Numerical Modelling of Twin-screw Pumps Based on Computational Fluid Dynamics

Transcription

1 Numerical Modelling of Twin-screw Pumps Based on Computational Fluid Dynamics 6-8 th March 2017 Dr Sham Rane, Professor Ahmed Kovačević, Dr Di Yan, Professor Qian Tang, Centre for Compressor Technology, City, University of London, UK. State Key Laboratory of Mechanical Transmission, Chongqing University, China. 1

2 Contents 1. Introduction 2. CFD analysis based design of Screw type PD Pumps 3. SCORG STAR-CCM+ Integration 4. Case Study Liquid Screw Pump 5. Other applications 6. Future Work 2

3 Components of a Twin-Screw Pump 3

4 Working of a Twin-Screw Pump Wall Casing Discharge Port Chamber Chamber Suction Port Casing Wall 4

5 Typical Tooth Profiles of Multiphase Screw Pumps x y y YT c 1 b 1 d 1 c1d1 O 1 r 1 1 ab a b c bc 2 d cd y x r 2 O 2 2 a 1 b1c 1 a1b1 2 1 x f e a0 r 1 1 o r(1-x) b r(1+x) c r 1 1( x1 ) r2 1 x ( 2 ) A-type Profile YT B-type Profile d f e 1 a0 1 a b c c o XT C-type Profile 主杆 D-type Profile 从杆 5

6 Contents 1. Introduction 2. CFD analysis based design of Screw type PD Pumps 3. SCORG STAR-CCM+ Integration 4. Case Study Liquid Screw Pump 5. Other applications 6. Future Work 6

7 CFD analysis based design of Screw type PD Pumps Why do CFD Modelling? Experimental performance measurements expensive, slow and difficult to control. Important phenomenon such as cavitation and leakages are difficult to observe and measure. Design parameters Pressure field Velocity field Fluid forces Mass flow rates Cavitation zones Leakage volume and efficiency Power Dynamic losses 7

8 Challenges in CFD modelling of Screw Pumps Positive Displacement Operation Rotor geometry is complex Pumping chamber formed between rotors and housing is highly deforming during operation Representative CFD grid has to capture core volume as well as leakage gaps, aspect ratio of the order of 1000 Cavitation might occur Small time step size demand due to highly transient flow structures Flow in the leakage gaps not very well understood Turbulence Modelling and Multiphase modelling required 8

9 Contents 1. Introduction 2. CFD analysis based design of Screw type PD Pumps 3. SCORG STAR-CCM+ Integration 4. Case Study Liquid Screw Pump 5. Other applications 6. Future Work 9

10 A tool for Screw Rotor Grid Generation Developed at City, University of London since 1999 Algebraic and Differential grid generation implementation Thermodynamic Chamber modelling Rack as a unique partitioning curve 10

11 SCORG STAR-CCM+ Integration Serial solver integration 2012 Extended to parallel solver 2014 User Defined Nodal Displacement model C++ User library, Windows / Linux platform Compatible with all STAR-CCM+ physics models 11

12 Contents 1. Introduction 2. CFD analysis based design of Screw type PD Pumps 3. SCORG STAR-CCM+ Integration 4. Case Study Liquid Screw Pump 5. Other applications 6. Future Work 12

13 Case Study Summary A type rotor profile Single phase liquid pump operation Two rotor profile designs A and D Performance comparison with measurements 3 Liquids with widely different molecular viscosity Design comparison and selection Cavitation influenced operation D type rotor profile Performance comparison with and without cavitation model Influence of Rotational speed and Discharge pressure compared 13

14 Case Study Liquid Screw Pump Single phase liquid pump operation Male Rotor Female Rotor Grid of D1-type Profile 329 mm rotor length Grid of D2-type Profile 200 mm rotor length Grid of A-type Profile 200mm rotor length Designed to match flow 14

15 Case Study Liquid Screw Pump Single phase liquid pump operation A numerical mesh used in this study compromises cells of which cells represent the fluid domain between the rotors, cells represent the two ports while cells represent the extension pipes. 15

