Patricia A. Tatem

On water mist fire suppression mechanisms in a gaseous diffusion flame

Abstract This is work is motivated by the urgent need to find an alternative to the banned halogen-based fire suppressing agents. Fine water mist is a contending alternative especially in total flooding applications. To obtain an optimum design of a water mist fire suppression system, one needs to understand the mechanisms of fire suspension by water mist. This paper presents a study of the relative contributions of the suppression mechanisms in a gaseous diffusion flame. A modified Wolfhard–Parker burner was used to measure temperature drops in a 2D methane diffusion flame, when various quantities of nitrogen, steam and water mist were added independently in a co-flow arrangement. A simple model of the flame was used to estimate the heat generation and loss processes taking place in the flame when small amounts of various suppressants were added. The results of both experiments and the analysis show that in a co-flow arrangement the addition of small quantities of fine water mist has more gas phase cooling effect on the flame than oxygen dilution effect.

Numerical modeling of water mist suppression of methane-air diffusion flames

Abstract This paper describes a numerical model for studying the suppression of co-flow diffusion flames by fine water mist. A two-continuum formulation is used in which the gas phase and the water mist are both described by equations of the eulerian form. The model is used to obtain a detail understanding of the physical processes involved during the interaction of water mist and flames. The relative contribution of various mist suppression mechanisms is studied. The effect of droplet diameter, spray injection density and velocity on water mist entrainment into the flame and flame suppression is quantified. Droplet trajectories are used to identify the regions of the flame where the droplets evaporate and absorb energy Finally, the model is used to determine the water required for extinction, and this is reported in terms of the ratio of the water supply rate to the fuel flow rate.

Numerical modelling of methanol liquid pool fires

The focus of this paper is on numerical modelling of methanol liquid pool fires. A mathematical model is first developed to describe the evaporation and burning of a two-dimensional or axisymmetric pool containing pure liquid methanol. Then, the complete set of unsteady, compressible Navier-Stokes equations for reactive flows are solved in the gas phase to describe the convection of the fuel gases away from the pool surface, diffusion of the gases into the surrounding air and the oxidation of the fuel into product species. Heat transfer into the liquid pool and the metal container through conduction, convection and radiation are modelled by solving a modified form of the energy equation. Clausius–Clapeyron relationships are invoked to model the evaporation rate of a two-dimensional pool of pure liquid methanol. The governing equations along with appropriate boundary and interface conditions are solved using the flux-corrected transport algorithm. Numerical results exhibit a flame structure that compares w...

The Effects of Droplet Size and Injection Orientation on Water Mist Suppression of Low and High Boiling Point Liquid Pool Fires

Abstract This paper presents the results of an experimental parametric study of water mist suppression of large-scale liquid pool fires. The experiments were conducted with 50cm diameter pan heptane and JP8 pool fires. Mist was injected into the fire from the base at 90° and 45° and from the top at 90°. The results show that base injection of droplets enhanced their suppression effectiveness by as much as two times. Secondly, optimum suppression effectiveness is obtained with small droplets injected at the base of the fire. This is because the droplets evaporated quickly within the lower region of the fire where a greater effect of oxygen dilution and water vapor higher heat capacity is fully realized. Finally, a comparison of the results with the two fuels show that water mist is more effective in suppressing the JP8 fires than the heptane fires. It is concluded that the additional effects of direct surface cooling contributed significantly to the observed difference.

Development and Validation of Corridor Flow Submodel for CFAST

The modeling of fire and smoke spread is an evolvingfield. As knowledge is acquired and resources become available, models are enhanced to make their predictions more accurate and/or their computations faster. This paper will discuss the Consolidated Fire and Smoke Transport (CFAST) zone fire model, developed by the National Institute of Standards and Technology (NIST), and a recent addition to that model, referred to as the Corridor Flow Submodel. The goal of this new submodel is to more accurately predict the flow of smoke down a corridor which has an impact on fire protection issues such as detection and escape time. Prior to the addition of this new submodel, CFAST assumed that smoke traveled instantly from one side of a compartment to another. Development of the submodel will be discussed and then the enhanced CFAST, Version 4.0.1 (executable dated 3/8/00), will be used to model a real-scale experiment conducted onboard the ex-USS SHADWELL, the Navys R&D Damage Control platform.

