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This paper proposes a Laminar Smoke Point (LSP)-based soot model, incorporating (as opposed to previously developed LSP-based models) soot surface growth. The latter is indeed believed to be dominant in soot formation. Simple reactions are also introduced to account for the conversion of fuel and oxygen in soot evolution mechanisms. The proposed and a reference LSP-based soot models have been implemented in OpenFOAM-v2006 and assessed against a wide variety of laminar flames (16 flames). A calibration-evaluation procedure is defined in which some flames are involved in the calibration of the constants, and the majority are utilised in an independent evaluation stage. The results show that the newly added features to the LSP-based soot modelling approach allow for a better agreement over a wider range of conditions, e.g. diluted and highly sooty flames. It is shown that although the proposed model is more accurate for buoyant flames, it performs significantly better than the reference model for non-buoyant flames. [ABSTRACT FROM AUTHOR]

Progress in fire safety science strongly relies on the use of Computational Fluid Dynamics (CFD) to simulate a wide range of scenarios, involving complex geometries, multiple length/time scales and multi-physics (e.g., turbulence, combustion, heat transfer, soot generation, solid pyrolysis, flame spread and liquid evaporation), that could not be studied easily with analytical solutions and zone models. It has been recently well recognised in the fire community that there is need for better modelling of the physics in the near-wall region of boundary layer combustion. Within this context, heat transfer modelling is an important aspect since the fuel gasification rate for solid pyrolysis and liquid evaporation is determined by a heat feedback mechanism that depends on both convection and radiation. The paper focuses on convection and reviews the most commonly used approaches for modelling convective heat transfer with CFD using Large Eddy Simulations (LES) in the context of fire-driven flows. The considered test cases include pool fires and turbulent wall fires. The main assumptions, advantages and disadvantages of each modelling approach are outlined. Finally, a selection of numerical results from the application of the different approaches in pool fire and flame spread cases, is presented in order to demonstrate the impact that convective heat transfer modelling can have in such scenarios. [ABSTRACT FROM AUTHOR]

The paper presents a comprehensive set of numerical simulations performed to examine the current Computational Fluid Dynamics (CFD) capabilities in the prediction of the interaction of a water mist spray with a vertical upward jet of hot air within an Eulerian-Lagrangian framework. The experimental tests considered herein are described by Zhou [Proceedings of the Combustion Institute, 2015]. The spray is a 30° full cone water mist spray emerging from a nozzle that delivers a water flow rate of 0.084 lpm at a pressure of 750 kPa. The vertical jet of hot air at 205 ∘ C is issued from a 72 mm-diameter nozzle placed at 560 mm below the water spray nozzle. Three exit velocities of 3.3, 4.2 and 5.3 m/s were examined. Gas phase simulations (described in the companion paper, Part I) have allowed to determine a set of parameters (e.g., cell size of 4 mm and modified Deardorff model for the turbulent viscosity) that are suitable for the water mist spray simulations. Moreover, it is shown here that a prescribed complex spray pattern with a full discharge angle of 60° is required in order to match water spray profiles in the nozzle near-field. The three regimes of spray-jet interaction (i.e., water spray dominated, vertical jet dominated or equal influence of the spray and the vertical jet) are qualitatively well captured by the numerical simulations. However, the location of the interaction boundary is underestimated by up to 26%. This could be partially attributed to modelling aspects related to, for example, turbulent dispersion or turbulence inflow conditions of the droplets. Uncertainties in the experimental measurements must also be considered. [ABSTRACT FROM AUTHOR]

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 / 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. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive set of numerical simulations for the characterization of a water mist spray emerging from a nozzle positioned at 2.2 m from floor level and operating at a pressure of 1.0 MPa. The droplet volume-median diameter is about 90 μm and the spray half-angle is around 42 °. The spray shape is visualized using a laser sheet and the water flux density distribution on the ground is measured with a 'bucket' test. An initial comprehensive numerical study using the Fire Dynamics Simulator (FDS) has been carried out by varying several numerical and physical models and parameters (e.g., cell size and turbulence modelling). The simulated sprays were very narrow (in comparison to the actual spray), yielding overestimations of the peak water flux density at floor level by about 430% (on average). Subsequently, it was found that there is a significant impact of the drag force modelling because the spray at hand is dense (near the nozzle). An ad-hoc reduction of the drag coefficient to a constant value leads to better results. The current study calls upon new developments for droplet aerodynamic modelling in dense sprays. [ABSTRACT FROM AUTHOR]

