Combined experimental and computational study of laminar, axisymmetric hydrogen-air diffusion flames

We investigate the structure of two-dimensional, axisymmetric, laminar hydrogen-air flames in which a cylindrical fuel stream is surrounded by coflowing air, using laser-diagnostic and computational methods. Spontaneous Raman scattering and coherent anti-Stokes Raman scattering (CARS) are used to measure the distributions of major species and temperature. Computationally, we solve the governing conservation equations for mass, momentum, energy, and species, using detailed chemistry and transport. The fuel is diluted with nitrogen (1: 1) to reduce heat transfer to the burner, to match the zero temperature gradient at the fuel exit. Three average fuel exit velocities are studied: 18, 27, and 50 cm/s. Comparisons of the measured and computed results are performed for radial profiles at a number of axial positions, and along the axial centerline. Peak major species mole fractions and temperatures are quantitatively predicted by the computations, and the axial species profiles are predicted to within the experimental uncertainty. In the radial profiles studied, base-case computations excluding thermal diffusion of light species were in excellent agreement with the measurements. While the addition of thermal diffusion led to some discrepancy with the measured results, the magnitude of the differences was no more than 25%. The computations predicted the axial centerline profiles from the burner exit to the maximum temperature well, though the experimental temperatures in the downstream mixing region decreased somewhat faster than the computed profiles. Radiative losses are seen to be negligible in these flames, and changes in transport properties and variations in initial flow velocities generally led to only modest changes in the axial profiles. The results also show that the detailed axial profiles of major species and temperature at different fuel jet velocities scale quantitatively with the jet velocity. (c) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved

ON THE ROLE OF TRANSPORT IN THE COMBUSTION KINETICS OF A STEADY-STATE PREMIXED LAMINAR CO+H2+O2 FLAME

The role of mechanistic steps, diffusion, and their interrelation is explored in a steady-state premixed laminar CO + H-2 + O2 flame using a numerical model. Sensitivity coefficients and Green's functions calculated for this system offer systematic characterization of the role of diffusion and exothermicity in carbon monoxide oxidation kinetics. The results reveal that the uncertainties in transport parameters are as important to the model predictions as those in the kinetic steps. The rate controlling steps of the CO + H-2 + O2 reaction are found to be different for adiabatic and nonadiabatic premixed flames, and also for systems with and without transport. In particular, the reactions of the hydroperoxyl radical with hydrogen, oxygen, and hydroxyl radicals are found to be important at all temperatures in the fuel lean (40 torr) adiabatic flame studied here. The diffusive mixing of chemical species from the low and the high temperature portions of the flame and the larger heats of reaction associated with the hydroperoxyl radicals are found to be responsible for the increased importance of these reactions. (C) 1994 , Inc

Dynamics of laminar triple-flamelet structures in non-premixed turbulent combustion

In the spirit of laminar-flamelet modelling of non-premixed turbulent combustion, a diffusion flamelet is studied. However, the flamelet is also taken to end at a finite position. Such an end of a diffusion flame exhibits fuel-rich and fuel-lean elements as well as the diffusion flame sheet—a structure that is known as a \emph{triple-flame} and which has the property of being able to propagate. A counterflow geometry with shear becomes the most relevant situation in which to picture ends of diffusion flames in a turbulent flow. In an equidiffusive system, the speed of propagation of the end-point is demonstrated to be positive only for relatively limited values of strain or scalar dissipation rate and becomes large and negative towards the higher finite value at which a diffusion flame would extinguish uniformly. The implications of these findings for the behaviour of turbulent diffusion flames are discussed