Technical Report

NREL/TP-550-45804

May 2009

Buoyancy-Driven Ventilation of 

Hydrogen From Buildings: 

Laboratory Test and Model 

Validation 

C.D. Barley and K. Gawlik
National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393

303-275-3000 • www.nrel.gov

NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency and Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

Contract No. DE-AC36-08-GO28308

Technical Report

NREL/TP-550-45804

May 2009

Buoyancy-Driven Ventilation of 

Hydrogen From Buildings: 

Laboratory Test and Model 

Validation 

C.D. Barley and K. Gawlik

Prepared under Task No. H274.7110
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iii

Abstract 

Hydrogen gas leaking from a hydrogen-powered vehicle in a residential garage may form a 

flammable mixture with air. Passive, buoyancy-driven ventilation is one approach to limiting the 

concentration to a safe level. We explored the relationship between leak rate, ventilation design, 

and hydrogen concentration through laboratory testing, an algebraic analysis, and CFD 

modeling. We used helium to test slow, steady, low-velocity leaks in a full-scale test room under 

nearly isothermal, steady conditions, and we report the results in sufficient detail that other 

modelers can use them. The results show the importance and variability of stratification. Our

algebraic and CFD models agree very well with the experimental results. We describe our CFD 

approach in sufficient detail for use by others. We tested under nearly isothermal conditions, but 

also discuss indoor-outdoor temperature difference as an important risk factor. Information 

about realistic leakage scenarios is needed to apply these results as safety recommendations.
iv

Acknowledgments

This work was funded by the Hydrogen Safety, Codes and Standards program of the U.S. 

Department of Energy’s (DOE) Office of Hydrogen, Fuels Cells and Infrastructure Technologies 

at NREL’s Hydrogen Technologies and Systems Center, in collaboration with NREL’s Buildings 

and Thermal Systems (BTS) Center. DOE program director Antonio Ruiz and NREL project 

managers Jim Ohi, Chad Blake, and Carl Rivkin provided valuable guidance and support. The 

support of BTS managers Ron Judkoff and Ren Anderson was also essential. NREL technician 

Matthew Post and test engineer Ed Hancock, with Mountain Energy Partnership, were helpful in 

crafting and installing the laboratory apparatus. 
v 

Nomenclature 

c = Concentration of hydrogen, by volume (dimensionless, 0-1 in formulas, 0%-100% in tables 

and graphs)

g = Acceleration of gravity = 9.81 m/s2

h = Height between vents, m

y = Vertical distance above floor, m

A = Vent area (top = bottom), m2

A’ = Vent area (top = bottom) adjusted for test room leakage, m2

ADR = Air density ratio = (local atmospheric pressure) / (sea level atmospheric pressure) 

(dimensionless)

CFM = Cubic feet per minute, ft3

/min

D = Vent discharge coefficient (dimensionless, 0-1)

D1 = D based on Equations 6 and 7 (dimensionless, 0-1)

D2 = D based on Equations 6 and 8 (temperature compensated) (dimensionless, 0-1)

F = Vent sizing factor (dimensionless)

L# (1-2) = Laboratory test number using the linear diffuser

LE = Entrainment length, m

LPM = Liters per minute at local conditions, L/m

NLPM = Liters per minute at NTP (20°C and 1 atm), L/m 

NTP = Normal temperature and pressure (20°C and 1 atm)

P = Total pressure (Pa)

P# (1-6) = Laboratory test number using the point diffuser

Q = Volumetric flow rate through a vent, m3

/s

S = Source rate of hydrogen (leak rate), m3

/s in formulas, LPM in discussions

T = Temperature, K

δ = Ratio of densities of hydrogen or helium to air at NTP (dimensionless) = 0.0695 for 

hydrogen, 0.1368 for helium

θ = Thermal buoyancy ratio (dimensionless)

ρ = Density, kg/m3

φ = Stratification factor = cT/cavg (dimensionless) 

Δ = Difference

Subscripts:

1 to 9 Sequence of gas sampling points from floor to ceiling

avg Spatial average over the height between vents inside the test room

i Inside test room

o Outside of test room, within building

x Transition height

B Bottom vent

Leak Leakage through cracks in test room

S Source of vehicle leakage or gas injection in the test room

T Top vent
vi 

Contents

Abstract ......................................................................................................................................... iii

Acknowledgments ........................................................................................................................ iv

Nomenclature .................................................................................................................................v

1.0 Introduction .........................................................................................................................1

1.1 Leak Rates .........................................................................................................................2

1.2 Leak Velocity ....................................................................................................................2

1.3 Wind Effects ......................................................................................................................2

1.4 Temperature Effects ..........................................................................................................3

1.5 Scope of Laboratory Test ..................................................................................................3

2.0 Apparatus and Procedures.................................................................................................4

3.0 Test Results ..........................................................................................................................9

3.1 Flow Visualization ............................................................................................................9

3.2 Leak Testing ......................................................................................................................9

3.3 Ventilation Measurements ...............................................................................................11

4.0 Simplified Model ...............................................................................................................14

4.1 Leakage Correction .........................................................................................................16

5.0 Entrainment and Stratification .......................................................................................17

6.0 Experimental Error ..........................................................................................................19

7.0 Computational Fluid Dynamics Model Validation ........................................................20

8.0 Conclusions ........................................................................................................................25

9.0 References ..........................................................................................................................26
1 

1.0 Introduction

This study addresses the safety issue of hydrogen gas leakage within a building, as from a 

hydrogen-powered vehicle parked in a residential garage. Any leakage of unignited hydrogen 

will mix with indoor air and may form a flammable mixture. The most widely accepted 

flammability range is 4.1% to 75% hydrogen by volume [1]. To provide a safety margin, many 

standards [e.g., 2,3] specify a safe limit of 25% of the lower flammability limit (LFL), which for 

hydrogen amounts to 1% by volume. Passive (buoyancy-driven) and active (mechanical)

ventilation strategies may be used to prevent the accumulation of hydrogen at flammable 

concentrations; both approaches have limitations. Researchers need to understand the 

relationship between leakage scenario, ventilation design, and hydrogen concentration to develop 

safety standards and codes to address this risk. 