The paper presents a numerical study on an 18 cm-diameter liquid pool fire in a reduced-scale and mechanically-ventilated enclosure. The enclosure walls are made of a calcium silicate layer and a steel layer on five sides, the remaining sidewall is made of heat-tempered glass. The ventilation system is extracted mechanically with free air inlet. Three air renewal rates (ARR) and open atmosphere conditions are considered in the numerical study. Computational Fluid Dynamics (CFD) simulations are carried out with Fire Dynamics Simulator (FDS 6.7.5). The default evaporation model in FDS (i.e., forced convection approach) is compared to a natural convection approach implemented in the source code. The results show that the oscillatory combustion, occurring at ARR = 15 h−1, is best captured by the natural convection approach. It is also found that prescribing the fuel Auto-Ignition Temperature (AIT) is important to capture the fire oscillations and (local) flame quenching. At ARR = 8 h−1, extinction occurs. It is found that prescribing the fuel AIT allows to prevent spurious oscillatory combustion. At ARR = 22.5 h−1, there is a stable steady-state burning regime for which (as well as for open atmosphere conditions) prescribing the fuel AIT is not crucial. • Fire burning regimes are well predicted in a wide range of ventilation conditions. • Oscillatory combustion is captured by natural convection for evaporation modeling. • Prescribing Auto-Ignition Temperature is important in capturing fire oscillations. • Prescribing AIT prevents spurious fuel re-ignition in under-ventilated conditions. [ABSTRACT FROM AUTHOR]

Air cavities and gaps between material layers are common in construction systems, e.g. ventilated façades. Air cavity may provide a pathway for smoke and flame spread in case of fire. Performing physical testing to investigate different systems and fire scenarios is resource demanding. Fire Dynamics Simulator (FDS version 6.7.0) was used to simulate fire driven flow between two parallel vertical walls. Flame heights, thermal impact to the interior wall surface and upward flow velocities were predicted with FDS and compared with experimental results. The fire source was a propane burner with 8 × 391 mm2 gas outlet area. Heat release rates were 6.6 kW and 12.4 kW and the distance between the parallel walls was 40 mm. Two different convective heat transfer coefficient sub grid scale models available in FDS were investigated. In this study the cavity width to mesh cell size ratio was equal or above 10, resulting in good predictions of flame heights, upward flow velocities and wall temperatures. 2 mm grid resulted in 25% lower HRR in locations near the burner gas inlet, compared to 4 mm grid, indicating the importance of well resolved gas outlet boundary. • Cavity width divided by mesh cell size should be greater than 10 in FDS simulations. • Predicted flame heights and flow velocities corresponded well with experiments. • Refining the grid in FDS resulted in lower cumulative HRR in the flame region. • FDS models for heat transfer at surfaces must be revised for transitional flows. [ABSTRACT FROM AUTHOR]

This paper presents a detailed experimental and numerical study on the interaction, at a reduced scale, between a turbulent buoyant propane flame of about 15.5 kW and a water mist spray with a flow rate of 0.43 L/min from a nozzle positioned at 1 m above the burner. The water spray has been characterized experimentally without a fire, by measuring the water mass flux and droplet size distributions at the level of the burner surface. The whole assembly was installed under a hood and the following three parameters were measured: (1) the chemical heat release rate (HRR) (using the oxygen consumption method), (2) the rise in gas temperature at the top, and (3) the radiative heat flux at 0.72 m from the axis of the burner and at a height of 0.05 m. Reductions of about 40% and 30% were recorded for, respectively, the gas temperature (in the hood) and the radiative heat flux, while the chemical HRR did not change. Computational Fluid Dynamics (CFD) simulations with the Fire Dynamics Simulator (FDS, version 6.7.0) predicted relatively well the gas temperature, without a reduction in the HRR, but, in contrast to the experiments, the radiative heat flux did not change. • A gas analysis system is used to accurately characterize the heat balance. • The cooling power of the spray is estimated using a single temperature measurement. • The water spray is well characterized experimentally and numerically. • The cooling power of the spray is underpredicted by about 34%. • The reduction in the radiative heat flux to the surroundings has not been predicted. [ABSTRACT FROM AUTHOR]