Natural and Forced Ventilation of
Buoyant Gas Released in a Full-Scale
Garage : Comparison of Model
Predictions and Experimental Data
Kuldeep Prasad, William Pitts, M. Fernandez, and
Jiann Yang
Fire Research Division
National Institute of Standards and Technology
Gaithersburg, MD 20899.
ICHS4 2011
e-mail : kprasad@nist.gov
What is the problem?
Fire safety in partially confined spaces
Hydrogen Powered Systems
– Rising energy demands, Environmental degradation
– Potential for fires and explosions (unintended releases).
Predicting temporally evolving concentration is
challenging :
–
–
–
–
Unknown release rates, release location, orientation.
Release duration (pressure of tank, size of release port)
Unknown compartment leak sizes, locations.
Effect of wind, thermal effects, forced ventilation.
Predicting the effect of mitigation techniques
– Reduce envelope of flammable concentrations as quickly
as possible.
Fire safety in partially confined spaces
Motivation :
– Literature Review :Limited studies on buoyant gas released
under an automobile in a realistic, full-scale garage (clutter).
– Methodology to predict hydrogen volume fraction in partially
enclosed compartments : Safety of hydrogen fueled
applications ( improving codes and standards)
Approach:
– Detailed experimental, numerical and analytical modeling
study to better understand hydrogen safety in partially
enclosed compartments.
– Development of a simple and validated analytical model
(based on compartment over-pressure).
– Study natural and forced dispersion of a buoyant gas.
– Compare / contrast results, Recommendations.
Experimental Setup
Garage : 6.8 m x 5.4 m x 2.4 m
Manual Door : 2.8 m x 2.1 m
Two Doors : 1.5 m x 1.1 m
Mid-size passenger car parked
centered over the release location.
Car windows rolled up and hood /
trunk closed.
Helium released under the car to simulate release from a hydrogen fueled car.
Flow rate monitored with a dry test meter (5 kg of hydrogen over 4 hours).
Release into the garage from a box 0.3 m x 0.3 m x 0.15 m (diffuser ,
wiremesh, crushed stone).
Experimental Setup: Air-leakage Test
Helium volume fraction : Thermal
conductivity sensors :
Estimate Air-leakage rates
(ASTM E779-03 standard):
INFILTEC Model E3 Blower door
fan coupled with a digital micromanometer DM4. Pressure
differential 10 Pa-70Pa.
Mitigation Strategy : Used the builtin fan of an INFILTEC duct
Leakage tester to provide forced
ventilation in garage.
Flow stratification vs. well-mixed
Flow-field characterized by Fr
Number. Low-momentum jet –
buoyancy controlled –
stratification.
High momentum jet induces
mixing in the compartment.
High pressure release under an
automobile – turbulent mixing,
break up into multiple jets –
Hydrogen escapes from the
wheel wells and perimeter as
multiple plumes (well mixed
compartment)
Phenomena observed in CFD,
experiments.
Problem Formulation
• Model the flow through vents.
• Bernoulli equation.
• Pressure varies hydrostatically.
• Discharge coefficient.
P1  g(H h i )  Pc
v1  2
P3  Pc
v3  2
g(H h i )  Pc
0
Pc
Qj  a j * vj *cj

Vertical pressure gradient inside compartment is lower than the
gradient outside the compartment. Difference between these
pressure gradients leads to a buoyancy driven flow through the
vents.
Conservation Equations

d YH 2 (H h i ) S )
Conservation of Hydrogen
in upper layer
dt
H2
 YH 2  (a 3 v3c3 )
d (H  h i ) S ) 
 Q p  a 3 v 3c 3
dt
Conservation of total mass
in upper layer
Constraint Equation
  M
VH 2  a1v1c1  a 3v3c3
Plume Modeling
Classical Plume Mixing Model – self similar plume solution
6
Qp 
5
Buoyancy Flux
 9 