Previous studies of indoor hydrogen leaks have employed simplified analyses, computational 

fluid dynamics (CFD) modeling, and laboratory testing. Swain and Swain [4] modeled transient 

accumulation of hydrogen within a cloud above a leak in a passively ventilated room, with leak 

rates of 10 to 1000 LPM. Breitung et al. [5] modeled transient hydrogen cloud formation 

resulting from pulsed releases of small amounts of hydrogen (insufficient to form a combustible 

concentration if fully mixed in the enclosure). Venetsanos et al. [6] conducted a CFD intercomparison exercise in which 12 participants modeled an experimental transient release of 

hydrogen into a garage-sized enclosure, with pressure-relief vents only. Those comparisons 

emphasize the sensitivity of the results to the modeling approach; the key variables are 

turbulence model, time and space resolution, and discretization scheme. Swain et al. used 

helium to study leakage scenarios experimentally [7] and tested hydrogen ignition and 

combustion scenarios [8,9,10]. Helium is useful in experimental work because (1) it avoids the 

hazard of possible ignition during the experiment, (2) its physical properties are similar to those

of hydrogen,1

Our study specifically addresses a slow, sustained, indoor hydrogen gas leak that, over time, may

develop a flammable mixture throughout a large portion of the garage volume. We have 

previously reported [17] analysis results, which include a simplified, one-dimensional, steadystate analysis of buoyancy-driven ventilation (Section 4), and CFD modeling (Section 7). The 

simplified model yields an algebraic formula relating hydrogen concentration to vent size, 

height, and discharge coefficient; leakage rate; and stratification factor. However, the 

stratification factor depends on dynamics that are beyond the scope of the simple model. The 

CFD model provides additional detail, including the stratification factor. In addition to our 

discovery of parametric relationships among the variables, we believe our study is the first to 

and (3) it is straightforward to account for the differences between hydrogen and 

helium in the models. Gupta et al. [12] tested transient releases of helium in a garage-sized 

enclosure with a pressure-relief vent only. Papanikolaou and Venetsanos [13] modeled three of 

the helium experiments performed by Swain et al. Wilkening and Baraldi [14] used CFD to 

simulate and compare outdoor releases of hydrogen and methane gas from a pierced pipeline. 

Venetsanos and Bartzis [15] modeled outdoor releases of liquid hydrogen and compared CFD 

predictions with experimental results. Schmidt et al. [16] modeled outdoor release of gaseous 

hydrogen and compared results obtained with the FLUENT® software—which we also use—

with those of other CFD software and with experimental results.

 

1 Although helium is almost twice as dense as hydrogen, the buoyancy relative to the density of air is 93% for hydrogen and 86% 

for helium. The diffusivities in air are 0.756 cm2

/s for hydrogen and 0.697 cm2/s for helium at NTP [11].
2 

describe the significant thermal effect that occurs when the outdoor air temperature is higher than 

the indoor air temperature, so that thermally driven air circulation opposes the effect of hydrogen 

buoyancy. The isothermal condition is an important reference case that we address in this 

article, which describes our laboratory testing and comparison of experimental results with

model predictions. 

1.1 Leak Rates 

Because the hydrogen-powered vehicle is an emerging technology, we do not yet have a 

statistically significant set of historical data describing accidental leak rates. Schefer et al. [18] 

present equations for calculating leak rates in various flow regimes, based on leak diameter. If 

these equations were coupled with physical descriptions of vehicle component failure modes 

(which are not yet available), corresponding leak rates could be calculated. Draft Standard SAE 

J2578 [19] tentatively prescribes a maximum allowed leak rate of 0.15 LPM for a standard 

passenger vehicle leaving a factory. However, the possibility of higher leak rates resulting from 

faulty maintenance, collision damage, or other degradation of components after the car leaves the 

factory should also be considered. Parsons Brinkerhoff [20] cites a hypothetical “on-board 

computer that is capable of shutting down hydrogen flow upon receiving a signal detecting a 

larger than 20 CFM [566 LPM] flow, ‘leak,’ when the vehicle is dormant.” Thus, we may 

consider the range of “slow” leaks as roughly 0.15 to 566 LPM. A much higher leakage rate of 

about 30,000 LPM may occur when a pressure relief device is triggered; however, it does not 

seem practical to design a building ventilation system that can prevent a combustible mixture 

under this condition. We used leak rates ranging from 2.4 to 50 LPM in laboratory testing. 

1.2 Leak Velocity

Leakage of hydrogen from a fuel system with expected pressures of about 35 to 70 MPa may 

occur at a high velocity in an arbitrary direction. Such a high-velocity leak may agitate and mix 

room air with hydrogen, minimizing stratification. However, if the leak occurs within the body 

of a vehicle, hydrogen may emerge from the vehicle at a much lower velocity.2

1.3 Wind Effects 

 Another lowvelocity leak scenario is boil-off of liquid hydrogen. A low-velocity leak minimally agitates

room air and thus enables more hydrogen stratification. For a given vent size and leak rate, more 

stratification leads to higher concentrations of hydrogen in the room (Section 4). Thus, for 

passive ventilation as a safety approach, a low-velocity leak is worse than a high-velocity leak. 

We used low leak velocities in our analyses and testing to explore worst-case conditions. A 

possible exception to this generalization might be a leak jet directed toward an inlet vent, so that 

the leak momentum opposes buoyancy-driven influx of outside air. 

Wind pressure on a building may affect buoyancy-driven ventilation of hydrogen. The wind 

may help or hinder ventilation, depending on the wind direction relative to the inlet and outlet 

vent locations. One approach to minimizing the likelihood of adverse wind effects is to locate 

the inlet (bottom) vent on the prevailing upwind side of the building and the outlet (top) vent on 

the prevailing downwind side. In this configuration, the prevailing winds are more likely to 

augment than to oppose the buoyancy-driven flow. Another approach, where architecturally 

feasible, is to distribute the high vent area among several outside walls of different orientations, 

 

2 The accumulation of hydrogen inside the body of the car poses an additional risk.
3 

so that wind in any direction would drive cross-ventilation, thus assisting the buoyancy-driven 

ventilation. Assuming that negative wind effects can be avoided architecturally, the absence of 

wind is a worse case than windy conditions, so we conducted our modeling and laboratory

testing in the absence of wind. 

1.4 Temperature Effects 

Buoyancy-driven ventilation is very sensitive to temperature differences. For example, a 1% 

change in the density of air, which determines the driving force for vent flow, may be caused by 

a 1.07% concentration of hydrogen, 1.16% helium, or a 3 K difference in temperature (based on 

the ideal gas law at 300 K). Thus, in a residential garage application, outdoor air that is a few 

degrees warmer than the indoor air would have a significant effect on the hydrogen concentration 

resulting from a leak; such thermal conditions occur frequently in most climates. 

Thermocirculation (the stack effect) opposes the buoyancy-driven ventilation of hydrogen, and 

warmer outside air may enter the top vent while hydrogen accumulates near the ceiling. This 

effect, which we modeled in our previous paper [17], is a significant concern for the reliability of 

buoyancy-driven ventilation as a safety approach. However, the isothermal condition is an 

important reference case for model validation and other purposes, and we address this case in our 

testing. 

1.5 Scope of Laboratory Test

We tested buoyancy-driven ventilation of helium in a full-scale test room, about the size of a 

typical one-car garage. This room is shielded from the wind inside a conditioned laboratory 

building. We tried to test under isothermal conditions, although there was some temperature 

variation during the tests (Section 2). We tested low-velocity leakage at rates of 2.4 to 50 LPM, 

although cases lower than 9 LPM are not reported because of the interference of thermal effects

(Section 3.3). In addition to reporting the experimental data, we also show the correspondence 

of the data to both our simplified model and our CFD model, which we improved through this 

exercise. We describe in detail the CFD model features with which we achieved good 

agreement. 
4 

2.0 Apparatus and Procedures

The test room (Figures 1 and 2) is located inside the Thermal Test Facility building at the 

National Renewable Energy Laboratory (NREL) in Golden, Colorado. At 1,800 m above sea 

level, the nominal air density ratio (ADR) is 0.804 [21]. During our tests, we measured the 

atmospheric pressure with a mercury-column barometer attached to the outside of the test room. 

