Theoretical And Experimental Investigation On Mechanism Of Onset Of Flooding At Liquid Nitrogen Temperature

In the process of countercurrent gas-liquid tow-phase flow, flooding is the phenomenon in which part of the liquid is entrained by gas and carried to above the injection point. Flooding takes place in many industry equipments, including the heat pipes, compact heat exchangers and structured packing columns. The occurrence of flooding not only fails the process of normal two-phase flow, but also accompanied with a sharp rise of the pressure drop; hence it is harmful to the operation of the equipments. The existing achievements cannot describe the onset of flooding exactly, since the development of interfacial waves and the interaction between gas and liquid during the process is complicated.In addition, flooding is the key factor that limits the capacity of structured packing columns, which hinders the development of the large-scale cryogenic air separation in industry. The specific properties of cryogenic fluids, like liquid nitrogen and liquid oxygen, will make the mechanism of onset of flooding even more complex. Therefore, the applicability and accuracy of the commonly used empirical formulas that obtained from the experimental data of room temperature fluidsfor the cryogenic fluids need to be verified.The present work focuses on further developing the mathematical model for the onset of flooding and exploring the exact mechanism at liquid nitrogen temperature. The following contents are included:1、Proposal of the3D geometrical model used for the gas-liquid CFD simulation in the structured packing columns, while the development and characteristics of the two-phase flow before the onset of flooding are clarified.The rapid advances in computational fluid dynamics (CFD) have greatly accelerated the development of distillation equipment. Up to now, the volume of fluid (VOF) mathematical framework with its2D geometrical model has been commonly applied to characterize the two-phase flow inside structured packing. However, it cannot describe the real flow situations, while the geometrical complexity and computer resource requirements of the present3D models are definitely high, making it difficult to be solved. In the present work, the mirrored shape is adopted to propose the3D geometrical model consisting triangle channels, 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、Development of the three-dimensional mathematical model for predicting the onset of flooding in a vertical tube, while the internal relations between the interfacial instabilityand the non-axisymmetric perturbations, flow velocities, physical properties and tube diameter are announced.The linear stability theory has been widely accepted to predict the interfacial instabilities. For the prediction of onset of flooding in a vertical tube, all the existing computation based on this theory has the assumption that the interfacial waves are axisymmetric, leading to the underestimated results comparing with the experimental data. The fact of the non-axisymmetry of the disturbance makes this assumption fail, and the perturbations in the circumferential direction play an important role on the stability of the interface. In the present work, the 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、Development of the mathematical model for predicting the onset of flooding in an inclined tube, while the internal relations of flooding velocity with the tube diameter, inclination and fluid properties are announced.Inclined channels are the most common form for transmission in the industrial equipments. The existing models used for this situation are usually one-dimensional, or do not take the boundary conditions of the limited walls into account, so that they can only predict the onset of flooding for the horizontal or nearly horizontal cases. In the present work, the perturbation in the inclined tube is simplified reasonably based on the flow characteristics, as well as the relation between the liquid film thickness and the critical wave length. Then a mathematical model for predicting the onset of flooding in the tube with a great inclination is developed based on the linear stability theory. The results are in good agreement with the experimental data in the open literatures, and the effects of the tube and fluid parameters, especially the specific characteristics of the cryogenic fluids are taken into account. The density ratio of gas to liquid (RHO) is proposed here as the elementary measure of the flooding velocity, and its validity is proved. The flooding velocity for cryogenic fluids is found to be much lower than that of room temperature ones.4、Experimental investigation on the onset of flooding in an inclined tube with air-water and liquid nitrogen-nitrogen vapor as working fluids, while the validity and accuracy for both temperature regions of the mathematic model presented in this work is verified.Almost all the experimental investigations on flooding make room temperature fluids as working medium, and there is rare open literatures focus on the cryogenic fluids. Actually, the mechanism will be changed if the fluid properties are different. The visualization experimental facility for flooding in an inclined tube is designed and manufactured in this work.The flooding process of room temperature working pair air-water and cryogenic pair liquid nitrogen-nitrogen vapor are investigated and compared experimentally. 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. On the other hand, the unique mist flow is formed for the cryogenic working pair, making the pressure drop reduced. The larger inertial force of the gas flow that can totally break the interfacial waves is attributed to the reason, which comes from the larger density ratio of gas to liquid phase and the smaller viscosity. The much smaller flooding velocities for the cryogenic working fluids obtained from the experiments agree with the calculated results in this work. After discussing the accuracy of the typical existing formulas for the cryogenic fluids, the comparison shows that the model presented in this work can predict the onset of flooding both for room temperature and cryogenic working pairs.