發(fā)布時間：2018-12-17 09:15

【摘要】：鹽析是工業(yè)輸運鹽溶液過程中常見的現(xiàn)象。由鹽析造成的壁面結(jié)鹽,過流斷面減小等問題給相關的工業(yè)生產(chǎn)帶來了巨大的能源浪費和經(jīng)濟損失。離心泵作為輸運鹽溶液的主要動力機械之一,研究其內(nèi)部鹽析兩相流場對防結(jié)鹽有重要意義。針對泵內(nèi)伴有鹽析的兩相流動,傳統(tǒng)的數(shù)值模擬利用CFD中的多相流模型,將晶體顆粒視為擬流體,但這種方法不能合理地體現(xiàn)顆粒-顆粒以及顆粒-壁面間的行為。因此,本文在鹽析動力學理論以及實驗觀測的基礎上,首次運用DEM方法研究離心泵內(nèi)鹽析顆粒的運動,對其液固兩相流動進行了CFD-DEM耦合計算,準確預測了離心泵內(nèi)鹽析過程中的顆粒碰撞、聚并以及破碎行為。以此為基礎,本文以顆粒與壁面間碰撞次數(shù)為主要優(yōu)化指標,采用了正交試驗設計,對離心泵葉輪的幾何參數(shù)進行了優(yōu)化。得到的結(jié)果可為流體機械的防結(jié)鹽設計提供依據(jù)。本文研究的主要工作和創(chuàng)造性成果如下:1、總結(jié)了離心泵內(nèi)液固兩相流動和鹽析兩相流動的研究現(xiàn)狀,發(fā)現(xiàn)原有的多相流模型不能合理體現(xiàn)顆粒-顆粒、顆粒-壁面和顆粒-液體的相互作用,無法滿足本文的研究需要。結(jié)合DEM處理離散相的優(yōu)勢,應用CFD-DEM耦合計算離心泵內(nèi)鹽析兩相流場。2、根據(jù)晶體動力學理論,分析了鹽析流場中晶體聚并和破碎的主導機制。流體的運動導致鹽析晶體顆粒之間相互碰撞,此時,由于弱相互作用力(范德華力等)的存在,會發(fā)生顆粒粘附,形成聚并體;當碰撞沖擊力大于碰撞晶體顆粒的相互作用力時,則發(fā)生破碎現(xiàn)象,由此構(gòu)建了DEM顆粒模型。3、使用了體網(wǎng)格生成技術,連續(xù)相的湍流求解使用了可實現(xiàn)的k-epsilon湍流模型,同時采用混合處理的方式(All y+Wall Treatment)處理壁面邊界,得到了較低的壁面y+值。4、通過分析CFD-DEM耦合計算的結(jié)果,得到了顆粒的運動軌跡、分布狀況、聚并、碰撞、液固兩相互作用以及離心泵外特性等規(guī)律。5、以降低鹽析顆粒黏附于葉輪流道壁面概率,減緩壁面結(jié)鹽為優(yōu)化原則。選定顆粒與葉片碰撞的次數(shù)為主要優(yōu)化指標,確定了葉輪的五個優(yōu)化因素進行正交試驗設計。對9個試驗方案進行CFD-DEM耦合模擬,找到了影響顆粒與葉片碰撞次數(shù)和揚程指標的因素順序,得到了葉輪的最優(yōu)結(jié)構(gòu)參數(shù)。6、分析發(fā)現(xiàn)影響晶體顆粒與葉片壓力面發(fā)生碰撞次數(shù)的主要因素為葉片壓力面附近的低速流場。低速區(qū)域尺寸越小,更規(guī)則,可使顆粒更加順暢地離開葉輪,縮短顆粒在流道中的滯留時間,減少鹽析晶體顆粒與壁面接觸次數(shù),降低壁面結(jié)鹽的概率。
[Abstract]:Salting out is a common phenomenon in the industrial transportation of salt solution. The problems of salting on the wall caused by salting out and the reduction of cross section bring huge energy waste and economic loss to the related industrial production. As one of the main power machinery for transporting salt solution, it is important to study the internal salting-out two-phase flow field of centrifugal pump. For the two-phase flow with salting out in the pump, the traditional numerical simulation uses the multi-phase flow model in CFD to treat the crystal particle as a pseudo fluid, but this method can not reasonably reflect the behavior between particle and wall. Therefore, on the basis of the theory of salting-out dynamics and the experimental observation, the movement of salting-out particles in centrifugal pump is studied by DEM method for the first time, and the liquid-solid two-phase flow is calculated by CFD-DEM coupling calculation. The particle collision, coalescence and breakage behavior in the process of salting out in centrifugal pump are accurately predicted. Based on the above results, the geometric parameters of impeller of centrifugal pump are optimized by orthogonal design, taking the number of collisions between particles and walls as the main optimization index. The results can provide the basis for the design of anti-salting of fluid machinery. The main work and creative achievements of this paper are as follows: 1. The research status of liquid-solid two-phase flow and salt-out two-phase flow in centrifugal pump is summarized. It is found that the original multi-phase flow model can not reasonably reflect the particle-particle. Particle-wall and particle-liquid interactions can not meet the needs of this study. Combined with the advantage of DEM in treating discrete phase, the two-phase flow field of salting out in centrifugal pump was calculated by CFD-DEM coupling. 2. According to the theory of crystal dynamics, the dominant mechanism of crystal aggregation and fragmentation in salting-out flow field was analyzed. The movement of fluid leads to the collision of crystal particles in salting-out. At this time, due to the existence of weak interaction force (van der Waals force, etc.), particles will adhere to each other and form a coalescence. When the impact force is greater than the interaction force of the colliding crystal particles, the phenomenon of fragmentation occurs, and the DEM particle model is constructed. 3. The volume mesh generation technique is used, and the k-epsilon turbulence model is used to solve the continuous phase turbulence. At the same time, (All y Wall Treatment) is used to treat the wall boundary, and the lower value y. 4 of the wall is obtained. By analyzing the results of CFD-DEM coupling calculation, the motion trajectory, distribution, coalescence, collision of particles are obtained. The rule of liquid-solid interaction and the characteristics of centrifugal pump. 5. The optimization principle is to reduce the probability of salting-out particles sticking to the wall of impeller passage and to slow down the formation of salt on the wall. The number of collisions between particles and blades was chosen as the main optimization index, and five optimization factors of impeller were determined for orthogonal design. Through CFD-DEM coupling simulation of nine test schemes, the order of factors influencing the impact times and lift index of impeller was found, and the optimum structural parameters of impeller were obtained. It is found that the main factor affecting the number of collisions between crystal particles and the blade pressure surface is the low velocity flow field near the blade pressure surface. The smaller the size of the low-speed region is, the more regular the particles are, which can make the particles leave the impeller more smoothly, shorten the retention time of the particles in the channel, reduce the number of contact between the crystal particles and the wall surface, and reduce the probability of salt deposition on the wall.
【學位授予單位】：江蘇大學
【學位級別】：碩士
【學位授予年份】：2017
【分類號】：TH311

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