According to our simple model (Section 4), atmospheric pressure does not influence the 

relationship between design variables in volumetric terms. (The effect of density on the 

buoyancy force is offset by its effect on the pressure drops through the vents.) We represented 

the actual pressure in the CFD model and studied sensitivity to this variable (Section 7). We also 

used the pressure measurements to convert gas flow rates (S) from NLPM in the flow control 

valve to LPM in Table 1 (Section 3.3). 

Figure 1. Inside of test room, showing point diffuser, sampling column, vents, and door
5 

Figure 2. Outside of test room, showing vents and door. The distance between the test room and 

the building wall is 2.44 m.
6 

The test room is a rectangular prism measuring 4.29 m wide, 7.02 m long, and 2.74 m high. The 

two vents were open rectangles cut in 11.1-mm (7/16-in.) plywood panels located in one of the 

end walls (4.29 m wide). The vents were horizontally centered in the wall, with their centers 

36.8 cm above the floor and below the ceiling; thus, the height between the vent centers (h) was

2.01 m. The vent openings were 32.4 cm wide by 24.3 cm high. The plywood vent panels were 

installed in channels cut through the test room wall, 81 cm wide, 61 cm high, and 17.1 cm deep 

(Figure 2). These channels were sized so that 97.5% of the flow pressure drop occurred across 

the vent cutouts. Thus, the vent geometry approximated a rectangular hole in a large, thin wall. 

To minimize air leakage from the test room, we replaced acoustical ceiling tiles with acrylic light 

lenses and sealed the edges with metallic tape. We also taped all obvious cracks in the walls and 

floor, and applied weatherstripping to the door. The floor material is plywood. Three of the 

interior wall surfaces consist of aluminum-clad foam insulation of the type used for walk-in 

coolers, and the fourth wall surface is foam core. A glass window (2.31 m wide and 2.11 m 

high) in one of the side walls separates the test room from the instrument room. We tested the 

helium leakage rate from the room with the vents closed and corrected for this leakage in our 

calculations (Section 4.1). The effective leakage area was about 1% of the vent area. 

We released compressed helium gas from a tank outside the test room through a mass flow 

control valve (Porter model 202, factory calibrated for 0 to 100 NLPM of helium) into the test 

room through one of two low-velocity diffusers. The “point” diffuser consisted of an automobile 

oil filter element3

We used a thermal conductivity gas analyzer (Teledyne 2000A-EU) to measure helium 

concentration, drawing samples through a system of 1.59-mm (1/16-in.) inside diameter 

polyethylene tubing and manual valves from 11 locations in the test room. These locations 

included a column of nine sample points centered in each foot of height between the floor and 

ceiling,

 with a cylindrical height of 9.6 cm and diameter 8 cm (Figure 1; Figures 5 and 

6 in Section 3). The active area was the curved side only: 242 cm

2

. Thus, with the highest 

helium injection rate of 50 LPM, the injection velocity was only 3.45 cm/s. The point diffuser 

was located at variable height above the center of the test room floor (yS in Table 1, Section 3). 

The “line” diffuser consisted of a 1.83-m length of 17.5 mm (11/16-in.) outside diameter, porous 

rubber, garden-type soaker hose (Figure 3), positioned 61 cm (yS) above the floor for Cases L1 

and L2. The surface area of the soaker hose was 1,003 cm2

, of which 260 cm2was loosely 

wedged into the angle-aluminum support bar and 743 cm2was unobstructed. Thus, with our 

highest helium injection rate used with the line diffuser, 40.4 LPM, the injection velocity was 

only 0.91 cm/s, based on the unobstructed area. The linear release pattern might be more similar

to hydrogen escaping from under the body of a vehicle than the point release pattern. 

4

 and one sample point in the center of each vent (Figure 1). The sampling column was 

located halfway between the injection diffuser and the vents.5

 

3 We chose this object based on its size, shape, and porosity to provide a low leak velocity. This is not intended to 

represent an actual leakage path in a vehicle.

 We used a tank of compressed air 

for the reference gas and zero calibration, and calibrated the measurement scale (span) to a 

compressed sample certified as 3.99 ± 0.04% helium in air. We adjusted the linearity correction 

function in the instrument to match a second sample certified as 2.01 ± 0.02% helium in air, with 

quadratic interpolation between the three reference points (0.00%, 2.01%, and 3.99%). Before 

any helium was released into the room, the instrument indicated a significant nonzero 

4 These are numbered 1 to 9, from floor to ceiling, in Table 1.

5 According to our CFD model results (Section 7, Figure 10), the vertical concentration profile is rather uniform 

across the length and width of the test room.
7 

measurement of indoor air—the room zero offset (RZO)—which is correlated to humidity ratio 

in Figure 4. Values of RZO ranged from 0.04% to 0.19% in the test cases. Because the test 

room is enclosed in a building, background helium concentrations of 0.01% to 0.10% 

accumulated in the air inside the building and outside the test room. We corrected for the 

combined effects of humidity and background helium by sampling the air entering the bottom 

vent and subtracting this measurement from all 11 samples for each steady-state measurement 

shown in Table 1. Thus, each measurement at the bottom vent (column B in Table 1) is zero by 

procedure. We deemed conditions to be steady when changes in the concentration 

measurements, on the order of hundredths of a percent over several minutes, tended to oscillate 

rather than drift in one direction. The elapsed times to steady state ranged from 72 to 190 

minutes in the test cases; the longest times corresponded to thermal transients in the building. 

Figure 3. Line diffuser used to inject helium in Cases L1 and L2

We also tested the sensitivity of the measurement of the calibration gas to length of tubing, 

between the shortest (3.66 m) and longest (9.14 m) sample tube lengths, and observed no 

measurable sensitivity to length. This result rules out significant effects of flow resistance 

through the tubing, diffusion of helium through the tubing, or contamination of the sample gas 

by the tubing material.
8 

Figure 4. Measured effect of humidity on gas analyzer indication

We tried to maintain isothermal conditions between the test room and the surrounding building. 

This is both challenging and critical, because a temperature difference as slight as 0.3 K

corresponds to about 0.1% helium in density effect,6

Temporal variations. Because of its heat capacity, the test room lagged the building as the 

temperature varied between the heating and cooling control setpoints because of equipment 

cycling and changes in the weather, solar gains, and occupant behavior. To minimize these 

variations, we conducted testing overnight during steady, cool weather when practical. 

 which is a significant measurement 

concern. We used shielded thermocouples to monitor temperatures at the heights of each vent, 

both inside and outside the test room. The ΔT reported in Table 1 is the inside temperature 

minus the outside temperature, averaged over the two measurement heights. Ti is the average of 

the two inside temperatures. For an analysis of thermal effects, the temperature profile between 

the vent heights should be measured inside and outside the test room. The ΔT we measured 

provides only a rough indication of the relevant thermal condition. Two temperature control 

problems arose: 

Spatial variations. The part of the building just outside the test room vents was cooler than other 

parts of the building because of a nearby below-grade outside wall. By virtue of heat exchanges 

through the other test room walls, the test room was generally warmer than the air just outside 

the vents. 

