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Recent Research and Development on Unintended Releases of Hydrogen

William Houf, Greg Evans, Robert Schefer, Jeff LaChance

Sandia National Laboratories

2nd International Conference on Hydrogen SafetySan Sebastian, Spain

September 11-13, 2007

This presentation does not contain any proprietary, confidential, or otherwise restricted information

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Objectives

• Development of new hydrogen codes and standards needs a traceable technical basis:– characterize small-scale gaseous leaks, determine barrier wall

effectiveness– perform physical and numerical experiments to quantify fluid

mechanics, combustion, heat transfer, cloud dispersion behavior

– develop validated engineering models and CFD models for consequence analysis

– use quantitative risk assessment for risk-informed decision making and identification of risk mitigation strategies

• Provide advocacy and technical support for the codes and standards change process:– consequence and risk: ICC and NFPA– international engagement: HYPER (EU 6th Framework Program),

Installation Permitting Guidance for Hydrogen and Fuel Cell Stationary Applications

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We have completed experiments and analysis on large-scale high momentum leaks.

Nighttime photograph of 413 bar

(6000 psig) large-scale H2 jet-flame

test (dj = 5.08mm, Lvis = 10.6 m)

from Sandia/SRI tests.

11.3 m

• H2 jet-flame radiation model verified at source

pressures of 172 bar (2500 psig), 413 bar (6000 psig)

• Unignited jet concentration decay model verified

against natural gas data (source pressure 3.5 -

76 bar) and compressible Navier-Stokes

simulations for H2 (206 bar, (3000 psig))

• Experiments and safety distance modeling results

published in peer-reviewed papers(1) Houf and Schefer, “Predicting Radiative Heat Fluxes and Flammability

Envelopes from Unintended Releases of Hydrogen,” Int. Jour. of Hydrogen Energy, Vol. 32, Jan. 2007.(2) Schefer, Houf, San Marchi, Chernicoff, and Englom, “Characterization of

Leaks from Compressed Hydrogen Dispensing Systems and Related Components,” Int. Jour. of Hydrogen Energy,

Vol. 31, Aug. 2006.(3) Molina, Schefer, and Houf, “Radiative Fraction and Optical Thickness in

Large-Scale Hydrogen Jet Flames,” Proceedings of the Combustion Institute, April, 2006.(4) Houf and Schefer, “Rad. Heat Flux & Flam. Env. Pred. from Unintended Rel. of

H2,” Proc. 13th

Int. Heat Tran. Conf., Aug., 2006.(5) Schefer, Houf, Williams, Bourne, and Colton, “Characterization of High-

Pressure, Under-Expanded Hydrogen-Jet Flames,” In Press, Int. Jour. of Hydrogen Energy, 2007.(6) Houf and Schefer, “Predicting Radiative Heat Fluxes and Flammability

Envelopes from Unintended Releases of Hydrogen,” 16th NHA Meeting, Washington, DC, March

2005.(6) Schefer, R. W., Houf, W. G., Bourne, B. and Colton, J., “Turbulent Hydrogen-

Jet Flame Characterization”, Int. Jour. of Hydrogen Energy, 2005.(7) Schefer, R. W., Houf, W. G., Bourne, B. and Colton, J., “Experimental

Measurements to Characterize the Thermal and Radiation Properties of an Open-flame Hydrogen Plume”, 15th NHA

Meeting, April 26-30, 2004, Los Angeles, CA.(8) Schefer, “Combustion Basics,” in National Fire Protection Association (NFPA)

Guide to Gas Safety, 2004.

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We have completed a study of the hydrogen small/slow leak regime.

