68 research outputs found
Development and experimental validation of a novel CFD approach for the simulation of high-pressure accidental gas releases
No full textL'abstract è presente nell'allegato / the abstract is in the attachmen
Hydrogen leakages in a congested aircraft environment: a CFD simulation method
No full textThe option of using hydrogen as a fuel for propulsion of aircraft has been investigated in the recent past especially in combination with long endurance unmanned mission targets (Helinet, Helios). This application has proved to be challenging mostly due to the low volumetric density of hydrogen, which needs to be compressed at very high pressures to be confined in the narrow volumes allowed by aircrafts structures. Aerial, as well as automotive, applications of hydrogen pose also the issue of weight for the total storage solution adopted: high pressures may mean thick layers for vessels and consequently high weights for unit of mass of hydrogen stored. Composite materials have helped in reducing the weight but remain tough to be adopted for vessels large enough to store the hydrogen mass necessary for long trips. Liquid hydrogen has only been adopted so far for aerospace applications and just for boosting rather than for endurance. Instead, hydrogen can be efficiently used for fuelling auxiliary systems on board and for ground services, helping to reduce the environmental impact, also regarding the idling phase. Fuel cells that are supplied with hydrogen can provide the electricity needed by all the auxiliary equipment, from air conditioning systems, to controls and avionics, to lighting and security services. Although smaller quantities of hydrogen are needed on board to supply only the auxiliaries rather than for propulsion, still there is a need for pressurising the gas and so to have a pressurised feed line that runs into the congested environment of an aircraft where ventilation is anyway usually present. In view of the experience gained in the oil and gas offshore sector, where flammable and pressurised gases may be released due to failures in the feed lines, we propose an innovative approach to investigate the possible hazards deriving from the use of pressurised hydrogen in aeronautics. Hydrogen releases may happen due to failures all along the lines but, statistically, ruptures are more frequent in lower pressures sections that are potentially less protected. A hydrogen supply line can cover pressures that range from 350 bars of the storage vessel to the nearly ambient pressure when it supplies the fuel cells. This induces to take greater care of possible mid-pressure (10-15 bars) releases and of their consequences. Ruptures are seldom catastrophic, while more often they are represented by small diameter cracks. The release through these ruptures is supersonic and it soon slows down also due to the scattering with obstacles in the aircraft environment. Modelling of the entire phenomenon is a challenging task for Computational-Fluid-Dynamic analysts as some variables such as pressure, and therefore velocity, have too strong variations throughout the domain. Yet, CFD remains the best tool to predict the dispersion and possibly the dangerous (i.e. above the - very low- flammability limit of hydrogen) accumulation of gas. Our proposal is to split the phenomenon in two phases and to study them separately with a coupling based on the parameters that are more relevant to describe the evolution: velocity and concentration. First, the supersonic release of hydrogen from the rupture, and the consequent compressible effects, are modelled in a domain that is large enough to contain the deceleration of the gas up to dispersion-like rates: this domain is, however, smaller than the full domain where we wish to study the entire phenomenon. Second, data related to speed and concentration calculated on the surface of this domain are given as boundary conditions for the simulation of the dispersion phase. Preliminary applications of this method to hydrogen releases from 10 mm hole ruptures at a pressure of 10 bar have provided interesting results especially with the supersonic release phase, that is the most challenging for the CFD simulation due to the intrinsic characteristics of hydrogen as a very light gas. In particular, addressing the supersonic release phase may allow estimating the effect of impact of the jet release onto the first obstacle (in the form of thermal stresses). The final coupling of the two phases can provide a dispersion pattern within a congested environment that can be validated in field tests, in the same way as it is being done with this method applied to natural gas releases in offshore platforms
CFD modelling of an accidental pressurised gas release
No full textRisk assessment usually requires to simulate hundreds of different accidental scenarios in order to identify the most potentially critical events. The objective of this work is to improve and optimize the use of a Two Steps CFD model in order to minimise the number of simulations needed this is achieved thanks to a sensitivity analysis on the main parameters characterising a release event in a typical congested industrial environment
A novel approach to high-pressure gas releases simulations
No full textRisk-relevant plants like Oil & Gas (O&G) or nuclear ones are subjected to strict safety
regulations. Risk Assessment is mandatory for these plants, and the damage quantification is a
crucial step which has to be carefully addressed.
Nowadays the state of practice for consequences estimation entails the use of semi-empirical
methods which permit a fast evaluation of the large number of accidental scenarios needed for
a Quantitative Risk Assessment (QRA).
However, in case of large and congested industrial environments like offshore platforms or
equipment inside nuclear primary containment buildings, the aforementioned methods usually
lead to an overestimation of the damage areas, for example because they neglect the space
congestion which highly affects the accident evolution. A more accurate analysis can be
performed using the Computational Fluid Dynamics (CFD), nonetheless its high
computational cost represents an important drawback.
In this work a CFD approach, called Source Box Accident Model (SBAM), is presented. It
models high-pressure gas releases (> 10 bar) in congested environments guaranteeing a good
computational cost-accuracy compromise. The aim of SBAM is to permit a fast consequences
estimation (evaluation of flammable/toxic areas, etc.) via CFD, in order to have a simulations
time compatible with the plants design schedule. The long-term objective is to integrate the
CFD contribution in a safety driven design process.
