Opposed flow premixed flame

Fig. 17.10 Velocity profiles for opposed-flow premixed flames, using both the finite-separation and semi-infinite formulation. Both profiles have the same apparent strain rate of 1200 s 1.

Kee, R.J., Miller, J.A., Evans, G.H., and Dixon-Lewis, G., A computational model of the structure and extinction of strained, opposed flow, premixed methane-air flames, Proc. Combust. Inst., 22, 1479, 1988. 

Fig. 1.1 Illustration of a premixed flat-flame burner and an opposed-flow diffusion flame.

The right-hand panel of Fig 1.1 illustrates an opposed-flow diffusion-flame arrangement. Here the fuel and oxidizer flows are separated, only coming together at the flame. Both premixed and nonpremixed flames find use in practical combustion devices. Thus it is important to model and understand the behaviors of both types of flames, as well as combinations. The opposed contraction nozzles illustrated in the figure lead to a desirable flow similarity, which facilitates modeling and data interpretation. 

Fig. 17.9 Selected species profiles in opposed-flow, premixed, twin flames [214]. The solution in the upper panel is at a high strain rate, which is very near extinction, and that in the lower panel is far from extinction. Both are for a mixture of 9% methane in air. The flow is from left to right, with the symmetry plane on the right.

Fig. 17.11 Extinction behavior of strained, opposed-flow, premixed, methane-air flames. The left-hand panel shows the dependence of the maximum temperature at the symmetry plane as a function of the semi-infinite strain-rate parameter a, for five different mixture stoichiometries. The right-hand panel compares measured extinction strain rates [238] with predictions for both the semi-infinite and finite-gap model formulations. The nozzle separation distance is 7 mm (i.e., 3.5 mm from nozzle to symmetry plane).

The general idea of arc-length continuation is illustrated in the upper panel of Fig. 17.13. The illustration is motivated by the premixed, opposed-flow, twin-flame extinction. The maximum flame temperature (at the symmetry plane) is shown as a function of the inlet velocity U. This is essentially the same situation as shown in Fig. 17.11, although in Fig. 17.11 the reciprocal strain rate 1 /a, and not the inlet velocity, is used as the parameter. 

As illustrated, here a single variable (the maximum temperature) is chosen as a characteristic function of the solution. For the premixed twin flame, this is a good choice. However, in other circumstances, like an opposed-flow diffusion flame, the choice of a characteristic scalar is less clear. Vlachos avoids the need for a choice by using a norm of the full-solution vector to characterize the solution in the arc length [415,416], The Nish-... 

Opdiff, simulates opposed-flow premixed or diffusion stagnation-flame configurations. 

R.J. Kee, J.A. Miller, G.H. Evans, and G. Dixon-Lewis. A Computational Model of the Structure and Extinction of Strained, Opposed-Flow, Premixed, Methane-Air Flames. Proc. Combust. Inst., 22 1479-1493,1988. 

Barlow, R.S., Karpetis, A.N., and Frank, J.H., Scalar profiles and NO formation in laminar opposed-flow partially premixed methane/air flames, Combust. Flame, 127, 2102,2001. 

Partial premixing has been proposed as a means of NOj, reduction in gas turbine engines by Jayavant Gore at Purdue University. An experimental and computational study was conducted to observe NO behavior under the circumstances of moderate stretch rate, opposed-flow, partially premixed flames. The results show that the minimum NO emissions at an optimal level of partial premixing result as a consequence of decrease in CH radical concentrations. Partial premixing appears to be a possible practical immediate solution for NO remediation in gas turbines. 

The tendency of premixed flames to detach from the flame holder to stabilize further downstream has also been reported close to the flammability limit in a two-dimensional sudden expansion flow [27]. The change in flame position in the present annular flow arrangement was a consequence of flow oscillations associated with rough combustion, and the flame can be particularly susceptible to detachment and possible extinction, especially at values of equivalence ratio close to the lean flammability limit. Measurements of extinction in opposed jet flames subject to pressure oscillations [28] show that a number of cycles of local flame extinction and relight were required before the flame finally blew off. The number of cycles over which the extinction process occurred depended on the frequency and amplitude of the oscillated input and the equivalence ratios in the opposed jets. Thus the onset of large amplitudes of oscillations in the lean combustor is not likely to lead to instantaneous blow-off, and the availability of a control mechanism to respond to the naturally occurring oscillations at their onset can slow down the progress towards total extinction and restore a stable flame. 

