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

Large Eddy simulations of CH4 fire plumes

Large eddy simulations of large-scale CH4 fire plumes (1.59-2.61 MW) with two different CFD packages, FireFOAM and FDS, are presented. It is investigated how the vorticity generation mechanism and puffing behavior of large-scale fire plumes differs from previously studied iso-thermal buoyant plumes of the same scale. In addition, the predictive capabilities of the turbulence and combustion models, currently used by the two CFD codes, to accurately capture the fire dynamics and the buoyancy-generated turbulence associated with large-scale fire plumes are evaluated. Results obtained with the two CFD codes, typically used for numerical simulations of fire safety applications, are also compared with respect to the average and rms velocities and temperatures, puffing frequencies, average flame heights and entrainment rates using experimental data and well-known correlations in literature. Furthermore, the importance of the applied reaction time scale model in combination with the Eddy Dissipation Model is examined. In particular, the influence of the considered mixing time scales in the predicted centerline temperatures is illustrated and used to explain the discrepancies between the two codes

Large Eddy simulations of the ceiling jet induced by the impingement of a turbulent air plume

In this paper, a sensitivity study is performed with FireFOAM 2.2.x for a hot air jet plume impinging onto a flat horizontal ceiling. The plume evolution and the induced ceiling flow are considered. The influence of the level of turbulence imposed at the inlet, in terms of intensity and eddy length scale, is discussed. Also, the effect of the turbulence model constant is examined. For the case considered, the best results are obtained when no sub-grid scale (SGS) model is used. If a SGS model is used, the level of turbulence at the inlet and the choice of the turbulence model constant are shown to have a significant effect on the prediction of plume's spreading and the ceiling flow velocity. The eddy length scale at the inflow does not have significant impact on the results. Comparisons with the available experimental data indicate that FireFOAM is capable of predicting the mean velocity-field well. In the near field region, an under-estimation of the turbulent velocity fluctuations is observed, whereas reasonably good agreement is obtained in the far field

Influence of the particle injection rate, droplet size distribution and volume flux angular distribution on the results and computational time of water spray CFD simulations

The paper presents a detailed sensitivity analysis on the volume flux probability density function (PDF) to represent water spray patterns with computational fluid dynamics (CFD). The effects of the turbulent viscosity model and the cell size are also investigated. The test case considered herein is a 30 full cone water mist spray emerging from a nozzle that operates at a pressure of 750 kPa and delivers a water flow rate of 0.084 lpm. The errors solely induced by the limited number of computational droplets per second, N-P, are proportional to 1/root N-P, and could reach up to 35%. The computational time generally increases linearly with N-P. The paper illustrates also the better numerical performance of the lognormal-Rosin-Rammler droplet size distribution over the Rosin-Rammler distribution, especially in terms of reaching a converged volume-median diameter with increased N-P. Furthermore, a uniform angular distribution is shown to provide results in better agreement with experimental data than a Gaussian-type distribution for the case at hand. For a sufficiently fine grid, the dynamic Smagorinsky and the modified Deardorff models converge to similar radial profiles of the water volume flux at 300 mm from the nozzle, with a deviation of less than 6% from the experiments. The deviations for the volume-median diameter are about 50% in the core region of the spray

Experimental study of corner fires : part I : Inert panel tests

Corner fires are known to spread more intensely in comparison with single wall fires. In view of the challenges associated with prediction of such fire behavior, the fire growth in a corner configuration of Medium Density Fiberboard (MDF) panels is investigated to provide a set of experimental data, performing Single Burning Item (SBI) tests. First, though, test results with inert calcium silicate panels are discussed for three values of HRR (10, 30 and 55 kW), allowing to address the main physics involved. The experimental data for 30 kW, the default SBI HRR, is used for detailed discussion of the observations. The SBI testing methodology, materials, and set-up are described. The results of total Heat Release Rates (HRR) and Smoke Production Rates (SPR), as well as the panel temperatures and total heat fluxes at several characteristic locations are analyzed. Moreover, the puffing frequency of the corner fire is characterized thanks to Video Fire Analysis (VFA) of the experimental footage. Additionally, flame heights are discussed, including the concept of mirroring. A new correlation for mean flame height is introduced, using the hypotenuse of the triangle as characteristic length for entrainment of air into the fire plume, and expressing that the flame height increases proportional to the square root of the fire heat release rate. The 30 kW propane burner of the standard SBI test is shown to feature a mean flame height of nearly 0.9 m and a puffing frequency of 2  ±  0.3 Hz, and an average total heat flux exceeding 44 kW/m² near the burner early on in the test. The completeness of the dataset is expected to be useful for testing and development of CFD codes for corner fire scenarios