196,448 research outputs found
Analysis of multi-year near-surface ozone observations at the WMO/GAW “Concordia” station (75°06′′S, 123°20′′E, 3280 m a.s.l. – Antarctica)
No full textThis work focuses on the near-surface O3 variability over the eastern Antarctic Plateau. In particular, eight years (2006–2013) of continuous observations at the WMO/GAW contributing station “Concordia” (Dome C–DMC: 75°06′S, 123°20′E, 3280 m) are presented, in the framework of the Italian Antarctic Research Programme (PNRA). First, the characterization of seasonal and diurnal O3 variability at DMC is provided. Then, for the period of highest data coverage (2008–2013), we investigated the role of specific atmospheric processes in affecting near-surface summer O3 variability, when O3 enhancement events (OEEs) are systematically observed at DMC (average monthly frequency peaking up to 60% in December). As deduced by a statistical selection methodology, these OEEs are affected by a significant interannual variability, both in their average O3 values and in their frequency. To explain part of this variability, we analyzed OEEs as a function of specific atmospheric variables and processes: (i) total column of O3 (TCO) and UV-A irradiance, (ii) long-range transport of air masses over the Antarctic Plateau (by Lagrangian back-trajectory analysis – LAGRANTO), (iii) occurrence of “deep” stratospheric intrusion events (by using the Lagrangian tool STLEFLUX). The overall near-surface O3 variability at DMC is controlled by a day-to-day pattern, which strongly points towards a dominating influence of processes occurring at “synoptic” scales rather than “local” processes. Even if previous studies suggested an inverse relationship between OEEs and TCO, we found a slight tendency for the annual frequency of OEEs to be higher when TCO values are higher over DMC. The annual occurrence of OEEs at DMC seems related to the total time spent by air masses over the Antarctic plateau before their arrival to DMC, suggesting the accumulation of photochemically-produced O3 during the transport, rather than a more efficient local production. Moreover, the identification of recent (i.e., 4-day old) stratospheric intrusion events by STEFLUX suggested only a minor influence (up to 3% of the period, in November) of “deep” events on the variability of near-surface summer O3 at DMC. © 2018 The Author
Stochastic Dynamics of Coastal Dune Vegetation
No full textThis work introduces a physically based modeling framework to capture the spatio-temporal dynamics of dune vegetation under stochastic environmental disturbances. The model evaluates vegetation cover in response to random wind speed and runup within a cross-shore dimensionless framework. The wind speed is modeled as a compound Poisson process with Gamma-distributed properties, facilitating the computation of up-crossing times for various thresholds. The dune topography is represented by a swash zone with a Gaussian shape and a monotonic landward increase, parameterized by slope, wavelength, and height. Key disturbance conditions affecting vegetation, that is, runup-induced flooding in the swash zone and wind-induced scour on the backshore and crest, are addressed through threshold-based analysis. The model uses a state-dependent dichotomic process for vegetation dynamics, where growth and decay are influenced by external forcing and vegetation state. Analytical solutions of the master equation for the vegetation distributions reveal the impact of stochastic factors on vegetation growth and stability. Sensitivity analysis identifies dune steepness, forcing magnitude and variability, and relative roughness as critical parameters. These factors significantly affect vegetation distribution, with increased steepness leading to higher vegetation density at the backshore and reduced density at the shorefront. Validation is carried out against satellite imagery and high-resolution real elevation data from the U.S. coastline and confirms the robustness and accuracy of the proposed approach. The results enhance understanding of dune vegetation dynamics and offer a framework for coastal restoration strategies
Streamtube Model for Analysis of Vertical Axis Variable Pitch Turbine for Marine Currents Energy Conversion
No full textMarine currents may represent a renewable energy source characterized by a limited environmental impact. In Italy, the Strait of Messina seems to be suited for exploitation of this energy source. A vertical axis turbine, with blades oscillating about the pivotal axis according to the Voith–Schneider system, has been considered. This paper presents a preliminary theoretical investigation of the performance of this kind of turbine that may be employed to tap marine currents energy sources. The investigation is conducted by means of a simple momentum model based on the “single-disk single-streamtube” approach. The theoretical results are compared with experimental measurements. The adequate agreement between experimental and theoretical results shows that such a simple model may be able to predict the power coefficient and the operating range of the turbine
Performance of a mixed gas–steam cycle power plant obtained upgrading an aero-derivative gas turbine