SBAM is based on the splitting of the multi-physics and multiscale phenomena characterizing
the accident: the initial supersonic compressible release and the successive low speed
dispersion. The first one is simulated in a small domain called Source Box (SB) and the
second one in the case study domain.
The two simulations are coupled in a suitable way, through proper parameters which are
extensively discussed. This work presents a detailed description of SBAM and two different
analyses: a sensitivity study on the coupling parameters and a numerical benchmark which
uses a standard CFD simulation as reference. The sensitivity analysis shows that the coupling
is a crucial step of the method and the coupling parameters must be treated in the most
accurate way. The numerical benchmark shows that SBAM is not introducing significant
errors with respect to a standard CFD simulation and in addition, permits a relevant
simplification in the simulation setup and computational cost reduction
Chemical composition and yield of rhizome biomass of Arundo donax L. grown for biorefinery in the Mediterranean environment
No full textThe contribution of the rhizome to productivity of fermentable sugars and the detailed composition of rhizomes
were analyzed in three mature stands of Arundo donax L. cultivated in three locations of variable fertility in the
South of Italy. Although the average yearly aboveground dry biomass and rhizome amount showed large and
significant differences among sites, (15.3 and 2.6 Mg ha−1 year−1 of rhizomes in the most and less productive
sites respectively), rhizomes of all sites had more than 30% of the dry matter (DM) as non-structural carbohydrates
(NSC). Sucrose and starch were the most abundant NSC but measurable amounts of glucose, fructose,
galactose and of the valuable trisaccharide raffinose were also present. The amount of NSC in rhizomes affected
their content of dry mater, and water extractives. The ash content also varied significantly among cultivation
sites; the highest amount was recorded in rhizomes of the most productive site (Acerra). The abundance in cell
wall components of rhizomes was similar to that of published values for the above ground biomass. The present
results demonstrate that NSC content in rhizomes of mature stands is a conserved trait. Hence, rhizome biomass,
thanks to its quantity and high fermentable sugars content, should be considered as a relevant fraction of the A.
donax crop product whose utilization can increase the productivity and the environmental fingerprint of this
crop, in view of its biomass utilization in biorefinery
CFD Modelling of Pressurized Gas Releases: Sensitivity Analysis of Driving Parameters
No full textThe consequences analysis is a crucial aspect of the Risk Assessment, especially for Oil & Gas structures, where
hundreds of accidental scenarios must be simulated. This work investigates the accidental release of high-pressure
flammable gas in a congested offshore environment considering multi-physics and multi-scale nature of the
phenomenon. Initially, the flow results supersonic with compressible effects and then it evolves in a subsonic
dispersion. To handle this change of physics, a Computational Fluid Dynamics (CFD) two steps approach is
developed at the SEADOG laboratory in Politecnico di Torino. This approach imposes two simulations: the first
one considers the compressible phenomena in a small domain called Source Box (SB), the second one considers the
gas dispersion in the platform. The advantage is to use the results of the first simulation as an input for several
dispersion simulations. The aim is to compile a library of plausible SB and to evaluate the consequences of an
accidental scenario selecting the proper SB for the dispersion simulation, allowing a timesaving. This work is
focused on the optimization of the number of SB to construct a SB library. The objective of this work is to achieve
a sensitivity analysis on the input parameters (release pressure, hole diameter, distance and dimension of an obstacle
inside the SB) in order to optimize the number of SB to be simulated reducing the computational effort
Phosphoenolpyruvate carboxykinase and gluconeogenesis in grape pericarp
No full textGlycolysis from sugars is necessary at all stages of development of grape pericarp, and this raises the question as to why gluconeogenesis from malate occurs. Phosphoenolpyruvate carboxykinase (PEPCK) is required for gluconeogenesis in grape pericarp. In this study we determined the abundance of PEPCK protein and activity in different parts of grape pericarp during its development. Both PEPCK protein and activity were present throughout development, however, in both the skin and the flesh their abundance increased greatly at the start of ripening. This coincided with the onset of the decrease in the malate content of the berry. The location of PEPCK in the pericarp at different stages of development was determined using both immunohistochemistry and dissection. We provide a possible explanation for the occurrence of gluconeogenesis in grape pericarp
The organic acids that are accumulated in the flesh of fruits: occurrence, metabolism and factors affecting their contents - a review
No full textOrganic acids are abundant constituents of ripe fruits and are responsible for their sourness. In addition, they contribute to their flavour. In many fruits, the most abundant organic acids are malic and citric. The aims of this review are two-fold. The first is to provide a clear overview of malic and citric acids in the flesh of fruits. The abundance of different organic acids in commercially grown fruits is described. How this abundance changes during fruit development is outlined. The metabolic pathways used in the synthesis and dissimilation of malic and citric acids in fruits are described. The functions of malic and citric acids in the flesh of fruits are discussed. Secondly, how environmental and cultural practices can alter the organic acid content of fruits is considered
Ottimizzazione del trattamento con CPPU in pieno campo e valutazione degli effetti sull'equilibrio vegeto-riproduttivo della pianta e su alcuni aspetti anatomici e metabolici dei frutti in Actinidia deliciosa
No full textRocket seedling production on the International Space Station: Growth and nutritional properties
No full text- …
