1,720,986 research outputs found
Influence of boundary conditions in numerical simulation of free surface vortices
No full textA numerical evaluation of gas core length in free surface vortices had been performed, applying different approaches. In the past only partial satisfactory agreement with experimental results was obtained because of an excessive simplification of the computational domain. In this work, two-phase CFD simulations (with Volume Of Fluid method) have been performed using a computational domain without any simplification and the evolution of the vortex formation has been analyzed in term of gas core length. Different turbulence models have been tested. A mesh sensitivity analysis has been performed. The new numerical results obtained with the most advanced turbulence model (SST SAS-CC) show an improved agreement with experimental data, with respect to the previous results, confirming the importance of imposing proper boundary conditions to correctly simulate free surface vortices
Temperature effects on the elastic properties of hysteretic elastic media: Modeling and simulations
No full textDifferent materials with mesoscopic characteristics (e.g., defects or intergrain regions) have been shown to share a peculiar elastic behavior when subject to temperature variations. We present here a simple model based on the description of the mesoscopic features as bistate bond regions that connect elastic portions described by the traditional Landau thermoelastic theory. We apply our model to simulate resonance frequency experiments in order to analyze the effects of temperature changes on the elastic properties of materials and reproduce part of the phenomenology observed concerning the conditioning effect and the recovery of the system to its original state after a thermal shock
Numerical evaluation of gas core length in free surface vortices
No full textThe formation and evolution of free surface vortices represent an important topic in many hydraulic intakes, since strong whirlpools introduce swirl flow at the intake, and could cause entrainment of floating matters and gas. In particular, gas entrainment phenomena are an important safety issue for Sodium cooled Fast Reactors, because the introduction of gas bubbles within the core causes dangerous reactivity fluctuation. In this paper, a numerical evaluation of the gas core length in free surface vortices is presented, according to two different approaches. In the first one, a prediction method, developed by the Japanese researcher Sakai and his team, has been applied. This method is based on the Burgers vortex model, and it is able to estimate the gas core length of a free surface vortex starting from two parameters calculated with single-phase CFD simulations. The two parameters are the circulation and the downward velocity gradient. The other approach consists in performing a two-phase CFD simulation of a free surface vortex, in order to numerically reproduce the gas-liquid interface deformation.. Mapped convergent mesh is used to reduce numerical error and a VOF (Volume Of Fluid) method was selected to track the gas-liquid interface. Two different turbulence models have been tested and analyzed. Experimental measurements of free surface vortices gas core length have been executed, using optical methods, and numerical results have been compared with experimental measurements. The computational domain and the boundary conditions of the CFD simulations were set consistently with the experimental test conditions
Modelling of dust resuspension in Tokamak devices during an air inflow event
No full textIn a Tokamak-fusion reactor, the plasma interacts with the first wall generating dust of different materials. Dust can play an important, if not primary, role in determining safety and economic performance of this type of fusion machines, due to potential accident and maintenance requirements. In this paper, after a brief introduction, a description of dust explosion mechanism is reported with reference to several experiments used to create or validate theories and models. The safety concerns about dust are based on the assumption that, in case of air inflow events, this particulate can be resuspended in the vacuum vessel leading to a potential explosive cloud. In the paper the main literature models used to deal with dust transport and resuspension are reported: the VZFG model, the Rock n’ Roll model and the ECART model. Differences, pros and cons of each models are described to obtain a comparison between energy-based and force-based balance models. Finally, the numerical approach for the resuspension phenomenon is discussed and an application is shown. The simulation has been carried out with reference to a laboratory scale facility-like in 3D geometry. The simulated event is an air flow intake, as from a Loss of Vacuum Accident, and the results show the distribution of dust in the geometry after the first instants of intake, the mass concentration of particulate on the surfaces and the path lines of the flow field. The simulated geometry is only partially referred to an existing experimental facility, and the methodology could be useful to replicate the same conditions to obtain a validation of the results. More and finer simulations are foreseen to reproduce more accurately experimental data and real scenarios, with the aim to evaluate the risk of explosion by means of an accurate prediction of dust distribution inside a vacuum volume during resuspension due to air inflow
Experimental study on unstable free surface vortices and gas entrainment onset conditions
No full textFree surface vortices formation has been, for many years, a relevant issue in many engineering applications with hydraulic intakes, since strong whirlpools introduce swirl flow at the intake causing entrainment of floating matters and gases (air or other gases). The Gas Entrainment (GE) phenomenon has also been an important topic in nuclear industry in the last years, due to the possibility of free vortex formation at the surface, especially in sodium-cooled fast reactors (SFRs). This phenomenon may result in unlikely positive reactivity insertion accident, affecting the safety performances of the reactor itself. The GETS experimental facility (Gas Entrainment Test Section) has been built in the thermal-hydraulics laboratory of the DIAEE in order to study free surface vortices occurrence. The main purpose of this facility is to understand the influence of different parameters on free surface vortices formation and evolution. Experimental tests and preliminary observations, carried out, with cold water as working fluid, different water level in the tank, circulating flow rate and outlet tube diameter, are presented as occurrence maps in this paper. The observed vortices have been qualitatively classified in different “formation stages” and a through a dimensionless analysis the most important parameters that influence the physical phenomenon have been identified. Empirical correlations based on dimensionless parameters of the transition boundaries between stages, including the onset of gas entrainment, have been developed and discussed, and very preliminary considerations on the sodium-water similitude, based on the Froude and Weber numbers, are introduced