Temperature variations are discussed further in Section 3.3. 

 

6 See discussion in Section 1: Temperature effects, and Equation 1 (Section 3.3) with θ = 1.

y = -1033.4x2 + 52.395x - 0.2063

R2

 = 0.9956

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.004 0.005 0.006 0.007 0.008 0.009 0.010 0.011 0.012 0.013

Humidity Ratio (lbw/lbda)

Room Zero Offset (% He)
9 

3.0 Test Results

We conducted three phases of testing: 

• Flow visualization

• Test room leakage measurement with vents closed 

• Ventilation measurements with vents open. 

3.1Flow Visualization

Figures 5 and 6 illustrate a flow visualization test we conducted using smoke generated by two 

burning paraffin wicks (Regin 220 Smoke Pen) near the helium point diffuser. Figure 5 shows 

the undisturbed smoke streams before any helium was released. Figure 6 shows the deflection of 

the smoke with a release rate of 20 LPM, demonstrating (1) negligible radial displacement of 

helium away from the diffuser; (2) horizontal entrainment of air into the helium plume above the 

diffuser; and (3) laminar flow of the entrained smoke. Based on our observations at the time of 

the test, the lack of a visible smoke stream from the wick above the center of the diffuser was 

due to the high vertical velocity of the plume at that location. An additional test with a release 

rate of 100 LPM showed no visible difference from the 20 LPM case. The laminar appearance 

of the smoke stream supports the use of the laminar CFD model (Section 7). The relationship 

between air entrainment and helium stratification is discussed in Section 5. 

3.2Leak Testing

To test the rate of helium leakage from our test room with the vents closed, we initially planned 

to use the following procedure to perform a constant-concentration test: 

• Inject helium and mix the room air with a fan until a uniform target concentration is achieved. 

• Turn off the fan and adjust the helium injection rate to maintain a constant concentration. 

• Record the steady-state concentration and injection rate. 

However, the leakage rate was below the functional range of the flow control valve. Thus, we 

used this alternative decay test procedure: 

• Inject helium and mix the room air with a fan until a uniform target concentration is achieved. 

• Turn off the fan and the helium injection.

• Wait for a period of time.

• Record the elapsed time and the helium concentration profile. 

The test results are as follows:

• Initial concentration (uniform) 2.96% 

• Elapsed time 70 minutes

• Final concentration (average over height) 2.91% (φ ≈ 1)

• Volume of test room 82.7 m3

• Rate of helium loss 0.60 LPM 

We corrected the ventilation measurements for test room leakage as described in Section 4.1. 
10 

Figure 5. Smoke visualization with no helium release. Linear distance scales are in inches.
11 

Figure 6. Smoke visualization with 20 LPM helium release. Linear distance scales are in inches.

3.3Ventilation Measurements

Results of the ventilation tests are shown in Tables 1 and 2. The letter “P” in the case number 

indicates use of the point diffuser, and the letter “L” indicates the line diffuser (Section 2). Table

1 shows direct measurements. These concentration data are corrected for humidity and 
12 

background helium as described in Section 2. Table 2 shows derived results that are discussed in 

the following sections. 

In order to gauge the significance of the thermal effects related to ΔT (Section 2; Table 1), we 

define the metric θ as the ratio of the effect on air density caused by the temperature difference to 

that caused by the helium concentration (with T in K): 

)1(

1

δ

θ −

+

+ −

=

avg

iTiB

oToB

c

TT

TT

(1) 

A value of θ = 0 indicates no thermal effect (isothermal conditions), and a value of θ = 1 

indicates a thermal effect that is equal to the helium effect. This comparison is only 

approximate, because the measurement of temperature at two heights does not capture the 

temperature stratification profile, which also affects the dynamics. The values of θ shown in 

Table 2 range from 0.05 to 0.40. The three test cases most closely approximating isothermal 

conditions (with the lowest values of θ) are P4, L1, and L2. We used Case P4 in the CFD model 

validation exercise (Section 7). Four additional test cases, not shown in Tables 1 and 2, yielded 

values of θ > 0.5. These cases include three with S < 9 LPM and one with ΔT = 0.8 K. The 

derived results for each of these cases stand out as anomalies that are not consistent with the 

other cases. We believe the cause of the anomalies is the relatively large deviation from 

isothermal conditions. A more detailed study of thermal effects is beyond the scope of this 

experiment, because of the limited temperature measurements we recorded.

Table 1. Measured Test Results

Case yS S Po ΔT Ti 

(m) (LPM) (mm-Hg) (K) (K) B T 1 2 3 4 5 6 7 8 9

P1 0.61 9.8 622 0.42 297.2 0.00 1.19 0.00 0.00 0.00 0.06 0.19 0.56 0.92 1.14 1.17

P2a 0.61 22.0 622 0.53 297.2 0.00 2.00 0.00 0.01 0.02 0.11 0.35 0.93 1.61 2.00 2.02

P2b 0.61 22.0 622 0.58 297.2 0.00 1.97 0.00 0.01 0.02 0.09 0.33 0.91 1.60 1.96 1.99

P3 0.61 40.3 622 Unk 296.4 0.00 2.86 0.01 0.02 0.03 0.09 0.47 1.42 2.43 2.85 2.87

P4 0.91 12.3 622 0.22 296.3 0.00 1.49 0.00 0.00 0.01 0.02 0.12 0.48 0.98 1.43 1.51

P5 0.91 24.5 619 0.61 296.0 0.00 2.47 0.01 0.01 0.02 0.04 0.14 0.57 1.45 2.34 2.56

P6 1.22 18.4 619 0.36 296.1 0.00 2.16 0.00 0.00 0.00 0.02 0.06 0.28 0.75 1.84 2.67

L1 0.61 22.0 625 0.14 295.6 0.00 1.66 0.08 0.17 0.40 1.26 1.58 1.64 1.65 1.66 1.66

L2 0.61 40.4 625 0.25 295.5 0.00 2.40 0.13 0.27 0.71 1.97 2.33 2.38 2.39 2.41 2.42

He Concentration (%)
13 

Table 2. Derived Test Results

Case cT φ yx LE D1 D2 θ F

(%) (--) (m) (m) (--) (--) (--) (--)

P1 1.19 2.96 1.70 1.09 0.650 0.549 0.40 2000

P2a 2.00 2.84 1.71 1.10 0.653 0.582 0.29 894

P2b 1.97 2.86 1.72 1.11 0.671 0.582 0.33 918

P3 2.86 2.79 1.68 1.07 0.690 0.607 [a] 515

P4 1.49 3.55 [b] 1.85 0.94 0.640 0.583 0.21 1559

P5 2.47 3.94 [b] 1.93 1.01 0.624 0.531 0.38 765

P6 2.16 5.45 [b] 2.18 0.96 0.674 0.579 0.36 1102

L1 1.66 1.37 [b] 0.93 0.32 0.603 0.590 0.05 824

L2 2.40 1.33 [b] 0.89 0.28 0.623 0.607 0.05 464

Notes: [a] ΔT was not measured.

