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 : email@example.com
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 micro-manometer DM4. Pressure differential 10 Pa-70Pa. Mitigation Strategy : Used the built-in 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. 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 in upper layer Conservation of total mass in upper layer Constraint Equation Plume Modeling Classical Plume Mixing Model – self similar plume solution Buoyancy Flux Effective origin
Quarter scale two-car residential garage Prior work :Reduced scale experiments • 6.1 m x 6.1 m x 3.05 m • Helium – surrogate gas. • Mass flow controller. • Release of 5 kg of hydrogen. • Time resolved concentrations. • Idealized leaks, 3 ACH at 4 Pa • Two square vents, 2.15 cm Sensors Vents 0.75 m Comparison with Analytical Model 1.5 m 1.5 m Release Chamber
Prior Work : CFD modeling Computational Domain Sensors • NIST Fire Dynamics Simulator • Low speed, chemically reacting fluid flow. • Low Mach number, LES model • 2nd order, multi-block rectilinear grid. Vents 3.0 m 6.0 m 6.0 m Release Chamber Comparison of CFD (symbols) with analytical model (line)
Summary of tests conducted Estimation of Leak Area (ASTM E779-03) Natural and Forced Ventilation Tests with Automobile
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 Vent Height Effect of Compartment Volume
Effect of Input Parameters Effect of Upper Vent Area Effect of Lower Vent Area
Assisting Wind Flow Prior Work :Wind Driven Ventilation Weak / Strong Opposing Wind Flow
Effect of Hydrogen Release Rate Hydrogen Volume Fraction Height of the Interface Volumetric Flow Rates Compartment Overpressure
Time required to empty a compartment Wind assisted venting Buoyancy driven flow
Release as a Distributed Source • Fully mixed hydrogen – air mixture in compartment. • Goal – predict hydrogen concentrations in compartment • Pressure varies hydrostatically with depth. Vent flows driven by pressure difference. • Release of hydrogen under an obstruction. • Cluttered environment, Multiple plumes. • Buoyant gas mixes rapidly with surrounding air..
Problem Formulation Conservation of hydrogen mass Constraint Equation
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 : firstname.lastname@example.org
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.
Quarter scale two-car residential garage Prior Work :Reduced scale experiments • 6.1 m x 6.1 m x 3.05 m • Helium – surrogate gas. • Mass flow controller. • Release 5 kg of hydrogen. • Time resolved concentrations. • Idealized leaks, 3 ACH at 4 Pa • Two square vents, 2.15 cm Sensors Vents 0.75 m Comparison with Analytical Model 1.5 m 1.5 m Release Chamber
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 • Model the flow through vents - Bernoulli equation. • Pressure varies hydrostatically with depth. 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 Constraint Equation Design of Idealized Vents Leak rates – described in terms of air changes per hour (ACH). Exchange across enclosure boundary - Effective Leak Area (ELA) of enclosure related to ACH varies widely for garages (recommended value ACH=3)
40 Mpa, 5 kg tank, 1 mm release port Jet Exit Conditions Tank Pressure Exit Velocity
Compartment Overpressure Release and Dispersion Volume Fraction
Height of Interface Vent Flow Rates
40 Mpa, 5 kg tank, 6 mm release port Jet Exit Conditions Exit Velocity Tank Pressure
Release and Dispersion Compartment Overpressure Volume Fraction
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 Compartment Overpressure Volume Fraction
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 high-pressure 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 : email@example.com Fire Research Division Engineering Laboratory National Institute of Standards and Technology Gaithersburg, MD 20899.