CD-Adapco: Analyzing complex pump hydraulics

Figure 1: Flowpath geometry of the pump (left) and computational mesh in Star-CCM (right) along center plane

Figure 1:
Flowpath geometry of the pump (left) and computational mesh in Star-CCM (right)
along center plane

Mechanical Solutions Inc. (MSI), headquartered in Whippany, New Jersey, specializes in fluid machinery design, fluid dynamic analysis, mechanical design and analysis. In the energy industry, MSI focuses on analysis, testing and troubleshooting of all kinds of rotating, reciprocating and turbomachinery, using sophisticated analysis and testing tools. These specialists are using Star-CCM, an industry-leading simulation package to tackle design challenges in water pumps.

Major design challenges of water pumps include the unsteadiness of the flow, cavitation and rotating machinery. Star-CCM offers features that can successfully tackle these difficult physics while offering a streamlined, robust numerical design process. Geometry generation, meshing, solving, post-processing and optimization can all be done in an efficient workflow.

Features like the unsteady flow solver, unsteady cavitation model, Rigid Body Motion (RBM) for rotating domains, automated unstructured meshing for complex geometry and parallel processing capability offer the ability to explore multiple water pump designs in an economical manner. The system allows designers to easily predict pump performance at design and off-design points and avoid damaging effects of cavitation and erosion.

Case study: Double suction pump
MSI was tasked by a customer to analyze a double suction pump design to verify whether the pump can meet their performance requirements. The pump was a complex, centrifugal design for enhanced cavitation performance. It included an inlet from the top with a splitter down the middle to mitigate radial loads and featured identical impellers on either side leading up to a volute. The flow entering the pump was equally split across the two impeller sections.

MSI used Star-CCM to simulate the complex physics of the pump including transient flow through 360 degrees, rotating geometry and unsteady cavitation. The flowpath was cut in half due to symmetry for simulation. The inlet, impeller and volute were modeled. The automated polyhedral meshing capability of this software was used to discretize the flow domain. A mesh for a single vane passage was created and cyclically patterned and fused to create the complete impeller mesh and ensure mesh uniformity. Prism layer cells were created automatically on surfaces to resolve the boundary layer and the final mesh had five million volume cells. Figure 1 shows the geometry and mesh of the flowpath.

Figure 2: Velocity contours shown as Line Integral Convolution (LIC) at center plane

Figure 2:
Velocity contours shown as Line
Integral Convolution (LIC) at center plane

The segregated flow solver in Star-CCM was used with a second order convection scheme and the SST k-ω turbulence model. The multiphase Volume of Fluid (VOF) model was activated to capture the interface between water and water vapor and the Rayleigh-Plesset cavitation model was selected for simulating cavitation. Total pressure conditions were given at the inlet via a reference point and mass flow was specified at both the inlet and exit. The rotating impeller was surrounded by an interface enclosing it and the domain was given a rotating speed matching the impeller speed using RBM. The unsteady time step and total time were chosen to simulate one complete rotation of the impeller and 20 inner iterations per time step were used. The simulation was run for several complete rotations until the residual monitors were settled and variable monitors for pressure, torque and mass flow showed cyclical behavior.

The velocity contours (Figure 2) showed stationary flow around the impeller, a nice uniform volute flow and recirculation near the splitter. Some recirculation was noticed near the inlet and volute due to the limited axial size of the pump. The nonuniformity of flow was deemed undesirable but had to be accepted because increasing axial space for more uniform flow was not possible with the limited axial space available.

Figure 3 shows contours of vapor fraction on the impeller with blue regions representing water and red representing water vapor. It can be seen that at a high inlet pressure (175 kPa), there is very minimal cavitation and as the inlet total pressure decreases, cavitation on the impeller increases.

Figure 3: Vapor fraction contours on the impeller showing cavitation at various inlet total pressure values

Figure 3:
Vapor fraction contours on the impeller showing cavitation at various inlet total pressure values

The simulation software is able to accurately capture the breakdown of pump performance at lower NPSH and lower inlet pressure and compares well with the experimental data. MSI was able to numerically create the NPSH breakdown curve for the pump at 70 %, 110 % and 150 % mass flow rates. The customer was able to make clear design and certification decisions including cavitation performance for the pump based on these simulations.

 

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