Type of Publication
Year of Publication
2007
Authors
C.D. Barley; K. Gawlik; J. Ohi; R. Hewett
Abstract

When hydrogen gas is used or stored within a building, as with a hydrogen-powered vehicle parked in a residential garage, any leakage of unignited H2 will mix with indoor air and may form a flammable mixture. One approach to safety engineering relies on buoyancy-driven, passive ventilation of H2 from the building through vents to the outside. To discover relationships between design variables, we combine two types of analysis: (1) a simplified, 1-D, steady-state analysis of buoyancy-driven ventilation and (2) CFD modeling, using FLUENT 6.3. The simplified model yields a closed-form expression relating the H2 concentration to vent area, height, and discharge coefficient; leakage rate; and a stratification factor. The CFD modeling includes 3-D geometry; H2 cloud formation; diffusion, momentum, convection, and thermal effects; and transient response. We modeled a typical residential two-car garage, with 5 kg of H2 stored in a fuel tank; leakage rates of 5.9 to 82 L/min (tank discharge times of 12 hours to 1 week); a variety of vent sizes and heights; and both isothermal and non-isothermal conditions. This modeling indicates a range of the stratification factor needed to apply the simplified model for vent sizing, as well as a more complete understanding of the dynamics of H2 movement within the building. A significant thermal effect occurs when outdoor temperature is higher than indoor temperature, so that thermocirculation opposes the buoyancy-driven ventilation of H2. This circumstance leads to higher concentrations of H2 in the building, relative to an isothermal case. In an unconditioned space, such as a residential garage, this effect depends on the thermal coupling of indoor air to outdoor air, the ground (under a concrete slab floor), and an adjacent conditioned space, in addition to temperatures. We use CFD modeling to explore the magnitude of this effect under rather extreme conditions.

Pagination
14 p.
Keywords