Heat release effects on laminar flame propagation in partially premixed flows are studied. Data for analysis are obtained from direct numerical simulations of a laminar mixing layer with a uniformly approaching velocity field. The structure that evolves under such conditions is a triple flame, which consists of two premixed wings and a trailing diffusion flame. Heat release increases the flame speed over that of the corresponding planar premixed flame. In agreement with previous analytical work, reductions in the mixture fraction gradient also increase the flame speed. The effects of heat release and mixture fraction gradients on flame speed are not independent, however; heat release modifies the effective mixture fraction gradient in front of the flame. For very small mixture fraction gradients, scaling laws that determine the flame speed in terms of the density change are presented.

Abstract-The structure of steady state diffusion flames is investigated by analyzing the mixing and chemical reaction of two opposed jets of fuel and oxidizer as a particular example. An Arrhenius onestep irreversible reaction has been considered in the realistic limit of large activation energies. The entire range of Damkohler numbers, or ratio of characteristic diffusion and chemical times, has been covered. When the resulting maximum temperature is plotted in terms of the Damkohler number (which is inversely proportional to the flow velocity) the characteristic S curve emerges from the analysis, with segments from the curve resulting from:(a) A nearly frozen ignition regime where the temperature and concentrations deviations from its frozen flow values are small. The lower branch and bend of the S curve is covered by this regime.(b) A partial burning regime, where both reactants cross the reaction zone toward regions of frozen flow. This regime is unstable.(c) A premixed flame regime where only one of the reactants leaks through the reaction zone, which then separates a region of frozen flow from a region of near-equilibrium.(d) A near-equilibrium diffusion controlled regime, covering the upper branch of the S curve, with a thin reaction zone separating two regions of equilibrium flow.Analytical expressions are obtained, in particular, for the ignition and extinction conditions.

The penetration of a wetting liquid in the narrow gap between two vertical plates making a small angle is analyzed in the framework of the lubrication approximation. At the beginning of the process, the liquid rises independently at different distances from the line of intersection of the plates except in a small region around this line where the effect of the gravity is negligible. The maximum height of the liquid initially increases as the cubic root of time and is attained at a point that reaches the line of intersection only after a certain time. At later times, the motion of the liquid is confined to a thin layer around the line of intersection whose height increases as the cubic root of time and whose thickness decreases as the inverse of the cubic root of time. The evolution of the liquid surface is computed numerically and compared with the results of a simple experiment.

In the present paper we investigate flame spread in laminar mixing layers both experimentally and numerically. First, a burner has been designed and built such that stationary triple flames can be stabilised in a coflowing stream with well defined linear concentration gradients and well defined uniform flow velocity at the inlet to the combustion chamber. The burner itself as well as first experimental results obtained with it are presented. Second, a theoretical model is formulated for analysis of triple flames in a strained mixing layer generated by directing a fuel stream and an oxidizer stream towards each other. Here attention is focused on the stagnation region where by means of a similarity formulation the three-dimensional flow can be described by only two spatial coordinates. To solve the governing equations for the limiting case in which a thermal-diffusional model results, a numerical solution procedure based on self-adaptive mesh refinement is developed. For the thermal-diffusional model, the structure of the triple flame and its propagation velocity are obtained by solving numerically the governing similarity equations for a wide range of strain rates.

We have studied flame propagation in a strained mixing layer formed between a fuel stream and an oxidizer stream, which can have different initial temperatures. Allowing the Lewis numbers to deviate from unity, the problem is first formulated within the framework of a thermo-diffusive model and a single irreversible reaction. A compact formulation is then derived in the limit of large activation energy, and solved analytically for high values of the Damkohler number. Simple expressions describing the flame shape and its propagation velocity are obtained. In particular, it is found that the Lewis numbers affect the propagation of the triple flame in a way similar to that obtained in the studies of stretched premixed flames. For example, the flame curvature determined by the transverse enthalpy gradients in the frozen mixing layer leads to flame-front velocities which grow with decreasing values of the Lewis numbers.The analytical results are complemented by a numerical study which focuses on preferentialdiffusion effects on triple flames. The results cover, for different values of the fuel Lewis number, a wide range of values of the Damkohler number leading to propagation speeds which vary from positive values down to large negative values. IntroductionFlame propagation in inhomogeneous mixtures occurs in most practical situations. For example, spatial non-uniformities in the enthalpy of the reactants are frequently encountered in unpremixed-combustion devices. Even when such non-uniformities are weak, their impact on the initiation process and the dynamics of burning is generally important. This is due to the typical large activation energies of the chemical reactions encountered in combustion, which make their rates very sensitive to the surrounding conditions. In many instances, composition and temperature inhomogeneities are essentially transverse to mixing layers, along which flames can propagate, as in lifted jet diffusion flames. Because the combustible mixture varies from lean to rich across a mixing layer, triple flames, consisting of two premixed branches and a trailing diffusion flame, are expected. Therefore, they have been the subject of a number of experimental, analytical and numerical studies [1][2][3][4][5][6].The main purpose of the present investigation is to determine how the propagation of the triple flame is influenced by transverse enthalpy gradients in the fresh mixture and by differential diffusion. We shall select for definiteness the strained mixing layer configuration as a frame for the investigation, and adopt additionally the constant-density approximation [7] to make the analytical description tractable. The configuration of the study is sketched Oxidizer side Figure 1. The strained mixing layer configuration. The fuel stream has temperature 7p, a fuel mass fraction >>F,F and contains no oxidizer. The oxidizer stream has temperature 7b, an oxidizer mass fraction >>o.o and contains no fuel. The density being assumed constant, the velocity field considered is a two-dimensional stagna...