# Laboratory Modeling of an Axial Flow Micro Hydraulic Turbine
## 1 Introduction  
Hydropower, being the oldest area of renewable energy, currently provides a significant contribution to world electric energy generation [1]. The basis of this production is large-scale hydropower plants (HPPP), which are built on large rivers and require heavy capital expenditures on the creation of dams. Further broadening of hydropower is limited on the one hand because suitable water resources for the creation of large-scale hydropower systems are already fully exploited in many countries [2]. On the other hand, the construction of new big HPP in places where such resources exist is hampered by economic reasons or safety concerns [3]. At that time, it is recommended the attention to utilization of the hydropower potential of natural water sources in the form of small rivers and streams [4], as well as technical systems of irrigation and water supply [5]. According to the classification of hydro turbines used in the literature, the class of microhydrotubines (MHT) include devices less than 100 kW, which are considered often as an off-grid technology for the electricity generation [6, 7]. Development of MHT systems addresses the current global concerns on climate change demanding decarbonization of the energy generation sector and increasing use of renewable energy sources [8, 9, 10, 11, 12]. Micro hydropower has been considered as a reliable electricity generation technology for remote communities [13, 11], living where extension of centralized power distribution networks is very expensive. Different hydro turbine types are employed nowadays depending on parameter range of available water resources in terms of head and flowrate [1, 11]. As moving towardsmicro hydropower applications requires reducing the scale of a power generation unit, it is important to keep low cost per kilowatts. This is becoming especially critical for pico hydroturbines with a capacity of below 5 kW for remote small communities [9; 11; 13]. In this regard, particular attention has been given to propeller-type axial hydraulic turbines [9; 14; 15; 16]. They feature a simple arrangement, moderate manufacturing costs, and high specific speed (high runner rotation speed). The high specific speed permits application of cheap power generators having common shaft with the impeller [17], i.e., avoiding use of additional transmitting gears. The necessity of MHT price reduction requires turbines with fixed blades since mechanisms for adjusting the guide and impeller vanes might be expensive and complicated [18]. It should be noted that propeller hydraulic turbines with fixed blade angles have a narrow range of efficient operation [11]. The use of power sources with a stable frequency of the output voltage at a variable turbine speed requires significant power electronics. The cheaper alternative is to manually adjust turbine operation to the available conditions of head and flow rate. The practical use of axial hydraulic units is associated also with reducing the flow passage size which increases the risk of clogging of blade elements with various debris often present in natural water bodies [10; 19]. This can lead to off-design turbine operation due to a change in flow rate with decreasing flow area or due to a deceleration of the runner by foreign object impact. The situation described above regarding propeller type MHTs that have favorable and disadvantageous features holds for all hydroturbine types. Therefore, a detailed analysis is required of different structural and regime conditions for the proper choice of the most efficient and reliable MHT technology. As for the equipment to be applied in large HPP, numerical simulation methods and model tests are employed at the hydroturbine design stage [20] with particular close attention paid to internal flow structure which determines the hydroturbine operation regime [21]. It should be noted that despite tremendous progress and vast possibilities of numerical tools, experimental data is still needed for verification of calculation results.