With the current global trend of the wind turbines to be commissioned, the next generation of state-of-the-art turbines will have a generating capacity of 20 MW with rotor diameters of 250 m or larger. This systematic increase in rotor size is prompted by economies-of-scale factors, thereby resulting in a continuously decreasing cost per kWh generated. However, such large rotors have larger masses associated with them and necessitate studies in order to better understand their dynamics. The present work regarding the aeroelastic behavior of stall-controlled rotors involves the study of the frequency content and time evolution of their oscillatory behavior. A wide range of experiments were conducted to assess the effects of rapid variations on the rotor’s operational conditions. Various gust conditions were tested at different wind speeds, which are represented by pulses of different intensities, occurring suddenly in an otherwise constant wind regime. This allowed us to observe the pure aero-elasto-inertial dynamics of the rotor’s response. A reduced-order characterization of the rotor’s dynamics as an oscillatory system was obtained on the basis of energy-transfer principles. This is of fundamental interest for researchers and engineers working on developing optimized control strategies for wind turbines. It allows for the critical elements of the rotor’s dynamic behavior to be described as a reduced-order model that can be solved in real time, an essential requirement for determining predictive control actions.

In this paper, we focus on the Variable-Speed (VS) stall control method, a relatively new idea, which offers a promising perspective for future applications. As with the classical Fixed-Speed (FS) stall method, the elimination of the pitch mechanism, lowers the capital cost and reduces maintenance expenses, while at the same time, allows for a more efficient and precise control of power production. We present an analysis focused on the aeroe-lastic dynamic response of wind turbine rotors operating on the Variable-Speed stall control method. We conducted a wide range of experiments to assess the effects of rapid variations on the rotor’s operational conditions, like sudden gusts. Various gust conditions were tested for different wind speeds, represented by pulses of different intensity, occurring suddenly in an otherwise constant wind regime. Results for the aeroelastic dynamics of the rotor’s response, and the frequency content of its vibrations, are reported and analyzed.

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For falling film evaporation, the most important considerations from a thermal design standpoint are the onset of film dryout and the local heat transfer coefficients in partially and fully wet conditions. Previous methods developed for the prediction of (i) pool boiling heat transfer coefficient (HTCs), (ii) the onset of dryout, and (iii) falling film heat transfer coefficient consist of empirical, tube-specific constants which are quite difficult, if not impossible, to determine, and hence have limited utility. New methods to predict these parameters have been developed in the present study, which eliminate the special constants by incorporating dimensionless parameters that capture the effect of refrigerant properties and macro-level tube-geometry. The predictions of the new model have been found to be better than or comparable to those of the best available existing models.