Hydrogen gas dispersion studies for hydrogen fuel cell vessels II: Fuel cell room releases and the influence of ventilation

Results are presented for computational fluid dynamics (CFD) modeling for varying hydrogen leaks within a hydrogen vessel's Fuel Cell Rack inside a Fuel Cell Room. In the limiting case of no room ventilation, modeling shows that the flammable region produced by the hydrogen leak is initially limited by self-induced entrainment and recirculation of air caused by the buoyant rising of hydrogen. Locally and at shorter times (minutes), this effect can be even more influential in limiting the size of the flammable envelope than Fuel Cell Room ventila-tion. Interestingly, the more diffuse detectable (but sub-flammable) region is not self-limited. This indicates the recirculation pattern required for the self-limiting effect requires a sufficient concentration of hydrogen to establish and differentiate the rising hydrogen mass from the surrounding air, thereby establishing the recirculation pattern that self-limits the flammable region at short times. Modeling results with the Fuel Cell Room ventilation activated shows that several seconds after a hydrogen leak is initiated, the flammable region reaches a steady state, with only minor fluctuations due to the air currents created by ventilation. The expected trends with ventilation rate are found: for a given leak size, a decreasing flammable envelope is found as ventilation is increased and for a given level of ventilation, an increasing hydrogen leak rate produces a larger flammable region. For the cases and ventilation rates examined, flammable H2/air mixtures greater than 4% clear the Fuel Cell Room within 1.5 s after the hydrogen leak is turned off. The CFD modeling results for the detectable level of hydrogen that would trigger an alarm showed that higher ventilation rates might have the unintended consequence of making a hydrogen leak harder to detect, depending on the location of the gas detector in the Fuel Cell Room For the hydrogen leak rates considered in this study, we find that a ventilation rate of 15 ACH provides timely hydrogen evacuation while allowing the leak to be detected by the ceiling-mounted hydrogen monitor (for most monitor locations). (c) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Automated summary

This study investigates the effects of hydrogen leaks on the flammable region inside a fuel cell room using computational fluid dynamics modeling. The results show that the size of the flammable region is initially limited by self-induced entrainment and recirculation of air caused by the buoyant rising of hydrogen. Ventilation has an impact on the stability of the flammable region. By adjusting the ventilation rate and leak level, timely hydrogen evacuation and leak detection can be achieved.