Flow behavior impact on the suppression effectiveness of sub-10-μm water drops in propane/air co-flow non-premixed flames

Extinction concentrations in a non-premixed propane/air co-flow flame were measured for piezoelectrically generated water mist containing sub-10-μm drops. Based on comparison to flame extinction with nitrogen, it was determined that 66% of the total available water mist enthalpy (27% of available enthalpy of vaporization) is used in flame suppression. Flow field analysis, using particle image velocimetry, and simultaneous flame imaging show that the water drops evaporate well outside the flame front and primarily travel vertically past the flame. Sub-micrometer particle imaging data support the assumption that water vapor resulting from drop evaporation near the flame follows the entraining airflow and interacts with the flame. In concert, the extinction and imaging experiments suggest that, due to the proximity and flow direction of the drops relative to the flame as they evaporate, the flame experiences only a portion of the cooling effect induced by drop evaporation. The majority of the evaporating drops likely exchange heat with the neighboring bulk water mist and contribute to flame suppression primarily by providing water vapor as a heat sink.