Elspeth Mosedale

Pumps are the unsung heroes of many aspects of our lives. They enable our water and sewage systems, ventilation systems, cooling in energy plants and many more applications.

Pump design engineers face multiple challenges when creating new, better products. Pumps have to operate well over a range of conditions, meet government efficiency regulations, and run reliably over long lifetimes with minimal downtime for repairs. Traditional design methods have relied heavily on testing physical prototypes. This can be expensive and time consuming, with multiple cycles of design/re-design and testing for each new product. Creating a virtual prototype enables rapid investigation of multiple pump designs over a range of possible operating conditions, which not only speeds up the design process but also reduces the costs associated with physical testing.  Taco Comfort Solutions, a leading ISO 9001 manufacturer of heating and cooling equipment, accessories and systems, uses this approach to bring better pumps to market faster.

 Figure 1: Example Taco Comfort Solutions Circulator Pump.[/caption]

Taco uses STAR-CCM+ software from Siemens PLM Software in their design process. This engineering simulation software enables them to create a digital prototype of their design and simulate the three-dimensional flow in the pump using computational fluid dynamics (CFD).

The engineering simulation software served as a “virtual lab” prior to creating and testing physical prototypes. After importing the geometry, all of the meshing, simulation and analysis is then performed in engineering simulation software tool, as shown in figure 2.  For each pump design the CFD simulation gives a full, three-dimensional representation of the flow and pressures within the pump housing. This validation of the CFD model offers a high level of confidence in the CFD results, along with the predictive insight into performance characteristics such as head, efficiency and BEP flow rate.

 Figure 2: A pump model is designed in SolidWorks and imported into STAR-CCM+ (a), where it is meshed (b) and simulated (c).[/caption]

Once there are three or four designs which yield acceptable performance in the engineering simulation software tool, it is time to move on to building and testing the physical prototypes. Even with the ability to rapidly prototype (via 3D-printing) the stereolithography (SLA) impellers, each one costs up to $2,500.00 to build and test.  Exploring the performance of numerous digital prototypes in engineering simulation software brings significant cost and time savings, enabling a high probability of success in a single phase of prototype testing,to ensure a short development time window.

Engineers can now run steady state simulations on each design, testing multiple flow points to establish the Best Efficiency Point (BEP) flow rate. In an ideal world, pumps would always run near the BEP but this is not always the case. To prevent field problems and meet customer needs it is important to look at off-design flow to be sure the pump will be robust over the complete operating curve. On the left hand side of the pump curve you can get an excessive droop, which is not acceptable. The BEP lies in the center and on the right hand side you need to examine the operating conditions at high flow rates. Users can perform unsteady simulations in engineering simulation software in an attempt to examine the entire pump curve.

In this process, the user initializes the CFD model with a steady state run at BEP flow rate, then changes the boundary conditions so the flow changes towards the extreme flow rate. In figure 3 is an example of a case which showed an underperforming design on the left hand side of the performance curve. Figure 4 shows an instance of where at off design flow rates the pump flow can become highly unsteady. If there is too much drop in pressure on the left hand side of the pump curve there could be problems with operating a parallel pumping setup.

 Figure 3: Example performance curve for two test cases. Dotted lines show STAR-CCM+ results, solid lines lab testing. Case A showed a large drop in pressure (ΔA) between 50% BEP (max head) and 10% BEP. Redesigning the impeller (case B) decreased this pressure drop (ΔB).[/caption]

 Figure 4: When the pump is operated at flow rates far off-design (left), the flow in the pump clearly becomes highly unsteady with significantly more recirculation visible compared to the flow at the BEP (right).[/caption]

Streamlining workflows and vision for the future

Using engineering simulation for virtual testing can speed up the design workflow and reduce the costs associated with physical testing. It eliminates the “start over” process and accelerates the complete design cycle. Improvements to engineering simulation software offers data to correlate with CFD results and provides a library of lab test data which validates its results and knows the best model settings to use to achieve accurate results.

Engineering teams are now working with engineering simulation todevelop a quick, multi-design testing process, using leading engineering simulation part-swapping capability. Once the model has been set up and run in STAR-CCM+, part-swapping enables users to replace just the impeller or volute with an alternative design, while the CFD model settings and boundary conditions remain unchanged, as in figure 5. The engineering simulation tool then rapidly regenerates the mesh, and computes a new solution.

 Figure 5: Part swapping allows quick changes of impeller, keeping the rest of the geometry the same. Results are easily comparable as all mesh and physics settings are identical[/caption]

Looking to the future with the goal to build on recently gained knowledge of design exploration, companies are relying on the strategic use of CFD simulation to beat the competition to market with better performing pumps in an effort to increase market share. Engineering simulation shortens the iterative design process in a highly competitive industry. Companies want to know that when they invest in lab testing efforts or part tooling, the results will have a high likelihood of success. Engineering simulation provides that confidence.

Engineers want to eliminate the feedback loop between lab testing and the design stage, so that each pump design has as few lab tests as possible. They prefer to explore digitally and confirm physically rather than the other way around. In the near future companies will no longer use SLA testing but go straight from engineering simulation to metalimpeller testing. It might seem to be a risk, but the reward will result in time and cost savings, enabling companies to bring new, more efficient and reliable pumps to market faster.

Elspeth Mosedale

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