[b] Stratification not fully developed; yx estimated from point of inflection.
14 

4.0 Simplified Model

We have previously presented [17] a simplified, one-dimensional, steady-state model of 

buoyancy-driven ventilation under isothermal conditions, leading to this vent-sizing equation:

( ) ( )

( )

2

1

3

2

2

1

1

111 2

















−

−+−− ≡ =

T

T T

c

c c gh S

AD F

δ

δ φ (2) 

Figure 7 compares our experimental results (points are based on Table 2) to the simple model

(curves and curve segments are based on Equation 2). The figure shows how the equation 

correctly tracks the relationship between the dimensionless vent size (F), stratification factor (φ), 

and concentration at the top vent (cT) over significant ranges of these variables. The range of 

stratification factors shown in Table 2 and Figure 7 is discussed in Section 5. For this 

comparison, we calculated values of the vent discharge coefficient (D) from the test data. First, 

we calculated the volumetric flow rate through the top vent, QT, based on the continuity equation 

for helium: 

TT = ScQ (3) 

T T = / cSQ (4) 

Then, based on continuity of both air and helium, 

TB −= SQQ (5)

Then, as developed previously [17], we calculated: 

2

1

2

2 2

)(2 















−

+ =

o avg

BoTT

ghA

QQ D

ρρ

ρρ (6) 

This formulation assumes equal values of D for the top and bottom vents, and it assumes that the 

flow through the vents is driven only by the static pressure difference, rather than by momentum 

or other effects. The symbol D1 denotes the apparent value of D given by Equation 6 with 

experimental values of the densities based on helium concentration: 

o [ ] c −−= δρρ )1(1 (7) 

Because D1 is calculated using the assumptions of the simple model, the agreement between the 

simple model and the test data is exact when D1 is inserted into Equation 2, as shown in Figure 7. 

The validity of the simple model can then be evaluated by comparing values of D1 with 

established values of D for this type of vent. The values of D1 listed in Table 2 range from 0.603 

to 0.690. These numbers are somewhat higher than the reference value 0.60, corresponding to a 

resistance coefficient reported by Idelchik [22] for an orifice in a thin wall with infinite surface 
15 

area, which approximates our geometry. Possible reasons for the higher values in our 

experiment are momentum and thermal effects.

Figure 7. Comparison of simple model (curves and curve segments based on Eqn. 2) with 

experimental data (points from Table 2)

Momentum generated by the buoyant upward motion of the plume is deflected horizontally by 

the ceiling. The component of that momentum in the direction of the top vent may augment the 

buoyancy force in driving flow through the top vent. We designed a test to measure this effect. 

As shown in Figure 8, we installed a foam-board baffle to block the line-of-sight path from the 

top of the plume to the top vent. The baffle, hinged at the ceiling, is 91.4 cm wide, 62.2 cm high, 

and offset 1.09 m from the vent. In Case P2a, the baffle was removed from the flow path by 

taping it flat against the ceiling. Then, the tape was removed so the baffle deployed to the 

position shown in Figure 8. After waiting again for steady-state conditions, Case P2b was 

recorded. The values of D1 are nearly identical for the two cases (Table 2), ruling out significant 

momentum effects on vent flow, at least for this case. (The slight increase in D1 with the baffle 

deployed is attributed to the change in thermal conditions between the two measurements, as 

shown by ΔT in Table 1.) 

To compensate for thermal effects, we modified Equation 7 as follows (T in K): 

[ ] 



 





+

+ ⋅−−=

iTiB

oToB

o TT

TT

c δρρ )1(1 (8) 

The values of D2 shown in Table 2 are based on Equation 6 with ρavg calculated by Equation 8. 

These values range from 0.531 to 0.607. Although the four measurement points (oB, oT, iB, and 

iT) only approximate the average temperatures over the vent height inside and outside the test 

room, this comparison indicates the sensitivity of D to the deviation from isothermal conditions 

0

500

1000

1500

2000

2500

1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00

cT = H2 Concentration by Volume, %

F = Vent Sizing Factor

P4

P1

P6

P2a

L2

P5

L1

P3

φ=1.00

φ=6.00 φ=2.96

φ=3.55

φ=1.37 φ=2.84

φ=5.45

φ=1.33

φ=3.94

φ=2.79
16 

in our tests. The differences between the experimental values of D and the literature reference 

value are of similar magnitude as the thermal effects.

Figure 8. Vent baffle used to test momentum effect in Case P2b

The simple model is a good predictor of steady-state concentration under isothermal conditions. 

However, the stratification factor φ must be determined by some other means (Section 5).

4.1Leakage Correction

The rate at which helium escapes from the test room through vents or cracks depends on the 

concentration (cT) and stratification (φ), as shown by Equation 2. In the leakage test (Section

3.2), both the concentration and stratification were different from those in the ventilation test 

cases, because (1) the concentration varied among the test cases and (2) the stratification depends 

on vent flow. To correct for leakage in the ventilation tests, we used the simple model to 

calculate the equivalent vent area of the cracks based on the leakage test results, the height 

between vents, and an assumed value of D = 0.60. From Equation 2: 

( ) ( )

( )

2

1

3

2

2

1

1

111

2 















−

−+−− =

T

T T

Leak

c

c c

ghD

S A

δ

δ φ (9) 

Equation 9 yields ALeak = 7.83 cm2. Based on the physical dimensions of the vents, the area of 

each open vent is A = 791.7 cm2. To account for the leakage area, we adjusted the vent area 

using 

A’ = A + ALeak (10) 

Thus, A’ = 791.7 + 7.8 = 800 cm

2

 or 0.0800 m2. We used this value of A’ in our simple model 

calculations and in the CFD model. 
17 

5.0 Entrainment and Stratification

Previous researchers have described the relationship between the entrainment of fluid into a 

plume and the resulting stratification pattern for buoyancy-driven displacement ventilation. 

Linden et al. [23] showed, through analysis and experiments with dyed liquid, a stratification 

pattern consisting of two rather uniform layers sharply divided by an interface plane. The plane 

occurs at the height at which the amount of fluid entrained in the plume is equal to the flow rate 

through the inlet vent; thus, all the mass transport across the plane is conveyed by the plume, and 

the vertical velocity through the remainder of the plane is zero. Lane-Serff et al. [24] presented a 

thorough description of buoyant plume geometry for cases in which “the density difference 

between the plume and the ambient fluid is relatively small,” which is not the case here. 

Lowesmith et al. [25] numerically solved the coupled differential equations involving continuity 

of mass, momentum, and buoyancy in their model for leaking methane-hydrogen mixtures. 

Our test results show three distinct layers in a fully developed stratification pattern (e.g. Case P3, 

Figure 9): 

• The bottom layer with near-zero helium concentration 

• The top layer with a rather uniform nonzero helium concentration 

• The middle layer with a gradual transition between the helium concentrations below and 

above. 