Burning rate distributions for boundary layer flow combustion of a PMMA plate in forced flow

Abstract Solutions of Navier-Stokes (NS) equations were obtained for burning rate Nu and temperature distributions for a flat, PMMA plate using an iterative method to impose steady-state, pyrolysis kinetics at the surface. The NS solutions show that Nu depends on both Reynolds number Re and air velocity U unlike the classical solutions, which include the boundary layer (BL) approximations. However, at large values of Re (Re >1000) and U (U >120 cm/s), the NS solutions can be represented by Nu = e +0.1 Re1/2, where the intercept e increases with U and the slope is identical to that given by the classical BL solutions. The NS solutions are compared with experiments, in which short (10 cm) PMMA plates were ignited uniformly and burnt for different lengths of time. The comparisons show that the steady-state surface pyrolysis approximation holds in the middle region of the pyrolysis zone, where the NS solutions agree with the data for Nu. Near the leading edge, where the heat feedback is high, the NS solutions over-predict the measurements as the initially flat surface becomes curved (concave) and forms a valley due to the moving boundary. As the valley size increases with time, the deviations between the NS solutions and data increase and extend to increasing distance from the leading edge. Far from the leading edge, where the heat feed back is low, NS solutions also over-predict the data due to transient effects caused by in-depth pyrolysis. As the melt approaches the pyrolysis temperature, the data approach NS solutions with increased burn times. Therefore, the curvature and in-depth heat transfer/pyrolysis effects are significant within the pyrolysis zone at large and small burn times, respectively.

A Network Fire Model for the Simulation of Fire Growth and Smoke Spread in Multiple Compartments with Complex Ventilation

There is a need for fire modeling tools capable of rapid simulation of fire growth and smoke spread in multiple compartments with complex ventilation. Currently available tools are not capable of simulation of complex ventilation arrangements in a timely manner. To address this problem, a new fire model called Fire and Smoke SIMulator (FSSIM) has been developed. FSSIM is a network model whose core thermal hydraulic routines are based on MELCOR. FSSIM capabilities include remote ignition, multilayer heat conduction, radiation streaming, arbitrarily complex HVAC systems, detection, suppression, oxygen-limited combustion, and simple control systems.

Modeling pool-like gas flames of propane

Abstract Turbulent, buoyant pool-like propane flames with heat release rates of 15.8, 22.9 and 37.9 kW are numerically modeled. The model assumes a parabolic flow field, α κ–ϵ turbulence model, and an eddy-dissipation concept for the interaction of the chemistry and turbulence. Radiative heat transfer is incorporated by the flux model with the absorption and emission coefficients evaluated using a temperature-weighted gray gas model. Predictions are made for the flame shapes, axial velocity, axial mean temperature and various scalar properties along the centerline, the radial distribution of temperature and velocity at various axial heights, and the air entrainment behavior. The overall agreement between the predicted and experimental flame behavior is seen to be good; however, the radial expansion of the flame is underestimated in the combustion zone.

Numerical Predictions for a Simulated Methane Fire

Abstract A buoyant, turbulent methane flame with a base diameter of 0.25mm and a heat release rate of 28 kW was numerically modeled. Soot formation was included in the model by a phenomenological soot formation scheme. Gas radiation was treated by a weighted-sum gray-gas model. A non-isothermal, non-homogeneous field approach was utilized and the thermal radiation was included by incorporating a four flux radiation model into a finite-difference scheme. The methane fire did not show appreciable soot concentrations to the extent that the radiation was significantly affected. The radiation present was predominantly due to gaseous species. The centerline flame properties such as the axial velocity, mean temperature, and entrainment behaviors are generally well reproduced by the theory. However, the radial expansion of the flame is underestimated near the flame base because of the neglect of the elliptic behavior in the present approach. An analysis of the thermal radiation behavior revealed a non-uniform heat feedback flux distribution. Unlike in sooting flames, where the flux maximizes usually midway between the centerline and flame edge, we observe the maximum flux at the pool center in the methane fire. In sooting flames, this behavior arises because of radiative energy blockage by the cold fuel vapor and soot in the core. A reduced radiation blockage in the methane flame is a distinguishing feature of the methane fire compared with sooting pool fires.

Pressurization with Nitrogen as an Extinguishant for Fires in Confined Spaces

Abstract Pressurization with nitrogen has been studied as a technique for suppressing fires in confined spaces. Liquid hydrocarbon-air fires of various sizes in a gastight experimental chamber were extinguished in approximately 30 seconds by increasing the total pressure of the enclosure from l atm. to 1.35 ± 0.03 atm. The post-fire atmospheres were found to contain sufficient oxygen for support of normal human activity, while the buildup of hazardous combustion products was well below established safety levels.