**General**

276 Mass Transfer in Chemical Engineering Processes

that up to the ignition surface-temperature the combustion proceeds under the "weak" COoxidation, that at the temperature the combustion rate abruptly changes, and that the

Figure 8(b) shows a similar plot in airflow of 820 s-1. Because of the lack of the experimental data, as well as the enhanced ignition surface-temperature (*T*s,ig 1810 K), which inevitably leads to small difference between combustion rates before and after the ignition of COflame, the abrupt change in the combustion rate does not appear clearly. However, the

It may informative to note that a decrease in the combustion rate, observed at temperatures between 1500 K and 2000 K, has been so-called the "negative temperature coefficient" of the combustion rate, which has also been a research subject in the field of carbon combustion. Nagel and Strickland-Constable (1962) used the "site" theory to explain the peak rate, while Yang and Steinberg (1977) attributed the peak rate to the change of reaction depth at constant activation energy. Other entries relevant to the "negative temperature coefficient" can be found in the survey paper (Essenhigh, 1981). However, another explanation can be made, as explained (Makino, et al., 1994; Makino, et al., 1996; Makino, et al., 1998) in the previous Sections, that this phenomenon can be induced by the appearance of CO-flame, established over the burning carbon, thereby the dominant surface reaction has been altered

Since the appearance of CO-flame is anticipated to be suppressed at high velocity gradients, it has strongly been required to raise the velocity gradient as high as possible, in order for firm understanding of the carbon combustion, while it has been usual to do experiments under the stagnation velocity gradient less than 1000 s-1 (Matsui, et al., 1975; Visser & Adomeit, 1984; Makino, et al., 1994; Makino, et al., 1996), because of difficulties in conducting experiments. In one of the Sections in Part 2, it is intended to study carbon

In this monograph, combustion of solid carbon has been overviewed not only experimentally but also theoretically. In order to have a clear understanding, only the carbon combustion in the forward stagnation flowfield has been considered here. In the formulation, an aerothermochemical analysis has been conducted, based on the chemically reacting boundary layer, with considering the surface C-O2 and C-CO2 reactions and the gas-phase CO-O2 reaction. By virtue of the generalized species-enthalpy coupling functions, derived successfully, it has been demonstrated that there exists close coupling between the surface and gas-phase reactions that exerts influences on the combustion rate. Combustion response in the limiting situations has further been identified by using the generalized coupling functions. After confirming the experimental fact that the combustion rate momentarily reduces upon ignition, because establishment of the CO-flame in the gas phase can change the dominant surface reaction from the faster C-O2 reaction to the slower C-CO2 reaction, focus has been put on the ignition of CO-flame over the burning carbon in the prescribed flowfield and theoretical studies have been conducted by using the generalized coupling functions. The asymptotic expansion method has been used to derive the explicit ignition criterion, from which in accordance with experimental results, it has been shown that ignition is facilitated with increasing surface temperature and oxidizer concentration, while suppressed with

"strong" CO-oxidation prevails above the temperature.

general behavior is similar to that in Fig. 8(a).

from the C-O2 reaction to the C-CO2 reaction.

combustion at high velocity gradients.

**6. Concluding remarks of part 1** 

decreasing velocity gradient.


Mass Transfer Related to Heterogeneous Combustion of Solid Carbon

~ nondimensional or stoichiometrically weighted

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ISBN 0-12-020002-3, New York.

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*j j*=0 and 1 designate two-dimensional and axisymmetric flows, respectively

