Number Lewis

Figure 12.2. Real part of the dominant eigenvalue versus Lewis number for steady state E of Figure 12,1, (From Hite and Jackson CSl].)...

The curious shape of the computed parts of the curves suggests that there is, in each case, a discontinuity of slope. However, examination of the results shows that there is, in fact, a switch in the dominant eigenvalue as the Lewis number changes. Above a certain value of the Lewis number this is real and moves to the right as decreases. But eventually it crosses with a pair of complex eigenvalues moving to the left and these, which become the dominant eigenvalues for smaller values of, cause... 

Figure 23-19 is one of several by Weisz and Hicks Chem. Eng. ScL, 17, 263 [1962]). Although this predic ts some very large values of T in some ranges of the parameters, these values are mostly not reahzed in practice, as Table 23-5 shows. The modified Lewis number is Lw =... 

The dimensionless number Le is called the Lewis number (m Russian literature it is called the Luikov number). The Lewis number incorporates the specific heat capacity of humid air pCp (J/m C), the diffusion factor of water vapor in... 

The diffusion factor D, which is contained in the Lewis number, is inversely related to the total pressure ... 

The Lewis number for air is approximately 1 (see Example 6), so with good... 

Comparing this value with the a) and b) point results ot Example 7, we discover that the line of constant enthalpy lies between the determination line of wet bulb temperature and the adiabatic humidification line. The nearer the Lewis number is to 1, the nearer the wet bulb temperature is to the adiabatic humidification temperature. 

The dimensionless parameter Dpc / is called the Lewis number, which is the ratio of the diffusion coefficient of a gas through the mixture divided by the thermal diffusion coefficient of the gas mixture. 

The value of the Lewis number for H2 is 2.4 while the value for CO and benzene are less than 1.0. Thus if the gaseous mixture contains much H2,... 

Calculate the resulting jet surface temperature if the Lewis number is 90. Neglect heat transfer between the water and the gas,... 

The flow structures of lean limit methane and propane flames are compared in Figures 3.1.2 and 3.1.3. The structure depends on the Lewis number for the deficient reactant. A stretched lean limit methane flame (Lepreferential diffusion, giving it a higher burning intensity. Hence, the flame extinction limit is extended. On the other hand, for a stretched lean limit propane flame (Le>l), the same effect reduces the burning intensity, which can... 

It can also be noted that the slope of the S , .,f-Ka plot reflects the combined effect of stretch rate and non-equidiffusion on the flame speed. Figure 4.1.6 clearly shows that the flame response with stretch rate variation differs for lean and rich mixtures. In particular, as Ka increases, the S for stoichiometric and rich mixtures increases, but decreases for the mixture of equivalence ratio = 0.7. This is because the effective Lewis numbers of lean w-heptane/ air and lean /so-octane/air flames are... 

Finally, we come to the effects of the Lewis number. Figure 4.2.14 shows the intensified images of vortex ring combustion of lean and rich propane/air mixtures. Since the flame is curved and stretched at the head region, the mass and heat is transferred through a stream tube. 

Frankel, M. L. and Sivashinsky, G. L, On effects due to thermal expansion and Lewis number in spherical flame propagation. Combustion Science and Technology, 31,131-138, 1983. 

J. Buckmaster and M. Matalon, Anomalous Lewis number effects in tribrachial flames, Proc. Combust. Inst. 22 1527-1535, 1988. 

The Lewis number, Le, is that of the deficient species (fuel or oxidant) in the mixture. In their analysis, Clavin and Williams used the simplifying approximation that the shear viscosity, the Lewis number, and the Prandtl numbers are all temperature-independent. They also showed that, at least for weak flame stretch and curvature, the change in local flame speed due to stretch and curvature is described by the same Markstein number ... 

A plof of fhe real part of the relative heat release response for three Lewis numbers is shown in Figure 5.1.10. This plot was calculated for a reduced activation energy y3 = 10 and a burnf gas femperafure of 1800 K, represen-fative of a lean hydrocarbon-air flame. Note fhaf fhe order of magnitude of fhe relative response of fhe flame is only a little more than unity. This is a relatively weak response. For example, a sound pressure level of 120 dB corresponds to a relative pressure oscillation p /p = 2 X10 so fhe fluctuation in the heat release rate will be of fhe same order of magnifude. 

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