# Laboratory Modeling of an Axial Flow Micro Hydraulic Turbine
## 4 Conclusions  
The current work presents an example of implementation of a novel experimental modeling approach for a micro hydraulic turbine. Use is made of atmospheric air as the working medium and 3D printing for precise and rapid fabrication of test section elements. By this means, the detailed structure of the flow passing through the model propeller-type micro hydraulic turbine has been surveyed for the operating conditions covering range from part load to overload. The experimental data acquired with the aid of nonintrusive optical technique on the basis of computerized LDA include the fields of the time-mean and fluctuating velocity components and spectra. Emphasis was placed on the development of the flow in the axial direction. The data show that when the operating regimes changes from design to off-design, the flow contains a significant amount of swirl. In the underload case that was studied, the flow swirl level exceeds the critical level when the vortex breakdown starts with subsequent formation of a central recirculation region and higher flow near the draft tube walls. This flow structure gives rise to hydrodynamic instability in the form of a precessing vortex core. The PVC effect clearly shows up as a pronounced central maximum in the distributions of the tangential velocity RMS. The presence of the PVC is also clearly seen in the spectra of tangential velocity pulsations containing a discrete frequency that coincides with the PVC precession frequency.  
In contrast, in overload cases, the generation of a centrally located vortex with a concentrated vorticity has been observed, as pointed out by the tangential velocity maxima approaching to the channel axis. The central vortex remains coherent along the draft tube length and causes velocity pulsations that can be seen as the central maxima in profiles of the tangential velocity RMS.  
In order to quantify the extent of the regime shift away from the design point and analyze the subsequent flow rearrangement, it has been proposed an application of a physical parameter, swirl number \(S\), relying on integral quantities representing axial and angular momentum fluxes. It has been confirmed that the deviation of the regime from nominal conditions can be interpreted in terms of critical swirl number at which the vortex  
Figure 10: Flow swirl number: comparison of experimental data, Formula (2), and the calculation based on the analytical dependence (3) at \(z/D=1.30\).  
breakdown occurs. This finding can assist the design of practical micro hydro turbine installations by determining the range of swirl number that does not lead to breakdown.  
Conceptualization, S.S. and D.W.; Formal analysis, E.G. and S.S.; Funding acquisition, D.W.; Investigation, D.S.; Methodology, I.L. and D.W.; Project administration, D.W.; Resources, I.L. and S.S.; Software, I.L. and E.G.; Visualization, D.S.; Writing--original draft, D.S. and I.L.; Writing--review and editing, E.G., S.S. and D.W. All authors have read and agreed to the published version of the manuscript.  
The work including measurements of flow characteristics and data analysis was carried out with the state support of a contract with the Ministry of Education and Science of the Russian Federation (Agreement No. 075-15-2019-1923). The design and installation of the experimental setup was carried out partly within the framework of a state contract with IT SB RAS (Project No. 121031800229-1).  
Not applicable.  
The authors declare no conflict of interest.