**3. Experimental Verification of Proposed Modifications**

For practical verification, the test impeller for this particular PAT was manufactured according to the design proposed by the calculation model. It was necessary to adapt the related flow parts at the same time (i.e., the volute casing and the draft tube). The fundamental change was to reduce the width of the impeller at the inlet from the original value of *B* = 16 mm to the width of *B* = 10 mm proposed by the model. The next change concerned the outlet cross section of the impeller. This was, on the contrary, necessary to increase the diameter (*D*<sup>2</sup> = 65 mm) for the original pump to the newly calculated value (*D*<sup>2</sup> = 84 mm). The impeller blades were also changed. A machine with completely new geometry was created, referred to as "Turbine" in the following text—see Figure 6 on the right. The original unmodified pump (META Plus 5 Czech Trade mark) is hereafter referred to as "Original" and is shown in Figure 6 on the left.

**Figure 6.** Cross-section of the Original PAT (**left**) and the Turbine after modification (**right**).

Photos of the impellers tested on the hydraulic circuit are shown in Figure 7. The unmodified original impeller on the left was made of cast iron in a sand mould. These types of impellers are used by the manufacturer as a standard for META Plus 5 pumps. The photo on the right is the impeller of the Turbine modification, with the geometry designed by the calculation model. To manufacture it, the hub and the rear shroud from the original impeller were used. The front shroud was made of copper sheet created by cold forming. All impeller blades were made from metal alloy by casting, and they were glued between the rear and the front shroud.

The numerical values of the geometry of both impeller variants are given in Table 1. The Turbine impeller has twice the number of blades compared to the Original. At the same time, the blades are almost half the length of the pump blades. This means a smaller wetted surface and, therefore, fewer hydraulic losses.


**Table 1.** Overview of the impellers' geometry.

**Figure 7.** Impellers of tested variants—Original on the left, Turbine on the right.

An overview comparison of the geometry proposed by the calculation model with other Francis turbine impellers with corresponding specific speeds is given in Table 2. These are relative values related to the size of the impeller—outer diameter *D*1. The results of the calculation model for the Turbine variant are shown in the left column of the table. The parameters of the test impeller of the Francis turbine F99 at NTNU Trondheim, Norway, are shown in the middle column [22]. The last column on the right presents the parameters of the Francis turbine impeller at Štˇechovice hydroelectric power plant on the Vltava River, Czech Republic [18].


**Table 2.** Comparison of model results with the geometry of Francis turbines.

As the table above indicates, the basic geometric characteristics proposed by the calculation model correspond to the parameters of standard Francis turbines. Any differences may be caused by slightly different values of the specific speeds of individual types of impellers.
