Dynamics of water droplets in a counterflow field and their effect on flame extinction: Fid-Bau Portal

Dynamics of water droplets in a counterflow field and their effect on flame extinction

Lentati, A.M. / Chelliah, H.K.

Automatic water sprinklers, with a typical mass median drop size of about 750 microns, are extensively used as fire-suppressing devices in fires like building fires. Recently, fine-water droplets or water-mist systems with drop sizes less than 100 microns are being considered as a replacement for recently banned halon 1301 (bromotrifluoromethane). The rapid transition from halon 1301 to water-mist system, has forced researchers to rush into applications without detailed understanding of the relative physical effects due to the water droplet size, thermal effects due to heat capacity and latent heat of evaporation, and chemical effects due to enhanced overall three-body recombination reactions and shift in water-gas reactions. Studied is the effect of fine-water droplets in extinguishing steady, laminar counterflow methane-air nonpremixed flames using a numerical approach. A two-phase model using a hybrid Eulerian-Lagrangian formulation for the gas-droplet flow is developed. It can avoid the singularity associated with the droplet number density equation in a consistent manner by using a Lagrangian equation for droplet flux fraction. The gas phase is described by a detailed model involving full chemical kinetics and transport, whereas droplet evaporation and heat transfer are modeled assuming quasisteady conditions. Application of the model to several monodisperse sizes of water droplets, ranging from 5 to 50 microns, revealed an interesting nonmonotonic dependence of the flame extinction strain rate on droplet size. This is attributed to the droplet dynamics in the counterflow field considered here and to the resulting nonmonotonic heat sink associated with mass evaporation observed at the oxygen consumption or radical production layer of the flame.