Modelling of H2 Dispersion and Combustion Phenomena Using CFD Codes

Paper 100071 – Paillère et al.International Conference on Hydrogen Safety,ICHS, Pisa, September 8-10, 2005

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Modelling of H2 Dispersion and Combustion Phenomena Using CFD

Codes

H. Paillère, E. Studer, A. Beccantini, S. Kudriakov, F. Dabbene and C. Perret*

CEA Saclay – *CEA Grenoble


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Outline

• Introduction• Description of phenomena related to H2 risk issues• CFD modelling at CEA• Dispersion / Distribution model validation• Combustion model validation• Mitigation model validation• Outline of a validation matrix ?• Necessity for new experiments in support of code

validation• On-going activities and conclusions


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Introduction (1/2)

• CEA has more than 10 years experience in the field of hydrogen safety, mainly in the field of nuclear safety (e.g. TMI accident in USA, 1979, 400kg H2 burn inside containment)

• Since 2001, CEA is actively engaged in the development of new energy technologies including hydrogen & fuel cell systems (over 250 engineers and technicians)

• Safety has been recognized as an important issue to ensure the success of these technologies

• CEA is a member of the HYSAFE Network of Excellence

H2+1/2O2


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Introduction (2/2)• In many industrial fields, safety demonstrations

have relied mainly on expertise, supported by experiments.

• Increasingly, numerical tools, in particular, Computational Fluid Dynamics (CFD) codes, are being used for safety assessment, e.g. to predict explosive cloud formations or to assess consequences of explosions or flames

• Requirements on use of CFD results for safety assessment:– Code validation on representative experimental data (need to identify relevant phenomena & associated test data)– Expertise of code users– Application / knowledge of Best Practice Guidelines

• An international consensus on a CFD code validation matrix would be an additional factor to support CFD for H2 safety

CEA tests of H2 tank crash


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Description of phenomena related to H2 risk issues

• Review of accident scenarios and identification of main phenomena (on-going PIRT exercise for example, within HYSAFE) List of phenomena (also identification of state of the art in modelling, existence of experimental data)

• Accidents involving H2 usually involve:– Release (gaseous or liquid)– Dispersion into environment, confined, semi-confined or open atmospheres– Action of passive or active mitigation systems to reduce the risk– In case of ignition, and depending on geometry & other parameters

• Diffusion flames

• Jet fires

• Slow deflagrations

• Flame acceleration

• DDT

• Very wide range of flow regimes involving chemical & heat transfer processes, from nearly incompressible buoyant flow to fully compressible reactive flow

• Challenge for physical models & numerical algorithms

• Validation of models requires a very extensive effort


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• Mainly relies on CAST3M code, which is an in-house research code, used for many different applications

• Not a “black box” code knowledge of models & methods… and their weaknesses

• Development of numerical algorithms “best suited” to the physics– Pressure based methods for dispersion calculations– Density based methods (shock capturing) for explosion modelling

• Trying to apply BPG (grid sensitivity, parametric studies, etc) … but very often beyond code & current computers’ capabilities

• Participation to benchmarks (e.g. HYSAFE)• Other codes are being/will be assessed & used by CEA

(e.g. FLUENT as well as two-phase flow codes)

CFD modelling at CEA


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Dispersion / Distribution model validation

• Main physical models:– Laminar diffusion– Turbulence– Buoyancy forces

• Test cases: – Russian-2 experiment (HYSAFE benchmark)– AECL Large Scale Gas Mixing Facility Helium tests– CEA MISTRA Helium tests MH1 and MH2

• All confined atmosphere cases – in 3D geometries with no internal obstaclesBuoyant Gas Volume Fully confined Duration Physical phenomena

Russian-2 H2 20m3 Yes 250mn jet release during 1mn followed by diffusion phaseAECL LSGMF He 1000m3 No (p=Ct) 10mn jet release during 10mn

MISTRA He 100m3 Yes 120mn jet release during 30mn followed by diffusion phase


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Russian-2 experiment (HYSAFE bench., see 120004)

• Experimental data not of very high quality (no repeatability, no information on wall temperatures)

• Grid and model (mixing length) sensitivity studies were performed

• Under-prediction of diffusion in the lower part of the facility (as with most codes)

• Effect of buoyancy forces due to temperature effects?