Figure 9. Stratification patterns for three test cases, from data in Table 1

Convective mixing and diffusion of helium across the interface plane are both significant causes 

of the transition layer (see Section 7). For analysis, we define the transition height (yx) as the 

location where the concentration of helium is halfway between the values in the top and bottom 

layers. In some of our test cases (e.g. Case P5, Figure 9), the transition layer terminates at the 

ceiling and the top layer does not occur. In Cases L1 and L2 (Figure 9), the transition layer 

terminates at the floor and the bottom layer does not occur. In these cases, we identify yx at the 

0.0

0.5

1.0

1.5

2.0

2.5

0.0 0.5 1.0 1.5 2.0 2.5 3.0

He Concentration, %

Height, m

P3

P5

L2
18 

point of inflection in the stratification curve, based on the symmetry observed in the fully

developed cases. We further define the entrainment length (LE) as the vertical distance between 

the injection diffuser (the leak) and the transition height. As shown in Table 2, LE ≈ 1 m over a 

variety of test conditions with the point diffuser. With the line diffuser, LE ≈ 0.3 m. Thus, over 

the range of cases we tested, leak geometry is the primary variable affecting LE. The 

combination of source height (yS) and entrainment length (LE) determines the transition height 

(yx). The stratification factor (φ) depends on the positions of the vents within the stratification 

profile.
19 

6.0 Experimental Error

The sources of error in our measurements include: 

• Tolerance on the 4% calibration gas sample: ±0.04% helium 

• Nonlinearity of gas analyzer between calibration points: ±0.02% helium 

• Factory calibration of mass flow valve used for helium injection: ±1 NLPM

• Humidity overcorrection. We corrected for the effect of humidity on the gas analyzer by 

subtracting the measurement at the inlet (bottom) vent from all the measurements 

(Section 2). At the highest measured helium concentration (3% in the room leakage test), 

the humidity ratio was diluted to 97% of its air-only value. Thus, we overcorrected by 

3% of humidity offset. The largest humidity offset in any of the test cases was 0.20% 

helium. Therefore, this error is no more than 0.006% helium. 

• Leakage from the test room: 0.60 LPM under leak test conditions. We corrected for this 

leakage (Section 4.1), so the remaining error should be negligible compared to the flow 

valve error. 

• Temperature control and measurement (see below). 

• Nonsteady conditions during measurements (see below). 

The largest source of error, with regard to model validation, is the deviation from isothermal 

conditions, along with the coarseness of our thermal measurements (only two measurement 

points inside the test room and two outside) and nonsteady thermal conditions. An estimate of 

the total error is reflected in the ranges of our derived values of D1 and D2 (Table 2):

• D1 ranges from 0.603 to 0.690, average 0.648 

• D2 ranges from 0.531 to 0.607, average 0.579, range 0.076. 

The difference of 0.069 between the D1 and D2 averages (12% of 0.60) indicates the approximate 

magnitude of thermal effects on vent flow and thus cT (per Equation 3). The range of 0.076 in 

the temperature-corrected values of D2 (0.57 ± 7%) indicates the approximate magnitude of 

nonsteady effects and the coarse resolution of our temperature measurements.
20 

7.0 Computational Fluid Dynamics Model Validation

We have previously [17] developed a CFD model for the slow leak/natural ventilation scenario 

under study. We used the FLUENT version 6.3.26 CFD software [26], with an implicit, 

pressure-based solver and first order discretization of convection and time for speed and stability. 

We explored both the renormalization group (RNG) k-ε turbulence model with the differential 

viscosity option enabled (see Section 12.19.5 in reference [27]) and the laminar model. The 

results indicated turbulence intensities lower than 1% in more than 85% of the domain in all the 

cases we ran. Other researchers have found that k-ε turbulence models tend to overpredict 

mixing of gas released from slow leaks [13], thus underpredicting stratification, and the use of a 

laminar model leads to a more stratified hydrogen distribution [5]. Based on Equation 2 and 

Figure 7 (Section 4), higher stratification corresponds to higher hydrogen concentrations for a 

given vent size and leak rate. Thus, the laminar model is more conservative for safety 

engineering, and we used the laminar model for the ventilation study. The flow visualization test 

described in Section 3.1 supports the choice of the laminar model.

In our previous work, the model domain corresponded to the interior of the enclosure, with 

atmospheric pressure boundary conditions at the inlet and outlet vents. This configuration led to 

overprediction of flow through the vents, because the loss in total pressure on the outside of the 

vents was neglected. We modified our model by including a space outside the vents in the 

domain. The outside space is physically bounded by the floor and wall of the outer building 

(Figure 2), and analytically bounded by atmospheric pressure boundary conditions on the other 

surfaces. This model agreed very well with test results, as described below. The model 

determines static pressure differences caused by the hydrostatic head outside the domain, based 

on the specified ambient outdoor air density. We used the incompressible ideal gas option7

 in 

FLUENT to determine density as a function of the air-hydrogen (or -helium) mixture within the 

domain. Time steps were changed during the simulation to speed convergence. Initially, the 

time step was 0.1 seconds to capture the transient behavior as the leak started. When the 

iterations to convergence at each time step dropped below 10, the time step was increased. The 

largest time step used was 0.4 to 0.5 seconds.8

We previously used a polyhedral

 

9

 

7 With the incompressible ideal gas option, operating pressure only is used in the density calculation, and this is 

suitable for low Mach number flows. Density would be calculated as a function of operating pressure and local 

relative pressure in a compressible, high Mach number flow. See Section 8.3.5 in reference [27].

 mesh for computational economy. However, for improved 

resolution of flow through the vents, we used a tetrahedral mesh in the current study. We 

arranged the experiment and the model with bilateral symmetry, so the model domain for half the 

space represents each half. A sensitivity study showed that the solution is insensitive to grid 

density at about 150,000 to 200,000 cells, over the 72.2 m3

 domain (41.3 m3 inside the test room 

and 31.0 m3

 outside). The computations reported in this article were performed with 205,215 

cells. Thus, the average size of a grid cell was 0.35 L (350 cm3), with a higher cell density near 

the leak and the vents where gradients are high, and a lower cell density where gradients are 

lower. Because we were studying steady-state conditions, we could reduce computation time by 

8 The Courant number was not monitored during the simulations, because the stability of the implicit solution 

method does not rely on this indicator, as would an explicit solution method. In retrospect, the value of the Courant 

number was typically 0.2.

9 The FLUENT software generates a mesh consisting of polyhedra with a variable number of faces.
21 

using the final conditions from one computation as the initial conditions for the next one, 

decreasing the process time to steady state. With this approach and the indicated mesh density, 

the computation time was typically 24 to 48 hours per case on a Dell Precision 670 dualprocessor computer with 8 GB of memory, running the Red Hat Enterprise Linux operating 

system. However, some cases required up to a week for the model to reach a stable steady-state 

solution. 

For model validation, we selected the point diffuser test case that came closest to isothermal 

conditions, based on the smallest value of θ—Case P4. We modeled the atmospheric pressure 

measured at the test room under assumed isothermal conditions. Figure 10 shows a mole 

fraction contour map of the steady-state solution. Figure 11 shows a comparison of the measured 

and modeled stratification profiles at the measurement points. In the laboratory, steady 

conditions were observed after about 190 minutes. The CFD model reached steady state in less 

time10 but was continued for 355 minutes to verify the steady solution. Although we did not 

record the transient response of concentration versus time with the CFD model, the behavior was 

similar to that shown in our previous paper (see Figure 4 in reference 17). Concentration 

increased monotonically, with no significant oscillation. The shapes of the profiles in Figure 11 

agree very well, although the modeled profile is offset about 20 cm below the measured profile. 

