The scope of work for this project includes safe building design, vehicle leak in residential garage, continual slow leak, passive, buoyancy-driven ventilation (versus mechanical), and steady-state concentration of hydrogen versus vent size

Barley, C. D.

Gawlik, K.

Hewett, R.

Ohi, J.

UNT Digital Library

Analysis of Buoyancy-Driven Ventilation of Hydrogen from BuildingsC. Dennis Barley, Keith Gawlik, Jim Ohi, Russell HewettNational Renewable LaboratoryU.S. DOE Hydrogen Safety, Codes & Standards ProgramPresented at 2nd ICHS, San Sebastián, SpainSeptember 11, 2007NREL/PR-550-42289Scope of Work• Safe building design• Vehicle leak in residential garage• Continual slow leak• Passive, buoyancy-driven ventilation          (vs. mechanical)• Steady-state concentration of H2 vs. vent sizePrior Work• Modeling and testing with H2 and He• Transient H2 cloud formation__________Swain et al. (1996, 2001, 2003, 2005, 2007) Breitung et al. (2001)Papanikolaou and Venetsanos (2005)Our Focus / New Findings• Slow continual leaks• Steady-state concentration of H2• Algebraic equation for vent sizing• Significant thermal effect (high outdoor temp)Range of “Slow” Leakage Rates• Low end: 1.4 L/min per SAE J2578       (vehicle manufacture quality control)• High end: 566 L/min automatic shutdown     (per Parsons Brinkerhoff for CaFCP)• Consider: Collision damage or faulty maintenance • Parametric CFD modeling:                           5.9 to 82 L/min (12 hr to 7 days/5 kg)Methods of Analysis• CFD modeling (FLUENT)• Simplified, 1-D, steady-state, algebraic analysisU.S. Depar tment  of  EnergyPulte Homes, Las Vegas, NVVolume of garage is 146 m3Volume of 5 kg of H2 is 60 m341% mixture is possibleWell within flammable rangeSample CFD Model ResultCFD modeling used to study H2 cloud. Half of garage is shown. Leak rate is 5 kg/24 hours (41.5 L/min). Vent sizes 790 cm2. Elapsed time = 83 min.  Full scale is 4% H2 by volume.H2 concentration at top vent increases monotonically and reaches a steady value in about 90 minutes.  A flammable mixture does not occur in this case.Sample CFD Model ResultSimulation Setup• FLUENT version 6.3• Poly mesh for computational economy• Grid density study showed solution invariant at approx. 40,000 cells (Avg. ~1.8 L/cell)• High mesh density near inlet, outlet, gas leak• Laminar flow model used (more conservative than turbulent models)• No diffusion across vents at model boundarySimulation Setup• Hydrogen concentration at outlet monitored to determine steady state• 5 kg discharge times from 12 hours to 1 week• Low speed leak from 8-cm-diameter sphere• Leak ~1 m above floor, one model near ceiling• Vent sizes and height variedConcept of 1-D ModelTypical H2 stratification determined by CFD model(steady-state condition)1-D Parametric AnalysisPressure Loop / BuoyancyΔP1-2 + ΔP2-3 + ΔP3-4 + ΔP4-1 = 0ΔP1-2 + ΔP3-4 = g h ρair cavg (1-δ)P = Total pressureh = Height between ventsc = Concentration of H2, by volumeρ = Densityg = Acceleration of gravityδ = Density of H2 / density of airρ142 −Δ= PADQQ = Volumetric flow rateA = Vent areaD = Discharge coefficient(Similar at bottom vent)Vent Flow vs. Pressure1-D Parametric AnalysisSteady-State Mass BalancesQT cT = SQ = Volumetric flow ratecT = H2 concentration at top vent, by volumeS = Volumetric H2 source rate1-D Parametric Analysiswhere:F = Vent sizing factor, dimensionlessA = Vent area (top = bottom), m2CT = H2 concentration at top vent, by volume (0-1)D = Vent discharge coefficient (0-1)S = Source rate of H2 (leak rate), m3/sg = Acceleration of gravity = 9.81 m/s2h = Height between vents, mδ = Ratio of densities of H2/Air = 0.0717φ = Stratification factor = CT/Cavg (Cavg = average over height)( ) ( )( )21322111112 ⎥⎥⎦⎤⎢⎢⎣⎡−−+−−=≡TTTCCCghSADF δδφIsothermal Vent-Sizing Equation:1-D Parametric AnalysisCurves illustrate isothermal vent-sizing equation.  Points 1-7 are CFD results.010002000300040005000600070000.