Two-dimensional Direct Numerical Simulation of Opposed-jet Hydrogen/air Flames: Transition from a Diffusion to an Edge Flame
نویسندگان
چکیده
In an opposed-jet diffusion flame experiment, under certain conditions, after the extinction of the diffusion flame, an edge flame can be obtained. This ring-shaped edge flame was first reported in 1959 by Potter and Butler but received little attention. It was reported again recently in a numerical and an experimental work and is responsible for an interesting transition between two distinct burning flames (multiple solutions). Motivated by our previous numerical results, obtained with simplified kinetics and some recently reported experimental data, we performed direct numerical simulations of this transition to investigate the underlying physical mechanisms. The appearance of an edge flame after the extinction of the diffusion flame, the hysteresis reported in the experiments, and the existence of multiple vigorously burning flames at identical conditions are all captured by our simulations. Our numerical results show that, in the absence of an inert coflow curtain, when the diffusion flame disk is extinguished, an edge flame forms and propagates in the mixing layer. After the formation of this edge flame, even when the applied strain rate is reduced to the initial subcritical value, the diffusion flame disk does not reappear, because the local fluid velocity still exceeds the propagation speed of the edge flame. This hysteresis has significant implications in the common submodel that utilizes the strain rate as a parameter to determine local reignition in flamelet models; it indicates that a subcritical strain rate is not a sufficient condition for the reignition of a diffusion flame. Further investigation of this phenomenon is clearly needed to refine submodels of local extinction and reignition in the flamelet models for turbulent diffusion flames. The opposedjet configuration provides a convenient platform to analyze edge flames which are stabilized aerodynamically in a two-dimensional geometry, thus making matching two-dimensional direct numerical simulations effective.
منابع مشابه
Structure and Propagation of Triple Flames in Partially Premixed Hydrogen–Air Mixtures
The characteristics of triple flames in a hydrogen–air mixing layer are studied using direct numerical simulation with detailed chemistry. Triple flames are initiated by imposing a temperature ignition source in the center of a scalar mixing layer of nonuniform thickness, thereby forming a pair of freely propagating triple flames. Two different fuel streams are studied: pure hydrogen and hydrog...
متن کاملDirect Numerical Simulation of a Turbulent Lifted Flame: Stabilisation Mechanism
A turbulent lifted slot-jet flame is studied using direct numerical simulation (DNS). A single step chemistry model is employed with a mixture-fraction dependent activation energy to quantitatively reproduce the dependence of laminar burning rate on equivalence ratio that is typical of hydrocarbon fuels. It is observed that the leading flame edge exhibits a single branch close to the stoichiome...
متن کاملDNS of auto–ignition in turbulent diffusion H2/air flames
Direct numerical simulation (DNS) is used to study auto–ignition of turbulent diffusion flames. A novel, all–Mach number algorithm developed by Doom et al is used. The chemical mechanism is a nine species, nineteen reaction mechanism for H2 and Air from Mueller at el. 2 Simulations of three dimensional turbulent diffusion flames are performed. Isotropic turbulence is superimposed on an unstrain...
متن کاملQuantification of differential diffusion in nonpremixed systems
Most attempts to quantify differential diffusion (DD) are based on the difference between different definitions of the mixture fraction. This paper presents a general method for evaluating differential diffusion in premixed or nonpremixed systems based on conservation equations for the elemental mass fractions. These measures form a basis for analysing differential diffusion. Casting these in t...
متن کاملSimulation of Cup-Burner Flames in Microgravity
The extinction process of cup-burner flames under normal-gravity conditions were previously studied. As the low-speed diffusion flames behave differently in microgravity compared to those on earth, it is important to understand the structure of cup-burner flame and its extinction characteristics under 0g conditions. A numerical study was performed in the present paper using a time-dependent, ax...
متن کامل