16 Case Study Liquid Screw Pump Single phase liquid pump operation Implicit unsteady segregated flow scheme second-order upwind discretization Gauss-Seidel node relaxation scheme The main rotor rotates 2.4 per step. The time-step is defined as follows: t = DPTS 6 RPM whereby, DPTS is the degree per time step, RPM is the rotation speed of male rotor. Pressure Inlet Pressure Outlet K-epsilon Turbulence Model 16

17 Case Study Liquid Screw Pump Single phase liquid pump operation A-type Profile: Pressure distribution in the working domain of the screw pump 17

18 Case Study Liquid Screw Pump Single phase liquid pump operation A-type Profile Effect of the clearance size on leakage losses Effect of the viscosity on leakage losses 18

19 Case Study Liquid Screw Pump Single phase liquid pump operation A-type Profile D1-type Profile D2-type Profile 19

20 Case Study Liquid Screw Pump Single phase liquid pump operation A-type Profile D1-type Profile D2-type Profile 20

21 Case Study Liquid Screw Pump Single phase liquid pump operation Performance Comparison 21

22 Case Study Liquid Screw Pump Single phase liquid pump operation Performance Comparison 22

23 Case Study Liquid Screw Pump Cavitation influenced operation Mechanism: Evaporation of liquid (Local pressure below saturated vapor pressure) Releasing of the dissolved and undissolved gas (seeding) inside the liquid Modelling: Volume of Fluid multiphase model Assumes that all immiscible fluid phases present in a control volume share velocity, pressure, and temperature fields Cavitation Model:Rayleigh-Plesset equation Bubble Growth Rate: R ሶ = dr dt = 2 P B P 3 ρ l Vapour production rate: dα dt = 1 α 4πn 0 R Rሶ 3 n 0πR 3 R seed radius P B Pressure at Bubble Boundary P Ambient cell pressure n 0 average seed density of bubble Vapour volume fraction transport equation: dα dt + αv = dα dt + α v = 1 α ρ l n 0 d 1 α ρ l + αρ v n 0 3 πr3 dt 4 3 πr3 23

24 Case Study Liquid Screw Pump Cavitation influenced operation A type rotor with CD40 lubricating oil 0.85 MPa Discharge Pressure, 630 rpm 0.85 MPa Discharge Pressure, 2100 rpm 24

25 Case Study Liquid Screw Pump Cavitation influenced operation Vapour volume fraction distribution in the working domain of the screw pump during start-up operation During running condition 25

26 Case Study Liquid Screw Pump Cavitation influenced operation Mass flow rate comparison Rotor torque fluctuation comparison The average mass flow rate under cavitation is similar but fluctuation amplitude without cavitation is 0.62kg/s while under cavitation it is 1.03kg/s Cavitation causes an increase of 3.17N-m (26.04%) in average female rotor torque 26

27 Case Study Summary A type rotor profile Single phase liquid pump operation Two rotor profile designs A and D Performance comparison with measurements 3 Liquids with widely different molecular viscosity Design comparison and selection Cavitation influenced operation D type rotor profile Performance comparison with and without cavitation model Influence of Rotational speed and Discharge pressure compared 27

28 Contents 1. Introduction 2. CFD analysis based design of Screw type PD Pumps 3. SCORG STAR-CCM+ Integration 4. Case Study Liquid Screw Pump 5. Other applications 6. Future Work 28

29 Twin Screw PD Compressors and Expanders Compressible gases Large thermal effects Oil injected and Liquid refrigerant operations 29

30 Gear Pumps Spur and Helical Gears Cavitation Pressure solution 30

31 Internal Screw Compressors Conical Rotors Compressible gases and oil/water injection 31

32 Non-Conventional Twin Screw Compressors Variable Lead Rotors Conical Rotors 32

33 Multi-Rotor screw machines Design failure case 33

34 Contents 1. Introduction 2. CFD analysis based design of Screw type PD Pumps 3. SCORG STAR-CCM+ Integration 4. Case Study Liquid Screw Pump 5. Other applications 6. Future Work 34

35 Future Work Validation of cavitating flow conditions Eulerian Multiphase with compressible gases Co-Simulation to exchange CFD results with Structural and Thermal analysis Fluid Structure Interaction to capture gap size changes during operation 35

36 Thank You 36