Validation of free surface vortex analytical models
No full textDuring the last fifty years, several studies have been performed on free surface vortex formation and evolution, and different analytical models have been developed. In a previous work, three different models were analyzed and discussed to evaluate limits and strong points of each one. In this paper the same three models have been considered and compared with experimental results obtained for different horizontal planes by means of Particles Image Velocimetry technique. Mean velocity fields have been measured and discussed for each test case, highlighting the vortex flow field differences at different heights and flow rates. Experimental results have been compared with three analytical vortex models in terms of tangential velocity and vorticity profiles; two parameters needed by the models - the circulation and the vortex core radius - are derived from the experiments. Further considerations on the circulation distribution in axial and radial directions are included
Comparative CFD simulations of a hydrogen fire scenario
Get PDFHydrogen leakage and fire ignition and propagation are safety concerns in several industrial plants. In a nuclear fusion power plants the separation of hydrogen and tritium takes place in different steps, among which one or more electrolyzers are foreseen. A fire scenario could take place in case of leakage of hydrogen. In such cases, it is important to prevent the spreading of the fire to adjacent rooms and, at the same time, to withstand the pressure load on walls, to avoid radioactivity release in the surrounding environment. A preliminary study has been carried out with the aim of comparing CFD tools for fire scenario simulations involving hydrogen release. Results have been obtained comparing two codes: ANSYS Fluent© and FDS. The two codes have been compared both for hydrogen dispersion and hydrogen fire in a confined environment. The first scenario is aimed to obtaining of volume fraction 3D maps for the evaluation of the different diffusion/transport models. In the second scenario, characterized by a double-ended guillotine break, the fire is supposed to be ignited at the same time of the impact. Simulations have been carried out for the first 60 seconds. Hydrogen concentration, temperature and pressure fields are compared and discussed
Preliminary evaluation of the expansion system size for a pressurized gas loop: Application to a fusion reactor based on a helium-cooled blanket
Get PDFSome considerations to preliminarily design the size of the Expansion Volume (EV) and the relief pipes for a Vacuum Vessel Pressure Suppression System, to be adopted in a fusion reactor based on a helium cooled blanket, are presented. The volume of the EV depends on the total energy of the cooling system and it can be sized based on a required final pressure at equilibrium, by a simple energy balance. Two different EV solutions have been analysed: a “dry” EV and a “wet” EV. In this last, a certain amount of water could be mixed (by spraying or discharging in a pool) with the discharged helium, to reduce its temperature and allowing a lower size of the EV with respect to the “dry” solution. The pressure peak in vacuum vessel (VV) depends mainly on break area and flow area of the relief pipes and a simple formula to be used to size these pipes is suggested. The computer code CONSEN has been used to perform sensitivity analyses and to verify the methodology
Preisach-Mayergoyz approach to fatigue-induced irreversibility
No full textThe Preisach-Mayergoyz (PM) approach has been widely used to describe hysteresis in different fields. Among these, various types of reversible hysteretic elastic behavior have been succesfully modeled, inclusing quasi-static, fast and slow dynamics, modulation, and so on. Here, we propose an approach to extend the PM formulation to also treat irreversible phenomena, such as fatigue and damage progression. For this purpose, we introduce a multilevel scheme based on nested PM descriptions. The mathematical formulation is outlined and a phenomenological application presente
Velocity profiles in bathtub vortices: validation of analytical models
No full textFree surface vortices in water are often observed around a drain outlet in sinks and bathtubs, and different researchers deal with the problem of how the bathtub vortex is generated. In a previous work [1], some analytical free surface vortex models were discussed and analyzed, and their limits were underlined. In the present paper, the vortex models of Burgers, Hite and Chen are considered. Burgers’ model was derived as a result of purely analytical hypotheses, while the other two were developed modifying the Burgers’ vortex model according to experimental observations. In all these models, two parameters - the circulation Γ and the vortex core radius rm - have to be defined. In order to validate these analytical models, experimental measurements have been carried out for different horizontal and vertical planes by means of optical methods (Particle Image Velocimetry - PIV), reproducing free surface vortex formation in the GETS (Gas Entrainment Test Section) experimental facility. The two main parameters to be used in the models have been derived from experimental measures. Velocity profiles and vorticity distribution, calculated from the analytical models, have been compared with experimental velocity and vorticity fields. As a result, it is showed that Hite’s model seems to be the most accurate model compared to experimental data
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