# Laboratory Modeling of an Axial Flow Micro Hydraulic Turbine
## 2 Research Methods  
The experiments were carried out employing an automated experimental setup with air used as a working medium. This significantly reduced the difficulties in manufacturing experimental sections (problems with sealing joints, structural strength, and providing optical access) [24]. As shown in the technical reports [25; 26] from the USBR group in the USA and in the USSR energy community handbook [27], results obtained with air and water test sections showed satisfactory correlation. The investigation of most of the hydrodynamic phenomena can be transferred from water to air [28]. This simplifies the experimental procedure significantly because no cavitation occurs in the flow. Nevertheless, we only suggest a concept of the experimental finding of optimal operation of the microturbine, which should be tested on water as well. Air is supplied to the model using a blower fan. An expansion section, a settling chamber with damping screens, and a contraction produce uniform flow at the inlet to the test section. The air flow rate is changed by means of a frequency converter connected to the fan motor. The flow rate is determined by measuring the velocity at the outlet of the contraction, i.e., in front of the test section. A model of propeller-type turbine (Figure 1) was chosen as a prototype of microhydroturbine device [17; 29]. The flow enters the working section through the inlet opening and passes through the first stationary vane swirler (guide vane). It is followed by a rotating swirler--an impeller with a streamlined body attached to it. The rotating swirler is driven by a shaft connected to an external servo drive. The shape of the vane swirlers and the order of their arrangement allows to simulate the velocity distribution at the outlet of a real hydraulic turbine [30]. The propeller turbine has high specific speed among other types of turbines. This allows high rotational speeds of the runner at low flow rates. High turbine speeds result in faster and therefore lighter and cheaper power generators. The high specific speed makes it possible to connect the turbine directly to the generator. All parts of the model section were manufactured using rapid prototyping technology. The diameter of the inlet part of the cone is \(D=100\) mm, the outlet part is1.2\(D\). The cone length is \(H=280\) mm, the cone angle is \(4^{\circ}\). Figure 1: _Cont._  
In experiments, the Reynolds number \(Re=4 DQ/\tau v(D^{2}-d^{2})\) ranged from \(3\times 10^{4}\) to \(9\times 10^{4}\) (v is the kinematic viscosity of air), which corresponds to the range of air flow rate \(Q\) from 0.02 to 0.08 m\({}^{3}\)/s, therefore, the flow was turbulent. It is supposed that the flow characteristics are unchanged when the impeller speed changes proportionally to the flow rate. The runner speed was maintained at a constant \(\omega_{n}=238\) rad/s. Determining the shape of the swirlers was carried out according to the method described in [30]. This computation showed that the value of \(\omega_{n}\) for a flow rate \(Q_{n}=0.049\) m\({}^{3}\)/s corresponds to the point with maximum efficiency (Best Efficiency Point, BEP). The studied range of micro hydraulic turbine operation was in the range from part load (\(0.6Q_{n}\)) to high overload (\(1.8Q_{n}\)). For the operation of a micro hydraulic turbine of this scale in water at the design flow rate \(Q_{n}\) and a pressure head \(H_{n}=2\)-\(3\) m water column, the available power in the flow is \(P_{\text{tot}}=Q_{n}H_{n}\)\(\sim\)1.1-1.4 kW and the power developed by the turbine at a typical 70% efficiency is \(P_{\text{turb}}\)\(\sim\)0.8-1 kW [11; 16]. The design point with maximal efficiency is maintained when the velocity triangles before and after the runner are similar (\(\omega/Q\) is constant). When the operating point moves along the line of constant running speed (off-design conditions), the turbine efficiency decreases.