Fire and Explosion Research Centre, Kingston University, UK

NUMERICAL STUDY OF SPONTANEOUS IGNITION OFNUMERICAL STUDY OF SPONTANEOUS IGNITION OF

PRESSURIZED HYDROGEN RELEASE INTO AIRPRESSURIZED HYDROGEN RELEASE INTO AIR

B. P. XuB. P. Xu11, L. EL Hima, L. EL Hima11, J. X. Wen, J. X. Wen11, S. Dembele, S. Dembele11 and V.H.Y. Tam and V.H.Y. Tam22

11Faculty of Engineering, Kingston UniversityFaculty of Engineering, Kingston University

Friars Avenue, London, SW15 3DW, UKFriars Avenue, London, SW15 3DW, UK22EPTG, bp Exploration, Chertsey Road, Sunbury-on-Thames, TW16 7LN, UKEPTG, bp Exploration, Chertsey Road, Sunbury-on-Thames, TW16 7LN, UK

Fire and Explosion Research Centre, Kingston University, UK

Analysis of Hydrogen Accidental Database Compiled Analysis of Hydrogen Accidental Database Compiled by Kingston Universityby Kingston University

Table 1 Frequency of occurrence of ignition sources

Ignition source Hydrogen incidents Number Percentage

Arson 3 0.44% Collision 29 4.29%

Flame 58 8.58% Hot surface 57 8.43%

Electric 44 6.51% Not identified 419 59.98% No ignition 66 9.76%

TOTAL 676

Fire and Explosion Research Centre, Kingston University, UK

Postulated Ignition MechanismsPostulated Ignition Mechanisms(Astbury & Hawksworth, 2005)(Astbury & Hawksworth, 2005)

Reverse Joule-Thomson Reverse Joule-Thomson effecteffect

Static electricityStatic electricity

Sudden adiabatic Sudden adiabatic compressioncompression

Hot surface ignitionHot surface ignition

Mechanical friction and Mechanical friction and impactimpact

Diffusion ignitionDiffusion ignition

Self-ignition and explosion during discharge of high-pressure hydrogen, Toshio Mogi, Dongjoon Kim, Hiroumi

Shiina, Sadashige Horiguchi, J of Loss Prevention, 2007.

Fire and Explosion Research Centre, Kingston University, UK

Diffusion IgnitionDiffusion Ignition(Wolanski and Wojcicki, 1973)(Wolanski and Wojcicki, 1973)

SuddenSudden rupture of a pressure rupture of a pressure boundaryboundary

Shock wave (primary)Shock wave (primary)

Shock-heated air or oxidizer Shock-heated air or oxidizer (behind the shock wave) (behind the shock wave)

Cooling hydrogen – flow Cooling hydrogen – flow acceleration or divergenceacceleration or divergence

Formation of combustible Formation of combustible mixture at contact surface – mixture at contact surface – molecular diffusionmolecular diffusion

Ignition (after a delay time)Ignition (after a delay time)

Te

mp

eratu

reS

ch

lieren

De

nsity

Fire and Explosion Research Centre, Kingston University, UK

Mixing at Contact SurfaceMixing at Contact Surface

Mass and energy exchange Mass and energy exchange between shock-heated air and between shock-heated air and cooled hydrogen through cooled hydrogen through molecular diffusionmolecular diffusion

Molecular diffusivity is Molecular diffusivity is inversely proportional to local inversely proportional to local pressure and proportional to pressure and proportional to local temperaturelocal temperature

The thickness of the contact The thickness of the contact surface – extremely thin and surface – extremely thin and increasing with release timeincreasing with release time

For the release case of 100 bar

through a 1mm hole at t=3us

Fire and Explosion Research Centre, Kingston University, UK

Ignition & its Delay TimeIgnition & its Delay Time

Mixing timeMixing time

the time for the the time for the temperature of the temperature of the mixture to reach the mixture to reach the autoignition temperature autoignition temperature

Chemical delay timeChemical delay time due to the slow due to the slow

hydrogen combustion hydrogen combustion rate under low rate under low temperature temperature

An increase in the release pressure will lead to a rapid decrease in both mixing time and chemical delay time

Numerical MethodsNumerical Methods

2-D2-D Unsteady Compressible Navier-Stokes equations solved with an Unsteady Compressible Navier-Stokes equations solved with an ILESILES method method

DetailedDetailed chemical-kinetic scheme - 8 reactive species and 21 chemical-kinetic scheme - 8 reactive species and 21 elementary steps –third body and “fall off” behavior considered elementary steps –third body and “fall off” behavior considered (Williams 2006)(Williams 2006)

Multi-componentMulti-component diffusion approach for mixing - thermal diffusion diffusion approach for mixing - thermal diffusion

ALEALE numerical scheme: convective term solved separately from numerical scheme: convective term solved separately from diffusion termsdiffusion terms

In Lagrangian stage, In Lagrangian stage, 22ndnd-order Crank-Nicolson-order Crank-Nicolson scheme scheme + + 22ndnd-order -order central differencingcentral differencing

In rezone phase, In rezone phase, 33rdrd-order Runge-Kutta method-order Runge-Kutta method + + 55thth - order upwind - order upwind WENO schemeWENO scheme

Numerical diffusion resulting from the use of lower order schemes could lead to overprediction of the contact surface thickness, and hence artificially increase the chance of autoignition.

