Laminar jets

Many different configurations of diffusion flames exist in practice (Fig. 4). Laminar jets of fuel and oxidant are the simplest and most well understood diffusion flames. They have been studied exclusively in the laboratory, although a complete description of both the transport and chemical processes does not yet exist (2). 

Few experimental data exist on laminar jets (see Gutfinger and Shinnar, AJChE J., 10, 631-639 [1964]). Theoretical analysis for velocity distributions and entrainment ratios are available in Schhcht-ing and in Morton Phy.s. Fluids, 10, 2120-2127 [1967]). 

Nonpremixed edge flames (a) 2D mixing layer (From Kioni, P.N., Rogg, B., Bray, K.N.C., and Linan, A., Combust. Flame, 95, 276, 1993. With permission.), (b) laminar jet (From Chung, S.H. and Lee, B.J., Combust. Flame, 86, 62,1991.), (c) flame spread (From Miller, F.J., Easton, J.W., Marchese, A.J., and Ross, H.D., Proc. Combust. Inst., 29, 2561, 2002. With permission.), (d) autoignition front (From Vervisch, L. and Poinsot, T., Annu. Rev. Fluid Mech., 30, 655, 1998. With permission.), and (e) spiral flame in von Karman swirling flow (From Nayagam, V. and Williams, F.A., Combust. Sci. Tech., 176, 2125, 2004. With permission.). (LPF lean premixed flame, RPF rich premixed flame, DF diffusion flame). 

The best fit of velocity exponent n in Hp °c ug (Figure 4.3.11) for pure propane (n-butane) is n = 4.733 (3.638), corresponding to Sc = 1.37 (1.61) from n = (2Sc-l)/ (Sc -1), which agreed well with the suggested value of Sc = 1.376 (1.524). The experimental liftoff height data are shown in Figure 4.3.12 for various nozzle diameters and partial air dilutions to fuel [53]. It can be observed that the air dilution to fuel does not alter Ypst and S° sf The results substantiated the role of tri-brachial flames on flame stabilization in laminar jets. As mentioned previously. Equation 4.3.5 limits the maximum velocity Ug for Sc > 1, which corresponds to blowout condition. 

J. Lee, S. H. Won, S. H. Jin, S. H. Chung, O. Fujita, and K. Ito, Propagation speed of tribrachial (triple) flame of propane in laminar jets under normal and micro gravity conditions. Combust. Flame 134 411M20,2003. 

A. Linan, E. Frenandez-Tarrrazo, M. Vera, and A. L. Sanchez, Lifted laminar jet diffusion flames. Combust. Sci. Technol. 177(5-6) 933-953,2005. 

Roper, F.G., The prediction of laminar jet diffusion flame sizes Part I. theoretical model. Combust. Flame, 29, 219, 1977. 

Miller, J.A. and Kee, R.J., Chemical nonequilibrium effects in hydrogen-air laminar jet diffusion flames, /. Phys. Chem., 81, 2534,1977. 

Takahashi, R, Mizomoto, M., and Ikai, S., Structure of the stabilizing region of a laminar jet diffusion flame, /. Heat Transfer, 110,182,1988. 

In Chapter 8.1, F. Takahashi presents candle and laminar jet diffusion flames highlighting fhe physical and chemical mechanism of combustion in a candle and similar laminar coflow diffusion flames in normal gravity and in microgravity. This apparently simple system turns out to be very complex, and thereby its study is of greaf importance for the understanding of diffusion flame fundamentals. 

Laboratory reactors for studying gas-liquid processes can be classified as (1) reactors for which the hydrodynamics is well known or can easily be determined, i.e. reactors for which the interfacial area, a, and mass-transfer coefficients, ki and kc, are known (e.g. the laminar jet reactor, wetted wall-column, and rotating drum, see Fig. 5.4-21), and (2) those with a well-defined interfacial area and ill-determined hydrodynamics (e.g. the stirred-cell reactor, see Fig. 5.4-22). Reactors of these two types can be successfully used for studying intrinsic kinetics of gas-liquid processes. They can also be used for studying liquid-liquid and liquid-solid processes. 

