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Chemkin 4.1.1 Chapter 2: Combustion in Gas-phase Processes 2.6.2.3 Project Results The solution plots of a reactor network are difficult to interpret because the reactor number does not necessarily correspond to its actual location in the flow field. In our case, reactor C1_R6 actually represents the outer edge of the fuel jet and should be physically located right next to the fuel jet nozzle in an upstream region. On the other hand, reactors C1_R5 and C1_R7 are the flame zone and the post-flame region, respectively, and are both located downstream from reactor C1_R6. Therefore, when we look at the solution plots as a function of the reactor number, we should ignore the sudden changes corresponding to reactor C1_R6. The predicted temperature distribution is shown in Figure 2-39. The adiabatic flame temperature of the stoichiometric CH 4 -air mixture is also given on the plot for reference. We can see that reactors C1_R3, C1_R4, C1_R5, and C1_R7 have temperatures close to the adiabatic flame temperature, which is a good indication for the flame zone and post-flame region. The reactor temperature dropping when coming from reactor C1_R5 to reactor C1_R7 is appropriate, since C1_R7 is in the post-flame region. We can further confirm this speculation by checking the solutions of other variables. We also think the jet flame is slightly lifted from the fuel jet nozzle because the outer edge of the fuel jet (reactor C1_R6) stays at a relatively low temperature. Figure 2-39 Jet Flame Network—Temperature Distribution Figure 2-40 gives the CO mole fraction distribution among the reactors. Again, the CO mole fraction at the adiabatic flame condition is provided to show where the combustion zone ends. The CO profile shows a spike starting from reactor C1_R3 to reactor C1_R7. We ignore reactor #6 because it does not connect either to reactor © 2007 Reaction Design 62 RD0411-C20-000-001
Chapter 2: Combustion in Gas-phase Processes <strong>Tutorials</strong> <strong>Manual</strong> C1_R5 or reactor C1_R7. This CO spike resembles the one we observe in a typical flame zone and makes us believe that the flame zone ends before reaching reactor C1_R7. The predicted CO mole fraction in reactor C1_R7 is also lower than its adiabatic flame value and is consistent with our observation for the post-flame region. Figure 2-40 Jet Flame Network—CO Distribution One of the advantages of using a reactor network is the ability to predict NO formation without running a full CFD calculation with detailed chemistry. The NO profile in Figure 2-41 shows that NO starts to form in the flame zone (reactors C1_R3, C1_R4, and C1_R5) and continues to rise in the hot post-flame region (reactor C1_R7). We can find out which NO formation mechanism is responsible for the NO increase in the high temperature region. In the absence of fuel nitrogen, NO in our jet flame can be formed via prompt NO and thermal NO mechanisms. 19 Since the prompt NO mechanism is characterized by the existence of radicals such as CH and HCN, we can plot the profiles of these radicals to find out the region where thermal NO is the dominant NO formation mechanism. From the profiles in Figure 2-42, we can see that all prompt-NO–related radicals disappear in the post-flame region (reactor C1_R7). so we can conclude that thermal NO is the main NO formation mechanism in the hot post-flame region. The reactor model also shows that, unlike gas temperature and CO mole fraction, the “exit” NO mole fraction is far below its equilibrium value. This is in accord with the fact that the characteristic chemical time scale of NO is greater than the fluid mechanical time scale of our jet-flame system. 19. Miller and Bowman, Prog. Energy Combust. Sci., 15:287-338 (1989). RD0411-C20-000-001 63 © 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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Chemkin 4.1.1 4 4 Materials Problem
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Chapter 4: Materials Problems Tutor
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Chemkin 4.1.1 5 5 Chemical Mechanis
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Tutorials Manual 4.1.1 Index Index
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