Vertical release of hydrogen in a partially enclosed compartment: Role of wind and buoyancy

The natural and wind driven mixing and dispersion of hydrogen released in an accidental manner in a partially enclosed compartment with two vents is investigated using theoretical tools. A simple analytical model is constructed to predict the entrainment of air in a buoyant turbulent hydrogen plume and the properties of the resulting two-layer stratification that drives the flow through the vents. Air flows in through vents below the position of neutral buoyancy and exits from vents above it. CFD simulations are conducted in a full-scale geometry to confirm the physical phenomena and to compare with the analytical results. Analytical results are also compared with experimental data from a 1/4 scale two-car residential garage. The analytical model is used to understand the important physical processes involved during hydrogen release as a vertical plume, and dispersion in a compartment with vents at multiple levels, with and without a steady wind. Parametric studies are conducted to study the effect of hydrogen release rate on location of the interface between the two layers and hydrogen volume fraction in the upper layer. Analytical model results indicate that for a given hydrogen release rate, the hydrogen concentration in the upper layer reaches a maximum under wind conditions that oppose the buoyancy induced flow, and that this maximum value can be as much as 70% higher than the case with no wind effects. Results also indicate that blowing outdoor air into the lower vent is an effective strategy for reducing the flammable volume of hydrogen gases in a compartment, following an accidental release. Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu.