25 research outputs found
A unified coordinate system for solving the three-dimensional Euler equations
Two general coordinate systems have been used extensively in computational fluid dynamics: the Eulerian and the Lagrangian. The Eulerian coordinates cause excessive numerical diffusion across flow discontinuities, slip lines in particular. The Lagrangian coordinates, on the other hand, can resolve slip lines sharply but cause severe grid deformation, resulting in large errors and even breakdown of the computation. Recently, Hui et al. (J. Comput. Phys. 153,596 (1999)) have introduced a unified coordinate system which moves with velocity hq, q being velocity of the fluid particle. It includes the Eulerian system as a special case when h = 0 and the Lagrangian when h = I and was shown to be superior to both Eulerian and Lagrangian systems for the two-dimensional Euler equations of gas dynamics when It is chosen to preserve the grid angles. The main purpose of this paper is to extend the work of Hui et al. to the three-dimensional Euler equations. In this case, the free function h is chosen so as to preserve grid skewness. This results in a coordinate system which avoids the excessive numerical diffusion across slip lines in the Eulerian coordinates and avoids severe grid deformation in the Lagrangian coordinates; yet it retains sharp resolution of slip lines, especially for steady flow. (C) 2001 Academic Press
Shallow water flow computation using the unified coordinates
Two general coordinate systems have been used extensively in computational fluid dynamics: the Eulerian and the Lagrangian. The Eulerian coordinates cause excessive numerical diffusion across flow discontinuities, slip lines in particular. The Lagrangian coordinates, on the other hand, can resolve slip lines sharply but cause severe grid deformation, resulting in large errors and even breakdown of the computation. Recently, Hui et al. has introduced a unified coordinate system which moves with velocity h=0 being the velocity of the fluid particle. It includes the Eulerian system as a special case when h=0, and the Lagrangian when h=1, and is shown for the multi dimensional Euler equations of gas dynamics to be superior to both the Eulerian and the Lagrangian systems. The main purpose of this paper is to adopt this unified coordinate system to solve the shallow water equations. It will be shown that computational results using the unified system are superior to existing results based on either the Eulerian system or the Lagrangian system in that it (a) resolves slip lines sharply, especially for steady flow, (b) avoids grid deformation and computation breakdown in Lagrangian coordinates
A new formulation of a spray dispersion model for particle/droplet-laden flows subjected to shock waves
International audienceA new analytical model is derived based on physical concepts and conservation laws, in order to evaluate the post-shock gas velocity, the gas density and the spray dispersion topology during the interaction of a shock wave and a water spray in a one-dimensional configuration. The model is validated against numerical simulations over a wide range of incident Mach numbers and particle volume fractions . Two regimes of shock reflection have been identified depending on , where the reflected pressure expansion propagates either opposite to the incident shock-wave direction for weak incident Mach numbers or along with it for strong Mach numbers. The numerical simulations reveal the presence of a particle number-density peak for and with particle diameters of the order of . The formation of the number-density peak is discussed and a necessary condition for its existence is proposed for the first time
Combustion modeling in large scale volumes using EUROPLEXUS code
International audienceMost of the numerical benchmarks on combustion in large scale volumes for hydrogen safety, which were performed up until today have demonstrated, that current numerical codes and physical models experience poor predictive capabilities at the industrial scale, both due to under-resolution and deficiencies in combustion modeling. This paper describes a validation of the EUROPLEXUS code against several large scale experimental data sets in order to improve its hydrogen combustion modeling capabilities in industrial settings (e.g. reactor buildings). The code is based on the Euler equations and employs an algorithm for the propagation of reactive interfaces, RDEM, which includes a combustion wave, as an integrable part of the Reactive Riemann problem, propagating with a fundamental flame speed (being a function of initial mixture properties as well as gas dynamics parameters). Validation of the first combustion model implemented in the code is based on obstacle-laden channels, interconnected reactor-type compartments, vented enclosures and covers all major premixed flame combustion regimes (slow, fast and detonation) with an aim to obtain conservative results. An improvement of this model is found in a direction of transient interaction of flame fronts with regions of elevated integral length scales presented in the velocity gradient field due to e.g. interactions with geometrical non-uniformities and pressure waves
