Ramagopal Ananth

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.

Selection of a length scale in unconstrained dendritic growth with convection in the melt

For the first time the free growth of fully developed succinonitrile, SCN, dendrites is studied experimentally with carefully controlled, well defined, forced convection velocities, U∞, in the ultrapure melt up to 1 cm/s, which is about 40 times larger than the velocity due to natural (thermal) convection, UN, and is 300 times larger than the growth velocity of the dendrite, ν. Therefore thermal convection and advection have a negligible effect on our experimental data. The selection parameter, σ∗ = 2αd0⧸ν R2, increases by over 50% as the ratio, U∞⧸ν, of the forced convection velocity, U∞, to the growth velocity, ν, increases from 3 to 300. This result is opposite to the prediction of microscopic solvability theory for a pure material. Our result also is opposite to that reported for binary experiments which support solvability theory and indicate that σ∗decreases as U∞/ν increases up to values of about 19.

Dendritic growth with thermal convection

Abstract Dendritic growth is inherently nonlinear and three-dimensional, and thermal convection, no matter how weak, always accompanies it and is counter-intuitive in nature because convection becomes more important as the driving force for it decreases. Moving boundary solutions of the nonlinear Navier-Stokes and energy equations are given which describe the three-dimensional axisymmetric growth of a shape preserving isothermal parabolic dendritic tip. New theoretical results show that the tip radius is the proper length scale if one includes the nonlinear interaction between fluid flow and energy transfer. Predicted values of growth velocity and tip radius are in good agreement with experiments over the entire range of available experimental data. It is shown that the Grashof number, which arises as a new parameter and enables one to compare theory with experiments without ad hoc hypotheses, increases as one decreases the subcooling, and convection becomes increasingly important in succinonitrile experiments as the subcooling is decreased below 1.65 K. The approximate analogy between thermal and forced convection, Gr = Re2, is useful only when thermal convection is weak. It overpredicts the growth velocity and underpredicts the tip radius with increasing error as the intensity of thermal convection increases.

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.

Mitigation of TNT and Destex explosion effects using water mist

The effects water mist has on the overpressures produced by the detonation of 50 lb equivalent of high explosives (HE) TNT and Destex in a chamber is reported. The overpressures for each charge density were measured with and without mist preemptively sprayed into the space. A droplet analyzer was placed in the chamber prior to the detonation experiments to characterize the water mist used to mitigate the explosion overpressures. The impulse, initial blast wave, and quasi-static overpressure measured in the blast mitigation experiments were reduced by as much as 40%, 36%, 35% for TNT and 43%, 25%, 33% for Destex when water mist was sprayed 60s prior to detonation at a concentration of 70 g/m(3) and droplet Sauter Mean Diameter (SMD) 54 microm. These results suggest that current water mist technology is a potentially promising concept for the mitigation of overpressure effects produced from the detonation of high explosives.

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.

THE EFFECT OF CONVECTION ON AXISYMMETRIC PARABOLIC DENDRITES

Abstract Moving boundary solutions of the Navier Stokes and energy equations are obtained for shape-preserving isothermal paraboloids of revolution by using the Oseen viscous flow approximation. The theoretical results are in good agreement with the precise data of Glicksman et al. on succinonitrilc. The intensity of thermal convection as indicated by the Grashof number is shown to increase approximately as the inverse of the square of the super-cooling which may have important implications in microgravity experiments.

Ultra-Fine Water Mist Extinction Dynamics of a Co-Flow Diffusion Flame

Computations are performed to examine the effectiveness of mono-disperse water droplets in extinguishing a co-flow, propane diffusion flame by injecting the droplets into the air stream. The calculations show that the droplets entrained into the reaction kernel at the flame base are crucial for extinction. The reaction kernel detaches from the burner rim and blows-off when the droplet concentration is increased to a critical value (extinction concentration). At the critical value, the maximum chain-branching reaction (H2 + O = OH + H) rate in the reaction kernel was found to be reduced by a factor of 5 in our computations. A large decrease in the reaction rate indicates that the maximum heat generations rate and Damkholer number are too low to sustain the flame, and cause the flame blow-off. Large drops are more effective than small drops, and the extinction concentration of water increases from 10.5% to 15% by mass as the size is reduced from 32 to 4 μm. This is because of competition between the degree of penetration and the rate of evaporation of the water drops. The large drops penetrate the reaction kernel at the flame base better than the small drops, which evaporate completely before reaching the 600 K isotherm located well outside the reaction kernel as shown by our computations. As the droplet diameter is increased further (> 32 μm), the trend will reverse as the evaporation rates get too small, despite increased penetration of flame core by the drops.