1,720,971 research outputs found
Effects of hydrofoil shape and turbine solidity on the wake energy recovery in cross-flow turbines
No full textCross-flow turbines (CFTs) are arousing a growing interest to harvest both off-shore wind and tidal currents. A promising characteristic of CFTs could be a high power density in case of multi-device clusters or farms, achievable by shortening the distance between arrays as allowed by the fast energy recovery observed inside the wakes. However just few studies, only concerning symmetrical airfoils/hydrofoils, are found in the literature. By means of 3D Unsteady Reynolds Averaged Navier-Stokes (URANS) simulations and a momentum budget simplified approach, this author investigated the effects of blade profile and turbine solidity on the blade tip vortex generation and then on the mixing mechanisms supporting the reintroduction of streamwise momentum into the wake. Results indicate that: (a) pairs of counter-rotating vortices occurs in the wake, which rotation direction depends on blade profile and it is such as to generate positive vertical advection for camber-out profiles, but negative vertical advection for camber-in profiles; (b) camber-out profiles are much more effective in supporting the wake energy recovery due to the massive vertical advection induced by tip vortices; (c) for camber-in profiles the tip vortices poorly contribute to the wake recovery, that appears delayed and promoted by turbulent transport; (d) higher solidity implies stronger tip vortices and higher turbulent transport, therefore, a faster wake recovery
Low-order CFD simulation of a ducted wind turbine in realistic hilly environments
No full textIt is common to place wind turbines over smooth hills to exploit the speed up effect resulting from the hill slope. On the other hand, another way to increase the power output of small turbines consists in the adoption of a diffuser. Therefore, it would be interesting to investigate the union of these two techniques, however, since the computing resources required by traditional fully 3D CFD simulations would be prohibitive, other less expensive approaches are needed. Using CFD, the performance of a 50kW wind turbine positioned on the top of a hill and equipped with a symmetrical convergent-divergent diffuser was predicted. To save computation time, the fluid dynamic interaction with the rotating blades is mimicked by adopting the momentum source low-order approach, that in our case is based on the Virtual Blade Model (VBM). The diffuser behavior has been analyzed for three hill profiles characterized by the same height but different shapes that are representative of realistic environments. The results show that the diffuser not only allows a general massive increase in power but also that its use is especially advantageous, compared to the bare turbine, in cases of flows made yawed by the slope of the hill or the geometry of the cliff. This happens because the converging section of the diffuser is able to realign the skewed flows normally to the turbine rotor. © 2024 IEEE
Tidal Farms: Optimising site-specific layouts by combining analytical methods and fluid dynamic simulations
No full textTidal energy is a promising source of renewable energy that countries should consider including in their energy mix. This paper presents a simple and efficient methodology for the analysis of tidal farms. Specifically, we have used a Mixed Integer Quadratic Programming (MIQP) model to optimise the farm layout using a discrete approach. The farm is made up of HATTs (Horizontal Axis Tidal Turbines). We compare the results obtained with a conventional discretisation based on literature assumptions to a site-specific adapted discretisation. The latter is obtained through 3D fluid dynamic simulations performed in series using the SHYFEM software, which is based on the Shallow Water equations. SHYFEM is an open-source marine circulation code and has been equipped with turbines parameterized by momentum source terms: this allows to reduce the computational effort while maintaining the accuracy of the turbine behaviour predictions thanks to the Blade-Element\Momentum (BEM) approach. In addition, the BEM approach allows interactions between devices to be captured, making the code well suited for farm applications. The results show a +6.7% increase in power production achieved by using the site-adapted discretisation for the optimisation process. © 2024 IEEE
Fluid dynamic mechanisms for the wake energy recovery in cross-flow turbines: effects of hydrofoil shape and turbine solidity: Paper 613
Full text linkCross-flow turbines (CFTs) are arousing a growing interest to harvest both off-shore wind and tidal currents. A promising characteristic of CFTs could be a high power-density in case of multi-device clusters or farms, achievable by shortening the distance between arrays as allowed by the fast energy recovery observed inside the wakes. However just few studies, only concerning symmetrical airfoils/hydrofoils, are found in literature. By means of 3d-URANS simulations and the momentum budget approach we investigated the effects of blade profile and turbine solidity on blade tip vortex generation and then on the mixing mechanisms supporting the reintroduction of streamwise momentum into the wake. Results indicate that: (a) a pair of counter-rotating vortices occurs in the wake at the turbine top and bottom ends, which rotation verse depends on blade profile and it is such as to generate positive vertical advection for camber-out profiles, but negative vertical dvection for camber-in profiles; (b) camber-out profiles are much more effective in supporting the wake energy recovery due to the massive vertical advection induced by tip vortices; (c) for camber-in profiles the tip vortices poorly contribute to the wake recovery, that appears delayed and promoted by turbulent transport; (d) higher solidity implies stronger tip vortices and higher turbulent transport, therefore a faster wake recovery
Numerical Investigation of an Innovative low pressure Direct-Injection System for hydrogen Engine
No full textThe paper shows CFD activity concerning an innovative hydrogen direct-injection system,
characterized by low fuel rail pressure.
