Experiment Study And Numerical Simulation Of A Novel Annular Spouted Bed With Multiple Air Nozzles

As a branch of fluidization, spouted beds technology has been applied initially to dryness of coarse particles, and gradually expand to a wide variety of chemical processes related to dryness, comminution, prilling, gasification, combustion, pyrolysis, petroleum thermal cracking etc. With the extension of spouted beds technology, many modified spouted bed designs have been proposed by researchers. These modified spouted beds have been applied to various situations. Base on the mechanism research of spouted bed technology, a novel annular spouted bed with multiple air nozzles has been proposed in this thesis, which combines spouting and coal usage. The flow characteristics of the novel spouted beds have been studied systematically by experiment method and numerical simulation. Visual experiments and pressure fluctuation signal analysis are carried out to investigate the spouting behaviors in the bed. It can provide some evidences of industry application. In numerical simulation, particle phase is considered to"quasi-fluid". The two-fluid model is adopted to simulate the gas-particle velocity field and pressure field. The spouting mechanism and spouting process are analysis by the quantization of characteristic information.Present research mainly consists of following sections:(i) design and investigation of feeding system for the novel annular spouted bed with multiple air nozzles; (ii) spouting characteristics in the spouting bed; (iii) flow patterns and transitions of the novel annular spouted bed; (iv) pressure fluctuation signal analysis in the bed and correlation among nozzles; (v) numerical simulation study based on flow equation of gas and particle phase in the bed.In the novel spouted bed, as feeding equipment, rotating cone is adopted to ensure equably distribution for experimental materials. To obviate some disadvantages for the rotating cone, such as short residence time and'passive'layer for particles, adding the upright ring wall and the stirrer will ensures suitable residence time and all particles remain in'active'layer. It will helpful to improve on particle mixing and transportation. Experimental results show that the dynamic hold-ups and the residence time of particles are inversely proportional to the cone rotational frequency and the areas of the overflow gate. The dynamic hold-ups increase with the increasing of feed rate, while the residence time of particles in the rotating cone tends to decrease with the increasing of feed rate.Dry materials were used to study flow characteristics in the three-dimension spouted bed. The results show that there exist three different zones for the particle flow in the annular spouted bed. With the increase of nozzle diameter or particle size, the maximum spoutable bed height tends to decrease. The effects of the static bed height, nozzle diameter and particle size on the minimum spouting velocity and the maximum spouting bed pressure drop are studied by experimental method. The minimum spouting velocity and the maximum spouting bed pressure drop were determined experimentally and correlated empirically. The calculated data is similar to the experimental data.By the digital CCD camera, experimental study on the flow patterns and transitions in the novel annular spouted bed with multiple air nozzles was carried out. Three distinct stable flow patterns, i.e. internal jet, jet-spouting and fully developed spouting were identified. In addition, two transitional flow patterns and flow instabilities, single internal jet, single jet-spouting, and bubbling or slugging were found. Schematic diagrams and typical flow pattern images obtained by a digital CCD camera were presented for classifying these flow patterns. Typical flow pattern map at various static bed heights and spouting gas velocity were plotted for describing the transitions between flow patterns.Under different operating conditions (spouting velocity, static bed height, measuring point height), the pressure fluctuation signals in a novel annular spouted bed were studied by statistical and frequency analysis. Experimental results show that the total bed pressure drop appears similar characteristics observed in a conventional spouted bed, with the increasing of the spouting velocity. Initially, the whole bed pressure drop increase with the increasing of the spouting velocity. After the spouting formation, the mean value of total bed pressure drop decline to a low value, and almost keeps constant but with fluctuations around this mean value. For all measuring point, The pressure fluctuation standard deviation increases with the increasing of the spouting gas velocity or the static bed height, while it decreases with increasing measuring point height. According to the power spectral density determined by Fast Fourier Transformation (FFT) of pressure fluctuation signals, there exist three pressure fluctuation sources, namely, the spouting jet, interaction among all nozzles and particle interaction. The pressure fluctuation major frequency decreases when increasing the spouting gas velocity, the static bed height or the measuring point height.Two statistical methods, partial correlation coefficient and cluster analysis are firstly introduced to assess the correlation and similarity of pressure fluctuation signals. There are less correlation among nozzles, which shows the novel spouted bed can adopted to industry process due to the equalization and stabilization of supplied gas for all nozzles. The correlation model for nozzles is found by the artificial neural network (ANN).Compared to the experimental results, the model have better application. In addition, by the cluster analysis, all nozzles are distributed to different group according to different similarity. Based on the results of cluster analysis, it can improve on the design of the novel spouted bed, and decrease the unbalance among all nozzles.Combined to the dynamics theory of particle, finally, the dissertation held a cold-state numerical simulation for the gas-solid two phases flow in the novel spouted bed with FLUENT CFD software. Three spouting velocity and three static bed height is used as the simulating condition. The numerical simulation displayed completely the formation, development, broken up of the bubble as well as the formation, development, disappearment of the ejection until the gas-solid steady flow in the spouted-fluidization bed. The numerical simulation results are basically in accordance with the experiment. With the spouting gas velocity increases, bulk concentration of particles in the center of spouting zone of bed, while the velocity of particle increases. In the dense-phase zone of bed, the velocity of gas decreases with the increasing of the spouting gas velocity. The higher static bed height, the bigger resistance of bed materials to flow, result in more spouting gas diffuse to the dense-phase. The jet velocity in the center of spouting zone is less for a spouted bed with high static bed height than for low static bed height, while the gas flow velocity in the dense-phase zone is bigger.