A simplified model for the shielding of fire thermal radiation by water mists

Abstract

A solution for the complete problem of attenuation of fire radiation by water mist is presented. This solution is based on simplified approaches for the spectral radiative properties of water droplets, the radiative transfer in the absorbing and scattering mist, and transient heat transfer taking into account partial evaporation of water mist. A computational study of the conventional model problem indicates the role of the main parameters and enables one to formulate some recommendations to optimize possible engineering solutions. The method developed is also applied to more realistic case study of a real fire. It is suggested to decrease the size of supplied water droplets with the distance from the irradiated surface of the mist layer. The advantage of this engineering solution is confirmed by numerical calculations. Potential possibility of microwave monitoring of water mist parameters is analyzed on the basis of Mie theory calculations.

An infrared scattering by evaporating droplets at the initial stage of a pool fire suppression by water sprays

“…A comparison of approximation ((6a), (b)) with Mie theory calculations for water droplets at two typical infrared wavelengths can be found in paper [8]. As one can expect, the approximate relations are sufficiently accurate for both Q a and Q tr s in the spectral range of a weak absorption.…”

“…We use the known P 1 approximation of the spherical harmonics method [8,49,50] to estimate the effect of radiation scattering by water droplets on thermal radiation field in the flame. In our case, this approach leads to the following boundary-value problem for the modified Helmholtz equation: to the boundary of the computational region.…”

“…A similar model has been recently used in analysis of water mist curtains [8,9]. This simplified evaporation model is also sufficient for qualitative estimates of the present paper because of very fast heating of moving droplets and large molar fraction of water vapour in the flame.…”

“…The exact Mie calculations are time-consuming, especially for large droplets with x ) 1. Therefore, as it was done in recent studies [8,9], we use the following analytical approximation suggested in [30] for semitransparent particles: …”

“…This optical effect, which is commonly observed in the visible for the natural mists, has been recently considered as a promising way to improve shielding of fire radiation by multi-layered water sprays [8,9]. It is important that the gaseous combustion products in the fire do not scatter the radiation.…”