Numerical validation of pressure peaking from an ignited hydrogen release in a laboratory-scale enclosure and application to a garage scenario

This work focuses on the overpressures arising from the rapid ignited release of hydrogen in an enclosure, specifically the peak in overpressure that may result in the initial period of the release, dependent on the level of ventilation. Two volumes are considered: a 1 m³ laboratory scale enclosure for which experimental data exists, and a real scale residential garage. Various vent configurations are considered for each scenario for leak rates typical of those from a fuel cell (laboratory scale enclosure) and from onboard hydrogen storage tanks through a thermally activated pressure relief device (TPRD) in the garage-like enclosure. A validation study has been performed for the laboratory scale enclosure and the modelling approach which gives optimum results has been identified. The influence of heat transfer on the pressure peak has been highlighted, particularly, the importance of radiation in predicting the pressure peak. The validated modelling approach has been applied to a range of experiments and garage scenarios. Both the laboratory and real scale simulations demonstrate the complex relationship between vent size and release rate and indicate the significant overpressures that can result through pressure peaking following an ignited release in an enclosure. The magnitude of the pressure peak as a result of an ignited release has been found to be two orders of magnitude greater than that for the corresponding unignited release. The work indicates that TPRDs currently available for hydrogen-powered vehicles may result in a dangerous situation for the specific scenario considered which should be accounted for in regulations, codes and standards. The application of this work extends beyond TPRDs and is relevant where there is a rapid, ignited release of hydrogen in an enclosure with ventilation.     

Large scale experiments and model validation of Pressure Peaking Phenomena-ignited hydrogen releases

The Pressure Peaking Phenomena (PPP) is the effect of introducing a light gas into a vented volume of denser gas. This will result in a nonequilibrium pressure as the light gas pushes the dense gas out at the vent. Large scale experiments have been performed to produce relevant evidence. The results were used to validate an analytical model. Pressure and temperature were measured inside a constant volume, while the mass flow and vent area were varied. The analytical model was based on the conservation of mass and energy. The results showed that increasing the mass flow rate, the peak pressure increases and with increasing the ventilation area, the peak pressure decreases. Peak pressure was measured above 45 kPa. Longer combustion time resulted in higher temperatures, increasing an underpressure effect. The experimental results showed agreement with the analytical model results. The model predicts the pressures within reasonable limits of+/-2 kPa. The pressure peaking phenomena could be very relevant for hydrogen applications in enclosures with limited ventilation. This could include car garages, ship hull compartments as well as compressor shielding. This work shows that the effect can be modeled and results can be used in design to reduce the consequences.     

Pressure effects of an ignited release from onboard storage in a garage with a single vent

A numerical study has been performed comparing the hazards, in particular overpressures, arising from the sustained unignited and ignited release from an onboard hydrogen storage tank at 700 bar through a 3.34 mm diameter orifice, representing a thermally activated pressure relief device (TPRD) in a small garage with a single vent equivalent in area to small window. It has been demonstrated how the overpressure predicted in the case of an unignited release using both CFD and an analytical model is in the region of 0.55 kPa and thus unlikely to cause structural damage. However, the overpressure predicted for the ignited release is two orders of magnitude greater, reaching over 55 kPA in less than 1 s and thus potentially causing destruction of the structure.It has been shown that whilst the overpressures resulting from the unignited release are unlikely to cause harm, the garage is engulfed by a flammable atmosphere in less than 1 s and the oxygen is depleted to levels dangerous to people within this time. In the case of the ignited release, whilst the resultant overpressures are the primary safety concern, it has been shown how the thermal effects resulting from the release extend almost 9 m from the jet in 1.5 s.     

Hydrogen jet fires in a passively ventilated enclosure

This paper describes a combined experimental, analytical and numerical modelling investigation into hydrogen jet fires in a passively ventilated enclosure. The work was funded by the EU Fuel Cells and Hydrogen Joint Undertaking project Hyindoor. It is relevant to situations where hydrogen is stored or used indoors. In such situations passive ventilation can be used to prevent the formation of a flammable atmosphere following a release of hydrogen. Whilst a significant amount of work has been reported on unignited releases in passively ventilated enclosures and on outdoor hydrogen jet fires, very little is known about the behaviour of hydrogen jet fires in passively ventilated enclosures. This paper considers the effects of passive ventilation openings on the behaviour of hydrogen jet fires. A series of hydrogen jet fire experiments were carried out using a 31 m³ passively ventilated enclosure. The test programme included subsonic and chocked flow releases with varying hydrogen release rates and vent configurations. In most of the tests the hydrogen release rate was sufficiently low and the vent area sufficiently large to lead to a well-ventilated jet fire. In a limited number of tests the vent area was reduced, allowing under-ventilated conditions to be investigated. The behaviour of a jet fire in a passively ventilated enclosure depends on the hydrogen release rate, the vent area and the thermal properties of the enclosure. An analytical model was used to quantify the relative importance of the hydrogen release rate and vent area, whilst the influence of the thermal properties of the enclosure were investigated using a CFD model. Overall, the results indicate that passive ventilation openings that are sufficiently large to safely ventilate an unignited release will tend to be large enough to prevent a jet fire from becoming under-ventilated.     

