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Forced Time-Varying Methane Diffusion Flame - Yale University

Forced Time-Varying Methane Diffusion Flame - Yale University

Forced Time-Varying Methane Diffusion Flame - Yale

Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute, 1998/pp. 693–702COMPUTATIONAL AND EXPERIMENTAL STUDY OF A FORCED, TIME-VARYING, AXISYMMETRIC, LAMINAR DIFFUSION FLAMERAHIMA K. MOHAMMED, MICHAEL A. TANOFF,* MITCHELL D. SMOOKE, ANDREW M. SCHAFFERand MARSHALL B. LONGDepartment of Mechanical EngineeringYale UniversityNew Haven, CT 06520-8284, USAForced, time-varying flames are laminar systems that help bridge the gap between laminar and turbulentcombustion. In this study, we investigate computationally and experimentally the structure of an acousticallyforced, axisymmetric laminar methane-air diffusion flame in which a cylindrical fuel jet is surroundedby a coflowing oxidizer jet. The flame is forced by imposing a sinusoidal modulation on the steady fuelflow rate. Rayleigh scattering and spontaneous Raman scattering of the fuel are used to generate thetemperature profile. Particle image velocimetry (PIV) is used to measure the fuel tube exit velocity overa cycle of the forcing modulation. CH flame emission measurements have been done to predict the excitedstateCH (CH*) levels. Computationally, we solve the transient equations for the conservation of totalmass, momentum, energy, and species mass with detailed transport and finite-rate C 2 chemistry submodelsto predict the pressure, velocity, temperature, and species concentrations as a function of the two independentspatial coordinates and time. The governing equations are written in primitive variables. Implicitfinite differences are used to discretize the governing equations and the boundary conditions on a nonstaggered,nonuniform grid. Modified damped Newton’s method nested with a Bi-CGSTAB iteration isutilized to solve the resulting system of equations. Results of the study include a detailed description ofthe fluid dynamic-thermochemical structure of the flame at a 20-Hz frequency. A comparison of experimentallydetermined and calculated temperature profiles and CH* levels agree well. Calculated molefractions of species indicative of soot production (C 2 H 2 , CO) are compared against those levels in thecorresponding steady flame and are observed to increase in peak concentration values and spatial extent.Analysis of acetylene production rates reveals additional significant production in the downstream regionof the flame at certain times during the flame’s cyclic history.IntroductionMost practical combustion systems, such as gasturbines and industrial furnaces, employ diffusionflames as the basic combustion element. These combustionsystems are unsteady, high-speed, three-dimensional,turbulent reacting systems. A completedetailed understanding of the dynamical complexityof non-premixed turbulent reacting flows is beyondthe capability of present computational models andexperimental diagnostics. The last decade, however,has yielded significant progress in the study of multidimensionallaminar steady diffusion flames bothnumerically and experimentally [1]. Time-varyinglaminar diffusion flames are another class of nonpremixedcombustion bridging the gap betweensteady laminar combustion and turbulent combustion.Time-varying flames offer a much wider rangeof interactions between chemistry and flow field thancan be examined under steady-state conditions. The*Present address: The W.K. Kellogg Foundation, 2Hamblin Ave. East, Battle Creek, MI 49016-3232.complex coupling between chemistry and fluid flowin time-varying laminar flames effectively samplesdifferent regimes of temperature, mixture fraction,residence time, strain, and scalar dissipation ratesthan are observed under steady conditions.A specific type of unsteady flame, in which a periodicfluctuation in time is imposed on the fuel flowrate of a steady laminar flame, is known as a forced,time-varying flame. The study of these flames helpsin understanding the interactions between fluidtransport and heat and mass transfer in practicalcombustion systems. Fundamental studies of theseinteractions including detailed combustion chemistryare critical to the understanding of the pollutantformation processes and to the modeling of turbulentdiffusion flames through the concept of laminarflamelets.A number of investigations have considered laminar,time-varying diffusion flames. Some of thesestudies focused on naturally occurring flickeringflames [2–6] and others on systems in which the fueland air coflow were forced mechanically [7–16].These investigations have been experimental693

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