• Goal - Provide technical information on small/slow leaks from hydrogen-based systems

• Slow leaks may occur from• Low pressure electrolyzers• Leaky fittings or O-rings with large amounts of pressure drop• Vents from buildings or storage facilities containing hydrogen

• Previous work focused on the high-momentum leak regime where the effects of buoyancy on the flow were small

• In the slow leak regime both momentum and buoyant forces are important

• Buoyant forces affect the trajectory and rate of entrainment• Significant curvature can occur in jet trajectory• Concentration decay and the distance to mean lower

ignition limit • The ratio of momentum to buoyant forces for the leak can be

characterized by the exit densimetric Froude number • Fden = Uexit /(gD(rhoamb- rhoexit)/rhoexit)1/2

• Approach• Experimentally characterize slow leaks (leak size and geometry)• Develop validated engineering models• Use engineering models to generate safety information

• Safety distance to (mean) lower concentration ignition limit

Flowrate = 20 scfm, Hole Dia. = 9.44 mm

Exit Mach Number = 0.1 (Unchoked Flow)

Fden = 117

*Photograph from: Dr. Michael Swain, (Univ. of Miami) Fuel Cell Summit Meeting June 17, 2004

Jet Flame from an Ignited H2 Slow Leak*

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Rayleigh scattering system

CCD camera

laser sheet

Rayleigh scattering is used to map concentration contours of small/slow leaks.

Experimentally measured centerline concentration

decay rates in vertical buoyant jets

Instantaneous H2 mole fraction images

in unignited vertical jet

Instantaneous H2 mole fraction images

in unignited horizontal jet

gg

0

5

10

15

20

25

0 100 200 300 400 500

1/Xcl_CL Decay_all.qpam

He (Pitts,1991)CH4 (Pitts,1991)C3H8 (Pitts,1991)CH4 (Present study)H2 (Present study)

1 /

Xcl

z/d

C3H

8

CH4

He

H2

5% m.f.

4% m.f.

10% m.f.

20% m.f.

6.67% m.f.

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6Mole Fraction

0.2

0.4

0.6

0.8

H2 jet at Re=2,384; Fr = 268

H2 flammability limits:LFL 4.0%; RFL 75%

CH4 flammability limits:

LFL 5.2%; RFL 15%

CH4 jet at Re=6,813; Fr = 478

Ignitable gas envelope is significantly larger

in H2 jets than CH4 jets.

Comparison of ignitable gas envelopes for hydrogen and methane jets.

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Buoyancy effects are characterized by Froude number.

Horizontal H2 Jet (dj=1.9 mm)• Time-averaged H2 mole

fraction distributions.

• Froude number is a measure of strength of momentum force relative to the buoyant force

• Increased upward jet curvature is due to increased buoyancy at lower Froude numbers.

Fr=99

Fr=152

Fr=268Fr=268

Fr=152

Fr=99

Mole Fraction

0.2

0.4

0.6

0.8

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We have developed an engineering model for the buoyant jet from an unignited small/slow hydrogen leak.

• Profiles of jet velocity and scalars assumed Gaussian in zone of established flow (for example)

uucl exp( r2 /B2 )

U

g

JetAxis

S

0

Z

X

r

DischargePoint ofLeak

• Engineering model for buoyant jet from hydrogen slow leak

• Jet trajectory• H2 concentration decay along jet trajectory• Distance to lower flammability limit (LFL)

• Based on integral jet model in streamline coordinates

• Continuity

• Horizontal Momentum

• Vertical Momentum

• Concentration

• Trajectory

s

urdrd0

E0

2

s

u2 cosrdrd0

00

2

s

u2 sinrdrd0

( )grdrd0

0

2

0

2

s

u(y y)rdrd0

00

2

s

(x) cos

s

(z) sin

• Simulation executes in a few seconds on a workstation compared to several hours for Navier-Stokes calculations of the same problem

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Comparison of model with data from the Sandia slow-leak experiments for

buoyant H2 plumes

The engineering model has been validated against data for buoyant slow leaks.