 10
2
1/ 3

 B01/ 3 hi  z0 5 / 3

 0   H 2

B0  VH 2 * 
 0

 * g

Effective origin
Prior work :Reduced scale experiments
Vents
Quarter scale two-car residential garage
• 6.1 m x 6.1 m x 3.05 m
Sensors
• Helium – surrogate gas.
• Mass flow controller.
• Release of 5 kg of hydrogen.
0.75 m
• Time resolved concentrations.
• Idealized leaks, 3 ACH at 4 Pa
1.5 m
1.5 m
• Two square vents, 2.15 cm
Release Chamber
Comparison with Analytical Model
Prior Work : CFD modeling
Computational Domain
Sensors
Vents
3.0 m
6.0 m
6.0 m
Release Chamber
•NIST Fire Dynamics Simulator
• Low speed, chemically reacting
fluid flow.
• Low Mach number, LES model
• 2nd order, multi-block rectilinear
grid.
Comparison of CFD (symbols) with analytical model (line)
Summary of tests conducted
Estimation of Leak Area (ASTM E779-03)
No.
1
2
3
4
5
6
7
8
Test
Pressurization Test Side Door,
Rear door un-sealed
De-pressurization Test Side Door,
Rear door un-sealed
Pressurization Test Side Door,
Rear door sealed
De-pressurization Test Side Door,
Rear door sealed
Pressurization Test Rear Door,
Side door un-sealed
De-pressurization Test Rear Door,
Side door un-sealed
Pressurization Test Rear Door,
Side door sealed
De-pressurization Test Rear Door,
Side door sealed
C
0.022
n
0.64
Q4Pa
0.0522
ACH
2.1
ELA
0.0202
0.025
0.75
0.0699
2.8
0.0271
0.017
0.64
0.0423
1.7
0.0164
0.024
0.72
0.0658
2.7
0.0255
0.031
0.50
0.0622
2.5
0.0241
0.038
0.61
0.0894
3.6
0.0346
0.012
0.67
0.0305
1.2
0.0118
0.013
0.86
0.0438
1.8
ELA  Qenc 
0.0170
Qenc  V 
Natural and Forced Ventilation Tests with Automobile
No.
Release Rate
(m3/s)
Release Duration
(s)
Natural / Forced
Forced Flow
Rate (m3/s)
Fan Start Time
(s)
1
0.004468
14396
Natural
-
2
0.004326
13622
Natural
-
3
0.004434
3628
Forced
0.0910
125
4
0.004239
3599
Forced
0.0922
101
5
0.004273
3618
Forced
0.1066
101
6
0.004283
3597
Forced
0.1071
0

2 P
ACH
3600
;
Natural Ventilation Tests
Garage Measurements
61.0 cm
30.5 cm
Car Measurements
Under car center
Top of engine
Wheelwell
Engine Compartment
DomeLight
Trunk
Forced Ventilation Tests
Garage Measurements
Car Measurements
Under car center
Top of engine
Wheelwell
Engine Compartment
Summary and Conclusions
•
•
•
•
•
A detailed experimental, numerical and analytical modeling
study to better understand and improve the safety of
hydrogen fueled applications in passively and actively
ventilated spaces.
Validation of analytical model with CFD and reduced scale
expt. (allows it to be used for improving hydrogen safety
codes and standards)
Models results over-predicted the experimental data by 0.4%
for natural ventilation conditions and 1.0% for forced
ventilation conditions.
Parametric studies to understand the effect of release rates,
vent size and location on the predicted helium volume
fraction.
Analytical model does not predict the pockets of buoyant gas
at concentrations that are significantly higher than the LFL,
does not predict the seepage of helium inside the vehicle.
Effect of Input Parameters
Effect of Compartment Volume
Effect of Vent Height
Effect of Input Parameters
Effect of Lower Vent Area
Effect of Upper Vent Area
Prior Work :Wind Driven Ventilation
Assisting Wind Flow
Weak / Strong Opposing Wind Flow
Forced Venting of Hydrogen : Results
Effect of Hydrogen Release Rate
Hydrogen Volume Fraction
Compartment Overpressure
Height of the Interface
Volumetric Flow Rates
Wind Driven Ventilation-Steady State Results
Time required to empty a compartment
dhi Q