The system allows combining the benefits of hydrogen direct injection, mainly consisting in
high specific power and in backfire prevention, with the typical ones of indirect injection,
mainly consisting in the almost complete utilization of the on-board stored hydrogen, as well
as in low cost and reliability.
The injection is split in two steps: first an electroinjector meters hydrogen and feeds a small
intermediate chamber, then the hydrogen stored in the chamber is injected into the cylinder
with proper timing by a mechanically-actuated valve.
A prototype was derived from a production single-cylinder motorcycle engine on the basis of
previous CFD predictions and, despite important limitations in design freedom, proved to run
correctly, without the typical drawbacks of hydrogen engines. Maximum power was higher
than with gasoline.
Present CFD activity is aimed at designing a new engine originally intended for hydrogen.
Axisymmetric analyses have been carried out for several valve and seat-valve profiles,
proving that proper geometrical details allow quite satisfactory fuel distribution at the ignition
time for every engine speed and load
Engine Experimental Results with hydrogen Two-Step Direct Injection
No full textIn hydrogen-fuelled internal combustion engines, port fuel injection, compared to direct
injection, allows higher range and is less expensive and more adaptable to existing engines,
but leads to lower specific power and may cause backfire and pre-ignition. Conversely, direct
injection is more complex and expensive, but provides even higher specific power than with
gasoline, prevents backfire and makes pre-ignition less probable.
The benefits of direct injection are added to those of port fuel injection in an innovative lowpressure
(12 bar rail pressure) direct-injection system that is shown in this paper. The
injection is accomplished in two steps: first an electroinjector meters hydrogen and feeds a
small intermediate chamber, then the hydrogen stored in the intermediate chamber is timely
injected into the cylinder by a mechanically actuated valve.
The prototype was obtained modifying a production single-cylinder 650 cm3 engine for
motorcycles.
Experimental results show proper engine operation, without pre-ignition, knocking or
roughness even with stoichiometric or slightly rich mixtures. Maximum power was higher
than with gasoline
Studio fluidodinamico sulla formazione della miscela nel condotto d’aspirazione di un motore alimentato con iniettore-pompa pilotato in frequenza
No full textMediante il codice fluidodinamico KIVA3v è stato simulato il comportamento di un motore monocilindrico per usi stazionari alimentato con un sistema d’iniezione indiretta pilotato in frequenza.
Obiettivo dello studio è stato di valutare le discrepanze tra il titolo realizzabile e quello desiderato ed in particolare l’entità delle fluttuazioni di titolo tra un ciclo e l’altro, problemi peculiari di tale sistema d’iniezione e dovuti essenzialmente alla limitazione a poche unità del numero massimo di iniettate per ciclo del motore.
Sono stati indagati l’interazione tra spray e parete del condotto di aspirazione, la formazione del film liquido, l’evaporazione del film e delle gocce, la miscelazione tra l’aria e il vapore per diffusione locale e per trasporto convettivo. Sono state simulate differenti lunghezze del condotto di aspirazione e posizionamenti dell’iniettore ed è stata considerata l’ipotesi di adottare materiali porosi o dispositivi mediante i quali incrementare la superficie bagnata del condotto, al fine di ottenere un vantaggioso “effetto spugna” (o “effetto polmone”)
A simple model for deep dynamic stall conditions
No full textThe present study is focused on modeling of dynamic stall behavior of a pitching airfoil. The deep stall regime is in particular considered. A model is proposed, which has a low implementation and computational complexity but yet is able to deal with different types of dynamic stall conditions, including those characterized by multiple vortex shedding at the airfoil leading edge. The proposed model is appraised against an extensive data set of experimental (α,CL) curves for NACA0012. The results of an existing widely used model, having comparable complexity, are also shown for comparison. The proposed model is able to well reproduce not only the classic curves of deep dynamic stall but also the curves characterized by lift oscillations at high angles of attack due to the shedding of multiple vortices. Furthermore, the model appears to be robust to variations of its parameters from the optimal values and of the airfoil geometry. Finally, the model is successfully implemented in a commercial CFD software and applied to the simulation of a vertical axis wind turbine within the actuator cylinder approach. The accuracy of the prediction of the turbine power coefficient in the whole rotation cycle is very good for the optimal working condition of the turbine, for which the model parameters were calibrated. Fairly good accuracy is also obtained in significantly different working conditions without any further calibration
Two-Stroke GDI: Improvement of Engine Behaviour at Light Loads
No full textDirect fuel injection is becoming mandatory for
two-stroke S.I. engines, since it allows using only
air in the scavenging process, preventing fuel loss
from the exhaust port.
However, also combustion irregularity at light
loads is a problem of these engines, due to
excessive presence of residual gas in the charge.
For its solution, additional strategies must be
adopted. The two most suitable ones are charge
stratification and ATAC (Active Thermo
Atmosphere Combustion), which makes use of
residual gas energy to ignite fresh gas, turning the
effect of residual gas from negative to positive.
In previous papers, the possibility of combining
ATAC with different injection systems was proved.
This paper provides further insight on ATAC
combined with liquid high-pressure direct injection
and shows the studies and the first experimental
results relative to charge stratification
- …
