大尺度混合流與非定常流動(dòng)界面形成規(guī)律

邱利民 張小斌

摘 要：液泛是指氣液兩相逆流過(guò)程中，部分液體受氣體作用被攜帶至液體進(jìn)口以上的現(xiàn)象，是限制低溫規(guī)整填料精餾空氣處理量，實(shí)現(xiàn)大規(guī)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(guī)流體相比物性差別巨大，低溫液泛機(jī)理愈加復(fù)雜，常規(guī)采用的室溫流體液泛實(shí)驗(yàn)關(guān)聯(lián)式在低溫下的適用性、準(zhǔn)確性急待驗(yàn)證。該報(bào)告研究液氮溫區(qū)流體液泛機(jī)理，展開(kāi)了以下4個(gè)方面的研究工作：（1）提出了CFD計(jì)算的三維規(guī)整填料代表性幾何結(jié)構(gòu)，闡明了液泛發(fā)生前兩相流在規(guī)整填料結(jié)構(gòu)中的特點(diǎn)?；谒ο嗨圃?，提出由三角通道組成的三維模型，大大減少了網(wǎng)格數(shù)。計(jì)算分析了潤(rùn)濕面積、出口處液相流量波動(dòng)、液膜厚度的空間變化以及壓降情況。計(jì)算結(jié)果均與其他研究者的實(shí)驗(yàn)觀察相符合，從而對(duì)液泛發(fā)生前的兩相流動(dòng)特性和影響因素有了定量的認(rèn)識(shí)，為液泛形成的進(jìn)一步理論分析做好準(zhǔn)備。（2）建立了豎直圓管中液泛形成過(guò)程三維數(shù)學(xué)模型，揭示了界面波不穩(wěn)定性與環(huán)向傳播的內(nèi)在聯(lián)系。基于線性穩(wěn)定性理論，引入了環(huán)向波數(shù)，建立了相應(yīng)的環(huán)向質(zhì)量和動(dòng)量守恒方程，理論求解了豎直圓管中的三維界面波不穩(wěn)定機(jī)理，揭示了界面波不穩(wěn)定性與兩相流速、物性及通道直徑的內(nèi)在聯(lián)系。計(jì)算結(jié)果表明，環(huán)向波動(dòng)有利于遏制液泛發(fā)生，三維模型預(yù)測(cè)結(jié)果與實(shí)驗(yàn)結(jié)果吻合較好。（3）建立了傾斜圓管內(nèi)液泛形成過(guò)程的數(shù)學(xué)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(zhì)物性的內(nèi)在聯(lián)系根據(jù)傾斜圓管內(nèi)液膜流動(dòng)特點(diǎn)及液膜厚度與液泛臨界波長(zhǎng)間的關(guān)系，將圓管內(nèi)分層流動(dòng)的三維波進(jìn)行合理簡(jiǎn)化，利用線性穩(wěn)定性理論，建立了適用于計(jì)算大角度傾斜圓管內(nèi)液泛發(fā)生臨界條件的數(shù)學(xué)模型。該模型與已公開(kāi)文獻(xiàn)中的實(shí)驗(yàn)結(jié)果吻合良好，同時(shí)充分考慮了圓管結(jié)構(gòu)及工質(zhì)物性特點(diǎn)，尤其是低溫流體的特殊性質(zhì)對(duì)液泛發(fā)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(jì)算中發(fā)現(xiàn)低溫流體的液泛氣速遠(yuǎn)低于室溫流體。（4）進(jìn)行了液氮/氮蒸汽傾斜圓管內(nèi)液泛的實(shí)驗(yàn)研究搭建了傾斜圓管內(nèi)液泛可視化實(shí)驗(yàn)裝置，分別采用空氣-水以及液氮-飽和氮蒸汽進(jìn)行了對(duì)比實(shí)驗(yàn)研究。發(fā)現(xiàn)室溫流體（水/空氣）液泛發(fā)生的特征是管內(nèi)“柱塞”流，且液泛發(fā)生時(shí)界面波基本保持完整波形。而液氮流體發(fā)生液泛時(shí)，由于氣液密度比遠(yuǎn)大于室溫流體，同時(shí)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(dòng)，令通道內(nèi)的壓降小于室溫流體。實(shí)驗(yàn)測(cè)得低溫流體的極小液泛氣速與理論計(jì)算結(jié)論相符。

關(guān)鍵詞：液泛 線性穩(wěn)定性理論 液氮 規(guī)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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