They also proposed an empirical correlation for laminar jet breakup length ... 

There have been many recent studies of the mechanism of mass transfer in a gas absorption system. Many of these have been directed towards investigating whether there is a significant resistance to mass transfer at the interface itself. In order to obtain results which can readily be interpreted, it is essential to operate with a system of simple geometry. For that reason a laminar jet has been used by a number of workers. 

Cullen and Davidson ) studied the absorption of carbon dioxide into a laminar jet of water. When the water issued with a uniform velocity over the cross-section, the measured rate of absorption corresponded closely with the theoretical value. When the velocity profile in the water was parabolic, the measured rate was lower than the calculated value this was attributed to a hydrodynamic entry effect. 

The possible existence of an interface resistance in mass transfer has been examined by Raimondi and Toor(12) who absorbed carbon dioxide into a laminar jet of water with a flat velocity profile, using contact times down to 1 ms. They found that the rate of absorption was not more than 4 per cent less than that predicted on the assumption of instantaneous saturation of the surface layers of liquid. Thus, the effects of interfacial resistance could not have been significant. When the jet was formed at the outlet of a long capillary tube so that a parabolic velocity profile was established, absorption rates were lower than predicted because of the reduced surface velocity. The presence of surface-active agents appeared to cause an interfacial resistance, although this effect is probably attributable to a modification of the hydrodynamic pattern. 

Goodridge and Robb(14) used a laminar jet to study the rate of absorption of carbon dioxide into sodium carbonate solutions containing a number of additives including glycerol, sucrose, glucose, and arsenites. For the short times of exposure used, absorption rates into sodium carbonate solution or aqueous glycerol corresponded to those predicted on the basis of pure physical absorption. In the presence of the additives, however, the process was accelerated as the result of chemical reaction. 

FIGURE 9.24 Transmission electron micrograph of soot particles collected from a laminar jet diffusion flame burning kerosene in air. 

In fact, this provides a method for calibrating the laminar jet to determine (/ as a function of the metered gas flow rate. Substituting Eq. (32) in Eq. (31), and solving for U, we have... 

A technique for studying extraction after times of 10 -1 sec. has been developed 55), though the hydrodynamic conditions are not exactly known. The laminar jet also constitutes a means of obtaining low values of the liquid phase resistances, permitting relatively low values of 22/ to be detected. The rate of solution of isobutanol into a water jet (over contact times of 0.05-0.5 sec.) indicates a resistance 22/ of the order 80 sec. cm. , whether or not surface-active agents are present (56). 

The liquid jet formed by a pressure nozzle is inherently unstable. The breakup of the laminar jet occurs by symmetrical oscillation, sinusoidal oscillation, and atomization. 

As discharge velocity at the nozzle outlet increases, the following states appear in succession dripping, laminar jet breakup, wave disintegration, and atomization. These states of fiow are described in a pi space Re, Fr, Wep, whereby Wep = pv dp/a represents the Weber number formed by the droplet diameter, dp. To eliminate the fiow velocity, v, these numbers are combined to give... 

With higher discharge velocities, laminar jets are produced that disintegrate to droplets at a certain distance from the capillary. The transition from dripping to liquid jet disintegration occurs at higher Weber numbers ... 

Some of the methods for measuring molecular diffusion coefficients, together with a few recent references, are (a) diaphragm cell [60,61] (b) boundary layer interferometry [59] (c) shearing plate interferometry [58] (d) chromatographic peak broadening [60] (e) nuclear magnetic resonance and electron spin resonance [62, 63] (f) electrolyte conductance [64] (g) isotopic tracers [65] and (h) laminar jets [66]. 

Both phases in a continuous form packed columns thin-film contactors wetted-wall columns contactors with flat surface laminar jet absorber disc (sphere) columns... 

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