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0 10 20 30 40 50

Comparison of Simulation and DataFor Buoyant He Jet

He Data of Pitts

He Data of Keagy & Weller

Slow Leak Model1

/ Xcl

y/d

Simulation

Data

Comparison of model and data for concentration decay of vertical buoyant

He plumeThe buoyantly-driven flow model :

• uses a different entrainment law than our momentum jet model

• integrates along the stream line to capture plume trajectory

0

5

10

15

20

25

0 50 100 150 200

1/X

H2

Axial Distance Z/D

Fr = 99

Fr = 152

Fr = 268

Z/D

He H2

• Lower Froude number leaks are more buoyant• Buoyancy increases entrainment rate causing faster concentration

decay• New entrainment law adds buoyancy-induced entrainment to momentum

induced entrainment

Simulations

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Risk-Informed decision making should be used for separation distances.

• Current code separation distances are not reflective of future fueling station operations (e.g., 70 MPa)

• Facility parameters (e.g., operating pressure and volume) should be used to delineate separation distances

• Consequence-based separation distances (i.e., single event) can be large depending on pressure, leak size, and consequence parameter

• QRA insights are being considered by NFPA-2 to help establish meaningful separation distances and other code requirements

Radiative Heat Flux = 1.6 kW/m2

0

20

40

60

80

100

120

0 20 40 60 80 100 120

Pressure (MPa)

Se

pa

ratio

n D

ista

nce

(m

)

13.5

11.5

9.52

6.35

4.23

2.38

1.00

0.40

0.18

Pipe Leak Diameter = 13.5 mm

0

50

100

150

200

250

0 20 40 60 80 100 120

Pressure (MPa)

Se

pa

ratio

n D

ista

nce

(m

)

2% Hydrogen

4% Hydrogen

1.6 kW/m2

6% Hydrogen

4.7 kW/m2

8% Hydrogen

Flame Length

25 kW/m2

Leak Diameter

(mm)

Consequence Parameter

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We are studying barriers as a mitigation strategy to reduce safety distances.

• Characterize H2 transport and mixing near barrier walls through combined experiment and modeling

• Identify conditions leading to deflagration or detonation• residence time and ignition timing• magnitude of over-pressure and duration

• Develop correlations for wall heights dependency and wall-standoff distances

• Combine data and analysis with quantitative risk assessment for barrier configuration guidance

• Goal: determine if barriers are an effective jet mitigation technique since mixtures of H2 and air can ignite and potentially generate large overpressures.

• Collaborating with the HYPER project in Europe.H2

(a)

(b)

(c)

Unignited H2 Jets

Mixing

Axial Distance

Pre

ss

ure

Deflagration to detonationOver-pressure from ignition of premixed hydrogen / air

Over-pressure characterization

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• Characterize stabilization of H2 jet flame on and behind barrier

• Characterize thermal/structural integrity of barriers

• Use CFD modeling and validation for H2 jet flames to minimize the number of tests

• Develop correlations for wall height dependencies and wall stand-off distances

• Combine data and analysis with quantitative risk assessment for barrier configuration guidance

• We have initiated the barrier work by verifying the ability of our in house Navier-Stokes code (Fuego) to compute hydrogen jet flames and unignited jet concentration

H2

(a)

(b)

(c)

Stabilized flame

Radiometers

H2 Jet Flames

The behavior of H2 jet flames near barrier walls is also an issue of importance.

Barlow flame A (ref. Combustion and Flame, v. 117, pp. 4-31, 1999)

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We are performing Navier-Stokes calculations of hydrogen jet flames and jet flames on barriers.

Fuego 3-D Navier-Stokes Simulation of

H2 Jet Flame Wall Impingement

Sandia/SRI H2 Jet Flame

Wall Impingement Test (2500 psi)

625

- 625

X (cm)

Z (cm)767.3

0

Y (cm)

-122

625

Barrier

Jet Exit (0,0,0)

Symmetry Plane(Side)

Open Boundaries(Top and Sides)

No Slip Boundary(Bottom)

• Simulations Sandia/SRI H2 jet flame wall impingement tests• Applied Mach disk model to 6000 psi jet at 100 s when stagnation pressure was 1505 psig• 8 ft by 8 ft barrier with nozzle (dia. = 5.018 mm) 4 ft from wall (20.3 cm thick) on center• Computed Mach disk diameter = 61.9 mm• Simulated 1/2 domain (106 elements)• 1st order upwind with std k- or RNG k-

• Radiation loss and heat fluxes predicted with discrete ordinates participating media model• New barrier tests being performed at SRI Test site with pre-test CFD predictions

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We are investigating many different barrier jet flame impingement scenarios and barrier geometries with modeling and full-scale experiments.

