicting similar indicated power.


21

Full Range CFD Comparison


• Over the full range, Solver-2 is predicting higher Volumetric efficiency and lower

Specific power as compared to Solver-1 even at higher discharge pressure due to

the corresponding higher mass flow predictions.

Comparison - Conclusions

22

• Comparison with experimental data suggests that Solver-2 is giving

more accurate estimation of flow rates for the expected clearances.

• Both solvers are predicting indicated power close to the experimental

data.

• Flow predictions are highly sensitive to rotor clearances.

• Operational clearances change due to the change in temperature. In

order to obtain accurate predictions, this change should be

accounted for in the CFD models. This may require employment of

fluid solid interaction modelling.

• Calculation time required for Solver-2 to reach a cyclic solution is

about one third the Solver-1 and it is also less memory intensive.


Thank You


References 1. Fleming J. S., Tang Y., Cook G., 1998, The Twin Helical Screw Compressor, Part 1: Development, Applications and

Competitive Position, Part 2: A Mathematical Model of the Working process, IMechE, Journal of Mechanical Engineering

Science, Vol. 212, p. 369.

2. Hanjalic K., Stosic N., 1997, Development and Optimization of Screw machines with a simulation Model – Part II:

Thermodynamic Performance Simulation and Design Optimization. Transactions of the ASME 664 / Vol. 119

3. Kovacevic A., Stosic N., Smith I. K., 2000, Grid Aspects of Screw Compressor Flow Calculations, ASME Congress,

Orlando FL

4. Kovacevic A., Stosic N. and Smith I. K, 2002, Numerical Simulation of Fluid Flow and Solid Structure in Screw

Compressors, Proceedings of ASME Congress, New Orleans, IMECE2002-33367.

5. Kovacevic A., Stosic N. and Smith I. K., 2003, Three Dimensional Numerical Analysis of Screw Compressor

Performance, Journal of Computer Methods in Applied Mechanics and Engineering.

6. Kovacevic A, 2005: Boundary Adaptation in Grid Generation for CFD Analysis of Screw Compressors, International

Journal for Numerical Methods in Engineering (IJNME), vol. 63.

7. Kovacevic A., Stosic N., Smith I. K., 2006, Numerical simulation of combined screw compressor–expander machines for

use in high pressure refrigeration systems, Simulation Modeling Practice and Theory, Volume 14, Issue 8, Pages 1143–115

8. Kovacevic A, Stosic N and Smith I. K, 2007. Screw compressors - Three dimensional computational fluid dynamics and

solid fluid interaction, ISBN 3-540-36302-5, Springer-Verlag Berlin Heidelberg New York.

9. Thompson J. F, Soni B and Weatherill N. P, 1999. Handbook of Grid Generation, CRC Press.

10. Voorde John Vande and Vierendeels Jan, 2005, A grid manipulation algorithm for ALE calculations in screw

compressors. 17th AIAA Computational Fluid Dynamics Conference, Canada, AIAA 2005-4701.

11. Brummer A., Hutker J, 2009. Influence of geometric parameters on inlet-losses during the filling process of screw-type

motors. Developments in mechanical engineering, vol. 4, pp. 105-121.

12. CFX Berlin, (2013), Twin Mesh for positive displacement machines. http://www.cfx-berlin.de/software /stroemungsmech

anik/twinmeshgerm aschinen.html

13. Rane, S., Kovacevic, A., Stosic, N. and Kethidi, M. 2013, CFD grid generation and analysis of screw compressor with

variable geometry rotors, International conference on compressors and their systems, London, C1390/139.

14. Pascu M., Kovacevic A., Udo N. (2012) Performance Optimization of Screw Compressors Based on Numerical

Investigation of the Flow Behaviour in the Discharge Chamber. Proc. Int. Compressor Conf. at Purdue, pp. 1145.

15. Kovacevic A. and Rane S., 2013, 3D CFD analysis of a twin screw expander, 8th International conference on

compressors and their systems, London, p. 417.


Influence of approaches in CFD Solvers on Performance ...

Documents

Transcript of Influence of approaches in CFD Solvers on Performance ...