Fourier-Hermite decomposition of the collisional Vlasov-Maxwell system: implications for the velocity space cascade

Turbulence at kinetic scales is an unresolved and ubiquitous phenomenon that characterizes both space and laboratory plasmas. Recently, new theories, {\it in-situ} spacecraft observations and numerical simulations suggest a novel scenario for turbulence, characterized by a so-called phase space cascade -- the formation of fine structures, both in physical and velocity space. This new concept is here extended by directly taking into account the role of inter-particle collisions, modeled through the nonlinear Landau operator or the simplified Dougherty operator. The characteristic times, associated with inter-particle correlations, are derived in the above cases. The implications of introducing collisions on the phase space cascade are finally discussed.Comment: Special issue featuring the invited talks from the International Congress on Plasma Physics (ICPP) in Vancouver, Canada 4-8 June 201

Numerical Investigation of a Darrieus Rotor for Low-head Hydropower Generation

n/

A Gas-Steam Combined Cycle Powered by Syngas Derived from Biomass

n/

CFD analysis of the energy conversion process in a fixed oscillating water column (OWC) device with a Wells turbine

Abstract Oscillating Water Column (OWC) devices, both the fixed structures and the floating ones, are an important class of Wave Energy Converter (WEC) devices. In this work, we carried out a numerical investigation aiming to give a deep insight into the fluid dynamic interaction between waves and a U-shaped OWC breakwater, focusing on the energy conversion process. The U-OWC breakwater under consideration, represents the full-scale plant installed in the Civitavecchia (near Rome) harbour. The adopted numerical method is based on the solution of the unsteady Reynolds Averaged Navier-Stokes equations (URANS). The water-air interaction is taken into account by means of the Volume Of Fluid (VOF) model. A two-dimensional domain has been adopted to investigate the unsteady flow outside and inside the OWC device. In order to simulate the action of an air turbine of the Wells type, the air chamber has been connected to the atmosphere by means of a porous medium able to reproduce its linear relationship between pressure drop and flow rate of the air turbine. Several simulations have been carried out considering periodic waves of different amplitudes in order to analyze the performance of the plant and, in particular to analyze the resonance with incoming waves, when the U-OWC is expected to absorb more energy. In order to characterize the plant efficiency, we split the energy conversion process into three main steps, 1) the primary conversion from wave energy to hydraulic energy the water discharge flowing inside the U-duct; 2) the secondary conversion from the OWC inlet to the oscillating pneumatic power made available to the turbine and, finally, 3) the turbine mechanical power output. To this purpose, the simulations of three different cases, varying wave period and height, have been carried out to quantify the energy captured by the plant and the fluid dynamic losses both in the water and in the air

Performance optimization of a gas-steam combined power plant partially fed with syngas derived from pomace

Abstract In this paper a gas-steam combined-cycle, partially fueled by syngas (produced in an embedded downdraft gasifier fed with pomace), is considered. In addition, an auxiliary combustion system is directly fed by ligno-cellulosic biomass. The thermodynamic model of the entire system is developed by means of the Cycle-Tempo software. The gasification process is supposed to occur at ambient pressure and air is used as gasifying agent. An optimization process has been introduced by means of the Design of Experiment ( DoE ) technique. The design variables and their corresponding ranges have been chosen by using a heuristic criterion. The power plant performance is represented by the thermal efficiency, _ η I , the exergetic efficiency, η II , the cost of electricity, COE, and the net return, R net . The DoE technique provided the so-called Pareto barrier, which isolates all the non-dominated solutions

Thermo-economic Assessment of Small Scale Biomass CHP: Steam Turbines vs ORC in Different Energy Demand Segments☆

AbstractThe energy performance and profitability of CHP plants, and the selection of the optimal conversion technology and size, are highly influenced by the typology of energy demand (load-duration curve, temperature of heat demand, heat and electricity load patterns). In the small scale range, where CHP can be particularly promising to match local heat and power demand, the technologies based on boilers coupled to steam turbines (ST) and bottoming Organic Rankine Cycle (ORC) can be operated in flexible mode to match the energy demand. This is particularly important when high temperature heat is required (i.e. industrial end users). In the case of solid biomass fired CHP, the boiler + ST/ORC option could be competitive with the alternatives of boiler + Stirling engine, externally fired GT or gasification + ICE. In this paper, a thermo-economic comparison of the following biomass-CHP configurations is proposed: (A) boiler + ST + bottoming ORC, (B) boiler + ST, (C) boiler + ORC and (D) configuration (A) with option to switch on or off the bottoming ORC on the basis of the heat demand available. The focus is on a 1 MWt biomass boiler, and the plants are operated to serve residential (r), tertiary (t) and industrial (i) heat and power demand. The thermodynamic cycles are modeled by Cycle-Tempo, while the energy demand is modeled through simplified indicators (temperature of heat demand, equivalent thermal demand hours). On the basis of the results of thermodynamic simulations, upfront and operational costs assessment, and Italian energy policy scenario (feed-in tariffs for biomass electricity), the global energy conversion efficiency and investment profitability is estimated, for each CHP configuration and energy demand segment. The results indicate the optimal CHP configuration for each end user and the key technical and economic factors in the Italian legislative framework