The difference in concentration is 0.03% helium near the floor and 0.05% helium near the 

ceiling. This is very good agreement for safety engineering. The differences in concentration 

are in the direction of thermal effects that occurred in the laboratory.

11

Because our laboratory is located 1,800 m above sea level in Golden, Colorado, we studied the 

sensitivity of the CFD result to elevation. We adjusted the ADR from 1.000 at sea level to 0.804 

in Golden and the diffusion coefficient for helium in air from 0.733 cm2/s at sea level to 0.905 

cm

2

/s in Golden, based approximately on inverse proportion with respect to pressure. As shown 

in Figure 12, there is negligible sensitivity of the result to elevation.

 The value of D 

corresponding to the CFD result is 0.527, about 10% lower than the value of D2 for the test case.

12

Figure 13 shows a comparison of the CFD result with hydrogen versus helium as the leaking gas. 

This difference is important because models are typically validated against experimental results 

using helium and then adjusted for hydrogen to assess the risk in hydrogen leakage scenarios. 

Our results show a hydrogen concentration higher than the helium concentration by a difference 

of 0.2% near the ceiling. In a similar CFD comparison, Swain et al. [7] found that helium and 

hydrogen concentrations were very nearly equal near the ceiling, although the test conditions 

were different than ours in those cases. Possible causes of this difference include physical 

phenomena (such as the higher velocity in the hydrogen plume) and CFD modeling issues. 

 Because this result 

surprised us, we ran an additional comparison with the diffusion coefficient set to 0. This result, 

also shown in Figure 12, indicates a narrower transition layer. We interpret this result to mean 

that the transition layer is caused by both convective mixing and diffusion of helium across the 

interface plane. (See also discussion in Section 5.) 

 

10 Thermal transients extended the time to steady state in the lab but were not an issue in the isothermal CFD model.

11 With ΔT > 0, as shown in Table 1, thermal buoyancy augments helium buoyancy in driving vent flow, thus 

decreasing the steady-state concentrations. The CFD model, which is based on assumed isothermal conditions, is 

not influenced in this manner.

12 The CFD computations compared in Figure 12 used the same volumetric leakage rate (12.3 LPM), not the same 

mass leakage rate.
22 

Figure 10. Graphic result of CFD model for Case P4. Cross section of test room, through vents, is 

shown. Color scale is helium mole fraction; the full-scale value of 0.02 corresponds to 2% helium 

by volume. Elapsed time is 171 minutes (steady state). 
23 

Figure 11. Comparison of CFD result with test result for Case P4

Figure 12. Sensitivity of CFD result to elevation above sea level and diffusion coefficient

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

He Concentration, %

Height, m

Lab

CFD

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

He Concentration, %

Height, m

Golden

Sea Level

Zero Diff
24 

Figure 13. Comparison of CFD results for helium and hydrogen

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

Concentration, %

Height, m

He

H2 
25 

8.0 Conclusions

We have generated a set of experimental data, summarized in Tables 1 and 2, with sufficient 

documentation that others can use it to validate models of buoyancy-driven ventilation in the 

slow, low-velocity leak scenario under isothermal conditions with no wind effects. 

The test results confirm our simple model formulation, which indicates parametric relationships 

among the design variables. The agreement is indicated by (1) experimental values of D that 

agree well with the literature reference value for similar geometry, and (2) exact agreement of

the model with test results when these values of D are applied (Figure 7).

The measured stratification factors (φ) range from 1.33 to 5.45 (Table 2). The lower limit is 

1.00; however, there is no theoretical upper limit. φ depends on the leak height above the floor, 

the leak geometry, the height of the enclosure, and the locations of the vents within the 

stratification profile. Specific leak scenarios likely to be encountered in practice are needed to 

determine a realistic range for φ.

Comparisons of CFD model results with test results led to an improvement of our CFD model, in 

which the atmospheric pressure boundary condition was moved away from the vents, outside the 

enclosure. The improved model agrees well with test results (Figure 11). We have described our 

modeling approach in sufficient detail that others can use it. 

A significant uncertainty in applying our findings is the range of leakage rates that will occur 

with hydrogen vehicles. Our models can predict hydrogen concentrations for given leakage 

scenarios. However, further research is needed to ascertain the occurrence probabilities of 

various leakage scenarios before this research can be applied to safety engineering for buildings. 

Temperature differences between the inside and outside of the enclosure where leakage occurs 

can have a significant effect on the indoor hydrogen concentration. Thermocirculation may 

either augment or oppose ventilation driven by hydrogen buoyancy. In the laboratory, a ΔT as 

small as 0.3 K is equivalent to about 0.1% concentration, which is a measurement concern. In 

garages, a ΔT of 3 K is equivalent to about 1% concentration, which is a safety engineering 

concern. This effect can occur in nearly any climate because of steady-state or transient 

temperature differences between the garage and the ambient air, in either hot or cold weather. 

Further research is needed to evaluate the ranges of architectural and weather conditions under 

which this effect is dangerous.
26 

9.0 References 

1. Bain, A.; Barclay, J.A.; Bose, T.K.; Edeskuty, F.J.; Fairlie, M.J.; Hansel, J.G.; Hay, D.R.; 

Swain, M.R.; Ohi, J.M.; Gregoire Padró, C.E. (1998). Sourcebook for Hydrogen 

Applications, TISEC, Inc., Montreal, Canada. 

2. International Code Council. International Fire Code, Section 2211.7.1.1, 2006, 

Washington, D.C. www.iccsafe.org, last viewed 5 June 2007. 

3. National Fire Protection Association. Vehicular Fuel Systems Code, Section 9.3.3.5.4.1, 

2006, Quincy, MA. www.nfpa.org, last viewed 12 June 2007. 

4. Swain, M.R.; Swain, M.N. (1996). “Passive Ventilation Systems for the Safe Use of 

Hydrogen.” International Journal of Hydrogen Energy 21:823–835. 

5. Breitung, W.; Necker, G.; Kaup, B.; Veser, A. (2001). “Numerical Simulation if 

Hydrogen Release in a Private Garage.” Proceedings of Hypothesis IV, 9–14 September 

2001, Strahlsund, Germany, p. 368. 

6. Venetsanos, A.G.; Papanikolaou, E.; Delichatsios, M.; Garcia, J.; Hansen, O.R.; Heitsch, 

M.; Huser, A.; Jahn, W.; Jordan, T.; Lacome, J.-M.; Ledin, H.S.; Makarov, D.; Middha, 

P.; Studer, E.; Tchouvelev, A.V.; Teodorczyk, A.; Verbecke, F.; Van der Voort, M.M. 

(2009). “An Inter-Comparison Exercise on the Capabilities of CFD Models To Predict 

the Short and Long Term Distribution and Mixing of Hydrogen in a Garage.” 

International Journal of Hydrogen Energy, accepted. 

7. Swain, M.R.; Filoso, P.; Grilliot, E.S.; Swain, M.N. (2003). “Hydrogen Leakage Into 

Simple Geometric Enclosures. International Journal of Hydrogen Energy 28:229–248. 

