Study On Transition Conditions And Evolution Characteristics Of Saturated Liquid Nitrogen And Nitrogen Vapor Flow In Tube

Cryogenic two-phase flow widely exists in the cryogenic fluid equipment of the rapidly developing fields,such as air separation,aerospace(liquid propellant rocket),and the liquefied natural gas.The momentum exchange between the liquid phase and the gas phase leads to the dynamic evolution of the two-phase interface structure,and even triggers the flow pattern transitions.At the same time,the pressure drop,the characteristics of the heat and mass transfer between the two phases can also change accordingly.Therefore,unlike the single-phase flow,it is difficult for the stable flow to establish in a two-phase flow.In addition,compared with the room temperature fluids such as water,the surface tension and the liquid viscosity of the cryogenic liquids are commonly smaller by an order of magnitude,and the density ratio of liquid to gas is also much smaller.The differences in physical properties lead to the different critical conditions of the instability and also the evolution characteristics.The spatiotemporal morphology of the flow pattern and the mass,momentum,and energy exchange between the two phases are also more complicated.Until now,there are few reports on the flow pattern transition conditions and the evolution rules of the cryogenic two-phase flow,which makes it difficult to adequately provide reliable flow pattern criteria and the corresponding calculation methods of the heat and mass transfer for the design of cryogenic two-phase flow equipment.In response to the current situation,to deeply reveal the critical transition conditions and fluctuation characteristics of the cryogenic two-phase flow,the saturated liquid nitrogen(LN₂)-nitrogen vapor(VN₂)are taken as the working medium in this paper,and the interface stability theory,the numerical simulations of the two-phase flow and the visualization experiments are carried out:1.Based on the viscous Kelvin-Helmholtz instability theory(VKHI),a theoretical model is developed to predict the critical boundaries from the stratified flow to stratified flow+ripple and stratified flow+roll wave,and the dimensionless correlation of the wavelength for flow pattern transitions is also provided.For a stable gas-liquid two-phase flow with a relatively smooth interface,as the relative velocity between the liquid and gas phase increases,unstable phenomena such as ripples and roll waves appear on the interface in sequence.Kelvin-Helmholtz instability theory is often applied to solve the critical gas velocities when the interface becomes unstable.The viscosity of the saturated LN₂ is about 0.00016 Pa?s under normal pressure.Some theoretical studies have shown that when the viscosity is smaller than 0.1 Pa?s,the effect of the viscosity should be considered in the calculation of the interface stability.Based on VKHI theory,the experimental data of the flow patterns in room-temperature fluids are analyzed,and the dimensionless correlation of the wavelength for flow pattern transitions is obtained.The developed theoretical prediction model successfully predicts the critical boundaries from the stratified flow to stratified flow+ripple and stratified flow+roll wave under the room-temperature and cryogenic conditions.2.Based on the hyperbolicity of the Two-Fluid Model(TFM),assuming that the transition from stratified flow to slug flow is triggered by solitary waves,a theoretical model for predicting the transition from stratified flow to slug flow is developed.The TFM of gas-liquid two-phase flow is the second-order partial differential equations,and the solution of the eigenvalues of the coefficient matrix is evolved into the solution of hyperbolic equations.When the eigenvalue becomes an imaginary number,it is called hyperbolicity breaking.At this time,the wave information cannot be transmitted normally in the flow field,and it is considered to be the transition point of flow patterns physically.Therefore,the hyperbolicity of TFM establishes a bridge between the physical phenomenon of the flow pattern transition and the mathematical solution of hyperbolic equations.The transition from stratified flow to slug flow is different from the transition from stratified flow to stratified flow+ripple and stratified flow+rolling wave.The latter two are the instability of the interface caused by the disturbance wave,but the mechanism of the transition from stratified flow to slug flow has not yet reached a consensus.Many experimental studies have shown that the flow pattern transition is not directly caused by the disturbance wave.As a result,the assumption of the solitary wave induced is introduced.By comparing with the experimental data in room-temperature and cryogenic conditions,it is verified that when D>9 mm,the developed theoretical model can successfully predict the transition from stratified flow to slug flow.3.Based on the Euler-Euler model,a numerical model for the LN₂-VN₂-droplet three-phase unsteady flow is developed to clarify the fluctuation characteristics and the droplet entrainment mechanism of the flow pattern transition.Under the Euler-Euler framework,three phases of the liquid film,the gas and the droplet are modeled with the complex momentum exchange.Based on the Algebraic Interfacial Area Density Model for calculating the drag force between the phases,the droplet entrainment model is developed.Through comparison with experimental results,it is confirmed that the developed numerical model can capture the fluctuation characteristics in the process of flow pattern transition.It is found that the“ligament breakup”is the main droplet entrainment mechanism in the stratified flow+roll wave and slug flow,while no droplet entrainment can be observed in the stratified flow+ripple.In addition,by comparing the characteristics of water-air and LN₂-VN₂,the results show that at the same superficial liquid velocity,droplets are more easily entrained in LN₂-VN₂(the smaller superficial gas velocity).4.The visual experiment research is performed to verify the theoretical and numerical results,using the LN₂-VN₂ as the working fluid.Based on the LN₂-VN₂ flow visualization experimental set-up,a total of five working conditions for the two-phase flow pattern maps are obtained,including the horizontal tube for D=12 mm,15 mm and 20 mm;and for D=15 mm at the inclination angles of 3.5°and 6°.The data greatly enriches the experimental data in cryogenic conditions.In the flow pattern map of LN₂-VN₂,a complete slug flow cannot be observed,attributed to that LN₂ has smaller surface tension and viscosity than water.Ligaments and droplets are constantly generated around the slug,forming a pseudo slug flow.The amplitude of the ripple is smaller than that of the roll wave,and it had more obvious periodicity and regularity.Within the range of the studied geometric parameters,the inclination angle and pipe diameter have a significant influence on the flow pattern transition.As the inclination angle increases or the pipe diameter decreases,the required superficial gas velocity is smaller under the same superficial liquid velocity.