Multi-scale Simulation Of Two-phase Flow And Bubble Dynamics During Boiling Period

Gas-liquid two-phase flow and boiling heat transfer processes widely exist in nature and in a variety of heat transfer equipment.It is of great importance to do researches on these phenomena among the areas of petroleum,chemical industry,aerospace,nuclear energy,electric power,metallurgy and environmental protection,etc.Comparing with single-phase flow,the mechanism of gas-liquid two-phase flow is more complicated because of the interactions between two phases and the topology changes at the interface.Simultaneously,the gas-liquid two-phase flow process is often accompanied with heat and mass transfer between the two phases,which makes the research of gas-liquid two-phase flow and boiling process to be more challenging.The study of bubble dynamics is one of the most important methods to investigate boiling phenomena.Therefore,the understanding of continuous bubble nucleation,growth and departure from the wall is the key to realize boiling heat transfer processes.Many researchers have investigated the evolution of bubble behavior and dynamic characteristics by a large number of experimental research and numerical simulations,however,due to the complexity of the gas-liquid two-phase flow and boiling process,the detailed mechanism of two-phase flows and boiling phenomena has not been completely clarified in theory.Traditional macro-scale numerical methods can describe some obvious interface movements between the two phases,however,it is difficult to trace a large number of small and dispersed interfaces.Simultaneously,a lot of assumptions have been implemented in the macro-scale simulations,and the details of the two-phase flow cannot be described.Moreover,the range of macro-scale numerical simulations is rather limited.When the grid size become small,the stability of the numerical calculation will be challenged.With the deepening of the research,macro-scale numerical methods cannot fully meet the requirements of research and development.Although microscale-based modeling is capable of directly modeling microstructures to reflect the complex mechanisms of heat flow processes,the number of molecules involved in the modeling is so large that it can lead to a large computer memory overhead,which exceeds the current computational power and cannot be achieved on the study of actual complex two-phase flows.Lattice Boltzmann method,one of the mesoscale numerical methods,has the characteristics of microscopic particles,it can easily describe the interparticle interaction between different phases and can capture the interface clearly.Therefore,this method has been developed into an effective numerical tool for the phase transition problems and multi-phase flows.Meanwhile,based on the multi-scale feature of two-phase flow and boiling phenomena,simulation from a single scale is not suitable for these problems.Therefore,multi-scale simulation of complex problems has been gaining widespread attention by researchers.In recent years,a new framework called equation-free method has been proposed and developed.This computational framework is built around the central that allows the researchers to perform macroscopic tasks by acting on the microscopic models directly,circumventing the derivation of macroscopic-level descriptions.In this way,this method provides a new way for multi-scale simulation of complex processes and systems.Based on the lattice Boltzmann method,the separation process of gas-liquid two-phase and the dynamic characteristics of bubbles are discussed in this paper.The contents of this paper mainly include the following aspects:1)The multiphase lattice Boltzmann pseudo-potential model uses an interaction force directly to impose the interactions between different phases.The selection of the interaction force term will have a direct impact on the results of the model.In this paper,the commonly used force terms in multiphase pseudo-potential model are discussed.Inspired by Gong and Cheng's improved model,a new scheme for the interparticle interaction force term is proposed.And the stability and the application range of the proposed pseudo-potential model are further improved.2)Based on the characteristics of the lattice Boltzmann method and the equation-free multiscale method,an equation-free multiscale model is introduced for simulating gas-liquid two-phase separation processes,in which the mesoscopic LBM model is used as the fine-scale model.Through the simulation of the gas-liquid phase separation process with different equation of state,it is found that the multiscale simulation method can well simulate the gas-liquid phase separation process.Moreover,the proposed multiscale method can improve the computational efficiency while ensuring the accuracy,which reflects its advantages in the simulation of complex processes or systems.3)Based on the single-component pseudo-potential model,the phase transition in the boiling model is determined by the equation of state incorporated in the model.Although the equation of state has a competent theoretical basis for reflecting the coexistence mechanism of the gas-liquid two-phase system.The phase transition cannot be well controlled due to the deviation of the pressure calculated by the equation of state and the actual pressure at the interface.The deviation can lead to the subcooled phenomenon in the fluid region during the simulation and the accuracy of the simulation is affected.In order to solve this problem,the source term related to phase transition in the temperature equation has been improved.The latent heat of phase transition is directly introduced into the source term,so that the heat absorbed in the unit fluid can retain a constant during the phase transition process,avoiding the errors and instabilities caused by the calculation of equation of state.With the improved phase-change heat transfer model,the bubble dynamics during the pool boiling and flow boiling period are simulated,respectively.In addition,a coupled method between the LBM and the finite volume method is used in this paper.The LBM model is adopted in the region near the heating surface,and the finite volume method is adopted in the area far from the heating surface.Then,the multi-scale simulation is carried out to simulate the bubble growth and departure process.In this thesis,useful explorations have been made for the multiscale simulation of multiphase flows and boiling phenomena.The dynamic characteristics and the mechanism of multiphase flows and boiling processes are revealed by establishing a more practical model.