Different Leak Scenarios

• Vertical Wall - 90 deg impingement

• Vertical Wall - +45deg impingement

• Vertical Wall - Jet centerline aligned with top of wall

60o

Side View

Top View

• Source pressure• Leak to barrier spacing• Leak trajectory• Leak location• Barrier geometry

• 3 Wall Configuration (135o between walls)

• 60o Tilted Wall

Different Barrier Geometries

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Barriers can reduce the exposure from jet flame radiation heat flux as well as reduce jet flame impingement hazard.

H2 Jet Flame Impinging on Barrier

H2 Free Jet Flame

d

d

No barrier(vertical jet flame)

With barrier

• Barrier deflects horizontal issuing jet flame vertically• The flame extends above the barrier

• Radiation at a distance (d) behind the barrier from the deflected flame is less than that from a free vertical jet flame with no barrier

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Full-scale jet flame impingement experiments provide valuable insight on barrier behavior as well as modeling validation data.

• Tests performed at SRI Corral Hollow test site

Jet Centerline Aligned with Center of Barrier

Jet Centerline Aligned with Top of Barrier

Pre

ss

ure

(p

sig

)

Time (msec)

Instrumentation Layout

Overpressure from Ignition

In front of barrier

Behind barrier

2.4m x 2.4m wall with jet centered

• Time to ignition - 135 msec

Spark (igniter)Pencil Pressure TransducerPressure Transducer

Radiometer (Heat Flux)

T1 Thermocouple

Radiometer (300 s response)

Dosplacement sensor

H2 Jet T1 T2 T3

T4

T9-T12 (Depth: 1/8,1/4,1/2,1”)

8 ft (2.4 m)

T6

T7

T8

T5

Visible Video

IR Video

High Speed Video

18” 18”

12”

24”

34.5”

52.75”

16”12”

54” 54.75”

17.75”

34.5”

R1

R2

R3

R4

R5

48”

??

Top View

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Summary

• Completed engineering model for buoyant plumes and reported at 2007 NHA meeting and SAE World Congress - Paper submitted to IJHE and in review

• QRA is being used to make risk-informed decisions regarding set-backs as part of the NFPA-2 activity

• Sandia staff are participating with the technical committee• QRA incorporates Sandia hydrogen release engineering

models• QRA methodology is vetted through international risk experts

as part of our involvement in IEA Hydrogen Safety Task 19

• Barrier walls are being characterized as a jet mitigation strategy for set back reduction

• Partnership with SRI (testing) and HYPER (analysis)• CFD best-practices working group

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Recent Publications

1. Houf and Schefer, “Small-Scale Unitended Releases of Hydrogen” , 2007 NHA Conference, San Antonio, TX, March 19-22, 2007.

2. Houf and Schefer, “Investigation of Small-Scale Unintended Releases of Hydrogen”, SAE World Congress, Detroit, MI, April 16-19, 2007, (invited paper).

3. LaChance, "Risk-Informed Separation Distances for Hydrogen Refueling Stations“, 2007 NHA Conference, San Antonio, TX, March 19-22, 2007.

4. Schefer and Houf, “Investigation of small-scale unintended releases of hydrogen: momentum-dominated regime”, submitted to International Journal of Hydrogen Energy.

5. Houf, Evans, and Schefer, “Analysis of Jet Flames and Unignited Jets from Unintended Releases of Hydrogen,” 2nd International Conference on Hydrogen Safety, San Sebastian, Spain, September 11-13, 2007.

6. LaChance, “Quantitative Risk Assessment, Safety Studies and Risk Mitigation,” 2nd International Conference on Hydrogen Safety, San Sebastian, Spain, September 11-13, 2007.