Fire and Explosion Research Centre, Kingston University, UK

Problem DescriptionProblem Description

Three hole sizes: Three hole sizes: 1mm, 3mm, 5mm1mm, 3mm, 5mm

Three release Pressures: Three release Pressures: 100 bar, 200 bar, 300 bar100 bar, 200 bar, 300 bar

Non-slip and adiabatic wall Non-slip and adiabatic wall boundaryboundary

Minimum cell size:Minimum cell size: 10 microns10 microns

Only Only early stageearly stage of release of release simulatedsimulated

Fire and Explosion Research Centre, Kingston University, UK

Overview of the Under-expanded jet Overview of the Under-expanded jet 1. Mach Number

3. Mass Fraction

2. Temperature

3. Density Schlieren

Fire and Explosion Research Centre, Kingston University, UK

The Very Early Release MomentThe Very Early Release Moment

1. Axial Velocity 2. Temperature 3. Density Schlieren

Fire and Explosion Research Centre, Kingston University, UK

Changing Patterns Changing Patterns for the release case for the release case of 300 bar through a 3mm hole at t=9usof 300 bar through a 3mm hole at t=9us

Fire and Explosion Research Centre, Kingston University, UK

Contour of OH Contour of OH for the release case of 300 for the release case of 300 bar through a 5mm hole at t=35usbar through a 5mm hole at t=35us

Fire and Explosion Research Centre, Kingston University, UK

Contour of Mach Number Contour of Mach Number for the for the release case of 300 bar through a 5mm hole at t=35usrelease case of 300 bar through a 5mm hole at t=35us

Fire and Explosion Research Centre, Kingston University, UK

Contour of Temperature Contour of Temperature for the release for the release case of 300 bar through a 5mm hole at t=35uscase of 300 bar through a 5mm hole at t=35us

Fire and Explosion Research Centre, Kingston University, UK

Contour of Mass Fraction Contour of Mass Fraction for the release case for the release case of 300 bar through a 5mm hole at t=35usof 300 bar through a 5mm hole at t=35us

Fire and Explosion Research Centre, Kingston University, UK

Density Schlieren Density Schlieren for the release case of for the release case of 300 bar through a 5mm hole at t=35us300 bar through a 5mm hole at t=35us

Fire and Explosion Research Centre, Kingston University, UK

Release through a 1mm, 3mm and 5mm holesRelease through a 1mm, 3mm and 5mm holes

Fire and Explosion Research Centre, Kingston University, UK

Grid Sensitivity - Grid Sensitivity - fine mesh (10 microns), fine mesh (10 microns), coarse mesh (20 microns)coarse mesh (20 microns)

Fire and Explosion Research Centre, Kingston University, UK

Concluding StatementConcluding Statement

The release of highly pressurised hydrogen into air could lead to spontaneous ignition depending on pressure, hole sizes, etc.   Current work demonstrated the conditions leading to autoignition. A flame was found to be sustained over a period of 50 s and still stable when the simulation was terminated due to limitation of computer power. 

Further work is underway to establish whether this flame could be maintained, leading to a jet fire, fire ball or an explosion.

Fire and Explosion Research Centre, Kingston University, UK

ConclusionsConclusionsDiffusion ignition has been numerically proved to be a possible mechanism for Diffusion ignition has been numerically proved to be a possible mechanism for spontaneous ignition of a sudden high pressure hydrogen release direct into spontaneous ignition of a sudden high pressure hydrogen release direct into airair

Ignition first occurs at the tip of a very thin contact surface and then moves Ignition first occurs at the tip of a very thin contact surface and then moves downward along the contact surfacedownward along the contact surface

The pressure at the contact surface is higher than the ambient pressure; this The pressure at the contact surface is higher than the ambient pressure; this high pressure produces a high heat release rate to overcomes the flow high pressure produces a high heat release rate to overcomes the flow divergence and sustain the very thin flamedivergence and sustain the very thin flame

As the jet develops downstream, the front of the contact surface becomes As the jet develops downstream, the front of the contact surface becomes distorted distorted

The distorted contact surface can increase the chemical reaction interface and The distorted contact surface can increase the chemical reaction interface and enhance the mixing process, and it may be the key factor for the final diffusive enhance the mixing process, and it may be the key factor for the final diffusive turbulent flameturbulent flame

Finally, an increase in the release pressure or the hole size can facilitate the Finally, an increase in the release pressure or the hole size can facilitate the ignition.ignition.

Fire and Explosion Research Centre, Kingston University, UK

ACKNOWLEDGEMENTACKNOWLEDGEMENT

B P XU’s Post-doctoral Fellowship is funded by the B P XU’s Post-doctoral Fellowship is funded by the Faculty of Engineering at Kingston University.Faculty of Engineering at Kingston University.

The authors would like to acknowledge EU FP6 The authors would like to acknowledge EU FP6 Marie Curie programme for funding hydrogen Marie Curie programme for funding hydrogen research at Kingston University through the HYFIRE research at Kingston University through the HYFIRE (HYdrogen combustion in the context of FIRE and (HYdrogen combustion in the context of FIRE and explosion safety) project. explosion safety) project.

BP and HSL are supporting groups for HYFIRE.BP and HSL are supporting groups for HYFIRE.