A Numerical / Analytical Model of Hydrogen Release and Mixing in

Partially Confined Spaces

Kuldeep Prasad, William Pitts and Jiann Yang

Fire Research Division

National Institute of Standards and Technology

Gaithersburg, MD 20899.

NHA 2010 Annual Conference

e-mail : kprasad@nist.gov

Fire safety in confined spaces

Hydrogen powered systems– Rising energy demands– Environmental degradation problem

Transition to a hydrogen economy poses new safety challenges

– Unique fire hazard - Hydrogen behaves differently than conventional fuels

– Lack technical data to support standard and code development – partially enclosed compartments.

What is the problem?

H2 Flammability, Detection, & Fire Safety

To study fire hazards associated with accidental release of

hydrogen in residential and commercial settings and to provide

sound technical data to support hydrogen fire safety code and

standard development.

● Hydrogen fire safety in partially confined spaces

● Release and mixing of hydrogen in confined space.

● Short and long term mixing and dissipatioin.

● Simple theoretical tools, CFD simulations, Experiment

● Hydrogen flammability.

● Standardization of test protocols for hydrogen detection.

● Code and standard activities (NFPA-2)

Hydrogen Release as a Point Source•Release of hydrogen as a point source.

• Un-cluttered environment

• Turbulent hydrogen plume entrains air.

• Plume reaches ceiling and spreads radially.

• Buoyant layer separated from ambient air by a density interface.

Problem Formulation

c3

ci1

PP

P)hH(gP

jjj c*v*ajQ

0

c3

0

ci1

P2v

P)hH(g2v

Conservation Equations

)cva())hH(

333i

22

2

HHH YMdt

SYd

333

i cva))hH(

pQdt

Sd

Conservation of Hydrogen in upper layer

Conservation of total massin upper layer

Constraint Equation

Plume ModelingClassical Plume Mixing Model – self similar plume solution

3/50

3/10

3/12

10

9

5

6zhBQ ip

gVB HH **

0

00

2

2

Buoyancy FluxEffective origin

333111 cvacva2

HV

Hole

Burner

1.5

m

0.75

m

1.5 m

Sensor 7 (0.65 m)Sensor 6 (0.56 m)Sensor 5 (0.45 m)Sensor 4 (0.37 m)Sensor 3 (0.28 m)Sensor 2 (0.19 m)Sensor 1 (0.09 m)

Reduced scale experiments

Comparison with Analytical Model

Full Scale CFD simulationsComparison with Analytical model

Hole

Burner

Effect of Hydrogen Release RateHydrogen Volume Fraction Height of the Interface

Compartment Overpressure Volumetric Flow Rates

Wind Driven Ventilation of Compartment

Assisting Wind FlowWind assists buoyancy driven flow

Opposing Wind FlowWeak wind opposes buoyancy driven flow

Opposing Wind FlowStrong wind opposes buoyancy driven flow

Wind Driven Ventilation-Steady State Results

Forced Venting of Hydrogen: Formulation

Forced Venting of Hydrogen : Results

Release as a Distributed Source

• Release of hydrogen under an obstruction.

• Cluttered environment, Multiple plumes.

• Buoyant gas mixes rapidly with surrounding air.

• Well mixed hydrogen air mixture in compartment.

Problem Formulation

)cva( 33322

2

HHH YM

dt

YdV

Conservation of hydrogen in compartment

Constraint Equation

333111 cvacva2

HV

Effect of Vent Area, Location

Multiple Vents

Conclusions and Future Work• Natural and wind driven ventilation of hydrogen

released in an accidental manner in a partially enclosed compartment.

• Development of simple analytical models– Validated with reduced scale experiments.– Validated with full scale detailed CFD simulations.

• Effect of Hydrogen Release Rate• Effect of vent cross-sectional area, distance between

vents, multiple vents, location of vents.• Role of assisting and opposing wind flows• Forced ventilation, thermal effects,• Effect of surrogate gases (helium).• Time to empty a compartment filled with hydrogen gas.