Ignition,flame Structure And Flame Stabilization In Laminar Boundary Layer Flows

Combustion phenomena in boundary layers occur widely in nature and industrial production,and in which there are often multiple factors coupled with each other.How-ever,the existing research on the topic still lacks in-depth theoretical analysis on the effects of these factors in actual physical processes.In this paper,we investigate sev-eral fundamental combustion phenomena in the laminar boundary layer both numerically and theoretically,involving combustion problems such as ignition,flame structure,and flame stabilization.It is focused on combustion dynamics under the influence of var-ious complex factors.The complex factors herein include multi-step radical reactions,non-constant boundary conditions,multi-layer boundary layer structures,non-monotonic flame liftoff and transitions,etc.The main contributions of this study are summarized as follows.To begin with,the ignition of premixed combustibles in the boundary layer is studied and two different types of ignition mechanisms,including radical-induced and thermal-induced ignition,are considered.We first analyze the radical-induced ignition of coun-terflowing hydrogen/oxygen mixtures versus heated nitrogen.The existing theoretical solution of the Z-shaped ignition limit in the pressure-temperature plot,which is only ap-plicable to the homogeneous situation,is extended to inhomogeneous flow field with the presence of convection and diffusion.The effects of the high diffusion coefficient of the hydrogen atom and the heat feedback mechanism of the reactions are further discussed.Then we analyze the thermal-induced ignition of a reacting boundary-layer flow over the hot surface with an unheated starting length.The influence of the unheated starting length,which is commonly found in experiments and engineering,on the ignition distance is ana-lyzed,and the theoretical relationship between the ignition distance and other parameters such as the starting length is obtained.Furthermore,two well-known evaporative diffusion flames in the fire research,namely Emmons flame and fire whirl,are studied.Firstly,the effects of the imperme-able leading segment on the Emmons flame structure and fuel evaporation are simulated and analyzed.The widely used Emmons boundary layer similarity solution is extended to the situation with the existence of an impermeable leading segment.As for the fire whirl,the Ekman boundary layer structure at its bottom is studied.Based on the fact that the fuel evaporation is mainly concentrated in the region near the edge of the fuel pool,the analytical equations of the important physical quantities such as the fuel consumption rate and flame height of the fire whirl are derived.It is also found that the product of the local evaporation rate and flame height in the boundary layer is an invariant,which does not depend on the Ekman number and radial distance.Finally,we further investigate the stabilization of the Emmons flame and fire whirl.By analyzing the dynamic balance of the propagation speed of the triple flame at the flame front and the local flow speed,the theoretical solutions of the flame liftoff position and the blowout limit for both the Emmons flame and fire whirl are obtained.The effect of the transversal velocity gradient on flame stabilization is also discussed.Compared with the monotonic response of the liftoff distance to the Damk?hler number for the Emmons flame,the liftoff of the fire whirl is non-monotonic and the response of the liftoff distance to the Damk?hler number has multiple branches.It is shown that these branches have different stability characteristics.According to our study,a fire whirl can transition to a blue whirl after being lifted to a certain extent and vortex breakdown is not a necessary condition for the creation of a blue whirl.We also propose a generalized definition of liftoff distance,which leads to a unified plot for the liftoff of the fire whirl and blue whirl.It further gives the respective existence regions and interconversion paths for these two types of whirling flames,which can provide guidance to the design of the experiment on fire whirl stabilization and blue whirl generation.