Laminar and Turbulent Flames

The characteristics of a reactive gas (a premixed gas) are dependent not only on the type of reactants, pressure, and temperature, but also on the flow conditions. When the flame front of a combustion wave is flat and one-dimensional in shape, the flame is said to be a laminar flame. When the flame front is composed of a large number of eddies, which are three-dimensional in shape, the flame is said to be a turbulent flame. In contrast to a laminar flame, the combustion wave of a turbulent flame is no longer one-dimensional and the reaction surface of the combustion wave is significantly increased by the eddies induced by the dynamics of the fluid flow. 

When the same chemical compositions of the reactants are used to generate both types of flame, the chemical reaction rate is considered to be the same in both cases. However, the reaction surface area of the turbulent flame is increased due to the nature of eddies and the overall reaction rate at the combustion wave appears to be much higher than that in the case of the laminar flame. Furthermore, the heat transfer process from the burned gas to the unburned gas in the combustion wave is different because of the thermophysical properties specifically, the thermal diffu-sivity is higher for the turbulent flame than for the laminar flame. Thus, the flame speed of a turbulent flame appears to be much higher than that of a laminar flame. 

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. 

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FIGURE 4-2. Sketch of differences in the local direction (upper) and flame front topography (lower) between a laminar and turbulent flame. 

Khitrin, L. N. et al. 1965. Peculiarities of Laminar- and Turbulent-Flame Flashbacks. Proe. 10th Sympos. (Inti.) on Combustion, pp. 1285-1291. 

In the previous discussion of laminar and turbulent flames, the effects of the physical and chemical parameters on flame speeds were considered and the trends were compared with the experimental measurements. It is of interest here to recall that it was not possible to calculate these flame speeds explicitly but, as stressed throughout this chapter, it is possible to calculate the detonation velocity accurately. Indeed, the accuracy of the theoretical calculations, as well as the ability to measure the detonation velocity precisely, has permitted some investigators to calculate thermodynamic properties (such as the bond strength of nitrogen and heat of sublimation of carbon) from experimental measurements of the detonation velocity. 

Figure 9. Comparison of direct photographs of a laminar and turbulent flame at the same flow rate, fuel-air ratio, and burner size...

Remember that the volume Flows and chemical reactions comprised three parts 1. Fluid media with a single component, 2. Reactive mixtures, and 3. Interfaces and lines, that the volume Flows and Chemical Reactions in Homogeneous Mixtures comprised 1. Pipe flows, 2. Chemical reactors, and 3. Laminar and turbulent flames, and that the volume Flows and Chemical Reactions in Heterogeneous Mixtures comprised 1. Generation of multi-phase flows, 2. Problems at the scale of a particle, 3. Simplified model of a non-reactive flow with particles, 4. Simplified model of a reactive flow with particles, and 5. Radiative phenomena. 

Figure 2.3 presents the values of visible velocity Sh of a single spherical source [2] and laminar flame velocity [27] for hydrogen-air mixtures. Later on, the double ignition method was used in [20] for measuring combustion velocities of both laminar and turbulent flames in lean hydrogen-air mixtures. 

A small number of experimental techniques is applicable for measuring a flame velocity in lean hydrogen mixtures. In such mixtures, effects requiring special attention to the arrangement of measurements of both laminar and turbulent flame velocities have been observed. 

Lately, efforts have been made toward the development of techniques to improve laminar and turbulent flame velocity measurements. 

B.E. Gelfand, O.E. Popov, V.P. Karpov, A.Y. Kusharin, G.L. Agafonov, Laminar and turbulent flame propagation in hydrogen-air-steam mixtures at accident relevant pressure-temperature conditions. Report IChPh-INR, 1995... 

Fig. 3.2 The pressure rise in the laminar and turbulent flames and the scheme of determining the expansion factor...

