The objective of this study is to investigate the extinction characteristics of propane/air flames under the influences of Lewis number, flow stretch, internal heat loss and external heat loss using activation energy asymptotics. We consider a steady, planar, premixed flame generated in a stagnation-point, two-phase flow in which the dispersed phase is simulated by a monodisperse, dilute and chemically inert spray (water spray). A completely prevaporized mode and a partially prevaporized mode of flame propagation are identified by a critical initial droplet size ( ) for the droplet to achieve complete evaporation at the premixed flame front. Internal heat loss associated with water vaporization increases with increasing liquid loading or decreasing initial droplet size. The (positive) flow stretch coupled with Lewis number (Le), respectively, weakens and intensifies the burning intensity of the lean propane/air flame (Le>1) and rich propane/air flame (Le<1). Additionally, the external heat loss (K) weakens the burning intensity and has a significant influence on the flame behavior. The results show that the Le>1 flame can be extinguished with or without external heat loss. Flame extinction characterized by a C-shaped curve is dominated by the external heat loss or the flow stretch. For the Le<1 flame without external heat loss, no extinction occurs under the influence of flow stretch. However, the Le<1 flame with completely prevaporized water sprays enduring a small amount of flow stretch can be extinguished by the external heat loss and this behavior is characterized by a C-shaped curve. Note that the W-shaped extinction curve is mainly governed by the internal heat loss. Flame extinction characterized by a W-shaped curve occurs when the Le<1 flame with external heat loss endures a positive stretch and experiences a partially prevaporized water spray having sufficiently large liquid loading and droplet size.
ICLASS-2006, 27 August-1 September, 2006, Kyoto, Japan