Large eddy simulation and experimental measurements of the near-field of a large turbulent helium plume

Abstract

Large eddy simulations (LES) are conducted of a large, 1 m in diameter, turbulent helium plume. The plume instability modes and flow dynamics are explored as a function of grid resolution with and without the use of subgrid scale (SGS) models. LES results reproduce well-established varicose puffing mode instabilities as well as secondary “finger-like” azimuthal instabilities leading to the breakdown of periodically shed toroidal vortices. Simulation results of time-averaged velocity and concentration fields show excellent agreement with experimental data collected from Sandia’s FLAME facility using particle image velocimetry and planar laser induced fluorescence measurement techniques. For locations very near the base of the plume, i.e., X/Dp<0.5, the LES overpredicts the measured root-mean squared streamwise velocity and concentration and, in addition, is found to be highly sensitive to grid resolution. The cause of these discrepancies is attributed to unresolved buoyancy-induced vorticity generation on resolved scales of fluid motion that is currently not explicitly treated in the SGS turbulence models used for the LES.

Application of FDS and FireFOAM in Large Eddy Simulations of a Turbulent Buoyant Helium Plume

“…Overall, the quality of the results is comparable to results previously published in the literature [11,13] with other CFD packages, which is encouraging for the use of FireFOAM in future work for simulations of fire-induced flows. A recent, more extensive, numerical study with FireFOAM, [20], supports this claim.…”

“…The mean streamwise velocities predicted, are close to the experimental uncertainty for both FireFOAM and FDS. The use of SGS model in FDS damps the flow, leading to a decrease in the mean velocity, as reported also in DesJardin et al [13]. Best agreement is observed by FDS with the use of SGS model.…”

“…When the SGS is applied, a reduction in the dynamic range of the streamwise velocity occurs. Similar behaviour is also reported in [13].…”

Large Eddy Simulations of CH4 Fire Plumes

“…For all the test cases considered in this study, the conditions at the source were laminar (inlet velocities of only a few cm/s), but very quickly buoyancy-generated turbulence makes the flow turbulent (within an inlet diameter distance). The triggering mechanism [8,9,18] is the generation of instabilities at the edges of the source due to baroclinic and gravitational torques. If coarse grids are used in the numerical simulations then small-scale mixing is not well captured and temperatures are typically under-predicted, resulting in smaller density gradients.…”

“…The specific motivation of the present numerical study is three-fold: first, to study the dynamics and oscillatory behavior of large-scale fire plumes and examine how the vorticity generation mechanism and puffing frequency of such plumes differ from previously studied iso-thermal buoyant plumes [8,9] of the same size; second, to evaluate the predictive capabilities of the turbulence and combustion models, currently used in the fire community, of accurately capturing the fire dynamics and the buoyancy-generated turbulence associated with large-scale fire plumes; third, to apply two well-known CFD codes, typically used for numerical simulations of fire safety applications, and evaluate the simulation results of transient and mean flow dynamics of large-scale fire plumes against experimental data. The comparison of the two CFD packages is expected to be useful for fire safety engineers working on real-life applications of fire scenarios.…”