| Flameless combustion technology is one of prospective combustion technologies. It is a new type combustion strategy for improving the efficiency of energy utilization and decreasing the NOx emission. Especially for its great advantages of combing with the oxyfuel combustion technology, this technology also helps to deal with the climate change. Recently, many studies were focused on the characteristics for critical changes form conventional combustion to flameless combustion, the organizations of flameless combustion and so on. Most of these topics were belonged to the studies of category of fluid dynamic for flameless combustion. However, the characteristic of chemical dynamic for flameless combustion was barely disscused. In the present work, this topic is systematically discussed under air and oxyfuel combustion condition.A special opposed flame model is set up to analysis the difference of flameless combustion under air and oxyfuel combustion condition. Firstly, the critical change condition from air combustion to flameless condition is analysed, i.e. the disappearance of prolytic region. Secondly, combustion regions are divided to explain the difference of flameless combustion under these two conditions. Meanwhile, on the premise of flameless or MILD combustion, the relationship between the fuel dilution, preheated temperature and equiverlence ratio is studied and the difference for the mixmum heat for sustaining the flameless or MILD combustion are furtherly analysed. Also, the reaction pathways for CH4oxidation and NH3conversion are also detailedly studied.Meanwhile, a diffusive opposed-flow flame model is set to investigate the thermal, chemical and diffusion effect of CO2and H2O on the flame under oxyfuel combustion condition. The flame temperature and Emission Index of CO (EICO) are chose to show the results. It is shown that the thermal property of CO2greatly affected the flame temperature, both thermal and chemical properties greatly affect the EICO. Although the existence of H2O hardly affects the flame temperature, it reduces the EICO. It means that the existence of H2O accelerates the fuel oxidation rate. Besides, the reaction pathways of fuel under air, oxyfuel with dry recirculation and oxyfuel with wet recirculation combustion condition has also been detailedly analysed.The global combustion mechanisms were modified and optimized to be suitable for the numerical computation prediction for flameless combustion. For flameless combustion, the oxidation rates of fuel becomes slower than that under flammable combustion condition. Aiming at this particular point, the oxidation rates of CO and H2are modified for predicting the main species adaptively. Different global combustion mechanisms are compared with the Co-flow experimental data to optimize the best one for prediction of flameless combustion. Although the global combustion mechanism is suitable for the prediction of main species, it is not used for the predictions of ignition delay and extinction of flamless combustion because of its small chemical reaction time scale. However, the detailed mechanism is also barely used for the CFD prediction of flameless combustion because it assumes a large amount of computation time. A skeletal mechanism or reduced mechanism is appropriate for sloving both of these problems. A newly developed methodogy called DRGEPSA is adopted to reduce the97species and778elementary reactions which developed by Glarborg et al. A skeletal mechanism which contained25species and168elementary is got by this method. Comparisions of ignition delay, flame propagation velocity, heat release rate and prediction of main species are conducted between the detailed mechanism and skeletal mechanism. The result shows that the skeletal mechanism was about95%accuracy compared with the detailed mechanism and it greatly saves the CPU computational time.Then, a numerical simulation for a20Kw flameless combustion is done. A reduced mechanism for QSS analysis of GRI3.0based on19reduced mechanism which developed by Lu and Law has been coupled with EDC turbulent-combustion by User Defined Functions (UDF). It shows that this strategy keeps the high numerical precision and saves a lot of computational afford. Also, the WD4global mechanism is also been used for this simulation. The result shows that the reduced mechanism and global mechanism are used to predict the steady state species well. However, the global mechanism fails to predict the characteristic of ignition for flameless combustion but the results for reduced mechanism are coincident with experimental investigation. It also shows that the ignition lag for flameless combustion existed and it is a volumetric combustion. The reduction of H is one of main causes for slowing of flameless combustion. The interaction of C1-C2chain is enhanced and the mass flow for C2chain was weaked. |
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