Computational Fluid Dynamics is thought to provide in the near future an essential contribution to the development of vertical-axis wind turbines, helping this technology to rise towards a more mature industrial diffusion. The unsteady flow past rotating blades is, however, one of the most challenging applications for a numerical simulation and some critical issues have not been settled yet.In this work, an extended analysis is presented which has been carried out with the final aim of identifying the most effective simulation settings to ensure a reliable fully-unsteady, two-dimensional simulation of an H-type Darrieus turbine.Moving from an extended literature survey, the main analysis parameters have been selected and their influence has been analyzed together with the mutual influences between them; the benefits and drawbacks of the proposed approach are also discussed.The selected settings were applied to simulate the geometry of a real rotor which was tested in the wind tunnel, obtaining notable agreement between numerical estimations and experimental data. Moreover, the proposed approach was further validated by means of two other sets of simulations, based on literature study-cases

Darrieus wind turbines are experiencing a renewed interest in the wind energy scenario, in particular, whenever small and medium-size installations are considered. In these contexts, the average wind speeds are generally quite low due to scale effects and therefore the most exploited design choices for the turbines are the H-shape configuration, as the entire blade can take advantage of the maximum rotational radius, and high chord to radius ratios, in order to ensure suitable Reynolds numbers on the airfoils. By doing so, the aerodynamic effects induced by the motion of the airfoils in a curved flowpath become more evident and the airfoils themselves have to be designed to compensate these phenomena if conventional design tools based on the blade element momentum (BEM) theory are used. In this study, fully unsteady 2D simulations were exploited to analyze a three-bladed H-Darrieus wind turbine in order to define the real flow structure and its effects on the turbine performance; in detail, the influence of both the virtual camber and the virtual incidence were investigated. Computational fluid dynamics (CFD) results were supported by experimental data collected on full-scale models reproducing two different airfoil mountings. Finally, the proper design criteria to compensate these phenomena are proposed and their benefits on a conventional simulation with a BEM approach are discussed.

Darrieus wind turbines are experiencing a renewed interest in the wind energy scenario, in particular whenever small and medium-size installations are considered. In these contexts, the average wind speeds are generally quite low due to scale effects and therefore the most exploited design choices for the turbines are the H-shape configuration, as the entire blade can take advantage of the maximum rotational radius, and high chord to radius ratios, in order to ensure suitable Reynolds numbers on the airfoils. By doing so, the aerodynamic effects induced by the motion of the airfoils in a curved flowpath become more evident and the airfoils themselves have to be designed to compensate these phenomena if conventional design tools based on the BEM theory are used. In this study, fully unsteady 2D simulations were exploited to analyze a three-bladed H-Darrieus wind turbine in order to define the real flow structure and its effects on the turbine performance; in detail, the influence of both the virtual camber and the virtual incidence were investigated. CFD results were supported by experimental data collected on full-scale models reproducing two different airfoil mountings. Finally, the proper design criteria to compensate these phenomena are proposed and their benefits on a conventional simulation with a BEM approach are discussed.

The reduction of pressure losses is one of the most important challenges for the efficiency increase of a reciprocating compressor. Since the absorbed power is strongly affected by the losses through pocket valves and cylinder ducts, an accurate prediction of these losses is essential. The use of computational fluid dynamics (CFD) simulation has shown great potential for the study of the entire reciprocating compressor, but is still limited by high computational costs. Recently, the authors have presented a simplified CFD approach: the actual valve geometry is replaced with an equivalent porous region, which has significantly increased the speed of calculation while ensuring accuracy as well. Based on this approach, a new methodology for the evaluation of pocket valve losses is presented. A set of CFD simulations using a parameterized geometry of the pocket valve was performed to evaluate the relationship between the losses of the pocket and its geometrical features. An analytical response surface (RS) was defined using the values of the geometrical dimensions as inputs and the pocket flow coefficient as output. Finally, the response surface was validated through a set of test cases performed on different geometries with the actual valve and the results have shown good predictability of the tool.

The working cycle of a reciprocating compressor is characterized by heat generation, mainly due to compression transformation and friction phenomena. The main consequences are a reduction of the volumetric efficiency and an increase in the gas discharge temperature. Current regulations such as API618 for reciprocating compressors require a cylinder cooling system. Therefore, a proper design of the cooling circuit is needed in order to achieve the best balance between refrigerating potential and system capacity. A systematic methodology for the evaluation of the heat transfer process is essential and since experimental characterization of the circuit is complex and case-dependent, the use of a numerical technique is the most favorable and generalizable approach. Within this scenario, 3D analysis shows a great potential although several phenomena must be accounted for in order to accurately model the system. In this paper, a conjugate heat transfer (CHT) analysis on a double-acting water-cooled reciprocating compressor cylinder is presented, where the three-dimensional flow field of the water circuit and the thermal conduction inside the solid metal are solved simultaneously. The best practice for the imposition of consistent boundary conditions for the metal body is given with special attention to the heat transfer coefficient values for the suction and discharge gas chambers, the compression chamber and the external ambient. The assessment of the numerical methodology is completed with an investigation on the influence of wall roughness and buoyancy effects.