Automatic Optimization of a Wastewater Pump

In the first part of this article on the design of a wastewater pump, we showed that there is scope for improvement in the baseline impeller especially in terms of meeting the solid-handling requirement.

In this second part, we reveal how the waste-handling capacity of this baseline impeller can be enhanced through a rapid automatic optimization without adversely impacting its efficiency, suction performance or structural integrity.

Optimization Workflow
Figure 1 displays the workflow used by the automatic optimization, where the blade loading and meridional geometry parameters are used to generate the blade shape in TURBOdesign1, and then the resulting performance parameters are fed into the optimizer which basically applies MOGA to drive the solution towards the optimum design.

 

Figure 1: Wastewater pump optimization workflow

Optimization Setup

The optimization process starts by specifying the range of variation for the input parameters, which are the blade loading parameters and the meridional geometry parameters because it also has an influence on the blade-to-blade passage size and therefore the solid-handling characteristics of the impeller, as shown in Figure 2.

 

Figure 2: Input parameter ranges used in wastewater pump optimization

Figure 3 shows the constraints that are imposed on the optimizer as follows:

• Geometrical constraints are selected to avoid high backsweep angle at the trailing edge and high bowing at the leading edge
• Secondary flow and profile loss are constrained to avoid high losses at the design point
• Overall diffusion ratio is limited to avoid large separation at lower flow rates
• Minimum surface pressure is constrained for cavitation performance
  
  

Finally, for the optimization objectives:

• Leading edge lean angle is minimized to avoid high blade stresses
• Minimum channel distances in 3D as well as in the 2D orthogonal projection are maximized to expand the narrow sections in the passage

 

 Figure 3: Constraints and objectives used in wastewater pump optimization

Optimization Results
Once the optimization run is complete, which is very fast and only takes about half an hour or so on a single core machine, the various designs can be seen on a scatter plot between any two objectives, in this case the minimum channel distance in 3D on the X-axis and the one in 2D on the Y-axis as shown in Figure 4. Here we have chosen to see only the feasible designs which respect all our specified constraints, and the position of the baseline impeller relative to the candidates may also be noted.

 

Figure 4: Scatter plot between minimum channel distances in 2D and 3D of feasible designs

Next as Figure 5 shows, we can choose to see the Pareto front of optimum designs from which it is possible to pick and analyse any design of interest. For the present study, we select a design which gives us a good trade-off between the minimum channel distances in 3D and 2D while still reporting a major reduction in leading edge lean, and so we use this design for further analysis and for comparison with the baseline impeller

 

Figure 5: Pareto front with the selected optimum design

Figure 6 reveals what changed as a result of the optimization. For the streamwise blade loading, compared to the baseline impeller, the optimizer recommends less aft-loading and some positive leading edge incidence at the hub and mid-span, along with some changes to the meridional shape as well. As a result, the optimized impeller clearly appears very different from the baseline, and the increase in minimum channel distance can be easily seen. Furthermore, the relative velocity distribution in TURBOdesign1 seems much smoother especially on the shroud suction side.

 

Figure 6: Comparison of baseline and optimized wastewater pump

The results for the optimized impeller are presented in Figure 7, where TURBOdesign1 parameters show that the minimum channel distance is significantly bigger and the leading edge lean and bow are reduced to almost half. When CFD performance map is generated for the new impeller, it is found that even though the peak efficiency is slightly reduced, the rating point efficiency is actually higher than the baseline pump. Moreover, the optimized impeller is able to generate a similar amount of head as the baseline.

 

                                                      Figure 7: CFD performance maps of optimized wastewater pump


Next, the 3” sphere test is repeated on the optimized impeller and this time it is found to be able to pass through the impeller passage quite easily, both near the leading and trailing edge regions as reported in Figure 8. This confirms that the new impeller does not present any risk of clogging and has achieved the desired waste-handling levels.

 

Figure 8: Optimized wastewater impeller solid-handling verification

Software Demo -  Automatic Optimization of Wastewater Pump Impeller with TURBOdesign1 

Cavitation CFD Analysis
Finally, in order to verify the suction performance, the baseline and optimized pumps are also analyzed by means of two-phase CFD analysis with the cavitation model, where the different NPSH values are achieved by changing the inlet total pressure. Figure 9 illustrates the cavitating regions based on the vapor volume fraction, where the new design has slightly bigger cavitation zone at low NPSH, but the NPSHr is similar to the baseline design and is quite low at the rating point which could be due to the large inlet dimension and low flow velocity.

 

Figure 9: Wastewater pump cavitation CFD analysis

Conclusion
Wastewater pumps typically present a complex trade-off between the efficiency, suction and waste-handling aspects of the impeller. Essentially, using automatic optimization on the blade loading and meridional geometry parameters, it is possible to optimize the impeller and achieve this trade-off in a matter of minutes. Moreover, this methodology involves very less computational resources in terms of CFD runs compared to conventional design methods. Finally, the optimum blade loading for dealing with a particular aspect (e.g. profile loss suppression and cavitation control) has generality, and can be applied regardless of the pump speed or size.