【摘要】：氣泡之間的聚并行為能夠顯著改變流場結(jié)構(gòu)和氣-液兩相界面,進(jìn)而深刻影響整個(gè)體系的特性,本文結(jié)合實(shí)驗(yàn)研究和數(shù)值模擬方法,對高黏體系中共軸線上升的低雷諾數(shù)雙氣泡接近及聚并的全過程進(jìn)行了系統(tǒng)的研究,并重點(diǎn)對聚并過程中所形成的連接型氣泡的動(dòng)力學(xué)行為及其對聚并的影響進(jìn)行了深入分析。在實(shí)驗(yàn)方面,本文主要采用高速攝影技術(shù)對六種實(shí)驗(yàn)體系(三種不同溫度的純甘油溶液、一種95%的甘油-水溶液及兩種加入了表面活性劑CDEA的甘油溶液)中當(dāng)量直徑Dc為6～10.5 mm的氣泡所形成的共軸線自由上升的雙氣泡體系的流體力學(xué)行為進(jìn)行了研究,液相莫頓數(shù)Mo為1.69～661.83,氣泡雷諾數(shù)Re范圍是0.190～2.998,涵蓋了氣泡生成、上升、碰撞、連接、液膜破碎實(shí)現(xiàn)聚并的全過程。研究中將連接聚并過程劃分為接觸階段、連接階段、排液階段三個(gè)階段,觀測雙氣泡在多種黏度和表面張力系數(shù)的粘性流體體系中的聚并行為以及聚并后的運(yùn)動(dòng)狀況,進(jìn)一步分析雙氣泡上升接觸過程中的速度和形變規(guī)律以及氣泡之間的相互作用規(guī)律,并對聚并條件和聚并時(shí)間進(jìn)行討論。此外,對連接型氣泡的形成、形態(tài)、動(dòng)力學(xué)行為也進(jìn)行了系統(tǒng)的研究。結(jié)果表明：連接型氣泡在排液階段以恒定外形做勻速上升運(yùn)動(dòng)直到發(fā)生聚并或滑移,不僅與單氣泡具有類似的運(yùn)動(dòng)特征且能夠用以描述聚并瞬間的氣泡狀態(tài)。同時(shí),分析得到了單氣泡和連接型氣泡各自最適合的曳力系數(shù)模型和上升速度模型,并用以計(jì)算連接型氣泡所受曳力和氣泡間的相互作用力。連接時(shí)間主要依賴于實(shí)驗(yàn)體系的黏度,而排液時(shí)間則呈現(xiàn)出較強(qiáng)的隨機(jī)性。隨著體系黏度的降低或表面活性劑的逐漸加入氣泡聚并愈發(fā)困難而是傾向于滑移。在模擬方面,本文使用最小網(wǎng)格尺寸為50μm的結(jié)構(gòu)化網(wǎng)格,采用VOF模型對等徑雙氣泡的直接聚并過程進(jìn)行了二維瞬態(tài)模擬,獲得雙氣泡的流場結(jié)構(gòu)和液膜厚度隨時(shí)間的變化規(guī)律。結(jié)果表明,本文的模擬結(jié)果無論是雙氣泡的形態(tài)還是聚并各階段的氣泡動(dòng)力學(xué)行為均與實(shí)驗(yàn)拍攝記錄相符。對兩氣泡相互靠近過程中氣泡四周的特別是兩氣泡間的流場進(jìn)行了深入分析,此過程中,氣泡兩側(cè)由四個(gè)小漩渦造成的低速區(qū)域的范圍逐漸縮小,并與外圍整體循環(huán)流動(dòng)融為一體；同時(shí),氣泡周圍的液相速度逐漸增大且分布趨于均勻,在中心軸線處存在最大值,并沿水平方向向兩側(cè)延伸下降至漩渦中心低速區(qū),之后又在漩渦外側(cè)的整體循環(huán)區(qū)域得到發(fā)展。兩氣泡接觸后的液膜厚度隨時(shí)間的推移呈現(xiàn)出先迅速變薄后緩慢變薄直至破碎的規(guī)律。
[Abstract]:The coalescence behavior between bubbles can significantly change the structure of the flow field and the gas-liquid interface, thus profoundly affecting the characteristics of the whole system. In this paper, the whole process of approaching and coalescence of low Reynolds number double bubbles in high viscosity system with coaxial rise is systematically studied, and the dynamic behavior of the connected bubbles formed in the process of coalescence and its influence on coalescence are analyzed in detail. In the aspect of experiment, this paper mainly uses high-speed photography technology to study six experimental systems (three kinds of pure glycerine solution at different temperature), A 95% glycerol-aqueous solution and two glycerol solutions with surfactant CDEA) were used to study the hydrodynamic behavior of a two-bubble system with a free-rising coaxial line formed by a bubble with an equivalent diameter of 610.5 mm. The liquid Morton number (Mo) is 1.69 ~ 661.83, and the bubble Reynolds number (Re) is 0.190 ~ (2.998), which covers the whole process of bubble formation, rising, collision, connection and film fragmentation. In the study, the coalescence process was divided into three stages: contact stage, connection stage and liquid discharge stage. The coalescence behavior and motion of the two bubbles in viscous fluid systems with various viscosity and surface tension coefficients were observed. The law of velocity and deformation and the interaction between bubbles in the process of double bubble ascending contact are analyzed, and the coalescence conditions and coalescence time are discussed. In addition, the formation, morphology and kinetic behavior of the connected bubbles are also systematically studied. The results show that the contiguous bubbles rise at a constant velocity in the discharge stage until the coalescence or slip occurs, which is not only similar to that of the single bubble, but also can be used to describe the bubble state at the moment of coalescence. At the same time, the drag coefficient model and the rising velocity model of the single bubble and the connected bubble are obtained, and the drag force and the interaction force between the bubbles are calculated. The connection time mainly depends on the viscosity of the experimental system, while the drainage time shows a strong randomness. With the decrease of the viscosity of the system or the gradual addition of surfactant, it is more difficult to agglomerate but tend to slip. In the aspect of simulation, a structured grid with the minimum mesh size of 50 渭 m is used in this paper, and the VOF model is used to simulate the direct coalescence of two bubbles with equal diameter. The flow field structure and the thickness of liquid film of the double bubble are obtained. The results show that both the morphology of the double bubbles and the dynamic behavior of the bubbles in each phase of coalescence are in agreement with the experimental records. The flow field around the bubble, especially between the two bubbles, during the process of two bubbles approaching each other is deeply analyzed. In this process, the range of the low velocity region caused by the four small swirls on both sides of the bubble is gradually reduced. And integrated with the peripheral circulation flow; At the same time, the velocity of liquid phase around the bubble increases gradually and tends to be uniform. There is a maximum at the center axis, and the velocity decreases along the horizontal direction to the low velocity region of the vortex center. Then the whole circulation area outside the vortex is developed. The thickness of liquid film after contact with two bubbles shows the law of thinning quickly and then thinning slowly and then breaking with time.
【學(xué)位授予單位】：北京化工大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2015
【分類號】：TQ021.1

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