dt S
Q  a1v1 c1  a3 v3c3
Buoyancy driven flow
Wind assisted venting
Release as a Distributed Source
• Release of hydrogen under an
obstruction.
• Fully mixed hydrogen – air mixture in
compartment.
• Cluttered environment, Multiple
plumes.
• Goal – predict hydrogen concentrations
in compartment
• Buoyant gas mixes rapidly with
surrounding air..
• Pressure varies hydrostatically with
depth. Vent flows driven by pressure
difference.
Problem Formulation
Conservation of hydrogen mass
d YH 2 
V
 M H 2  YH 2  (a 3 v3c3 )
dt
Constraint Equation
VH 2  a1v1c1  a 3v3c3
Effect of Vent Area, Location
Multiple Vents
Conclusions and Summary
• 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, buoyancy driven flow, thermal effects.
• Effect of surrogate gas (helium).
• Time to empty a compartment filled with hydrogen gas.
High Pressure Release and Dispersion
of Hydrogen in a Partially Enclosed
Compartment
Kuldeep Prasad*, Thomas Cleary and Jiann Yang
Fire Research Division
Engineering Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899.
Fuel Cell and Hydrogen Energy 2011
* Corresponding Author : kprasad@nist.gov
Current and Future Technologies
Hydrogen powered systems
– Hydrogen : Energy carrier for future vehicles.
– Driven by rising energy demands.
– Environmental degradation problem.
What is the problem?
Current technologies require :
– High pressure storage of hydrogen (70 MPa).
– Acceptable levels of vehicle driving range, storage
volume and weight requirements.
Risk associated with high pressure releases :
–
–
–
–
Damage to storage tank, piping or PRD failure.
Dispersion in partially enclosed compartments.
Effective mitigation techniques and requirements.
Support standard and code development.
Prior Work :Reduced scale experiments
Vents
Quarter scale two-car residential garage
• 6.1 m x 6.1 m x 3.05 m
Sensors
• Helium – surrogate gas.
• Mass flow controller.
• Release 5 kg of hydrogen.
0.75 m
• Time resolved concentrations.
• Idealized leaks, 3 ACH at 4 Pa
1.5 m
1.5 m
• Two square vents, 2.15 cm
Release Chamber
Comparison with Analytical Mode
Justification of well-mixed assumption
• Flow-field in compartment characterized by Froude Number.
• Low-momentum jet – buoyancy controlled – stratification.
• High momentum jet induces mixing in the compartment.
• High pressure release under an automobile – turbulent mixing,
break up into multiple jets – Hydrogen escapes from the wheel
wells and perimeter as multiple plumes (distributed sources).
• Phenomena observed in experiments performed in full scale
garages as well as CFD simulations.
Problem Formulation
Pj
• Model the flow through vents - Bernoulli equation. v j  2

• Pressure varies hydrostatically with depth.
P1  gH  Pc
P3  Pc
v1  2
v3  2
gH  Pc
0
Pc
Qj  a j * vj *cj

Vertical pressure gradient inside compartment is lower than the
gradient outside the compartment. Difference between these
pressure gradients leads to a buoyancy driven flow through the
vents.
Conservation Equations
Conservation of Hydrogen
V

d YH 2
dt
  M
H2
 YH 2  (a 3 v3c3 )
VH 2  a1v1c1  a 3v3c3
Constraint Equation
Design of Idealized Vents
Qenc
ELA  Qenc 

2 P
ACH
V 
3600
Qenc
; P  4Pa
Leak rates – described in terms of air changes per hour (ACH).
40 Mpa, 5 kg tank, 1 mm release port
Jet Exit Conditions
Tank Pressure
Exit Velocity
Release and Dispersion
Volume Fraction
Compartment
Overpressure
Height of Interface
Vent Flow Rates
40 Mpa, 5 kg tank, 6 mm release port
Jet Exit Conditions
Tank Pressure
Exit Velocity
Release and Dispersion
Volume Fraction
Compartment
Overpressure
Height of Interface
Vent Flow Rates
Compartment overpressure vs.
Diameter of release port
40 Mpa, 5 kg tank, 1 mm release port
Forced Flow Rate 0.1 m3/s
Volume Fraction
Compartment
Overpressure
Height of Interface
Vent Flow Rates
Forced Flow Rate
vs
Peak Volume Fraction
Forced Flow Rate
vs Duration of
flammable mixture
Conclusions and Summary
• Developed a simple analytical model to predict the risk
associated with accidental release of hydrogen from a highpressure system in a partially ventilated compartment.
• Assumed that the hydrogen released under an automobile
mixed rapidly with the surrounding air.
• Analytical model for natural and forced mixing and dispersion
of hydrogen released in a compartment.
• Ventilation of the compartment occurs through two idealized
holes in the compartment walls (ACH varied between 1-5).
• Examine conditions that lead to major damage of the
compartment due to overpressure.
– 6 mm diameter release port – Significant damage
– 1 mm diameter release port – Cosmetic damage
• Forced ventilation is a viable technique for reducing
dangerous levels of hydrogen concentration in compartment.
Conclusions and Summary (cont.)
• Model can be used to provide design guidelines for forced
ventilation requirements in a compartment
• Simple analytical models have been
– Validated with reduced scale and full scale experiments.
– Compared with detailed CFD simulations
• Effect of hydrogen release rate, vent cross-sectional area,
distance between vents, multiple vents, location of vents.
• Role of assisting and opposing wind flows.
• Forced ventilation, buoyancy driven flow, thermal effects.
• Effect of surrogate gas (helium).
• Time to empty a compartment filled with hydrogen gas.
Contact Information
Kuldeep Prasad*, Thomas Cleary and Jiann Yang
*Corresponding Author
Email : kuldeep.prasad@nist.gov
Fire Research Division
Engineering Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899.