Industrial-Scale Hydrogen Deflagration Simulations Using EUROPLEXUS code
International audienceDuring certain severe accident scenarios in a Nuclear Power Plant, hydrogen gas is released into the reactor building. In the case of ignition, various combustion regimes are possible depending on the local concentrations of hydrogen and steam, as well as pressure and temperature distributions. These regimes may include jet fires, slow or fast deflagrations and detonations. Therefore, in order to improve hydrogen risk management strategies one has to find means to estimate the severity of combustion processes involved in various regimes of flame propagation.In recent years, the use of CFD codes for combustion modelling at industrial scale becomes widely accepted. Due to difference in scales involved in flame propagation modelling in large geometries,validation of a CFD code becomes a necessary preliminary step in order to assure a high level of confidence. This paper describes a validation of the EUROPLEXUS code against several large and medium scale experimental data as well as a methodology for the code application for hydrogen deflagration simulation under typical containment conditions for nuclear safety
IR absorption measurements of the velocity of a premixedhydrogen/air flame propagating in a obstacle-laden tube
International audienceFlame acceleration and explosion of hydrogen/air mixtures remain key problems for severe accident managementin nuclear power plants. Empirical criteria have been developed in the early 2000s by Dorofeev and colleagues [1]providing effective tool to discern possible FA or DDT scenarios. A huge experimental database, composed mainlyby middle-scale experiments in smooth tubes and obstacle-laden ducts, has been used to validate these criteria. Inthese devices the position of the reaction front is usually detected by photo-diodes or photomultiplier tubes uniformlydistributed along the tube axis. As a result, only a coarse representation of the velocity profile can be achieved.In this paper we develop a new technique to track the flame position along the tube at any time. This method consistsin performing time-resolved IR absorption measurements by doping the fresh mixture with an alkane. The velocityprofile is then derived by measuring the variation of the extension in depth of the unburnt gas along the tube axis.Correction factors are eventually drawn from the comparison between longitudinal (IR absorption measurements) andcross-sectional (photomultiplier tubes) flame velocity diagnosis techniques. Finally, experimental results are comparedto numerical simulations [2] and analytical models proposed in the literature [3]
influence of initial pressure on hydrogen/air flameacceleration during severe accident in npp
International audienceFlame acceleration (FA) and explosion of hydrogen/air mixtures remain key issues for severe accidentmanagement in nuclear power plants. Empirical criteria were developed in the early 2000s byDorofeev and colleagues, providing effective tools to discern possible FA or DDT (Deflagration-to-Detonation Transition) scenarios. A large experimental database, composed mainly of middle-scaleexperiments in obstacle-laden ducts at atmospheric pressure condition, has been used to validate thesecriteria. However, during a severe accident, the high release rate of steam and non-condensable gasesinto the containment can result in pressure increase up to 5 bar abs. In the present work, the influenceof the unburnt gas initial pressure on flame propagation mechanisms was experimentally investigated.Premixed hydrogen/air mixtures with2 H et61539; close to 11percent were considered. From the literature we knowthat these flames are supposed to accelerate up to Chapman-Jouguet deflagration velocity in longobstacle-laden tubes at initial atmospheric conditions. Varying the pressure in the fresh gas in therange 0.6-4 bar, no relevant effects on the flame acceleration phase were observed. However, as theinitial pressure was increased, we observed a decrease in the flame velocity close to the end of thetube. The pressure increase due to the combustion reaction was found to be proportional to 0 p
Modeling of pressure loads during a premixed hydrogen combustion in the presence of water spray
International audienceThis paper describes the development of a simplified model for pressure evolution inside a closed volume during a combustion process in presence of a water spray. The model is based on empirical correlations available in the literature. These ingredients allow us to estimate the values for the main factors influencing the pressure evolution. The results of this model are used as a guideline for adjusting the parameters ofa three-dimensional hydrodynamic code based on CREBCOM combustion model,developed and validated for large-scale hydrogen combustion. The methodology is successfully assessed by comparing the computed results with the experimental data
Influence of initial pressure on hydrogen/air flame acceleration during severe accident in NPP
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Benchmark exercises related to the safety analysis of hydrogen / natural gas mixture transportation in pipelines
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