Fundamental, laminar, and turbulent burning velocities describe three modes of flame propagation (see the Glossary for definitions). The fundamental burning velocity, S, is as its name implies, a fundamental property of a flammable mixture, and is a measure of how fast reactants are consumed and transformed into products of combustion. Fundamental burning velocity data for selected gases and vapors are listed in Appendix C of NFPA68 (1998). 

Laminar Versus Turbulent Flames. Premixed and diffusion flames can be either laminar or turbulent gaseous flames. Laminar flames are those in which the gas flow is well behaved in the sense that the flow is unchanging in time at a given point (steady) and smooth without sudden disturbances. Laminar flow is often associated with slow flow from small diameter tubular burners. Turbulent flames are associated with highly time dependent flow patterns, often random, and are often associated with high velocity flows from large diameter tubular burners. Either type of flow—laminar or turbulent—can occur with both premixed and diffusion flames. 

A link between laminar and turbulent lifted flames has been demonstrated based on the observation of a continuous transition from laminar to turbulent lifted flames, as shown in Figure 4.3.13 [56]. The flame attached to the nozzle lifted off in the laminar regime, experienced the transition by the jet breakup characteristics, and became turbulent lifted flames as the nozzle flow became turbulent. Subsequently, the liftoff height increased linearly and finally blowout (BO) occurred. This continuous transition suggested that tribrachial flames observed in laminar lifted flames could play an important role in the stabilization of turbulent lifted flames. Recent measurements supported the existence of tribrachial structure at turbulent lifted edges [57], with the OH zone indicating that the diffusion reaction zone is surrounded by the rich and lean reaction zones. 

P. Clavin. D)mamic behaviour of premixed flame fronts in laminar and turbulent flows. Progress in Energy and Combustion Science, 11 1-59,1985. 

Strykowski, P. J., A. Krothapalli, and S. Jendoubi. 1996. The effect of counterflow on the development of compressible shear layers. J. Fluid Mechanics 308 63-96. Beer, J. M., N. A. Chigier, T. W. Davies, and K. Bassindale. 1971. Laminarization of turbulent flames in rotating environments. Combustion Flame 16 39-45. 

Mungal, M.G., L. Lourenco, and A. Krothapalh. 1995. Instantaneous velocity measurements in laminar and turbulent premixed flames using on-line PIV. Combustion Science Technology 106 239-65. 

After the bifurcation behavior is examined, the role of flame-wall thermal interactions in NOj is studied. First, adiabatic operation is considered. Next, the roles of wall quenching and heat exchange in emissions are discussed. Two parameters are studied the inlet fuel composition and the hydrod3mamic strain rate. Results for the stagnation microreactor are contrasted with the PSR to understand the difference between laminar and turbulent flows. 

Qb (for blow-off). For gradients less than gf, for example, line 1, the burning velocity is somewhere greater than the flow velocity, so the flame will flash back for gradients greater than g6, for example, line 2, the flow velocity is everywhere greater than the burning velocity, so the flame must blow off. Stability data for both laminar and turbulent flow may be correlated by gf and gb this is reasonable because in either case there is a laminar sublayer at the burner wall (23). 

Beer, J. M., N. A. Chigier, T. W. Davies, and K. Bassindale. 1971. Laminarization of turbulent flames in rotating environments. Combustion Flame 16 39-45. 

Horsley, M. E., Purvis, M. R. L, and Tariq, A. S. "Gonvective Heat Transfer From Laminar and Turbulent Premixed Flames." Heat Transfer 1982, vol. 3, edited by U. Grigull, E. Hahne, K. Stephan, and J. Straub, 409-15. Washington, DC Hemisphere 1982. 

The transition from laminar to turbulent flames occurs for Re 2,000 with the Reynolds number referring to the flame. It is smaller than in the unbumt mixture because the viscosity of gases rises with increasing flame temperature. The combustion process of a turbulent pre-mixed flame can be controlled well. However, for safety reasons it is not readily applied because flammable mixtures may accumulate and hence explode. 

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