Chuka C. Ndubizu

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.

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.

Influence of Complete Enclosure on Liquid Pool Fires

Abrtract This work advances the area o f zone modeling by the development of a five-zone theoretical model with time-incrementing capabilities to predict the results of liquid pool methanol fires in a gas-tight enclosure. The model consists of a liquid pool fuel hed, a developed flame above the fuel bed, a plume on top of the flame, the enclosure walls, and an ambient gaseous medium between the enclosure walls and the flame-plume boundaries. It assumes a temperature dependent chemical equilibrium between the combustion products and evaporated fuel in the flame. It also dictates the energy feedback within each defined zone and predicts the temperatures (ambient gas. flame, and plume), the mass evaporation rate of the fuel, the total pressure. and the oxygen concentration within the enclosure. The early effects of oxygen concentration on a con-fined methanol fire are well established in this work through a comparison with an open methanol fire model. Initially the rising ambient gas temperature causes the m...

SUPPRESSION AND EXTINGUISHMENTS OF FORCED-FLOW BOUNDARY LAYER COMBUSTION BY ULTRAFINE WATER MIST

Abstract Experiments were performed to quantify the effects of droplet concentration and air velocity on the suppression and extinguishment of a boundary layer flame with ultrafine water mist (UFM). Unlike the traditional nozzles, UFM has very small droplet size (Sauter mean diameter, 3 µm) and forms laminar flow with low momentum. The results show that the UFM mass fraction needed to extinguish the flame decreases linearly by a factor of 7 as the air velocity is doubled. About 12% UFM is needed to extinguish the flame when the data are extrapolated to zero air velocity. Below the extinction concentration, the flame temperature is reduced slightly, but the local burning rate is suppressed significantly. In contrast, the spray-nozzle-mist (SMD range 15 to 50 µm, high momentum) enhanced the local burning rate at water concentrations below the extinction limit. Furthermore, the extinction concentrations are significantly lower with the spray-nozzle-mist than with UFM. The precise reasons for this are not clear but it may be due to better penetration of high-momentum droplets into the flame than the UFM. Unlike the UFM, the nozzle spray induces turbulence in the flame. Despite these differences, the UFM and nozzle mist data show that the extinction concentration exhibits a shallow minimum in the range of 25 to 40 µm as the droplet size is increased.

Boundary layer flame spread over pmma within the quench zone: A moving boundary effect

This article focuses on the effects of moving boundary on countercurrent flame spread over polymethylmethacrylate (PMMA) near the flame extinction limits. Experiments were conducted with low oxygen concentration ( h 19.4% by volume) and/or high freestream velocities. At these conditions the flame was observed to retreat from the fuel leading edge after ignition and stabilize downstream, establishing a quench zone. While the flame was retreating, the entire sample surface was relatively flat. As the flame stabilizes, a small valley is formed near the flame leading edge and then the flame spreads upstream, decreasing the length of the quench zone with time. The results show that the flame could not be sustained within the quench zone when the surface was flat, molten, and pyrolyzing but will spread upstream after the surface has solidified. It appears that the presence of the valley stabilizes the flame, perhaps by creating a stagnation/recirculating zone that increases the Damkohler number and enables the flame to spread upstream. Flame spread in these experiments is chemically controlled and the spread rates were comparable to the regression rates. Both rates are observed to decrease with time as the valley enlarges.