No full textThe performance that can be achieved in a power plant obtained upgrading a typical aero-derivative gas turbine is analysed. The methodology is based on the off-design analysis of the gas generator (compressor and high pressure turbine) in the upgraded plant configuration and is applied to the design of a power plant based on the recuperative water injected cycle. The gas generator operating region and its boundary have been evaluated for the upgraded plant configuration; an optimization procedure has been established in order to show the maximum efficiency and power output that can be achieved
Energy Conversion OWC Devices With Additional Vertical Ducts
No full textIn breakwaters embodying an OWC connected to the sea through a vertical duct or through a small opening, the oscillations of the water column are due to the wave pressure acting on the outer opening of the vertical duct or on the small opening. In fact, in neither of the two cases waves can enter the plant, like it happens in conventional OWCs. The additional vertical duct extends along the wave-beaten wall, giving the device the characteristic form of a U-conduit; for this reason they were also named U-OWC. Experiments on a small-scale breakwater embodying a U-OWC were carried out in the natural laboratory of Reggio Calabria. The plant is a 1: 10 scale model of a breakwater suitable for the North-East Pacific coast. The paper describes new experiments on the U-OWC device connected to a monoplane Wells turbine. During an intensive measurement campaign, more than 260 sea states, 5 min long were recorded in order to characterize the energy conversion process. From the experiments, the analysis of the energy conversion shows that: 1) the system is able to absorb a share up 80 % of the incident wave energy; this result is similar to that obtained in previous experiments carried out without the turbine; 2) a large fraction of the energy entering the U-OWC is converted in pneumatic power acting on the turbine, being head losses in the water flow limited 3) the efficiency of conversion of the pneumatic power in turbine power is relatively low (about 36% of the pneumatic power) due to the small dimensions of the turbine that lead to low Reynolds number and large influence of secondary losses
The hydrodynamic genesis of linear karren patterns
No full textIn karst and alpine areas, the interactions between water and rocks give rise to a large variety of marvellous patterns. In this work, we provide a hydrodynamic model for the formation of dissolutional patterns made of parallel longitudinal channels, commonly referred to as linear karren forms. The model addresses a laminar film of water flowing on a rock that is dissolving. The results show that a transverse instability of the water–rock system leads to a longitudinal channelization responsible for the pattern formation. The instability arises because of a positive feedback within the channels between the higher water flow and the enhanced chemical dissolution. The spatial scales predicted by the linear stability analysis span different orders of magnitude depending on the Reynolds number. This may explain why similar patterns of different sizes are observed on natural rocks. Results also show that the rock solubility affects just the temporal scale of the instability and the rock inclination plays a minor role in the pattern formation. It is eventually discussed how rain is not strictly necessary for the appearance of linear karren patterns, but it may affect some of their features
Aero-Thermodynamic Simulation of a Double-Shaft Industrial Evaporative Gas Turbine
No full textA modeling study has been carried out in order to determine the behavior of evaporative industrial gas turbines power plants at part-load and for varying ambient temperature.
On-design and off-design performance have been analyzed by means of a computational program developed for the analysis of advanced cycles.
In order to verify the mathematical model and to evaluate the characteristics of up-to-date gas turbine technology, an industrial engine, presently available on the market, has been simulated. A double-shaft gas turbine for power generation has been considered. On-design performance and ratings vs. ambient temperature have been evaluated, with good accordance.
It is assumed that, in order to realize a Recuperated Water Injected (RWI) cycle, the industrial gas turbine could be modified, maintaining substantially unchanged the compression system and modifying the turbine blades.
The thermodynamic analysis of the cycle has been carried out in order to determine efficiency and power output as a function of the amount of water addition.
The RWI cycle gas turbine has been designed and the characteristic maps of the two new turbines have been evaluated.
The regulation is performed by means of the simultaneous manipulation of fuel flow rate, water rate, and position of the free turbine nozzle guide vanes (NGV). The regulation criteria, the interaction among the input variables, the safety of the operations (max. turbine inlet temperature, surge limits) and the optimization of the part-load efficiency, are examined and discussed.
Ratings as a function of the ambient temperature are examined. The possibility to manipulate the water rate and the position of the NGV in order to provide high efficiency and power output, even on hot days, has been examined.
