大尺度混合流与非定常流动界面形成规律

Annual Project Report on Interface Formation Principal of Large-scale and Unsteady Flow

液泛是指气液两相逆流过程中,部分液体受气体作用被携带至液体进口以上的现象,是限制低温规整填料精馏空气处理量,实现大规模低温空气分离的主要障碍之一。液氮、液氧等低温流体与常规流体相比物性差别巨大,低温液泛机理愈加复杂,常规采用的室温流体液泛实验关联式在低温下的适用性、准确性急待验证。该报告研究液氮温区流体液泛机理,展开了以下4个方面的研究工作：（1）提出了CFD计算的三维规整填料代表性几何结构,阐明了液泛发生前两相流在规整填料结构中的特点。基于水力相似原理,提出由三角通道组成的三维模型,大大减少了网格数。计算分析了润湿面积、出口处液相流量波动、液膜厚度的空间变化以及压降情况。计算结果均与其他研究者的实验观察相符合,从而对液泛发生前的两相流动特性和影响因素有了定量的认识,为液泛形成的进一步理论分析做好准备。（2）建立了竖直圆管中液泛形成过程三维数学模型,揭示了界面波不稳定性与环向传播的内在联系。基于线性稳定性理论,引入了环向波数,建立了相应的环向质量和动量守恒方程,理论求解了竖直圆管中的三维界面波不稳定机理,揭示了界面波不稳定性与两相流速、物性及通道直径的内在联系。计算结果表明,环向波动有利于遏制液泛发生,三维模型预测结果与实验结果吻合较好。（3）建立了倾斜圆管内液泛形成过程的数学模型,揭示了大倾角圆管中液泛气速与管径、倾斜角度及工质物性的内在联系根据倾斜圆管内液膜流动特点及液膜厚度与液泛临界波长间的关系,将圆管内分层流动的三维波进行合理简化,利用线性稳定性理论,建立了适用于计算大角度倾斜圆管内液泛发生临界条件的数学模型。该模型与已公开文献中的实验结果吻合良好,同时充分考虑了圆管结构及工质物性特点,尤其是低温流体的特殊性质对液泛发生的影响。提出了以气液相密度比（RHO）作为初步判断液泛气速值的量度,在计算中发现低温流体的液泛气速远低于室温流体。（4）进行了液氮/氮蒸汽倾斜圆管内液泛的实验研究搭建了倾斜圆管内液泛可视化实验装置,分别采用空气-水以及液氮-饱和氮蒸汽进行了对比实验研究。发现室温流体（水/空气）液泛发生的特征是管内“柱塞”流,且液泛发生时界面波基本保持完整波形。而液氮流体发生液泛时,由于气液密度比远大于室温流体,同时液体粘度极小,使得气体惯性力较大从而能够击碎界面波,形成特有的雾状流动,令通道内的压降小于室温流体。实验测得低温流体的极小液泛气速与理论计算结论相符。

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：⑴Based 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.⑵Using 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 theinternal relations between the interfacial instability and the non-axisymmetric perturbations, flow velocities, physical properties andtube diameter are announced. The results agree with the experimental data well, and prove that the circumferential perturbations can delay the onset of flooding.⑶A 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.