CFD modelling of the effect of mechanical ventilation on hydrogen leak and dispersion in a partially enclosed space

This study employed a Computational Fluid Dynamics (CFD) approach based on STAR-CD code to investigate the effect of mechanical ventilation on hydrogen gas leaks and diffusion in partially enclosed space. It is a case study of a homogenous charged compression ignition engine (HCCI) laboratory of the Mechanical Engineering Department, University College London (UCL). The 3-D modelling was based on the geometry as well as airflow designed for the test laboratory. Two turbulence models and three differencing schemes were employed on two grid refinement levels. All the differencing Schemes predicted a similar velocity profile and hydrogen concentration below 25% of the lower flammability limit (LFL) in most parts of the test laboratory. Although the predicted hydrogen mass fraction from the steady state simulation does not resolve the buoyant shape of the gas, the time-dependent solution captures the buoyant characteristic of hydrogen. It revealed that the hydrogen gas initially rises to a height 0.55cm above the exit towards the ceiling, from where it gradually diffuses in a radial pattern to a homogenous non-flammable concentration in the room. This predicted pattern of hydrogen gas dispersion is consistent with experimental data. Therefore, a small hydrogen leak of the type and at the airflow rates investigated in this study does not pose a risk of fire in most parts of the Engine Test laboratory; except in the region very close to the leak source.