Antonio Rossetti

Numerical Analyses of Cavitating Flow in a Pelton Turbine

Erosion and wear of hydraulic surfaces are frequent problems in hydraulic turbines, which lead to a decrease of the performance in time and/or in extreme cases to the rotor mechanical failure. These circumstances have negative repercussions on the annual produced power due to the decay of the efficiency, the delivered power, and to the off line periods as result of ordinary and extraordinary hydraulic profiles maintenances. Consistently, the study of this wearing process is an important step to improve the impeller design, and to avoid or minimize the rise of extraordinary maintenance. While mechanical damages are well documented and studied, little information can be found on cavitation in Pelton turbines. In this paper, a CFD model was applied to study the cavitation mechanics on a Pelton turbine. A Pelton runner affected by pitting cavitation was taken as a test case. The bucket geometry was modeled and analyzed using unsteady Reynolds averaged Navier-Stokes (RANS) multiphase analyses. Numerical results allowed us to highlight the different vapor productions during the cut-in water jet processes by the bucket. Furthermore, a simple procedure to identify the locations of higher damage risk was presented and verified in the test case runner. [DOI: 10.1115/1.4027139]

An optimum design procedure for an aerodynamic radial diffuser with incompressible flow at different Reynolds numbers

Abstract In this article a systematic procedure aimed at achieving the best compromise between flow deflection, static pressure recovery, and total pressure loss is proposed for aerodynamic diffusers with incompressible flow. Such a result was accomplished by using a neural network to generalize the radial diffusers performance data obtained by numerical analyses, a multi-objective approach based on the employment of fuzzy sets, and a swarm particle algorithm to find a good compromise between flow deflection, static pressure recovery, and total pressure loss. Useful design tools, obtained by collecting the results in proper design charts, are finally proposed to simplify the design of radial diffusers without resorting to expensive and time-consuming procedures of optimization. The influence of the Reynolds number on the overall performance was also taken into account.

Influence of the bucket geometry on the Pelton performance

The increasing share of hydropower in world electricity production requires the development of standardized and optimized design procedures leading to increasingly higher efficiency values. To date, despite a certain amount of support from computational fluid dynamics, Pelton turbines are still characterized by semiempirical design criteria that do not make it possible to optimize the jet–bucket interaction in order to maximize turbine performance. Based on an analysis of particle flow tracks, this paper presents a hybrid Eulerian–Lagrangian method to investigate the influence of bucket geometry on the Pelton efficiency at two different operating conditions. Jet–bucket interaction was numerically analyzed by means of a traditional mesh-based numerical approach, using a transient multi-phase homogeneous model. Subsequently, the numerical results were integrated using a predictor–corrector algorithm, combining a fourth order Adams-Bashforth method as predictor and a fourth order Adams-Moulton method as corrector, in order to determine the fluid particle trajectories on the rotating buckets. The particle flow tracks were analyzed in detail to evaluate the single-particle performance in terms of discharged kinetic energy, momentum variation, and total energy variation during the jet–bucket interaction. Moreover, on the basis of the particle discharging position, the contribution of the different bucket areas to the total torque of the turbine was investigated to determine the time-depending influence of the bucket geometry on the turbine energy exchange and to suggest possible design solutions for improving bucket performance.

Experimental and numerical analyses of micro rotary shaft pumps

This paper presents experimental and numerical results obtained with micro rotary shaft pumps (RSP). Impellers with a diameter of 2.5 mm, different outlet widths and blade number were coupled with semicircular volutes with different eccentricities. Experimental data for every impeller?volute couple were reported and include the flow rate, head and overall efficiency. Different rotational speeds were tested up to 24?000 rpm, obtaining pressure increases up to 5.7 kPa and flow rates up to 80 ml min?1. The non-dimensional performance was also computed obtaining the maximum head coefficient of 0.49 and the maximum flow coefficient of 0.138. Furthermore, experimental data were compared with 3D time-dependent CFD simulations. The focus of the simulation was to study the flow field structure inside the impeller and in the volute. Moreover, CFD data allowed for the calculation of the hydraulic efficiency of the pump and for the impeller to highlight the stator rotor interference influence on the pump characteristics, as well as to show the distribution of losses inside the volute.

A procedure for the design of radial cascade diffusers based on the maximum ratio between flow deflection and total pressure loss coefficient

Abstract A design procedure of radial cascade diffusers for incompressible flow, based on the maximum ratio between the flow deflection and the total pressure loss coefficient, is proposed. To achieve such a result, numerical investigations were carried out on several different diffusers at design and off-design operating conditions. Four space chord ratios, with seven blades and nine outlet flow angles were considered. For each diffuser, the optimum performance parameters were correlated to the incoming flow angle and the design variables of the aerofoil cascade. Reynolds number and mean camberline influence were also analysed. Design charts were then produced in order to attain an efficient preliminary design of aerodynamic diffusers without resorting to time-consuming approaches. Finally, auxiliary charts were proposed for predicting the diffuser performance at off-design operating conditions.