摘要
下排气旋风分离器具有单旋流流场结构,压力损失低,并能满足循环流化床锅炉π型布置的需要。但是,下排气旋风分离器对细颗粒央带严重,导致细颗粒分离效率稍低。为了提高分离效率,同时保持较低的压力损失,需进一步研究其对细颗粒的分离机理,从结构上进行优化设计,以提高分离性能。
本课题采用数值模拟和理论分析方法,对分离器内气固运动细节、分离机理、内部压力分布及压力损失进行研究。在此基础上提出能提高分离效率,降低阻力的优化方案。通过本文的研究和分析,得到了以下主要结论:
①采用渐缩进口时,气固混合物在渐缩管内加速,颗粒团的穿透能力增强,更多颗粒能够穿透分离器内的旋涡场而到达分离器壁面,分离效率提高:随着气流在管内加速后,旋涡更加强烈,压降增大。
②随着导流体直径的增加,分离器旋流场得到了强化,旋涡强度加剧,离心作用增强而旋转动能损失增大,可以提高分离效率,但是压力损失增加。
③排气管采用圆台入口时,合理设计圆台的倾角和底径,可以有效预防排灰斗内的固体颗粒沿排气管外壁上升进入排气管,从而提高分离效率。
④随着排气管不断下移,分离效率增幅减小,最终越来越低,说明导排管间距存在一最佳值。
⑤随着排气管直径的减小,分离效率提高,但气流进入排气管时压力损失增加;同时旋转气流的离心力场增强,局部压力损失也增加。
⑥排气管插入高度升高,阻力损失增加,同时排气管口远离排灰斗高湍流度、高含灰的气流,分离效率提高。
⑦下排气旋风分离器的涡流中心不在其几何中心,排气管偏移合理的角度和距离,可以提高分离效率。
⑧对下排气旋风分离器的实际结构进行改进,将进口改为渐缩型、排气管位置下移,并采用圆台入口。改进后的下排气旋风分离器对细颗粒的分离效率明显提高,当颗粒粒径小于于20μm时,相同粒径颗粒的分离效率最大提高37.75%,总分离效率也得到了提高,压降相应增加。
Cyclone separator with downward exhaust outlet has the flow structure of single swirl flow, and it has lower pressure loss. Cyclone separator with downward exhaust outlet is suitable for the theπtype arrangement of CFB boilers, but cyclone separator with upward exhaust gas has a severe entrainment of the fine particles, leading to a low separating efficiency of the fine particles. To increase the separating efficiency of the fine particles and keep pressure drop low, it needs to further study for the separating mechanism and improve its performance by optimization design of the structure.
The research adopts numerical simulation and theory analysis methods to sudy the gas-solid two-phase flow, separating mechanism, pressure distribution and pressure loss in details. Based on these study results, an optimization scheme to increase the separating efficiency and decrease the pressure loss of the Cyclone separator with downward exhaust outlet is presented in the project. According to the results and analysis, the main conclusions are as follows:
①When the Cyclone separator with downward exhaust outlet has the taper import, the gas-particle mixture is accelerated in the tapered duct, and the particles get a higher penetration ability in the flow. Then, more particles can penetrate the vortex to reach the Cyclone separator wall, and the separating efficiency increases. However, with gas accelerated, vortex gets more intensive, and the pressure loss also increases.
②When the diameter of diversion object increases, the swirl intensity of cyclone separator gets more acute, the function of centrifugal force is strengthed, but spin kinetic energy loss increases, so separating efficiency may be inhanced and pressure loss increases.
③When exhaust pipe uses the cone structure entrance, optimal design of the obliquity and bottom diameter can effectively prevent solid particles in ash bucket turning back into the exhaust pipe along the outside wall of exhaust pipe, consequently, separating efficiency increases.
④When the exhaust pipe moves dowmward, separating efficiency increases initially and then decreases. It proves that the distance between the diversion object and the exhaust duct has an optimal value.
⑤When the diameter of exhaust duct minishes, separating efficiency is enhanced, but pressure loss increases when gas flows into the exhaust duct. Moreover, centrifugal force field of rotary gas reinforces, local pressure also loss increases.
⑥When insertion height of exhaust duct increases, pressure loss increases. At the same time, the exhaust gas entrance is far away from high turbulence intensity area, so separating efficiency increases;
⑦The center of vertex in cyclone separator with downward exhaust gas is not at the center of its geometric center. When exhaust gas duct offsets in right angle with right distance, separating efficiency increases.
⑧The cyclone separator with downward exhaust gas is improved by adopting tapered entrance and cone shaped exhaust pipe entrance, moveing the exhaust pipe dowmward.The separating efficiency for fine particles increases evidencely , especially when the particle size less than 20μm, the separating efficiency for the same particle size increasing 37.75% at most.The total separating efficiency also increases as well as the pressure drop.
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