大尺度混合流與非定常流動界面形成規律

邱利民 張小斌

摘 要：液泛是指氣液兩相逆流過程中，部分液體受氣體作用被攜帶至液體進口以上的現象，是限制低溫規整填料精餾空氣處理量，實現大規模低溫空氣分離的主要障礙之一。液氮、液氧等低溫流體與常規流體相比物性差別巨大，低溫液泛機理愈加復雜，常規采用的室溫流體液泛實驗關聯式在低溫下的適用性、準確性急待驗證。該報告研究液氮溫區流體液泛機理，展開了以下4個方面的研究工作：（1）提出了CFD計算的三維規整填料代表性幾何結構，闡明了液泛發生前兩相流在規整填料結構中的特點。基于水力相似原理，提出由三角通道組成的三維模型，大大減少了網格數。計算分析了潤濕面積、出口處液相流量波動、液膜厚度的空間變化以及壓降情況。計算結果均與其他研究者的實驗觀察相符合，從而對液泛發生前的兩相流動特性和影響因素有了定量的認識，為液泛形成的進一步理論分析做好準備。（2）建立了豎直圓管中液泛形成過程三維數學模型，揭示了界面波不穩定性與環向傳播的內在聯系。基于線性穩定性理論，引入了環向波數，建立了相應的環向質量和動量守恒方程，理論求解了豎直圓管中的三維界面波不穩定機理，揭示了界面波不穩定性與兩相流速、物性及通道直徑的內在聯系。計算結果表明，環向波動有利于遏制液泛發生，三維模型預測結果與實驗結果吻合較好。（3）建立了傾斜圓管內液泛形成過程的數學模型，揭示了大傾角圓管中液泛氣速與管徑、傾斜角度及工質物性的內在聯系根據傾斜圓管內液膜流動特點及液膜厚度與液泛臨界波長間的關系，將圓管內分層流動的三維波進行合理簡化，利用線性穩定性理論，建立了適用于計算大角度傾斜圓管內液泛發生臨界條件的數學模型。該模型與已公開文獻中的實驗結果吻合良好，同時充分考慮了圓管結構及工質物性特點，尤其是低溫流體的特殊性質對液泛發生的影響。提出了以氣液相密度比（RHO）作為初步判斷液泛氣速值的量度，在計算中發現低溫流體的液泛氣速遠低于室溫流體。（4）進行了液氮/氮蒸汽傾斜圓管內液泛的實驗研究搭建了傾斜圓管內液泛可視化實驗裝置，分別采用空氣-水以及液氮-飽和氮蒸汽進行了對比實驗研究。發現室溫流體（水/空氣）液泛發生的特征是管內“柱塞”流，且液泛發生時界面波基本保持完整波形。而液氮流體發生液泛時，由于氣液密度比遠大于室溫流體，同時液體粘度極小，使得氣體慣性力較大從而能夠擊碎界面波，形成特有的霧狀流動，令通道內的壓降小于室溫流體。實驗測得低溫流體的極小液泛氣速與理論計算結論相符。

關鍵詞：液泛 線性穩定性理論 液氮 規整填料

Abstract：Flooding is the phenomenon in which part of the liquid is entrained by gas and carried to above the injection point in countercurrent gas-liquid tow-phase flow. The applicability and accuracy of the commonly used empirical formulas that obtained from the experimental data of room temperature fluids for the cryogenic fluids need to be verified. The present work investigates the mathematical model for the onset of flooding and exploring the exact mechanism at liquid nitrogen temperature. The following contents are included： （1）Based on the hydrohaulic similarity， a 3D geometrical model consisting triangle channels is proposed， in which the number of grids is reduced greatly. The simulation results of wetted area， liquid mass flow fluctuation at the outlet， spatial liquid film thickness variation and pressure drop are analyzed， and qualitatively agree with previous experimental observations. This simulation quantificationally reveals the characteristics and influencing factors of countercurrent gas-liquid flow before the flooding is onset， preparing for the further theoretical analysis. （2）Using the linear stability theory， a circumferential wave number is introduced into the governing equations to develop the three-dimensional mathematical model for predicting the interfacial instability and further the onset of flooding， while the internal relations between the interfacial instability and the non-axisymmetric perturbations， flow velocities， physical properties and tube diameter are announced. The results agree with the experimental data well， and prove that the circumferential perturbations can delay the onset of flooding. （3）A mathematical model for predicting the onset of flooding in an inclined tube is developed based on the linear stability theory. Thee results are in good agreement with the experimental data in the open literatures. The flooding velocity for cryogenic fluids is found to be much lower than that of room temperature ones. （4）A visualization experimental facility for flooding in an inclined tube is designed and manufactured. For the air-water pair， the “slug” flow in the tube is the feature the flooding process， while the interfacial waves can keep the shape on the whole. While， the unique mist flow is formed for the cryogenic working pair. The comparison shows that the developed model presented in this work can predict the onset of flooding both for room temperature and cryogenic working pairs.

Key Words：Flooding；Linear stability theory；Liquid nitrogen；Structured packing

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