Hydrogen combustion in partially confined areas presents unique risks due to the interaction between gas accumulation and ventilation. Unlike open environments, partially confined spaces like parking garages or tunnels can trap hydrogen, leading to localized concentrations that may exceed flammability limits. The behavior of hydrogen in such settings is influenced by its low density, high diffusivity, and wide flammability range (4–75% in air). Computational fluid dynamics (CFD) studies have been instrumental in understanding these risks by simulating gas dispersion, ignition, and flame propagation under varying conditions.

One critical factor is the buoyancy-driven flow of hydrogen. Due to its low molecular weight, hydrogen rises rapidly in air, but in partially confined spaces, ceiling obstructions or poor ventilation can hinder dispersion. CFD analyses reveal that even moderate leaks can lead to stratified layers of hydrogen-rich mixtures near the ceiling. For example, a study modeling a parking garage with a ceiling height of 3 meters showed that a leak rate of 1 gram per second could create a flammable layer within 5 minutes under low ventilation conditions (0.5 air changes per hour). The stratification increases the risk of delayed ignition, as hydrogen may accumulate unnoticed until an external spark triggers combustion.

The flammability range of hydrogen also complicates risk assessment. While the lower flammability limit (LFL) of 4% is well-defined, the upper limit (75%) is rarely reached in practical scenarios due to dilution. However, CFD simulations demonstrate that localized pockets can exceed the LFL even when average concentrations remain low. In tunnel environments, where longitudinal airflow exists, hydrogen tends to form elongated plumes. A study of a 100-meter tunnel with a ventilation velocity of 1 meter per second found that a 10-liter leak could create a flammable zone extending 15 meters downstream within 2 minutes. The plume’s shape and stability depend on ventilation rate, leak location, and tunnel geometry.

Flame acceleration and deflagration-to-detonation transitions (DDT) are additional concerns in partially confined areas. Hydrogen flames propagate faster than hydrocarbon flames, with laminar burning velocities up to 3 meters per second. In obstructed environments like parking garages with structural beams or vehicles, turbulence can accelerate flames to supersonic speeds. CFD models of a garage with 30% obstruction density showed flame speeds exceeding 100 meters per second, leading to overpressures of 0.5 bar—enough to cause structural damage. Tunnels, with their long, narrow geometry, are particularly prone to DDT if flame acceleration exceeds critical thresholds. Research indicates that a hydrogen concentration of 13–25% in tunnels poses the highest DDT risk, with overpressures surpassing 10 bar in worst-case scenarios.

Ventilation plays a dual role in risk modulation. While adequate ventilation dilutes hydrogen below flammable concentrations, uneven airflow can create recirculation zones where gas accumulates. CFD studies of parking garages with mechanical ventilation systems highlight that improperly placed vents may exacerbate stratification. For instance, a vent placed near a leak source can draw hydrogen into dead zones behind parked vehicles, creating isolated flammable pockets. In tunnels, natural ventilation from traffic-induced piston effects is unreliable; cross-sectional variations or bends can disrupt airflow, allowing hydrogen to pool in unexpected areas.

The ignition energy of hydrogen (0.02 mJ) is another critical factor. Electrostatic discharges or electrical equipment in partially confined spaces can easily ignite hydrogen-air mixtures. CFD simulations incorporating ignition sources show that even small sparks (0.1 mJ) can trigger combustion if they occur within a flammable pocket. The resulting flame front may propagate unpredictably due to interactions with obstacles and ventilation currents. In one simulation of a tunnel with intermittent ventilation, an ignition at the tunnel midpoint led to bidirectional flame propagation, with one front moving against the airflow at 50 meters per second.

Radiation effects from hydrogen fires are less pronounced than in hydrocarbon fires due to the absence of soot, but the high flame temperature (up to 2,300°C) poses direct thermal hazards. CFD analyses of garage fires indicate that nearby vehicles or structural elements can absorb and re-radiate heat, creating secondary ignition risks. In tunnels, the confined geometry intensifies radiative heat flux, with models predicting 10 kW/m² at 5 meters from the flame front—a level capable of igniting combustible materials.

Quantitative insights from CFD studies underscore the importance of leak rate and ventilation interaction. A parametric analysis of garage scenarios found that leak rates above 0.5 grams per second consistently produced flammable mixtures unless ventilation exceeded 2 air changes per hour. For tunnels, the critical ventilation velocity to prevent upstream flame propagation was determined to be 3 meters per second for hydrogen concentrations below 10%. However, these values are highly scenario-dependent, emphasizing the need for site-specific modeling.

The dynamic nature of hydrogen dispersion in partially confined spaces also affects sensor placement strategies. CFD results demonstrate that traditional ceiling-mounted sensors may fail to detect stratified layers if placed too far from leak sources. In tunnels, sensor spacing of less than 10 meters is recommended to account for plume variability, whereas garages may require dense arrays near potential leak points like fuel cell vehicle parking spots.

In summary, hydrogen combustion risks in partially confined areas are governed by complex interactions between gas behavior, ventilation, and geometry. CFD studies provide validated insights into dispersion patterns, ignition dynamics, and flame propagation, highlighting scenarios where traditional risk assessment methods may fall short. The data underscores the need for tailored safety designs in environments like parking garages and tunnels, where hydrogen’s unique properties demand specialized engineering solutions.