Daniel Catalin Stroita, Adriana Sida Manea, Anghel Cernescu - Blade Polymeric Material Study of a Cross-Flow Water Turbine Runner

This work presents an in-depth study on the system identification of a servo-valve controlled hydraulic cylinder operating under variable load. This research addresses the growing demand for improved control systems (enhancing time response, settling time, and precision) in variable load hydraulic actuators, such as those used in blade pitching systems of wind turbines. The paper begins by detailing the experimental setup, followed by the development of the system’s mathematical model, a fourth-order transfer function (TF). The experimental data collected by a proposed data acquisition system are used for the dynamic identification of the hydraulic setup using periodical signals as commands. All possible combinations of TFs up to order 8 are identified. After an initial visual preselection of the 15 most accurate ones, analyses comparing quality indicators between the measured (experimental) and the TF (simulated) step and sinusoidal responses are conducted to determine the most accurate TF. The paper concludes with the presentation and analysis of the dynamic model, identified as being a fourth-order TF, which replicates the system dynamics with the greatest fidelity. It provides an identification methodology with significant potential for industry practitioners aiming to improve, optimize, and enhance control strategies for variable load hydraulic actuators.

The degradation of material surface structures caused by cavitation is of a mechanical nature due to cyclic local fatigue loading. Its intensity depends on both the material’s mechanical properties and its surface microstructure. Evaluating the surface structure resistance to cavitation loading can be performed based on experimentally determined mechanical properties and/or through macro- or microscopic analysis of the eroded structure. Manufacturers, designers, and users of hydromechanical equipment operating under cavitation conditions are interested in materials whose properties and structures can withstand cavitation loading. For this reason, the current experimental research in the field focuses on establishing relationships that express the influence of either mechanical properties or surface microstructure on the resistance of material structures to cavitation erosion. The current paper aligns with this research direction and aims to determine statistical relationships between mechanical properties and the cavitation erosion resistance of aluminum-based alloys. The mechanical properties considered include ultimate tensile strength (R_m), yield strength (R_p0.2), surface hardness (HB), resilience (KCU), and elongation at fracture (A5). Cavitation resistance is evaluated using the parameter R_cav, defined according to the ASTM G32-2016 standard. The experimental results were obtained from cavitation tests conducted using a standard vibratory device that complies with ASTM G32 requirements.

The cross-flow turbines cover from the point of view hydraulic power the running domain of some well-known turbines such as Pelton, Francis or Kaplan. This type of turbine has a simple construction, long life and low execution cost, which makes it very suitable for on and off grid small to medium hydro power plants. It is quite difficult to establish an exact theoretical dynamic model for this type of turbines, due to the complex flow phenomenon (bi phase flow water and air). In order to obtain the exact dynamic behavior of the hydraulic machine, experimental dynamic identification will be done. In automation, the dynamic properties, represent the fundamental characteristic of the object which must be regulated. When the dynamic properties of the regulated object are obtained experimentally, we analyze the characteristics of the transient regime, which appears because of the application at the system inlet of some stochastic or deterministic signals (sine waves for our case). The hydraulic turbine is modeled as an informational quadrupole having the inlet parameters the movement of the wicket gate and the turbine head and outlet parameters the torque and the speed. In this paper it will be presented the frequency modelling of the cross-flow turbine and the validation of the mathematical model through experimental dynamic identification.

The use of bulk heat treatments to improve the resistance of the material structures to cavitation erosion remains an effective approach due to the beneficial modifications induced in the microstructure and physical-mechanical properties. Depending on the intensity of cavitation loading, various heat treatment regimes can be applied. Among these, artificial aging treatments are particularly suitable for non-ferrous alloys, especially aluminum, zinc, and magnesium-based alloys. The current study investigates the effect of artificial aging heat treatment performed at 250 °C with holding times of 12 and 24 h on the biodegradable magnesium-based AM50 alloy. Cavitation tests were carried out using the method with a stationary specimen on a standard vibratory device according to ASTM G32-2016 requirements. The analysis of cavitation-eroded surfaces through macro- and microstructural images, together with the interpretation of characteristic erosion curves and specific parameters (cumulative mass loss, erosion speed and cavitation resistance), revealed both similarities and significant differences governed primarily by surface hardness and microstructural features. Comparison with the initial (semi-finished) state and with previous studies on artificial aging treatments performed at 200 °C for 12 and 24 h confirms the similarly beneficial effect of the 250 °C aging regime on the cavitation erosion resistance of the AM50 alloy.