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Chemkin 4.1.1 Chapter 2: Combustion in Gas-phase Processes 2.7.1.2 Project Results Two simulations, one with both H-abstraction-C 2 H 2 -addition (HACA) and PAH condensation growth reactions and the other with HACA growth reactions only, are performed so that contributions of either soot mass growth mechanism can be identified. Results of the T JSR = 1630K and Φ = 2.2 case are presented in Figure 2-64 and Figure 2-66. As can be seen from these figures, predictions obtained by the Particle Tracking Module are in good agreements with experimental data. The Particle Tracking Module in general slightly underpredicts gas-phase species. There are many factors that can contribute to the discrepancies shown in the figures. For example, Marr did not provide details composition and temperature of the inlet gas mixture to the JSR. Since the temperature of the JSR is maintained by adjusting the N 2 fraction in the inlet gas stream, uncertainties in inlet condition, reactor heat loss, and reactor residence time will surely affect the simulation results in the PFR section behind the JSR. Comparison of the predicted and measured soot mass concentration profiles in the PFR is presented in Figure 2-65. While the HACA-only mechanism shows an excellent agreement with the data at the PFR inlet, the slope of the soot mass profile predicted by the HACA-only mechanism (dash-dot line) is much smaller than that of the experimental profile. This is an indication that the HACA growth mechanism alone gives a too-slow soot mass growth rate in the post-flame region. Since the present soot model underpredicts C 2 H 2 mole fraction in the PFR (Figure 2-64), it is possible that the lower C 2 H 2 concentration leads to lower HACA soot mass growth rate. It is also possible that another growth mechanism, possibly PAH condensation, might contribute equally to soot mass growth under this condition. The soot mass growth rate predicted by the HACA + PAH mechanism (solid line), on the other hand, shows a much better agreement to the experimental data that the HACA only mechanism does. Since the sticking coefficients of all the PAH considered here are within the range suggested by Marr 23 , the PAH contribution to soot mass growth should be reasonably predicted by the model. However, the HACA + PAH mechanism does overpredict the soot mass density at the PFR inlet. Note that experimental data indicate that soot mass density increases by about 4 x 10 -8 gm/cm 3 for the first 5 mini-seconds in the PFR. Since the residence time in the JSR is about 5 miniseconds and the temperature in JSR is only 10K higher than that of PFR, the soot mass density at the PFR inlet should be higher than the measured value. Of course, this assessment is based on the assumption that soot particles start to grow once they are created inside the JSR. © 2007 Reaction Design 82 RD0411-C20-000-001
Chapter 2: Combustion in Gas-phase Processes <strong>Tutorials</strong> <strong>Manual</strong> Figure 2-66 compares the contributions of HACA and PAH condensation to soot mass growth by using the Rate-Of-Production (ROP) analysis. PAH condensation accounts for about one-third of the soot mass growth. Note that PAH condensation reactions also contribute to part of the HACA growth by eliminating some H atoms on soot particle surface. The pure HACA growth effect is indicated by the dash-dot line of Figure 2-65. The evolution of average soot particle diameter inside the PFR is shown in Figure 2-67. The soot particle diameter increase along the plug flow reactor due to particle coagulation and mass growth. Note that the average particle diameter actually drops a little near the PFR entrance. This signifies that soot nucleation is still occurring as the gas mixture entering the PFR. Figure 2-64 Comparisons of Mole Fraction Profiles of Selected Gas Phase Species Inside the PFR for the 1630K and Φ = 2.2 Case of the C 2 H 4 /O 2 /N 2 JSR/PFR Experiment by Marr 23 . Symbols: data; Solid lines: predictions with HACA and PAH condensation growth mechanisms RD0411-C20-000-001 83 © 2007 Reaction Design
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Chemkin 4.1.1 Contents Table of Con
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Chemkin 4.1.1 Tables List of Tables
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Tutorials Manual 4.1.1 Figures List
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List of Figures Tutorials Manual 2-
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Chemkin 4.1.1 1 1 Introduction The
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Chapter 1: Introduction Tutorials M
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Chemkin 4.1.1 2 2 Combustion in Gas
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Chapter 2: Combustion in Gas-phase
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Tutorials Manual 4.1.1 Index Index
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