Design guidelines for H-Darrieus wind turbines: Optimization of the annual energy yield

H-Darrieus wind turbines are gaining popularity in the wind energy market, particularly as they are thought to represent a suitable solution even in unconventional installation areas. To promote the diffusion of this technology, industrial manufacturers are continuously proposing new and appealing exterior solutions, coupled with tempting rated-power offers. The actual operating conditions of a rotor over a year can be, however, very different from the nominal one and strictly dependent on the features of the installation site. Based on these considerations, a turbine optimization oriented to maximize the annual energy yield, instead of the maximum power, is thought to represent a more interesting solution. With this goal in mind, 21,600 test cases of H-Darrieus rotors were compared on the basis of their energy-yield capabilities for different annual wind distributions in terms of average speed. The wind distributions were combined with the predicted performance maps of the rotors obtained with a specifically developed numerical code based on a Blade Element Momentum (BEM) approach. The influence on turbine performance of the cut-in speed was accounted for, as well as the limitations due to structural loads (i.e. maximum rotational speed and maximum wind velocity). The analysis, carried out in terms of dimensionless parameters, highlighted the aerodynamic configurations able to ensure the largest annual energy yield for each wind distribution and set of aerodynamic constraints

Large Eddy Simulation of a Bluff Body Stabilized Lean Premixed Flame

The present study is devoted to verify current capabilities of Large Eddy Simulation (LES) methodology in the modeling of lean premixed flames in the typical turbulent combustion regime of Dry Low NOx gas turbine combustors. A relatively simple reactive test case, presenting all main aspects of turbulent combustion interaction and flame stabilization of gas turbine lean premixed combustors, was chosen as an affordable test to evaluate the feasibility of the technique also in more complex test cases. A comparison between LES and RANS modeling approach is performed in order to discuss modeling requirements, possible gains, and computational overloads associated with the former. Such comparison comprehends a sensitivity study to mesh refinement and combustion model characteristic constants, computational costs, and robustness of the approach. In order to expand the overview on different methods simulations were performed with both commercial and open-source codes switching from quasi-2D to fully 3D computations

Stand-alone wearable system for ubiquitous real-time monitoring of muscle activation potentials

Wearable technology is attracting most attention in healthcare for the acquisition of physiological signals. We propose a stand-alone wearable surface ElectroMyoGraphy (sEMG) system for monitoring the muscle activity in real time. With respect to other wearable sEMG devices, the proposed system includes circuits for detecting the muscle activation potentials and it embeds the complete real-time data processing, without using any external device. The system is optimized with respect to power consumption, with a measured battery life that allows for monitoring the activity during the day. Thanks to its compactness and energy autonomy, it can be used outdoor and it provides a pathway to valuable diagnostic data sets for patients during their own day-life. Our system has performances that are comparable to state-of-art wired equipment in the detection of muscle contractions with the advantage of being wearable, compact, and ubiquitous

Experimental assessment of a methodology for the indirect in-cylinder pressure evaluation in four-stroke internal combustion engines

Recent innovations in engine control and diagnostics are providing room for development of innovative combustion approaches (e.g., low-temperature combustion) able to minimize the creation of pollutants. To ensure the constant fulfillment of the prescribed thermodynamic conditions, however, a fast real-time monitoring of the in-cylinder pressure is needed. To this end, dynamic pressure sensors, flush-mounted on the cylinder head, are commonly used. With this approach, the measurement accuracy is high, but the durability is limited by the harsh working conditions. The installation on the cylinder head is also complex. The development of robust and effective indirect measurement systems could then represent the enabler of a further development of this technology. In the present study, an innovative methodology to measure the in-cylinder pressure has been conceived and extensively tested on a four-stroke single-cylinder engine. The proposed approach is based on the analysis of the mechanical stress on the engine studs by means of a piezoelectric strain washer. This solution allows the user for a rapid and cost-effective sensor installation, described in the paper along with the signal post-processing techniques. Results showed good accuracy and robustness of the methodology, making the results of practical use for engine control

Design and characterization of latent thermal energy storage systems (LTESS) using pure and metal-foam-loaded PCMs