8. Swain, M.R.; Filoso, E.S.; Swain, M.N. (2005). “Ignition of Lean Hydrogen-Air 

Mixtures.” International Journal of Hydrogen Energy 30:1447–1455. 

9. Swain, M.R.; Filoso, E.S.; Swain, M.N. (2007). “An Experimental Investigation Into The 

Ignition of Leaking Hydrogen” International Journal of Hydrogen Energy 32:287–295. 

10. Swain, M.R. (2001). “Fuel Leak Simulation.” Proceedings of U.S. DOE Hydrogen 

Program Review, Baltimore, MD, 17–19 April 2001, pp. 679–689. NREL Report No. 

CP-610-30535, www.nrel.gov/docs/fy01osti/30535.pdf. 

11. CRC Handbook of Chemistry and Physics. (2007). 87th edition, Boca Raton, FL: CRC 

Press.

12. Gupta, S.; Brinster, J.; Studer, E.; Tkatschenko, I. (2007). “Hydrogen Related Risks 

Within a Private Garage: Concentration Measurements in a Realistic Full Scale 

Experimental Facility.” Proceedings of the 2nd International Conference on Hydrogen 

Safety, 11-13 September 2007, San Sebastian, Spain. 
27 

13. Papanikolaou, E.A.; Venetsanos, A.G. (2005). “CFD Modeling for Helium Releases in a 

Private Garage Without Forced Ventilation. Proceedings of First International 

Conference on Hydrogen Safety, 8–10 September 2005, Pisa, Italy. 

conference.ing.unipi.it/ichs2005/index.html, last viewed 27 April 2007. 

14. Wilkening, H.; Baraldi, D. (2007). “CFD Modelling of Accidental Hydrogen Release 

From Pipelines.” International Journal of Hydrogen Energy 32:2206–2215. 

15. Venetsanos, A.G.; Bartzis, J.G. (2007). “CFD Modeling of Large-Scale LH2 Spills in 

Open Environment.” International Journal of Hydrogen Energy 32:2171–2177. 

16. Schmidt, D.; Krause, U.; Schmidtchen, U. (1999). “Numerical Simulation of Hydrogen Gas 

Releases Between Buildings. International Journal of Hydrogen Energy 24:479–488. 

17. Barley, C.D.; Gawlik, K.; Ohi, J.; Hewett, R. (2007). “Analysis of Buoyancy-Driven 

Ventilation of Hydrogen From Buildings.” Proceedings of the 2nd International 

Conference on Hydrogen Safety, 11–13 September 2007, San Sebastian, Spain. NREL 

Report No. CP-550-41081, www.nrel.gov/docs/fy07osti/41081.pdf. 

18. Schefer, R.W.; Houf, W.G.; San Marchi, C.; Chernicoff, W.P.; Englom, L. (2006). 

“Characterization of Leaks From Compressed Hydrogen Dispensing Systems and Related 

Components.” International Journal of Hydrogen Energy 31:1247–1260. 

19. Standard SAE J2578. (2008). Surface Vehicle Recommended Practice, Revised Proposed 

Draft, SAE International, Warrendale, PA. www.sae.org/servlets/index, last viewed 17 

December 2008. 

20. Parsons Brinkerhoff. (2004). Support Facilities for Hydrogen-Fueled Vehicles: 

Conceptual Design and Cost Analysis Study. West Sacramento, CA: California Fuel Cell 

Partnership. www.fuelcellpartnership.org/pdf/ResourceDocs/FacilitiesStudy08.2004.pdf. 

21. American Society of Heating, Refrigerating and Air-Conditioning Engineers. (2005). 

Handbook—Fundamentals, Chapter 6, Eqn. 3, Atlanta, GA. 

22. Idelchik, I.E. (1986). Handbook of Hydraulic Resistance, 2nd ed., Diagram 4-18. New 

York: Hemisphere Publishing Corp. 

23. P.F. Linden, G.F. Lane-Serff, D.A. Smeed, Emptying filling boxes: the fluid mechanics 

of natural ventilation, J. Fluid Mechanics 212 (1990) 309-335. 

24. G.F. Lane-Serff, P.F. Linden, M. Hillel M, Forced, angled plumes, J. Hazardous 

Materials 33 (1993) 75-99.

25. B.J. Lowesmith, G. Hankinson, C. Spataru, M. Stobbart, Gas build-up in a domestic 

property following releases of methane/hydrogen mixtures, Proceedings of the 2nd

International Conference on Hydrogen Safety, San Sebastian, Spain, 11-13 September 

2007. 

conference.ing.unipi.it/ichs/fileadmin/user_upload/CD/PAPERS/12SEPT/1.1.62.pdf. 
28 

26. FLUENT, Inc. www.fluent.com, last viewed 26 April 2007. 

27. FLUENT 6.3 User’s Guide, 

www.ipt.ntnu.no/manuals/Fluent.Inc/fluent6.3.26/help/index.htm, last viewed 2 April 

2009. 
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Buoyancy-Driven Ventilation of Hydrogen from Buildings: Laboratory 

Test and Model Validation

5a. CONTRACT NUMBER

DE-AC36-08-GO28308

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

C.D. Barley and K. Gawlik

5d. PROJECT NUMBER

NREL/TP-550-45804

5e. TASK NUMBER

H2747110

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory

1617 Cole Blvd.

Golden, CO 80401-3393

8. PERFORMING ORGANIZATION

REPORT NUMBER

NREL/TP-550-45804

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR'S ACRONYM(S)

NREL

11. SPONSORING/MONITORING

AGENCY REPORT NUMBER

12. DISTRIBUTION AVAILABILITY STATEMENT

National Technical Information Service

U.S. Department of Commerce

5285 Port Royal Road

Springfield, VA 22161

13. SUPPLEMENTARY NOTES

14. ABSTRACT (Maximum 200 Words)

Hydrogen gas leaking from a hydrogen-powered vehicle in a residential garage may form a flammable mixture with 

air. Passive, buoyancy-driven ventilation is one approach to limiting the concentration to a safe level. We explored 

the relationship between leak rate, ventilation design, and hydrogen concentration through laboratory testing, an 

algebraic analysis, and CFD modeling. We used helium to test slow, steady, low-velocity leaks in a full-scale test 

room under nearly isothermal, steady conditions, and we report the results in sufficient detail that other modelers can 

use them. The results show the importance and variability of stratification. Our algebraic and CFD models agree 

very well with the experimental results. We describe our CFD approach in sufficient detail for use by others. We 

tested under nearly isothermal conditions, but also discuss indoor-outdoor temperature difference as an important 

risk factor. Information about realistic leakage scenarios is needed to apply these results as safety 

recommendations.

15. SUBJECT TERMS

hydrogen; buoyancy; ventilation; cfd; computational fluid dynamics

16. SECURITY CLASSIFICATION OF: 17. LIMITATION 

OF ABSTRACT

UL

18. NUMBER 

OF PAGES

19a. NAME OF RESPONSIBLE PERSON

a. REPORT

Unclassified

b. ABSTRACT

Unclassified

c. THIS PAGE

Unclassified 19b. TELEPHONE NUMBER (Include area code)

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