Numerical Modeling of Radiant Heat Attenuation Through Water Mist

Abstract

This paper describes the development of a two-phase radiation model for the prediction of radiant heat attenuation in fires by water mist. It has been especially developed for incorporation in comprehensive field models of fire extinguishment by a water mist. The model predicts the opacity of the mist by summing over the absorption and scattering coefficients for each panicle size present. A novel approach is proposed to reconcile the Eulerian fonn of the radiation transport equations with the Lagrangian particle tracking procedure. The model is fast, applicable to cases where the mist is modeled following the Lagrangian concept, and it retains in evidence the physics of the problem while avoiding complex mathematics. Validation with experimental measurements has shown that the model reproduces the water mist/radiation interaction with sufficient engineering accuracy. Results also indicate that radiation is a primary mechanism in fire extinguishment. since it contributes to nearly half of the total heat removal by the droplets.

The interaction of thermal radiation and water mist in fire suppression

“…Nevertheless, it may be useful to develop an approximate transmissivity, which may be used qualitatively to assess the attenuation effectiveness of a mist. Moreover, such numbers provide a means to compare with earlier analyses (e.g., [12,13]) that report transmissivity, but do not consider in-scattering effects. For the purposes of comparing with the results of Ravigururajan [12], the transmissivity here is defined as the remaining total radiative energy at 3 m into a mist divided by the initial radiative energy.…”

“…They reported that the maximum attenuation is achieved when droplet radius is equal to the wavelength. In another study, Keramida et al [13] developed a twophase model to predict the radiation attenuation by water mist. Their model was developed to be incorporated into a multidimensional model for fire extinction by water mist.…”

Absorptance and transmittance of water spray/mist curtains

“…For example, Ravigururajan et al [11] modeled radiation absorption by water droplets using Lambert-Beer's law and determined spectral attenuation factors. In another study, Keramida et al [12] developed a two-phase model to predict radiation attenuation by water mists. In more recent studies [13,14], the effects of droplet diameter and number density have been investigated to determine the structure and extinction of premixed, freely propagating, stoichiometric, methane-air flames.…”

Investigation into the use of a water curtain over openings to prevent fire spread

“…The effect of water droplets on the suppression of fire and as a shielding mechanism has been the subject of many studies [7][8][9][10][11][12][13][14][15][16][17]. Kermida [7] found the ability of water droplets to suppress fires is due to the following four methods; gas phase cooling, oxygen dilution, fuel bed cooling, and radiation attenuation. This study utilized the ability of water to attenuate radiation to reduce the heat flux leaving the compartment.…”

“…This study utilized the ability of water to attenuate radiation to reduce the heat flux leaving the compartment. The main mechanisms involved are radiation absorption and scattering by water droplets [7][8][9][10][11][12][13]. Chow [14] and Choi [15] performed similar experiments in order to determine if a room could be protected from a fire in an adjacent room with the use of a water curtain.…”