Analysis of auto-ignition of heated hydrogen-air mixtures with different detailed reaction mechanisms

Auto-ignition processes of hydrogen, diluted with nitrogen, in heated air are numerically investigated by means of an unsteady laminar flamelet approach in mixture fraction space. The focus is on the auto-ignition delay time and the most reactive mixture fraction as obtained with five chemical mechanisms. Two strongly different levels of dilution, corresponding to experiments in the open literature, are considered. This concerns low-temperature chemistry at atmospheric pressure. The temperature of the air stream is much higher than the temperature of the fuel stream in the cases under study. We extensively investigate the effect of the co-flow temperature, the conditional scalar dissipation rate and the resolution in mixture fraction space for one case. With respect to the conditional scalar dissipation rate, we discuss the Amplitude Mapping Closure (AMC) model with imposed maximum scalar dissipation rate at mixture fraction equal to 0.5, as well as a constant conditional scalar dissipation rate value over the entire mixture fraction value range. We also illustrate that an auto-ignition criterion, based on a temperature rise, leads to similar results as an auto-ignition criterion, based on OH mass fraction, provided that the hydrogen is not too strongly diluted

Numerical study of the adhered smoke plume for fire in an atrium

In case of fire in an adjacent room to an atrium, one of the most important parameters in fire safety design is the smoke free height in the atrium. A recent article [1] reports on a series of small-scale experiments and the development of a new one-line equation for the mass flow rate of the spill plume. In our paper, we first describe a calculation method to determine the smoke layer interface height in CFD-simulations. A series of CFD-simulations is validated, based on the experiments of [1]. Large-scale atria are also simulated and discussed. The presence of a “more-dimensional” effect is detected and discussed

Analysis of the impact of the inlet boundary conditions in FDS results for air curtain flows in the near-field region

CFD results are discussed for planar jet flows, resembling configurations in use for air curtain flows in the context of smoke and heat control in buildings in case of fire. The CFD package FDS (Fire Dynamics Simulator), Version 6.0.1, is used. Special focus is given to the impact of the inlet boundary condition, in combination with the mesh size, on the flow field in the near-field region. Investigation of different slot configurations, including calculations inside a straight rectangular duct ahead of the air slot, reveals a small vena contracta effect when the slot is flush with a solid boundary, leading to an acceleration of the flow in the symmetry plane in the near-field region. More important is the effect of the duct length: starting from a top hat velocity profile, a duct length of about 15 hydraulic diameters is required for the flow to become fully developed at the slot. The vena contracta effect disappears if the co-flow at the nozzle exit is aligned with the jet. The FDS results capture the self-similarity in the far-field jet region, regardless of the inlet configuration

Application of FDS and firefoam in large eddy simulations of a turbulent buoyant helium plume

Large eddy simulations are conducted in the near-field region of a large turbulent buoyant helium plume. Such plumes are of relevance for fire safety research due to the similar flow features as in the buoyant (smoke) plumes above the fire source. The transient and mean flow dynamics are discussed with and without the use of a Smagorinsky-type subgrid scale (SGS) model. For this purpose, two different computational fluid dynamics (CFD) packages are used. Small-scale structures, formed at the edge of the plume inlet due to a baroclinic and gravitational mechanism and subject to flow instabilities, interact with large-scale features of the flow, resulting in a puffing cycle. This puffing cycle is recovered in the simulations. In general, the LES calculations reproduce the main features of the turbulent plume. Mean velocity results compare well with the experimental data. The mass fractions are overpredicted on the centerline though, and higher on the domain