Addition of a water mist on a small-scale liquid pool fire: Effect on radiant heat transfer at the surface

Abstract

It is well established that the use of water mist can be an attractive alternative to gaseous suppression agents to extinguish fires for specific scenarios. The fundamental mechanisms involved—gas-phase cooling and dilution—are largely described in previous papers. But much attention must be paid to a less predictable effect such as radiant heat-transfer change, especially regarding the energy balance at the fuel surface and, therefore, the rate of generation of flammable vapors. The objective of this work is to analyze, on the one hand, the perturbing influence of a mist addition as an opposed flow to a small-scale liquid (heptane) pool fire structure, especially at its base, the more interesting zone regarding the mechanisms of flame stabilization and extinction and, on the other hand, the effect on the surface radiant heat feedback. A mapping is presented of the two major parameters, temperature and extinction coefficient, that control the thermal radiation of the flame. A comparative analysis of the fields deduced from averaged values, without and with mist addition, evidences an overall cooling effect and a loss of the structure of the flame with a periodic short phase of disruptive expansion due to the rapid vaporization of water droplets. These fields also provide a database for use in subsequent radiation computation. The measurements result in a new and interesting information: during water mist addition the burning rate increases, signifying an increase in both convective and radiative heat flux to the surface. The droplets and the water vapor do not shield the pool surface from radiation. The evolution of flammable vapor being not reduced, it is argued that oxygen displacement plays the major role in the suppression effect.