Flow Behaviors And Critical Velocity Prediction Methods Of Flooding In Inclined Tubes With LN2-VN2

With cryogenic air separation plant becoming ultra-large, natural gas consumption growing fast, and cryogenic propellant getting popular in rockets propulsion, investigations of cryogenic two-phase flow therefore become increasingly important for technology upgrading in energy and aerospace industry. Flooding, as a special phenomenon widely occurring in two phase equipment, needs to be intensively investigated with emphasis on its triggering mechanism and critical velocity prediction method. Up to now, the existing findings of flooding are mostly derived from room-temperature fluids. However the different physical properties (i.e. surface tension and viscosity) between cryogenic fluids and room-temperature fluids lead to the different triggering mechanisms. Therefore the investigation of flooding with cryogenic fluids becomes one of the great concerns in the researches of two phase flow. To study the flooding mechanisms with cryogenic fluids and build prediction models of flooding velocities, this paper mainly carried out the research in the following four aspects:1. A theoretical model for predicting flooding velocity in an inclined tube was built based on the viscous Kelvin-Helmholtz instability theory, and influences of physical properties and geometry parameters on flooding velocities were investigated.The Kelvin-Helmholtz instability (KHI) theory has been widely used to predict interfacial instability. Researches have proved that the effects of liquid viscosity should be considered by adopting Viscous KHI (VKHI) theory to predict flooding velocities when the liquid viscosity is small. Because of liquid nitrogen with much lower viscosity than water, a theoretical model of flooding with cryogenic fluids in an inclined tube was developed based on VKHI theory by connecting two-phase flow instability and flooding under the assumption that the most dangerous waves with maximum growth factors trigger flooding. The accuracy of theoretical model for flooding was further proved by experimental results with both room-temperature and cryogenic fluids. Effects of physical properties (density, surface tension and viscosity) and geometry parameters were analyzed based on the theoretical model, which concludes that decrease in gas density and liquid viscosity, increase in surface tension, inclination and tube diameter can all increase flooding gas velocities.2. Experimental investigation on flooding with liquid nitrogen (LN2)-nitrogen vapor (VN2) and water-air as working fluids to verify the theoretical model, and study on the features of flow pattern during flooding were carried out.Flooding with cryogenic fluids differs much from that with room-temperature fluids because of the great differences in physical properties. However the existing researches on flooding were mostly carried out with room-temperature fluids as working fluids. An adiabatic visualization experimental system was built for flooding research with LN2-VN2 and water-air. Flooding velocities under different tube inclinations were measured and the flow pattern transition processes during flooding were observed. Experimental results illustrated that flooding gas velocities with LN2-VN2 were much lower than those with water-air. Observations of flow pattern transition during flooding show that liquid drops were formed by smashing the interfacial waves with LN2-VN2, while the flow reversal of complete interfacial wave and slug flow appeared with water-air. Experimental results also proved that theoretical model presented in this work could satisfactorily predict flooding velocity with cryogenic fluids. An empirical velocity correlation for flooding prediction for both cryogenic fluids and ambient-temperature fluids in error within 20% was further proposed.3. A CFD numerical model of flooding in an inclined tube was developed and the effects of surface tension and liquid viscosity on flow pattern were investigated.The wide range of spatial scales of interface structures and drastic momentum exchange between gas and liquid are the two main difficulties in numerical simulation of flooding. The present model adopted interface identification technique to catch multi-scale interfaces and Algebraic Interfacial Area Density (AIAD) technique for drag force calculation between phases. Effects of surface tension and liquid viscosity on flow pattern transition were investigated. Numerical results illustrate that surface tension acts as a stability force, and the liquid film is more easily to be torn into liquid drops with a smaller surface tension. Meanwhile, liquid viscosity acts as a cohesive force on the liquid film, and slug flow is more easily to form with higher liquid viscosity.4. Investigation on interfacial wave characteristics and effects of interfacial waves on flooding were announced.Interfacial waves, as the origin and prerequisite of flooding, need to be studied to help understand the triggering mechanism of flooding. Nevertheless, few researches focused on the interfacial wave characteristics during flooding. Based on the numerical simulation and experimental results in this work, FFT analysis and wavelet analysis of wall shear stress signals and pressure drop signals were carried out to compare the wave frequencies and flooding intensities between LN2-VN2 and water-air. Results show that the liquid film behaves drastic oscillation and the wave frequencies fall about 2.5 Hz at flooding. Meanwhile, it also concludes that with water-air, wave frequencies before flooding are higher than those at flooding, while with LN2-VN2, wave frequencies don’t show obvious changes before and at flooding.