Theoretical analysis of the liquid thermal structure in a pool fire

The paper presents a theoretical work on liquid heat-up in the case of a pool fire. It is assumed that the convective currents occurring within the upper layer of the liquid are induced by Rayleigh-Bénard instabilities that are caused by in-depth radiation. The upper layer depth has been estimated based on the analytical solution of a 1-D Fourier’s equation for the temperature with a source term for in-depth radiation. The model has been assessed against experimental data for a 9 cm – diameter methanol steady-state pool fire and three different liquid depths (18, 12 and 6 mm). The general trend, i.e., increase in the upper layer depth as the bottom boundary temperature increases, is well captured. In order to ensure that the well-mixed upper layer is at a temperature near the boiling point (as suggested by the experimental data), an improvement is proposed based on a radiative heat balance integral method. In addition to the above, a novel methodology is developed for the calculation of the ‘effective’ thermal conductivity as a means to circumvent detailed calculations of heat transfer within the liquid

Parametric numerical analysis of fire-induced pressure variations in a well-confined and mechanically ventilated compartment

The investigation of a fire in a well-confined and mechanically ventilated compartment is of primary importance for the nuclear industry. In normal operating conditions, a ventilation network system is set-up to ensure confinement via an appropriate pressure cascade. In the event of a fire, the subsequent pressure build-up alters the confinement level significantly and therefore changes the level of safety of the installation. The fire-induced pressure variations depend mainly on the: (1) HRR (Heat Release Rate) history of the fire, (2) heat losses to the walls, (3) leakage area, and (4) operating conditions of the fans. A numerical parametric analysis on the latter three parameters, using the Fire Dynamics Simulator (FDS 5.5.3), have shown that a change in the initial ventilation parameters (i.e. operating conditions of the fans and/or leaks), which can be sometimes difficult to determine, may lead to substantial changes in the pressure profiles. However, only a change in the thermal boundary conditions (i.e. presence or no of insulation) produces significant changes in the gas temperature

Assessment of heating and evaporation modelling based on single suspended water droplet experiments

The work described in this paper is undertaken with the purpose of providing a detailed assessment of the current modelling capabilities of the effects of fire suppression systems (e.g., sprinklers) in fire-driven flows. Such assessment will allow identifying key modelling issues and, ultimately, improving the reliability of the numerical tools in fire safety design studies. More specifically, we studied herein the heating and evaporation of a single water droplet. This rather 'simple' configuration represents the first step in a tedious and rigorous verification and validation process, as advocated in the MaCFP (Measurement and Computation of Fire Phenomena) working group (see https://iafss.org/macfp/). Such a process starts ideally with single-physics 'unit tests' and then more elaborate benchmark cases and sub-systems, before addressing 'real-life' application tests. In this paper, we are considering the recently published comprehensive and well-documented experimental data of Volkov and Strizhak (Applied Thermal Engineering, 2017) where a single suspended water droplet of initial diameter between 2.6 and 3.4 mm is heated up by a convective hot air flow with a velocity between 3 and 4.5 m/s and a temperature between 100 and 800 degrees C. In the present numerical study, 36 experimental tests have been simulated with the Fire Dynamics Simulator (FDS 6.7.0) as well as with an in-house code. The results show that the droplet lifetime is overpredicted with an overall deviation between 26 and 31%. The deviation in the range 300-800 degrees C is even better, i.e., 5-8%, whilst the cases of 200 and, more so 100 degrees C, showed much stronger deviations. The measured droplet saturation temperatures did not exceed 70 degrees C, even for high air temperatures of around 800 degrees C, whereas the predicted values approached 100 degrees C. A detailed analysis shows that the standard Ranz & Marshall modelling of the non-dimensional Nusselt and Sherwood numbers may not be appropriate in order to obtain a simultaneous good agreement for both the droplet lifetime and temperature. More specifically, the heat-mass transfer analogy (i.e., Nu = Sh) appears to be not always valid

Analytical modelling of the effect of in-depth radiation within a liquid layer in the case of a pool fire

In this paper, we present a ‘simplified’ approach for the numerical modelling of the convective currents that occur within a liquid fuel in the case of a pool fire and which are induced by in-depth thermal radiation. This approach is based on the concept of ‘effective’ thermal conductivity, which is calculated herein based on the analytical solution of a steady-state one-dimensional heat conduction equation including a source term for in-depth radiation. This solution leads to a temperature profile which displays a horizontal liquid layer (of a given depth) that is bounded by a temperature that is higher at its bottom than its top. This thermal structure generates Rayleigh-Bénard instabilities which enhance heat transfer within the liquid. This effect is modeled via an increase of the ‘actual’ thermal conductivity of the liquid by a dimensionless heat transfer number, namely the Nusselt number. The Nusselt number is calculated based on the ‘classical expression’ of the Rayleigh number for the case of a ‘horizontal cavity heated from below’. The paper provides the details of the derived solution for the ‘effective’ thermal conductivity along with examples of application to several fuels

Assessment of the burning rate of liquid fuels in confined and mechanically-ventilated compartments using a well-stirred reactor approach

The objective of this work is to provide a 'support tool' to assess the burning rate of a pool fire in a well-confined and mechanically-ventilated room using a single-zone model based on conservation equations for mass, energy and oxygen concentration. Such configurations are particularly relevant for nuclear facilities where compartments are generally sealed from one another and connected through a ventilation network. The burning rates are substantially affected by the dynamic interaction between the fuel mass loss rate and the rate of air supplied by mechanical ventilation. The fuel mass loss rate is controlled by (i) the amount of oxygen available in the room (i.e. vitiation oxygen effect) and (ii) the thermal enhancement via radiative feedback from the hot gas to the fuel surface. The steady-state burning rate is determined by the 'interplay' and balance between the limiting effect of oxygen vitiation and the enhancing effect of radiative feedback. An extensive sensitivity study over a wide range of fuel areas and mechanical ventilation rates shows that a maximum burning rate may be obtained. For the studied HTP (Hydrogenated Tetra-Propylene) pool fires, the maximum burning rate is up to 1.75 times the burning rate in open air conditions

Blind simulation of periodic pressure and burning rate instabilities in the event of a pool fire in a confined and mechanically ventilated compartment

Fire dynamics in a well-confined and mechanically ventilated enclosure (a configuration of interest to the nuclear industry) strongly depends on the interaction between the fuel burning rates and the intake and exhaust volume flow rates (delivered by the fans). Several experiments show that, in the under-ventilated regime, an oscillatory behavior may be established with frequencies in the order of a few mHz. This article reports one of the first comprehensive numerical analyses performed in order to study periodic pressure and burning rate instabilities for the case of a full-scale heptane pool fire, for which experimental data has not been published yet. In the numerical analysis, carried out with the fire dynamics simulator (FDS), periodic pressure and burning rate instabilities were predicted with a frequency of approximately 10 mHz. The analysis shows evidence of the correlation between the variation of ventilation flow rates (due to pressure instabilities) and the burning rate oscillations. The latter oscillations are directly linked to oscillations in the fuel burning area attributed to the ventilation-controlled conditions

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