Grid convergence study


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AECL LSGMF He tests

• Study of dynamics of buoyant jet

• Side opening in facility so constant pressure was maintained

• Comparison of different turbulent models was made, RNG k- vs. standard k

• Better accuracy was found using RNG k-

• Similar conclusion found using CFX (GRS)


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MISTRA He tests MH1 and MH2

• Release of He in purely confined geometry (small pressurisation) to study physics of stratification, and mixing by diffusion

• Very well instrumented facility (gas sampling, thermocouples, LDV) detailed field measurements, including temperatures which might play a role in additional buoyancy effects

• Good agreement between CFD calculations & experimental data (velocity & concentration profiles)


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Combustion model validation• Focus on confined/semi-confined H2, large scale experiments,

many of which performed in framework of nuclear safety studies:– HDR E12.3.2 Test, slow deflagration

– Battelle BMC Ex29 Test, slow deflagration

– RUT STH 06, fast deflagration

– RUT STM4 Test, detonation

• Model implemented in CAST3M code: Compressible Euler equations with CREBCOM model. Diffusion and turbulence effects taken into account through experimental-based correlations (flame speed) – predictability of such model?

• Validation efforts aimed at showing ability to calculate dynamic loads (conservative values) rather than detailed physics

Volume (order magnitude) [H2] Physical phenomena

HDR E12.3.2 500m3 12% Slow deflagration, with acceleration through ventBMC Ex29 100m3 10% Slow deflagration, with acceleration through ventRUT STH06 500m3 16.5% Fast deflagrations with shock wavesRUT STM4 500m3 24.8% Detonation


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HDR E12.3.2 test: slow deflagration

• Slow deflagration through interconnected volumes

• Ability to model flame acceleration and pressure effects by choosing appropriately model constants

• Grid sensitivity results show limitations of such simplified combustion model


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BMC Ex29 test: slow deflagration

• Slow deflagration through interconnected volumes (similar to HDR test)

• Ability to model flame acceleration and pressure effects by choosing appropriately model constants

• Grid sensitivity results show limitations of such simplified combustion model


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RUT STH06 test: fast deflagration

• Flame acceleration modelled using CREBCOM model

• Presence of shock waves (precursor and reflected shock waves)

• Good agreement with experimental data


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RUT STM4 Test: detonation

• Flame acceleration & transition not modelled• Simulate detonation entering canyon & reflection• Results not dependent on CREBCOM model parameters.

Code gives similar results with global Arrhenius model• Ability to capture pressure peaks if grid sufficiently fine &

second-order schemes used


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Mitigation model validation

• Mitigation effects can also be evaluated using CFD• For example: use of catalytic recombiners as a risk-reducing

measure in confined environments or to decrease H2 content of release (for ex. Boil-off system for LH2 engines)

• CEA has performed experiments to qualify recombiner systems & developed model incorporated in CAST3M code

• Model development for other types of mitigation devices & appropriate experimental validation to be carried out in HYSAFE


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Outline of a validation matrix?Name of experiment

Media Geometry, Scale

Open, semi-confined or confined atmosphere

Main phenomena Quality of experimental data (High – Medium - Low)

Features of validation

RUSSIAN-2 GH2 Cylindrical vessel, 5.5m high, 2.2m diameter, 20m3 vol.

confined Subsonic release near the top at a rate of 4.5l/s for 60s, then diffusion over a period of 250min

Low grid-dependence

AECL LSGMF

GHe Rectangular room, 1000m3

Confined Buoyant jet release of 3g/s at a speed of 8.6m/s for 600s

High turbulence model

MISTRA MH1-MH2

GHe Cylindrical vessel, 7m high, 4m diameter, 100m3

Confined Buoyant jet release of 1 g/s for 1800s, then diffusion over a period of 7000s

