# Analysis of Bulb Turbine Hydrofoil Cavitation
## 3 Experiment
### Turbine Measurements  
Measurements of efficiency were performed on a low-head axial and bulb turbines measuring station for low-head bulb turbines (Figure 5) in Kolektor Turboistitut (Lijubljana, Slovenia). The measuring station was built according to standard [18] and was used for the model, witness, and acceptance tests. In agreement with standard [18], all measurements and data acquisitions were automatic. The nominal runner diameter of the test rig is 350 mm, and the recirculating flow is generated using two radial pumps with frequency regulation. The flow measurement was performed by using an electromagnetic flowmeter with a diameter of DN 400 and an accuracy of \(\pm\)0.15%. The static pressure was measured using a differential pressure transducer (accuracy of \(\pm\)0.025%), with the measuring taps located on the inlet and the outlet. Absolute suction pressure was measured using an absolute pressure transducer with an accuracy of \(\pm\)0.025% to assure that the measuring station operated well above the incipient cavitation number. An electromagnetic incremental sensor was used to measure the rotational speed; the output was processed by a counter-timer module with an accuracy of \(\pm\)0.01%. The shaft torque was measured on a horizontal shaft using a rotating torque transducer with an accuracy of \(\pm\)0.01%. The friction torque was estimated by operation with air. Both bulb turbine runners were tested under similar conditions at six operating points. The blade angle was cases set to 25\({}^{\circ}\) in all cases (Figure 6). The blade angle was set using a control template. For comparison, both runners operated at the same specific energy E (J/kg) and the corresponding nondimensional energy numbers \(\psi\) were recorded. E was \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Modified Hydrofoil** & **Existing Hydrofoil** \\ \hline Hydrofoil chord length l & 60 mm & 60 mm \\ Maximum hydrofoil thickness d\({}_{\text{max}}\) & 2.55 mm & 2.55 mm \\ Maximum thickness position l\({}_{1}\) & 9 mm & 24 mm \\ Maximum thickness position ratio l\({}_{1}\)/l & 15\% & 40\% \\ Maximum hydrofoil curvature s\({}_{\text{max}}\) & 0.87 mm & 0.87 mm \\ Maximum curvature position l\({}_{2}\) & 18 mm & 27 mm \\ Maximum curvature position ratio l\({}_{2}\)/l & 30\% & 45\% \\ Radius at the Leading edge r\({}_{0}\) & 0.54 mm & 0.42 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Hydrofoil parameters. Figure 5: Turbine measuring station. determined from the total specific energy at the turbine inlet and the outlet cross-sections [7]. Energy numbers \(\psi\) were determined using the equation:  
\[\psi\ =\ \frac{2\cdot\mathrm{E}}{\pi^{2}\cdot\mathrm{N}^{2}\cdot\mathrm{D}^{2}} \tag{1}\]  
Here N (min\({}^{-1}\)) is the rotational frequency, while D (m) is the discharge ring diameter of the bulb turbine. The turbine rotational frequency N (1000 min\({}^{-1}\)) was the same for all measuring points. Both runners had the same nominal diameter D (m). The guide vane opening coefficient A\({}_{0}\) was the same for each pair of measuring points used in the comparison:  
\[\mathrm{A}_{0}\ =\ \frac{\mathrm{a}_{\mathrm{v}}\cdot\mathrm{z}}{\mathrm{D}_{ \mathrm{v}}} \tag{2}\]  
\(\mathrm{a}_{\mathrm{v}}\) (mm) is the guide vane's opening and z (/) is the number of guide vanes. D\({}_{\mathrm{v}}\) (mm) is the guide vane pivot diameter. Important manufacturing methods and characteristics were kept the same for reliable comparison between both runners. Control of runner tip gaps to the discharge ring and clearances of guide vanes to the guide vane pressure rings are very important for reliably estimating model turbine runners. Both runners were manufactured from the same material and using the same production methods and procedures. As such, for both runners, the tip gaps, guide vane clearances, and surface finishes were the same. The measuring station was of a closed type. To maintain the \(\psi\) fixed, the main water circulation pump's frequency was controlled.