Flame Reynolds number

Orifice Jlanies can be cliaracterized as either prcmi.xed or diffusion flmnes. In a premixed flame, die air for combustion is already nii.xed widi the fuel gas before it leaves the orifice or pipe. In a diffusion flame, fiiel e.xidng die orifice is pure and the air needed for combustion diffuses into die fuel gas from the surroundings. Orifice flames can also be cliaracterized by die flame Reynolds number. Tlie flame lengdi of a diffusion flame can be calculated by Jost s equadon. ... 

TurbulentPremixedFlames. Combustion processes and flow phenomena are closely coimected and the fluid mechanics of a burning mixture play an important role in forming the stmcture of the flame. Laminar combusting flows can occur only at low Reynolds numbers, defined as... 

Turbulent Diffusion FDmes. Laminar diffusion flames become turbulent with increasing Reynolds number (1,2). Some of the parameters that are affected by turbulence include flame speed, minimum ignition energy, flame stabilization, and rates of pollutant formation. Changes in flame stmcture are beHeved to be controlled entirely by fluid mechanics and physical transport processes (1,2,9). 

Consider the case of the simple Bunsen burner. As the tube diameter decreases, at a critical flow velocity and at a Reynolds number of about 2000, flame height no longer depends on the jet diameter and the relationship between flame height and volumetric flow ceases to exist (2). Some of the characteristics of diffusion flames are illustrated in Eigure 5. 

Because of fhe planar nafure of the cormterflow flame and the relatively high Reynolds number associated with the flow, the flame/flow configuration can be considered to be "aerodynamically clean," where the quasi-one-dimensional and bormdary-layer simplifications can be implemented in either analytical or computational studies. Useful insights into the thermochemical structure... 

Chomiak, ]., Dissipation fluctuations and the structure and propagation of turbulent flames in premixed gases at high Reynolds numbers. Proceedings of the Combustion Institute, 16, 1665-1673,1977. 

In the so-called "wrinkled flame regime," the "turbulent flame speed" was expected to be controlled by a characteristic value of the turbulent fluctuations of velocity u rather than by chemistry and molecular diffusivities. Shchelkin [2] was the first to propose the law St/Sl= (1 + A u /Si) ), where A is a universal constant and Sl the laminar flame velocity of propagation. For the other limiting regime, called "distributed combustion," Summerfield [4] inferred that if the turbulent diffusivity simply replaces the molecular one, then the turbulent flame speed is proportional to the laminar flame speed but multiplied by the square root of the turbulence Reynolds number Re. ... 

M.S. Wu, S. Kwon,J. Driscoll, andG.M. Faeth 1990, Turbulent premixed hydrogen-air flames at high Reynolds numbers. Combust. Set. Technol. 73(l-3) 327-350. 

Scatter plots of temperature atx/d = 15 in turbulent Cl-14/air jet flames with Reynolds numbers of 13,400 (Flame C) and 44,800 (Flame F). The stoichiometric mixture fraction is = 0.351. The line shows the results of a laminar counterflow-flame calculation with a strain parameter of a = 100 s and is included as a visual guide. (From Barlow, R.S. and Frank, J.H., Proc. Combust. Inst, 27,1087,1998. With permission.)... 

Turbulent nonpremixed flames contain a wide range of lengfh scales. For a given flame geomefry, fhe largest scales of furbulence are determined by the overall width of an unconfined jef flame or by fhe dimensions of the hardware that contain the flow. Therefore, the largest scales of turbulent motion are typically independent of Reynolds number. As the Re5molds number increases. 

Clemens, N.T., Paul, P.H., and Mungal, M.G., The structure of OH fields in high Reynolds number turbulent jet diffusion flames. Combust. Sci, Technol., 129,165,1997. 

Owing to the exponential increase in the computational power, today s DNS reaches fully turbulent Reynolds numbers and can use detailed chemical schemes, once affordable only for one-dimensional laminar flames computations. 

Bilger, R. W. (1982). Molecular transport effects in turbulent diffusion flames at moderate Reynolds number. AIAA Journal 20, 962-970. 

The flames themselves can alter the turbulence. In simple open Bunsen flames whose tube Reynolds number indicates that the flow is in the turbulent regime, some results have shown that the temperature effects on the viscosity are such that the resulting flame structure is completely laminar. Similarly, for a completely laminar flow in which a simple wire is oscillated near the flame surface, a wrinkled flame can be obtained (Fig. 4.41). Certainly, this example is relevant to <5L < /k that is, a wrinkled flame. Nevertheless, most open flames... 

For a better understanding of this type of flame occurrence and for more explicit conditions that define each of these turbulent flame types, it is necessary to introduce the flame stretch concept. This will be done shortly, at which time the regions will be more clearly defined with respect to chemical and flow rates with a graph that relates the nondimensional turbulent intensity, Reynolds numbers, Damkohler number, and characteristic lengths /. 

Below and to the right of this line, the Klimov-Williams criterion is satisfied and wrinkled laminar flames may occur. The figure shows that this region includes both large and small values of turbulence Reynolds numbers and velocity ratios (VISA) both greater and less than 1, but predominantly large Da. 

The figure shows U >. S L in this region and Da is predominantly small. At the highest Reynolds numbers the region is entered only for very intense turbulence, U > SL. The region has been considered a distributed reaction zone in which reactants and products are somewhat uniformly dispersed throughout the flame front. Reactions are still fast everywhere, so that unbumed mixture near the burned gas side of the flame is completely burned before it leaves what would be considered the flame front. An instantaneous temperature measurement in this flame would yield a normal probability density function—more importantly, one that is not bimodal. 

The flow of oxygen through the inner capillary of the burner (Fig. 1 b) is laminar. The estimated Reynolds number in this region for 1000 bar (Fig. 4) is about 200, much below the critical number for turbulence. This is also true for the other pressures investigated. The flames can clearly be considered as diffusion flames. Because of their conical shape the conventional simplified treatment of laminar diffusion flames can be applied [16 — 18]. According to Burke and... 

Figure 15.5 A Damkohler-Reynolds number plot revealing the turbulent combustion regimes characterizing flame propagation in the TC apparatus, identified by the region enclosed by the parallelogram...

The creation of eddies in a combustion zone is dependent on the nature of the flow of the unburned gas, i. e., the Reynolds number. If the upstream flow is turbulent, the combustion zone tends to be turbulent. However, since the transport properties, such as viscosity, density, and heat conductivity, are changed by the increased temperature and the force acting on the combustion zone, a laminar upstream flow tends to generate eddies in the combustion zone and here again the flame becomes a turbulent one. Furthermore, in some cases, a turbulent flame accompanied by large-scale eddies that exceed the thickness of the combustion wave is formed. Though the local combustion zone seems to be laminar and one-dimensional in nature, the overall characteristics of the flame are not those of a laminar flame. 

One can describe these phenomena through the Reynolds number (forced convection) and Rayleigh number (natural convection), but the reader can see immediately that the situations are so complicated that correlations in elementary texts on fluid flow are not easily applicable to predicting flame behavior. Reactive flows are among the most complex problems in modem engineering. 

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