The paper shows that maintaining constant the temperature at the power turbine exit, ratings decrease of 17% in power and 5% in efficiency
CFD Simulation of Humid Air Premixed Flame Combustion Chamber for Evaporative Gas Turbine Cycles
No full textEvaporative cycles, such as Recuperated Water lnjected (RWI) cycle, Humid Air Turbine (HAT) cycle, Cascaded Humidified Advanced Turbine (CHAT) offer the attractive possibility to increase plant efficiency without the use of a steam turbine, necessary for gas-steam combined cycles, appearing, therefore, as an interesting solution for industrial power applications such as electric utilities and independent power producers.
It is expected that water addition may contribute to reduce NOx emissions in premixed flame combustors. In order to analyse this solution, a lean-bum combustor, fed with an homogeneous mixture formed by methane and humid air, has been analysed through CFD simulations, in order to predict velocity field, temperatures and emissions. The study has been carried out under the hypothesis of a two-dimensional, axisymmetric combustion chamber assuming, as set of operation conditions, atmospheric pressure, inlet temperature of 650 K, fuel-air equivalence ratio of the methane-air mixture ranging from 0.5 to 0.7 and water-air mass ratio varying from 0% to 5%. In the simulation, the presence of turbulence in the flow has been taken into account using a RNG k-ε model, whilst the chemical behaviour of the system has been described by means of a five-step global reduced mechanism including the oxidation mechanism and the NOx formation mechanism.
The analysis of the results shows that the moisture in the premixed flow reduces both NOx and CO emissions at constant equivalence ratio; moreover the lean blow-out limit is shifted toward higher equivalence ratio. The main effect of the water seems to be the increase of the specific heat the mixture which causes a reduction in flame temperature, slowing the chemical reactions responsible of NOx formation.
The reasonable agreement has been found between the simulation results concerning NOx emissions and recent experimental results carried out on premixed flamed with humid air. A discussion is also provided about the adopted turbulence models and their influence on the emission results
Simplified prediction model of the discharging time of a shell-and-tube LHTES
No full textWe present a simplified theoretical model able to predict the discharging performance of a shell-and-tube latent heat thermal energy storage. The model is validated against two-dimensional axi-symmetric numerical simulations. Here, the heat exchange area, A, the whole PCM volume, V, and the heat exchange wall temperature have been kept constant. According to these constraints, the shell-and-tube shape depends on just one geometrical parameter. Thus, six values of the ratio between the external and internal radius of the PCM module, re/ri, in the range between 2 and 6, are considered. The simplified model matches the discharging time predicted by the numerical simulations. Details of the sensible and the latent heat contribution to the discharging time is provided with respect to the radius ratio. Hence, the latent heat contribution represents about the 70% of the overall discharging time in the range 2⩽re/ri⩽6. The results reveal a scaling law between the Fourier number related to the complete solidification and the radius ratio, Fo∝(re/ri)-5.5, supported by the numerical simulations. Moreover, the rescaled dimensionless time, Fo/(re/ri)-5.5, leads to the self-similar behaviour of the liquid fraction βl time series for all the geometries here investigated. Thus, it represents a promising prediction tool for the design of latent heat thermal energy storage in shell-and-tube configuration
Performance of gas turbine power plants controlled by the multiagent scheme
No full textIn recent years the idea of artificial intelligence has been focused around the concept of rational
agent. An agent is a (software or hardware) entity that can receive signals from the environment and act
upon that environment through output signals, trying to carry out an appropriate task. Seldom agents are
considered as stand-alone systems; on the contrary, their main strength can be found in the interaction
with other agents, constituting the so called multiagent system.
In the present work, a multiagent system was chosen as control system of a single-shaft heavy-duty
gas turbine in Multi Input Multi Output mode. The shaft rotational speed (power frequency) and stack
temperature (related to the overall gas turbine efficiency) represent the controlled variables; on the other
hand, the fuel mass flow (VCE) and the Variable Inlet Guide Vanes (VIGV) have be chosen as manipulating
variables.
The results will show that the multiagent approach to the control problem effectively counteracts the
load reduction (including the load rejection condition) with limited overshoot in the controlled variables
(as other control algorithms do) while showing good level adaptivity readiness, precision, robustness and
stability
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