High Preliminary calculations

NASA-6 LH2 40m Open, non congested

spill of LH2, evaporation, heat transfer ground, atmospheric dispersion, buoyancy forces

Medium On-going (coupling FLUENT- CAST3M codes)

Fh-ICT Balloon

GH2 80m Open, non congested

Deflagration of a 20m diameter half-sphere and propagation of pressure waves over a distance of 80m

Medium model parameters, grid dependence

HDR E12.3.2

GH2 + steam

Series of interconnected rooms of respective volumes 140m3, 75m3 and 330m3

Semi-confined (last room vented)

Ignition at far end of a hydrogen, steam and air mixture followed by flame propagation (deflagration) and acceleration

Low (designed for lumped-parameter codes)

model parameters, grid dependence

BATTELLE BMC Ex29

GH2 Series of two interconnected rooms of 41m3 vol. each

Semi-confined (second room vented)

Ignition at far end followed by flame propagation (deflagration) and acceleration

Low (same reason as above)

model parameters, grid dependence

RUT STH06 GH2 Two long channels (about 36m each) separated by large vol. (canyon), overall volume 480m3

Confined

Fast deflagration High grid dependence

RUT STM4 GH2 + steam

Same as above

Confined Fully developed detonation entering the canyon

High grid dependence, order schemes

KALI GH2 + steam

Steel vessel of 15.6m3

Confined Global H2 reduction through the use of a Passive Autocatalytic Recombiner

Low None

• Current identified gaps:– Separate Effect

Tests (eg. Pure diffusion)

– Low momentum release in confined atmospheres

– Release in obstacle-laden environments

– Release in partially / open atmospheres

– Combustion in presence of gradients

– Diffusion flames & jet fires

• Need for new experiments


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New tests at CEA: Diffusion column experiment• Some doubts as to the mixing

processes in Russian-2 test:– Other than molecular diffusion?

• GADIFAN Separate Effect Test • Study of diffusion mixing. No

injection, light gas separated from air by diaphragm that can be opened without disturbing the flow

• He or H2 can be compared• Column can be tilted at various

angles (to introduce buoyancy)


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New tests at CEA: detailed mixing experiment• GAMELAN test for study of He

injection & mixing• Use of non-intrusive

measurement techniques (LIF) for gas concentration measurements

• Also LDV measurements of velocity


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New tests at CEA: compartmented tests

• Objective: study mixing processes in a confined, compartmented geometry (public multi-storey car park)

• Additional physical processes:– Jet impinging– Additional turbulence

created by flow around obstacles

• Use of large scale (7m high, 100m3) facility MISTRA facility (developed for other applications)


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New tests at CEA & INERIS: garage experiments• Garage tests “à la Swain”, performed in the framework

of HYSAFE’s insHYde project, for typical European garage layouts

• Detailed measurements for CFD code validation (various concentration measurement techniques will be tested)

• He release (CEA) & H2 release (INERIS) experiments• Study of geometric configurations, effect of ventilation

H2

H2

Heor

Several vent locationswith possibility of mechanical ventilation

Simulated car

vent

ventVarying roof inclination

X

Hydrogen or Helium : various locations for injection

Large experimental hall forwell controled external boundary

conditions (stable temperature, no wind)


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On-going activities and conclusions

• Presentation of validation efforts performed at CEA for H2 risk assessment

• More work on model development & validation needed• Gaps in experimental data identified:

– Availability of existing (published) data

– need for better experiments (quality of experimental data)

– Need for new types of tests, including “Separate Effect Tests” & “Coupled Effect Tests”

• On-going experimental programme at CEA in support of code modelling & validation

• Main objective: stimulate discussions on an “internationally”-agreed validation matrix for CFD codes to be used for H2 safety assessment– Within HYSAFE Network of Excellence

– Among OECD / IEA Task 19 (H2 Safety) group of experts

Modelling of H2 Dispersion and Combustion Phenomena Using CFD Codes

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