Figure 27.1 Measurements and predictions of mole fractions of CH4, O2, and N2 as a function of distance from the fuel duct for diffusion (a) and partially premixed opposed flow flames with b = 2.2 (6) and 1.42 (c), Tair = 560 K and Ttuei = 321 K, Fair = 70, 60, and 50 cm/s, Ffuei = 70 cm/s, distance between ducts 1.5 cm...

As illustrated in Fig. 1.2, a premixed flow of acetylene, hydrogen, and oxygen issue from a flat burner face onto a parallel, flat surface. Mathematically there is very little difference between this situation and one in which two flat burners face each other, in an opposed-flow configuration. There are many commonly used variants of the opposed-flow geometry. For example, premixed, combustible, gases could issue from both burner faces, causing twin premixed flames. Alternatively, fuel could issue from one side and oxidizer from the other, causing a nonpremixed, or diffusion, flame. 

The Opposed-Flow Laminar Diffusion Flame Laminar diffusion flames can be more complicated chemically and physically than the corresponding premixed flames. This is especially so for a candlelike co-flowing situation. Because of the difficulties of adequately representing the two- or three-dimensional flow field, together with detailed chemistry, these flames are difficult to use as the basis for chemical-kinetic studies. How-... 

Opposed-flow configurations can be used to establish strained premixed flames. Like the diffusion-flame situation, there are several ways to create the opposed flow, including opposed porous plates [197] or opposed contraction nozzles [349]. As illustrated in Fig. 17.8, two opposed contraction nozzles form a symmetric flow. When the mixture stoichiometry, temperature, and flow rates are equal in both nozzles, twin flames are stabilized near the center. 

In addition to the low-strain limit, which can be used to determine laminar burning velocities, the opposed-flow configuration can also be used to determine high-strain-rate extinction limits. As the inlet velocities increase, the flame is pushed closer to the symmetry plane and the maximum flame temperature decreases. There is a flow rate beyond which a flame can no longer be sustained (i.e., it is extinguished). Figure 17.11 illustrates extinction behavior for premixed methane-air flames of varying stoichiometries. 

JET. The diameters of most jets used today are roughly 0.5 mm. This is because of the type of flame used. In a diffusion flame, as opposed to a flame in which the oxidizer and fuel are premixed, the rate of diffusion of the two gases controls the rate of burning. The velocity of the gas flow out the jet must be set to match the rate of diffusion. If the jet diameter is a little different from 0.5 mm, the detector can be optimized at a proportionately different flow. Much beyond a factor of two differences in diameter leads to some other effects, such as thermal transfer or flow stability problems. 

Recirculation of combustion products can be obtained by several means (1) by inserting solid obstacles in the stream, as in ramjet technology (bluff-body stabilization) (2) by directing part of the flow or one of the flow constituents, usually air, opposed or normal to the main stream, as in gas turbine combustion chambers (aerodynamic stabilization), or (3) by using a step in the wall enclosure (step stabilization), as in the so-called dump combustors. These modes of stabilization are depicted in Fig. 4.52. Complete reviews of flame stabilization of premixed turbulent gases appear in Refs. [66, 67],... 

Kostiuk, L. W., Bray, K. N. C. and Cheng, R. K. (1993). Experimental study of premixed combustion in opposed streams, Part I —Non-reacting flow field. Combustion and Flame, 92 377-395. 

This experimental investigation was motivated by the requirements of lean-premixed methane-air flames in modern gas-turbine combustors and the periodic extinction and relight observed close to the lean limit [1]. The first involves low equivalence ratios with possible dynamic effects, and the second involves a strain rate mechanism that may imply oscillations in bluff-body stabilized flames at all equivalence ratios. Opposed flames are used here to examine the nature of extinction, and to a lesser extent ignition to quantify extinction velocities and times and to determine limitations of this comparatively simple arrangement. The same arrangement was used in investigations of the corresponding isothermal flow [2]. 

Kostiuk, L.W., K.N.C. Bray, and R. K. Cheng. 1993. Experimental study of premixed turbulent combustion in opposed streams. Part II — Reacting flow field and extinction. Combustion Flame 92 396-409. 

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