Latent thermal energy storage systems (LTESSs) are innovative technologies to store thermal energy very useful when the heat production is decoupled by the heat request (i.e. when renewable energy is used in a thermal system). Heat storage systems which use phase change materials (PCMs) represent an efficient solution to store thermal energy thanks to the PCMs high energy storage density. Actually, these materials are widely used as components of building elements (walls, windows), in solar energy systems, heat exchangers and heat pumps storage systems. This thesis follows the research purpose of the CLIWAX project which is devoted to demonstrate the effectiveness of PCM-based energy storage solutions dedicated to HVAC systems

"Test-site operations for the health monitoring of railway ballast using Ground-Penetrating Radar"

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Modelling defects in Ni-Al with EAM and DFT calculations

We present detailed comparisons between the results of embedded atom model (EAM) and density functional theory (DFT) calculations on defected Ni alloy systems. We find that the EAM interatomic potentials reproduce low-temperature structural properties in both the γ and ${{\gamma}^{\prime}}$ phases, and yield accurate atomic forces in bulk-like configurations even at temperatures as high as  ~1200 K. However, they fail to describe more complex chemical bonding, in configurations including defects such as vacancies or dislocations, for which we observe significant deviations between the EAM and DFT forces, suggesting that derived properties such as (free) energy barriers to vacancy migration and dislocation glide may also be inaccurate. Testing against full DFT calculations further reveals that these deviations have a local character, and are typically severe only up to the first or second neighbours of the defect. This suggests that a QM/MM approach can be used to accurately reproduce QM observables, fully exploiting the EAM potential efficiency in the MM zone. This approach could be easily extended to ternary systems for which developing a reliable and fully transferable EAM parameterisation would be extremely challenging e.g. Ni alloy model systems with a W or Re-containing QM zone

an indirect in cylinder pressure measurement technique based on the estimation of the mechanical strength acting on an engine head screw development and assessment

Abstract The increasing application of new concepts for the combustion process in internal combustion engines, e.g. HCCI or RCCI, is mainly aimed at reducing pollutant emissions and fuel consumption. A typical drawback of these technologies is the difficulty of properly controlling the combustion process in the area of medium-high brake mean effective pressure (BMEP), where the thermodynamic conditions inside the cylinder promote a very fast combustion process. To this end, the availability of a fast real-time monitoring of the in-cylinder pressure is then becoming pivotal. This is commonly done by means of piezoelectric dynamic pressure sensors, which are indeed very accurate, but also extremely expensive and characterized by a limited durability due to the harsh working conditions. Moving from this background, the present study describes a new methodology to evaluate the in-cylinder pressure by correlating it with the mechanical stress measured by a strain washer installed on an engine head screw. The strain washer can indeed work in a much more favorable environment with respect to a dynamic pressure sensor flush-mounted on the cylinder head (with aggressive hot gasses and high pressure) with direct benefits for its durability and ease of installation. To assess the model capabilities, experimental tests have been carried out on a single-cylinder, 4-stroke engine and on a 2-stroke engine at the laboratory of internal combustion engines of the Universita degli Studi di Firenze. The results reported in the study show the direct comparison of the in-cylinder pressure, as a function of the crankshaft angular position, measured directly with a dynamic pressure sensor and indirectly by means of the strain washer. Sound agreement was found between the two, proving the effectiveness of the proposed methodology

Critical Analysis of Dynamic Stall Models in Low-Order Simulation Models For Vertical-Axis Wind Turbines

Abstract The efficiency of vertical-axis wind turbines (VAWTs) still lacks from those of horizontal-axis rotors (HAWTs). To improve on efficiency, more accurate and robust aerodynamic simulation tools are needed for VAWTs, for which low-order methods have not reached yet a maturity comparable to that of HAWTs' applications. In the present study, the VARDAR research code, based on the BEM theory, is used to critically compare the predictiveness of some dynamic stall models for Darrieus wind turbines. Dynamic stall, connected to the continuous variation of the angle of attack on the airfoils, has indeed a major impact on the performance of Darrieus rotors. Predicted lift and drag coefficients of the airfoils in motion are reconstructed with the different dynamic stall models and compared to unsteady CFD simulations, previously validated by means of experimental data. The results show that low-order models are unfortunately not able to capture all the complex phenomena taking place during a VAWT functioning. It is however shown that the selection of the adequate dynamic stall model can definitely lead to a much better modelling of the real airfoils' behavior and then notably enhance the predictiveness of low-order simulation methods