Performance prediction model of multistage centrifugal Pumps used as Turbines with Two-Phase Flow

Abstract Pump as Turbines (PaTs) can be used not only in hydraulic power generation but also in chemical processes, such as refinery, where fluids containing dissolved or undissolved gases or volatiles can be expanded from a higher to a lower pressure level for energy recovery. As the gas contained in the fluid is released from the solution during expansion, the flow rate increases and additional energy is delivered with respect to the case of incompressible flow. This higher power output is very attractive. In this work, a theoretical approach is proposed in order to predict the PaT performance with a two-phase flow whose expansion characteristics are known

Numerical prediction of the natural frequency of an Oscillating Water Column operating under resonant conditions

Among the different technologies developed in order to harness wave energy, the Oscillating Water Column devices are the most accredited for an actual diffusion. Recently, Boccotti has patented the REWEC1 (REsonant sea Wave Energy Converter solution 1), a submerged breakwater that performs an active coast protection, embedding an Oscillating Water Column device, which is capable of operating under resonant conditions with that sea state, which gives the highest yearly energy contribution. The REWEC1 dynamic behavior can be approximated by means of a mass-spring-damper system. According to this approximation, a criterion for evaluating the oscillating natural frequency of the REWEC1 has been derived. This criterion has been validated against both experimental results and computational fluid dynamics simulations, performed on a REWEC1 laboratory-scale model. The numerical simulations have shown a good agreement between measurements and predictions

Numerical Simulations of the flow field and chemical reactions of the Storage/Oxidation process within a NSC Pt - BaO Catalyst

A NOxStorage Catalyst (NSC) has been studied by means of reactive CFD simulations. In the scenario of automotive pollutant emission reduction, due to the stringent regulation, the detailed description of the chemical and physical phenomena within catalysts represents a key point in order to improve their conversion efficiency. The active part of the catalyst has been simulated as a porous medium. In this zone, surface reactions take place, which are modelled by means of an Arrhenius chemical kinetic approach, involving the Pt and BaO sites on the active surface of the matrix. Actually, two chemical mechanisms are considered, the simplest involves only BaO site, the other one includes both BaO and Ptsites. Both models are validated against data available in the literature and then applied to a real automotive catalyst geometry. Thus, a detailed description of the spatial distribution of the species is provided for both models. Lean condition is simulated in order to check the catalyst behaviour according to experimental results

Flame Describing Function analysis of spinning and standing modes in an annular combustor and comparison with experiments

This article reports a numerical analysis of combustion instabilities coupled by a spinning mode or a standing mode in an annular combustor. The method combines an iterative algorithm involving a Helmholtz solver with the Flame Describing Function (FDF) framework. This is applied to azimuthal acoustic coupling with combustion dynamics and is used to perform a weakly nonlinear stability analysis yielding the system response trajectory in the frequency-growth rate plane until a limit cycle condition is reached. Two scenarios for mode type selection are tentatively proposed. The first is based on an analysis of the frequency growth rate trajectories of the system for different initial solutions. The second consid- ers the stability of the solutions at limit cycle. It is concluded that a criterion combining the stability analysis at the limit cycle with the trajectory analysis might best define the mode type at the limit cy- cle. Simulations are compared with experiments carried out on the MICCA test facility equipped with 16 matrix burners. Each burner response is represented by means of a global FDF and it is considered that the spacing between burners is such that coupling with the mode takes place without mutual interac- tions between adjacent burning regions. Depending on the nature of the mode being considered, two hypotheses are made for the FDFs of the burners. When instabilities are coupled by a spinning mode, each burner features the same velocity fluctuation level implying that the complex FDF values are the same for all burners. In case of a standing mode, the sixteen burners feature different velocity fluctua- tion amplitudes depending on their relative position with respect to the pressure nodal line. Simulations retrieve the spinning or standing nature of the self-sustained mode that were identified in the exper- iments both in the plenum and in the combustion chamber. The frequency and amplitude of velocity fluctuations predicted at limit cycle are used to reconstruct time resolved pressure fluctuations in the plenum and chamber and heat release rate fluctuations at two locations. For the pressure fluctuations, the analysis provides a suitable estimate of the limit cycle oscillation and suitably retrieves experimental data recorded in the MICCA setup and in particular reflects the difference in amplitude levels observed in these two cavities. Differences in measured and predicted amplitudes appear for the heat release rate fluctuations. Their amplitude is found to be directly linked to the rapid change in the FDF gain as the velocity fluctuation level reaches large amplitudes corresponding to the limit cycle, underlying the need of FDF information at high modulation amplitudes