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00cT = H2 Concentration by Volume, %F = Vent Sizing Factor124  5  6  73φ  =  1.0,  1.5,  2.0,  3.0  left to rightComparison of Models1 2 3 4 5 6 7Leak-Down Time, hr/5 kg 168 72 48 24 24 24 12Vent Size, cm2 788 788 788 788 788 788 1576Vent Offset, cm 0.0 0.0 0.0 0.0 15.2 30.5 0.0Vent Height, m 3.650 3.650 3.650 3.650 3.345 3.040 3.599H2 Conc. at top vent, % Vol. 0.47 0.79 1.04 1.55 1.63 1.69 1.75Straification Factor (φ ) 1.65 1.67 1.67 1.52 1.58 1.59 1.88Discharge Coeff. (D*) 0.952 0.952 0.952 0.965 0.948 0.944 0.903CFD CaseSpecifications,           ResultsSeries of CFD CasesRanges of Parameters• Stratification factor (φ):                                  1.52 to 1.88• Apparent discharge coefficient (D*):         0.903 to 0.965D* higher than typical D (0.60 to 0.70)D* includes momentum effectsFurther study needed (experimental)Reverse ThermocirculationWhen outdoor temperature is higher than indoor (garage) temperature, thermal circulation opposes H2-buoyancy-driven circulation.Thermal Case StudyLeak rate = 5 kg/12 hours. Vent size = 1,580 cm2.Tamb-Tcond = 20°C.  Elapsed time = 3.3 min.Full scale = 4% H2 by volume.Leak rate = 5 kg/12 hours. Vent size = 1,580 cm2.Tamb-Tcond = 20°C.  Elapsed time = 11.7 min.Full scale = 4% H2 by volume.Thermal Case StudyLeak rate = 5 kg/12 hours. Vent size = 1,580 cm2.Tamb-Tcond = 20°C.  Elapsed time = 15 min.Full scale = 4% H2 by volume.Thermal Case StudyLeak rate = 5 kg/12 hours. Vent size = 1,580 cm2.Tamb-Tcond = 20°C.  Elapsed time = 33 min.Full scale = 4% H2 by volume.Thermal Case StudyLeak rate = 5 kg/12 hours. Vent size = 1,580 cm2.Tamb-Tcond = 20°C.  Elapsed time = 2.8 hr (steady state). Full scale = 4% H2 by volume.Thermal Case StudyLeak rate = 5 kg/12 hours. Vent size = 1,580 cm2.Tamb-Tcond = 20°C.  Thermal Case StudyA Perfect StormExtreme thermal scenarioGarage strongly coupled to house & groundGarage weakly coupled to ambientHot day, cool ground, low A/C setpointSmall vents—sized for 2% H2 max with 1-D modelA Perfect StormU.S. Depar tment  of  EnergyHeartland Homes, Pittsburgh, PAA Perfect StormAmbient conditions modeled• Ambient temp. = 40.6°C (Approx. max. in Denver)• Ground temp = 10°C  (Denver, mid-April)• A/C setpoint = 21.1°C (Rather low)Reverse Flow ScenarioH2 exiting through bottom ventCase 9.  Leak rate = 5 kg/7 days. Vent size = 494 cm2.Elapsed time = 31 hr (steady state). Full scale = 1.5% H2 by volume.A Perfect StormResults• Case 8 (1-day leak):  Vents from top, 2.3% max• Case 9 (7-day leak):  Vents from bottom, 1.0% max • Case 10 (3-day leak):  Vents from top, 4.8% max0123450 5 10 15 20 25Time (hr)H2 Concentration, % by volumeNo Thermal EffectsWith Thermal EffectsA Perfect StormWorst thermal case we modeledCase 10.  Leak rate = 5 kg/3 days. Vent size = 405 cm2.Conclusions1. The leakage rates that will occur and their frequencies are unknown.  Further study of leakage rates is needed to put parametric results into perspective.2. Our CFD model has not yet been validated against experimental data.  • Uncertainty in results• Future work3. The 1-D model ignores thermal effects, but otherwise provides a safe-side estimate of H2 concentration by ignoring momentum effects (pending model validation).4. Indicated vent sizes would cause very low garage temperatures in cold climates, for leak rates of roughly 6 L/min and higher (leak-down in 1 week or less).Conclusions5. Reverse thermocirculation:• Can occur in nearly any climate • The worst case we modeled increased the expected H2 concentration from 2% to 5%.  This is a significant risk factor, • Likelihood of occurrence may be low, judging by the lengths we went to in order to identify a significant example.Conclusions6. Mechanical ventilation is alternative approach to safety.  • H2-sensing fan controller is recommended.  • Research is needed to develop a control system that is sufficiently reliable and economical for residential use.Conclusions

Analysis of Buoyancy-Driven Ventilation of Hydrogen from Buildings

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Analysis of Buoyancy-Driven Ventilation of Hydrogen from Buildings

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