CFD Analysis of a Centrifugal Pump with Supercritical Carbon Dioxide as a Working Fluid

Transcription

1 KNS 2013 Spring CFD Analysis of a Centrifugal Pump with Supercritical Carbon Dioxide as a Working Fluid Seong Gu Kim Jeong Ik Lee Yoonhan Ahn Jekyoung Lee Jae Eun Cha Yacine Addad Dept. Nuclear & Quantum Eng., KAIST

2 C O N T E N T S 1. Introduction 2.1 Geometry and meshing 2.2 Fluid property 2.3 Problem setup 3. Results 4. Summary & Conclusion

3 1. Introduction *SFR (Sodium-Cooled Fast Reactor) - Generation IV reactor project to design an advanced fast neutron reactor. - Designed for management of high-level wastes(plutonium and other actinides). - Enhancement use of uranium resource by recycle of the spent fuel. *Supercritical CO 2 Brayton Cycle - Water/steam and CO 2 are being considered as the working fluids for the power conversion system in order to achieve high-level performances in thermal efficiency. - Above the 550 of the turbine inlet temperature, S-CO 2 Brayton cycle have higher cycle efficiency. - Chemical reaction of sodium-co 2 is much milder than the sodium-water reaction.

4 1. Introduction *Supercritical CO 2 Brayton Cycle - Fluid behavior of S-CO 2 in the supercritical state was not fully understood. -Major technical challenges exists in the turbomachinery design. - Research team is conducting a S-CO 2 pump experiment (SCO 2 PE facility) to obtain fundamental data for the advanced pump design and measure the performance. <SCO 2 PE Facility> *CFD analysis of a centrifugal pump -To develop a design method and obtain detail flow information of S-CO 2 pump, 3-D numerical analysis is required. - CFD results will be an important reference for development of a high efficiency S-CO 2 pump. - This research presents methodology of the 3D flow analysis of S-CO 2 for the pump by using ANSYS CFX code.

5 1. Introduction *Previous studies [1] <3D CFD study of radial compressor with S- CO 2 > Delft Univ. of Tech., Rene Pecnik et al Investigated geometry is the compressor tested in the SNL (Sandia National Laboratory) compression loop facility. - Spalart & Allmaras and k-w SST (Shear Stress Transport) turbulence model were used. - To simulate fluid property, multi-parameter equation of state (MPEoS) was used to generate two property tables(s-co 2 and 2-phase). - Numerical result of ideal head 2 coefficient( h U ) showed higher than 2 experimental data. - Computed impeller efficiency 45-70%.

6 1. Introduction *Previous studies [2] <Fluent CFD predictions of a compressor with S-CO 2 > Knolls Atomic Power Laboratory, T.A. Munroe. et al Realizable K-epsilon turbulence model was used for the analysis. - NIST real gas model based equation of states were applied to Pressure-based coupled solver(pbcs) of Fluent. - Incompressible fluid flow assumption used. - The Fluent results were compared against meanline code results. - Computed impeller efficiency 80-95%

7 2.1 Geometry and Meshing *Geometry - The closed impeller has rotating circular shroud and hub on either side. - Pump impeller and diffuser geometry provided by the manufacturer was utilized to perform CFD analysis. - Imported solid model was divided into rotor and stator part and converted into fluid domain. - Fluid domain was exported to ICEM CFD software and structured to the meshing elements. < Impeller geometry > < Compressor full geometry > < Fluid domain of rotor and stator>

8 2.1 Geometry and Meshing *Meshing - Volume meshing elements were generated in the ICEM CFD software by using geometry of 3D fluid domain. - The tetrahedron dominant mesh with 20 sheets of prism layer at the free surface has been produced. - 1,813,726 node, 4,867,877 elements. - To assure independence of grid size, grid sensitivity study was performed. - More than 1 million of grid size, compressor efficiency reported less variation. Number of nodes Number of elements Compressor efficiency [%] Toque in the rotor domain [N-m] 447,262 2,445, ,006,882 6,624, ,813,726 4,867, Reference mesh system 2,274,206 7,312,

9 2.2 Fluid Property *Fluid property - To simulate nonlinear behavior near the critical point of CO 2, TASCflow RGP(Real gas property) table was employed. - The CFX solver returns calculated property by using bilinear interpolation. - RGP tables were generated by an in-house code based on MATLAB program. - The program is automatically writes a RGP files in the range of selected pressure and temperature with specified resolution. - P, T range : K, MPa. - Resolution : 100x100, 300x300, 500x500, 1000x1000, 2000x2000.

10 2.2 Fluid Property *Size effect of property table - The finer resolution for the tabulation provides smaller error. - Making a large size table is consume too much computational time and resource. - Various size of tables were applied to analysis and property error was compared to NIST real property. - Density and Cp showed large error among the properties. - Maximum error is below 1.67% in case of table 2000 by 2000.