Pressure dynamics,self-ignition,and flame propagation of hydrogen jet discharged under high pressure

The self-ignition of hydrogen released from a high-pressure tank using extension tubes (2200 mm) with different diameters was studied. The processes of flame transition at a nozzle and jet flame development were characterized using a high-speed camera. The results indicated that the intensity of a shockwave and the Mach number decay faster in a 10-mm-diameter tube than that in a 15-mm-diameter tube. The pressure in a 15-mm-diameter tube was weaker than that in the 10-mm-diameter tube at the initial stage; however, it became higher in the later stage. Spontaneous ignition was more likely to happen in a 15-mm-diameter tube. The formation of a stabilized flame at the tube exit and Mach disk were observed during the transition of the flame to a jet fire. The stabilized flame showed a triangular shape because of the influence of a Prandtl–Meyer flow when a hydrogen jet entered a suddenly expanding environment. The formation and separation of a spherical flame were recorded during jet flame development. Large vortexes were formed in front of the flame because of the Kelvin–Helmholtz instability, which resulted in the separation of the spherical flame. The vortexes stopped rotating until the separated flame disappeared.     

Hydrogen non-premixed combustion in enclosure with one vent and sustained release: Numerical experiments

Numerical experiments are performed to understand different regimes of hydrogen non-premixed combustion in an enclosure with passive ventilation through one horizontal or vertical vent located at the top of a wall. The Reynolds averaged Navier–Stokes (RANS) computational fluid dynamics (CFD) model with a reduced chemical reaction mechanism is described in detail. The model is based on the renormalization group (RNG) k-ε turbulence model, the eddy dissipation concept (EDC) model for simulation of combustion coupled with the 18-step reduced chemical mechanism (8 species), and the in-situ adaptive tabulation (ISAT) algorithm that accelerates the reacting flow calculations by two to three orders of magnitude. The analysis of temperature and species (hydroxyl, hydrogen, oxygen, water) concentrations in time, as well as the velocity through the vent, shed a light on regimes and dynamics of indoor hydrogen fires. A well-ventilated fire is simulated in the enclosure at a lower release flow rate and complete combustion of hydrogen within the enclosure. Fire becomes under-ventilated at higher release flow rates with two different modes observed. The first mode is the external flame stabilised at the enclosure vent at moderate release rates, and the second mode is the self-extinction of combustion inside and outside the enclosure at higher hydrogen release rates. The simulations demonstrated a complex reacting flow dynamics in the enclosure that leads to formation of the external flame or the self-extinction. The air intake into the enclosure at later stages of the process through the whole vent area is a characteristic feature of the self-extinction regime. This air intake is due to faster cooling of hot combustion products by sustained colder hydrogen leak compared to the generation of hot products by the ceasing chemical reactions inside the enclosure and hydrogen supply. In general, an increase of hydrogen sustained release flow rate will change fire regime from the well-ventilated combustion within the enclosure, through the external flame stabilised at the vent, and finally to the self-extinction of combustion throughout the domain.     

Study on hazards from high-pressure on-board type III hydrogen tank in fire scenario: Consequences and response behaviours

Exploration of thermal performances of composite high-pressure hydrogen storage tank under fire exposure were critical issues to reduce the risk of tank rupture. Three bonfire tests of type III tanks of 210 L-35 MPa with full compressed hydrogen were exposed to a pool fire to study the response behaviours in fire scenarios. Detailed data on the tank wall temperature and inner pressure were presented in this work. Prototype bonfire tests for the type III tank indicated the failure pressure limits amounted to 41.1–41.8 MPa (average 41.4 MPa). Two consequences (rupture and hydrogen blowdown) will be caused when the inner pressure beyond this limits in fire scenario. The loading-bearing capacity of the tank reduced nearly 3 times under the prescribed fire condition when compared to its average burst pressure of 123.5 MPa conducted from the hydraulic burst test. Results also shown that fire resistance rating (FRR, time to rupture) of the three tanks were 784, 666, and 596, respectively. The FRR got shorter when the tank was exposed in the engulfing fire in advance at hydrogen blowdown case.     

Experimental and numerical investigation of localized fire test for high-pressure hydrogen storage tanks

Vehicle fires may cause localized fires on on-board high-pressure hydrogen storage tanks. To verify the safety performance of such tanks under localized fire exposure, a localized fire test was proposed in the Global Technical Regulation for Hydrogen Fuel Cell Vehicles. However, practicality and validity of the proposed test still require further verification. In this paper, this new fire test was experimentally investigated using the type 3 tanks. Influences of hydrogen and air as the filling media were studied. A three-dimensional computational fluid dynamics model was developed to analyze the effects of filling pressure and localized fire exposure time on the activation of thermally-activated pressure relief device (TPRD). The experimental results showed that temperature distribution on the tank surface was uneven around the circumference. The rising temperature of internal hydrogen or air contributed little to TPRD activation. The simulation results indicated that TPRD activation time was slightly affected by the variations of the filling pressures, but it increased when the localized fire exposure time was extended.