RESTRICTED AREVA CFD for atmospheric dispersion prediction in - - PowerPoint PPT Presentation

STAR Global conference AREVA - dispersion – March 17-19th 2014 - p.1

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Engineering & Projects Organization

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• B. Farges, P. Brocheny, C. Thelier, N. Goreaud

AREVA NP

CFD for atmospheric dispersion prediction in close range

STAR Global conference AREVA - dispersion – March 17-19th 2014 - p.3

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Outline of the presentation

• 1. Context
• 2. Methodology setup and validation
• 2a. Hydraulics
• 2b. Dispersion
• 3. Industrial application
• 4. Conclusion and outlook

STAR Global conference AREVA - dispersion – March 17-19th 2014 - p.4

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• 1. Context (1/2)

Some accidental or incidental scenarios can lead an industrial site to release gases or particules in the atmosphere Depending on the concentration, of the weather conditions and of the quantity, it will form a plume of a given size / concentration Utilities must demonstrate that these scenarios are properly handled and that the consequences are acceptable

For the public outside the facility fences => long range More and more : for the workers on site => close range

Some tools exist on the market dedicated to these issues, but

No or very limited capacity for detailed representation of a given

industrial site (buildings, complex obstacles…)

STAR Global conference AREVA - dispersion – March 17-19th 2014 - p.5

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• 1. Context (2/2)

AREVA has a long history in using CFD

Variety of problems Trained engineers Computation power available Validation file on numerous

applications

CFD used as a support to safety

analyses

For this reason : choice to use CFD for the modelling of near- field atmospheric dispersion

Detailed representation of site

geometry (buildings…)

Recirculations, turbulence,

buoyancy,… taken into account

STAR Global conference AREVA - dispersion – March 17-19th 2014 - p.6

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• 2. Methodology setup

Approach

Step by step : growing complexity

• a. Hydraulics only :
• On an empty domain
• With a single building
• Benchmark against litterature
• b. Addition of dispersion :
• On an empty domain first
• Validation against the Prairie Grass experiment
• c. Addition of reacting flows (not part of presentation today) :
• Possibilities to account for chemical reactions / power decay during dispersion

Illustration on the sizing of protection on an AREVA facility

STAR Global conference AREVA - dispersion – March 17-19th 2014 - p.7

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2.a. Methodology setup

Hydraulics on empty domain

Assumptions

Surface boundary layer Perfect gas Stationnary wind profile : Assumtion that meteo variation are slow compared to

dispersion transient

Computation assumptions

In accordance with best practices

Identified through a litterature survey

Large domain dimensions in order to

prevent side effects.

Turbulence model : standard

(was found to be the best compromise)

Dispersion in neutral conditions (classical

choice : Richards & Hoxey, Hargreaves, Vendel)

B.Blocken et al, 2007

ε − k

Equations of velocity , temperature and turbulent quantities profile relative to a neutral atmosphere prescribe at inlet.

STAR Global conference AREVA - dispersion – March 17-19th 2014 - p.8

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Profiles prescribed at domain inlet (from similitude theory from Monin-

Obukhov) Acceptable situation on empty domain

2.a. Methodology setup

Hydraulics on empty domain

Turbulent kinetic energy Turbulent dissipation Velocity Temperature

Atmosphéric profiles

) ( *

3

z z u + = κ ε

µ

C u k ² * =

) ln( * z z z u u + = κ

( )

z T z T ⋅ − = 0098 , Profiles maintained Local anomaly typical for standard CFD codes

STAR Global conference AREVA - dispersion – March 17-19th 2014 - p.9

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Behaviour of flow around an obstacle (square building)

Presence of turbulent structures Classical k-epsilon limitations (consistent with litterature) :

Results are consistent with the state of the art and judged acceptable Slight surestimation of turbulent kinetic energy upstream Slight underestimation of recirculation zone Slight surestimation of the recirculation zone downstream

2.a. Methodology setup

Hydraulics with a single building

10 m

STAR Global conference AREVA - dispersion – March 17-19th 2014 - p.10

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Reference experiment : Projet Prairie Grass (1956)

SO2 release in an open plain 68 tests about 10 of them in neutral conditions

• Three configurations chosen to validate the methodology

Two means to calibrate the results :

Calibration of a parameter : Sct => fast, provides order of magnitudes at the plume

centerline

Statistical correction => more complex, more reliable

2.b. Methodology setup

Dispersion

Vervecken et al. 2013

STAR Global conference AREVA - dispersion – March 17-19th 2014 - p.11

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Simulation of the Prairie Grass experiment

Calibrating Sct enables to get the right concentration in the axis of the

plume

But the shape of the plume is incorrect : overestimation in the axis of the plume and underestimation of its width. (Known phenomenon : Vervecken, Riddle ) Acceptable for an order of magnitude determination.

2.b. Methodology setup

Dispersion

STAR Global conference AREVA - dispersion – March 17-19th 2014 - p.12

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Simulation of the Prairie Grass experiment

Accounting of wind oscillations (litterature : Vervecken)

• Statistical fluctuations around the average wind direction : a statistical fluctuation

with standard deviation σα has been measured :

• Turbulence modelling already accounts for some deviation σm definition of an

additive deviation σe to reach σα

• Performance of several computations for several angles
• Gaussian ponderation of the results

2.b. Mise en place de la modélisation

Dispersion

Fluctuations de la direction du vent

(essai n°57) 0,02 0,04 0,06 58 90 122

Angle en degré Densité de probabilité

STAR Global conference AREVA - dispersion – March 17-19th 2014 - p.13

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Simulation of Prairie Grass experiment

Results with second method

• Shape of the plume and concentration of the plume are better predicted
• Drawback : several computations are necessary

Correction statistique sur les capteurs à 50 m

100 200 300 400 500 600 60 70 80 90 100 110 120 Angle en degré concentration de SO2 en mg/m3

Mesures test 57 Simulation 57 - Sct0,7 Simulation 57 - Sct 0,7 - correction statistique

2.b. Mise en place de la modélisation

Dispersion

STAR Global conference AREVA - dispersion – March 17-19th 2014 - p.14

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• 3. Industrial application

Milling facility Industrial problem

Ammonia stored on site under

pressure

If a leakage happens, need to ensure

acceptable doses are not overcome

First step

CAD repair inside STAR-CCM+ from

site data

Large domain generation Meshing

• Less than 5 million cells for the total

volume

• Enabling a fast and smooth transient run

STAR Global conference AREVA - dispersion – March 17-19th 2014 - p.15

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• 3. Industrial application

Results

Instantaneous velocity profiles on

site for a given wind direction

• First indication of zones of potential

hazard

Instantaneous concentration profiles

everywhere on the plant

• Information that enables to quantify the

dilution of the release and the size of the plume

Derived into the quantification of the

zones where reglementary threshold can be overcome

• Used for demonstration to regulators and

design optimisation

STAR Global conference AREVA - dispersion – March 17-19th 2014 - p.16

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• 4. Conclusion and outlook

AREVA has developped a method to assess the consequences of atmospheric dispersion in close range

Based on its expertise on CFD For its own purpose

This method is operational and proved to provide useful results on an industrial case Beyond safety demonstration, this method has a lot of potential :

Site layout verification / optimisation Strong wind sizing Wind power generation …