11 2.3 Problem Setup *Problem setup - The k-omega SST (Shear Stress Transport) model was used. - SST is widely used in numerical analysis of turbomachineries since it shows higher accuracy in case of have rotating frame and large swirling among two-equation models. - A turbulence intensity of 5% and length scale of 0.05m (inlet diameter) were determined. - Automatic wall function applied for prediction of viscous effect near the wall. - A high resolution scheme of CFX (second order upwind scheme) was used. - Inlet condition Case1 : 8.3MPa, 40. Case2 : 7.45MPa, Outlet condition : mass flow rate 0.6~2.6kg/s. *Convergence criteria - The converged solution is determined when RMS (root mean square) residual of momentum, mass, energy showed less than The imbalance of mass and energy in each domain shows less than 0.5% - Total iteration step of 2,000 to 15,000 for each converged solution takes about 4 to 30hr. - Workstation with two Intel Xeon processor (2.90GHz) containing 16 cores of physical CPU was used for computation.

12 3. Results *Results with water fluid - To evaluate the validity of the CFD analysis, results with water were compared to test data. - The experiment data with water provided by pump manufacturer. - Inlet condition of 101.3kPa and 7 was used. -Predicted efficiency results showed high accordance with experimental data. - The error of efficiency between CFD and test data is less than 2.2%. -Computed head curve showed 6.5~11% higher value than the test data.

13 3. Results *Results with CO 2 fluid Case1 (8.3MPa, 40 ) - Computed efficiency curve showed very close to the results of test Case1. - Results of pressure ratio were recorded about 2% higher than the test data.

14 3. Results *Results with CO 2 fluid Case2 (7.45MPa, 32.5 ) - In case2, CFD predicted efficiency and pressure ratio were reported less value than the test results. - Error of efficiency between the CFD result and test data is %. - Error of pressure ratio between the CFD result and test data is 3-4%.

15 3. Results *Contour plot at the impeller - Total and static pressure were plotted with streamwise locations in the impeller region. - The difference between static and total pressure have increased with radial streamwise length.

16 3. Results *Contour plot at the outlet pipe - Entrance region of the outlet pipe, showed maximum total pressure due to large mass flow rate and high velocity. - Along the outlet pipe, velocity and dynamic pressure have decreased and converted into static pressure.

17 4. Summary and conclusion *Summary - Geometry provided by the pump manufacturer was converted into the volume mesh and utilized to the CFD analysis. - To simulate a non linear behavior of CO 2 near the critical point, the high resolution RGP format tables were generated by an in-house code. - Numerical solution of flow field and contour plots have been successfully obtained with ANSYS CFX code. *Conclusion - CFD results predicted efficiency and pressure ratio of water and CO 2 Case 1 successfully with high accordance. - In the real facility, steel structure of pump is not an adiabatic wall and also mechanical losses (leakage, disk friction) exist. Thus, over estimated results of efficiency and pressure ratio in the CFD results can be understood. - In Case2, The error between predicted error and test data is not well understood yet. Large sized table also have errors more than 1.7%. In the CFD results, some part in the rotor domain showed low P & T below the critical point. It is assumed that properties of out of tabulated region caused related errors. *Future works - The error near the critical point and two-phase flow will be investigated and refined. - The enhanced geometry of impeller and diffuser will be designed. - More obtained data from SCO 2 PE facility will be compared continuously.

18 References Acknowledgement Authors gratefully acknowledge that this research is supported by the National Research Foundation of Korea(NRF) and funded by Korean Ministry of Science, ICT and Future Planning. [1] Tae W. Kim, Nam H. Kim, Kune Y. Suh, Seong O. Kim, Engineering Thermo-fluid Dynamics Analysis of Supercritical Carbon Dioxide Turbomachinery for Kalimer-600, AICFM, [2] Jekyoung Lee, Jeong Ik Lee, Yoonhan Ahn, Hojoon Yoon, KAIST, KUSTAR, Design Methodology of Supercritical CO2 Brayton Cycle Turbomachineries, Proceedings of ASME Turbo Expo 2012, [3] T.A. Munroe, M.A. Zaccaria, W.H. Flaspohler, R.J. Pelton, K.D. Wygant, O.B. Dubitsky, Knolls Atomic Power Laboratory, Concepts NREC, Fluent CFD Steady State Predictions of a Single Stage Centrifugal Compressor with Supercritical CO2 Working Fluid, Proceedings of the S- CO2 Power Cycle Symposium 2009, [4] Rene Pecnik, Enrico Rinaldi, Piero Colonna, Computational Fluid Dynamics of A Radial Co mpressor Operating with Supercritical Carbon Dioxide, Proceedings of ASME Turbo Expo 201 2, [5] ANSYS Inc., ANSYS CFX-Solver Modeling Guide Release 14.0, ANSYS Technical paper, 2011.