Numerical investigation of the effects of polydisperse water sprays on extinction conditions of counterflow methane non-premixed flames

Water, sprayed in the form of tiny droplets, has emerged as a potential fire suppressant after the halon compounds such as trifluorobromomethane (CF₃Br, Halon 1301) were banned by the Montreal protocol. The size distribution of the water droplet plays a crucial role in the effectiveness of the water spray in fire suppression. A numerical investigation of the influence of size distribution of a polydisperse water spray on extinction of counterflow diffusion flames is presented in this paper. This study uses laminar finite rate model with reduced CHEMKIN chemistry for numerical simulations. The discrete phase, namely the water spray, is simulated using Lagrangian Discrete Phase Modelling approach. In this work, the polydispersity of water spray is taken into account in the numerical simulation by a suitable Rosin–Rammler distribution. Results obtained from numerical simulation are validated with the experimental results reported in the literature. This study demonstrates that the representation of the polydisperse spray by a monodisperse spray (with droplet diameter same as the SMD of the polydisperse spray) in numerical simulations is not always justified and it leads to deviation from the experimental results. The effects of number mean diameter and spread parameter on the efficacy of flame suppression are investigated for polydisperse sprays. A comprehensive comparison is done between the effectiveness of monodisperse and polydisperse water sprays. An optimum droplet diameter is obtained for monodisperse sprays for which the effectiveness of the spray is maximum. The effects of evaporation Damköhler number and Stokes number of water droplets on flame suppression have also been explained.