The Design & Analysis of a Low NPSH Centrifugal Pump Featuring a Radial Inlet and Axial Inducer Using STAR-CCM+

Transcription

1 The Design & Analysis of a Low NPSH Centrifugal Pump Featuring a Radial Inlet and Axial Inducer Using STAR-CCM+ Edward M Bennett Travis A Jonas Mechanical Solutions 11 Apollo Drive Whippany, NJ March 19,

2 AGENDA Introduction and problem definition Design challenges Multiphase CFD analysis Design optimization Results Conclusions 2

3 Problem Statement Mechanical Solutions, Inc., an engineering consultancy specializing in turbomachinery engineering that is based in New Jersey, was commissioned to design a two-stage centrifugal pump for a major manufacturer This presentation will be concerned with the first stage, specifically the radial inlet and inducer 3

4 Fluid Dynamic Challenge The pump design is required to operate at low NPSH(Net Positive Suction Head) at two different flow rates. As the inlet pressure is lowered, vapor will form in the pump passages, and eventually the performance will deteriorate. Cavitation erosion and vibration can cause serious damage to blading. Cavitation head breakdown curve 4

5 Fluid Dynamic Challenge First stage pump components Radial inlet Axial inducer Centrifugal impeller Vaneless diffuser Crossover and de-swirl cascade First stage flowpath 5

6 Radial Inlet The radial inlet guides and turns the flow from radial to axial parallel to the rotating shaft. The challenge of the radial inlet design is to symmetrically distribute the flow through the pump Non-symmetric flow can destroy range, stability, and performance. Typical pump radial inlet 6

7 Axial Inducer Axial inducers are used in rocket turbopumps and high energy density industrial pumps Inducers act as boost pumps to the main head producing impeller. They can operate at higher cavitation levels since their power level is only a fraction of the main impeller. The large axial inlet facilitates lower velocities at the throat and can swallow more cavitating vapor Cavitating Inducer (Brennen(1994) 7

8 Introduction High energy industrial pumps Reducing the size and first cost of pump and driver by means of durable higher-speed machines with demonstrated durability. (Cooper, 2009 Texas A&M Pump Symposium) Industrial pumps generally have far greater life cycle requirements compared to rocket turbopumps and usually have much greater range requirements Typical inducer and centrifugal pump stage 8

9 Challenges of Inducers General performance Inducers have generally had reasonable design performance but unsatisfactory off-design performance. This can create problems, particularly in industrial applications where range is important. For the present case, the NPSH requirement is fixed on two flows, that are different by 37%. High performance rocket inducers have a maximum flow range from 65 to 125%. 9

10 Challenges of Inducers Low flow recirculation and instability Inducers often exhibit large tip recirculation at low flow rates. These instabilities can cause unsteady vibrations and damage to the system, in addition to reducing range. From AIAA , Ferguson, et al Design flow coefficient =

11 Challenges of Inducers In order to design inducers with lower NPSH capability, the flow coefficient must be reduced. The flow coefficient is the ratio of the incoming axial velocity/tip speed of inducer. This creates a compromise with the problem of flow instabilities and recirculation. Brumfield Criterion for inducers (Lobanoff and Ross) 11

12 Extra Inducer Challenges This pump design has a requirement for a large rotating shaft that forces the inducer design to utilize a large inlet hub/tip ratio and a higher inlet flow coefficient (0.156), which is not ideal for low NPSH operation. The inducer design will have a smaller throat passage area and thus internal velocities will rise. This lowers static pressure and can lead to premature cavitation. Radial inlets and inducers have normally not been as efficient due to the non-symmetries in the flow created by the radial inlet. 12

13 MSI Inducer and Pump Design Flowchart Preliminary Design CFturbo Detailed Blading Design CFturbo, BladeModeler Detailed Volute Design CFturbo CAD and FEA Inducer NPSH Analysis Cooper Streamline Code Component CFD and Cavitation Analysis STAR-CCM+ Detailed Stage Performance Analysis STAR-CCM+ 13

14 Radial Inlet Design MSI started the pump design by creating a radial inlet that minimized circumferential distortions around the inlet MSI radial inlet 14

15 Axial Inducer Design MSI designed an axial inducer that was skewed to the highest flow operating point, in order to achieve the necessary pressure boost. The design was optimized to reduce recirculation at lower flows MSI 4-vaned axial inducer 15

16 Physics of the Pump Stage Full unsteady stationary/rotating components nonsymmetric flow interaction Unsteady multiphase cavitation Turbulent, separating, recirculating flows 16

17 CFD Requirements Unsteady stator/rotor interaction Advanced turbulence models Sophisticated unstructured meshing capabilities of complex passages Unsteady cavitation models Massive parallelization STAR-CCM+ possessed all of the CFD requirements! 17

18 CFD Model Full 360 degree model was meshed in STAR-CCM+ Mesh created using polyhedrals and prismatic boundary layer elements Total mesh size was vertices and cells High order unsteady segregated model used for discretization Realizable k-epsilon model used for turbulence Unsteady cavitation model employed Calculation solved on 132 Linux cluster 18

19 CFD Model Boundary Conditions Mass flow inlet Mass flow exit Reference pressure set and sequentially lowered to meet NPSH requirements RPM selected Two volumetric flows 2700 gpm 3700 gpm Computational Mesh 19

20 First Prototype Results The first stage analysis was conducted at 2700 gpm. Cavitation breakdown began at NPSH = 22 feet(goal is 10 feet). There is an non-symmetric interaction between the inducer and impeller created by the non-multiple impeller vane count. Non-symmetric cavitation present in impeller 4 inducer vanes 7 impeller vanes 20

21 Impeller Revision The impeller vane count was changed to match the inducer vane count. The impeller was positioned circumferentially such that the leading edge of the impeller is situated between the trailing edges of the inducer to minimize wake /cavitation interaction. 21

22 2700 GPM Analysis Streamlines(No Inducer Recirculation) 22

23 2700 GPM Analysis Vapor Fraction at NPSH = 7.81 feet (Target is to be less than 10 feet) 23

24 2700 GPM Analysis Total Head vs NPSH (rke, 2700 gpm) Total Head (ft) NPSH (ft) Target NPSH is Achieved (10 feet) 24

25 3700 GPM Analysis Streamlines(No Inducer Recirculation) 25

26 3700 GPM Analysis Vapor Fraction at NPSH = 8.54 feet (Target is to be less than 10 feet) 26

27 3700 GPM Analysis Total Head vs NPSH (rke, 3700 gpm) Total Head (ft) NPSH (ft) Target NPSH is Achieved (10 feet) 27

28 Conclusions All fluid dynamic targets and goals have been achieved. Cavitation behavior successfully predicted across a wide range of flows. STAR-CCM+ was able to accurately mesh the flowpath using unstructured polyhedrals and prismatic elements, resulting in an efficient, economic workflow. STAR-CCM+ was able to facilitate the required unsteady, multiphase physics. The parallel capabilities of STAR-CCM+ enabled solution turnaround in quick order. 28