| In the process for the preparation of nanoparticles by gas-phase flame synthesis, the structure of the combustion chamber and the flame structure have great influence on the distribution of the thermodynamics and kinetics parameters which are required by the nucleation and growth of the particles. And then the thermodynamics and kinetics parameters decide the morphology, structure and the performance of final product. The design and optimization of the combustion chamber structure are always the basis and key of process research, but many parameters inside the combustion chamber are not easy to obtain through experiments, which brings a lot of inconvenience for the design of the chamber structure. Therefore, it is of great theoretic value and practical significance to predict the flow field inside the reactor through numerical simulation method.In this thesis, we design the multi-jet flame reactor by analyzing the air dynamics in the reactor and numerically simulate the flame inside the combustion chamber by using GAMBIT and CFD commercial software FLUENT. The main research work carried out includes:1. We use the GAMBIT software to build the physical model of the multi-jet combustion chamber and divide the grids. The grid structure is Tetrahedron and the total grid number is 6.45 million. According to the structure of the reactor, the operation conditions and the application scope of the mathematical models in the software FLUENT 6.3, we finally determine the reasonable mathematical model to simulate the combustion process inside the reactor. We use the standard k-εturbulent model, the EDC model to calculate the chemistry reaction rate and the DO model to calculate the gas heat radiation.2. We numerically investigate the effect of many engineering factors on the flame structure, temperature profiles, velocity profiles, species distribution and etc. The factors include the flame configuration, the shapes of the nozzle, the radiation heat transfer, the air film speed and different wall boundaries. By analyzing the simulation results, we get the following conclusions. The reaction temperature has a difference under different flame configuration. The effect of the radiation model on the heat transfer is significant and the flame peak temperature becomes lower by adding the radiation model. The temperature in the reactor decreases as the tangential purge gas enters the reactor. Compared to a single reaction step, the predicted temperature in the reactor is lower by using complex reactions, but the velocity field just changes little. When the reactor wall is set for convection wall, the temperature near the reactor wall is lower and the peak temperature along the center axis is also much lower than the adiabatic wall boundary, which mainly takes place in the lower temperature area.3. In the multi-jet flame reactor, anhydrous AICl3 powder was used as precursor, Al2O3 nano-particles were successfully prepared with different morphology and crystallinity. Based on the experiment, we simulate the flow field in the multi-jet combustion reactor and mainly investigate the flame temperature distribution, velocity distribution and species concentration in the combustion reactor under different flame configuration. At the same time, the effects of the flame radial temperature on the size of the particles and aggregates are also studied. The agreements of the temperature and major species are considered to be good between the predictions and the available measured date. When the fuel/air ratio equals to the stoichiometric ratio, the average temperature in the reactor reaches to the maximum. The results also indicate that the flame temperature field has a great influence on the size, size distribution and morphology of the final product. A flame of higher temperature more easily leads to spherical particles; the size of the particle aggregates becomes bigger with the long residence time. The data of simulation is confirmed with experimental result. |
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