Numerical Simulation On Characteristics Of Bubble Dynamics With Heat Transfer On Heated Surfaces Using Lattice Boltzmann Method

Bubble motion and deformation are extremely important phenomena in the field of gas-liquid two-phase flow. The investigations on characteristics of bubble thermokinetics are also important subject of scientific research in gas-liquid two-phase flow. Generally speaking, depending on the circumstances, gas-liquid two-phase flow can be divided into isothermal and non-isothermal system. In isothermal system, pressure leads to unsteady deformation of gas-liquid interface, which does not involve phase-change. In non-thermal system, phase-change leads to bubble deformation accompanying with heat and mass transfer at interface. Bubble phenomena exist extensively in nature and industrial processes, such as the cyclic process of refrigerant in refrigerating system, cloud cavitation in hydraulic system, exploit and transit of natural gas, boiling phenomenon and so on. The investigations on the characteristics of bubble dynamics and on the mechanism of heat and mass transfer are conductive to the industrial equipment design and operation. Therefore, the studies of two-phase flow have begun to receive more and more attention.As an emerging numerical method, lattice Boltzmann method (LBM) has strong parallel computing ability, clear physical image and terseness advantage in deal with complex boundary. Thus, it has advantages and applicability for simulation of gas-liquid two-phase flow. In this paper, LBM is used to numerically investigate the characteristics of bubble dynamics and the mechanism of heat and mass transfer in isothermal and non-isothermal system, respectively. The obtained results furnish a reference for generalizing LBM in relevant theoretical and experimental studies.At first, to verify the feasibility of LBM in research of two-phase flow in isothermal system, based on the free energy model, multiple bubbles rising in a quiescent viscous incompressible fluid is simulated. Due to the numerical instability caused by a large density ratio, eight-point and eighteen-point difference schemes are used to avoid numerical oscillation. The simulation results present flow characteristics and interaction as follows. The strength of influence between two bubbles depends on not only the distance and the relative position, but also the initial size. In addition, this method can be extended to non-isothermal system in consideration of the characteristics in capturing the interface.In order to further study on lattice Boltzmann thermal model, the characteristics of flow and heat transfer of fluid in the triangular and rectangular structure surfaces are investigated. Through the comparisons of average temperature, outlet temperature and average Nu, it is found that the rectangular structure surface possesses better heat transfer performance. This conclusion is analysed based on field synergy principle. The numerical results not only prove the feasibility of thermal model in research of convection heat transfer, but also lay the foundation for the studies of two-phase flow in non-thermal system.Combining the free energy model with the thermal model above, a hybrid LBM can be used to describe phase-change. This hybrid model is used to simulate the dynamics behaviour of vapor bubble growth on horizontal and vertical superheated wall. The simlated results shows micro-convection plays a crucial role during nucleate boiling process. While during the flow boiling, the main ways of heat transfer are nucleate boiling and forced convection. Multi-bubble growth on and departure form vertical wall is studied by present model. The simulated results show transition boiling is a combination of film and nucleate boiling alternatively existing on the superheated wall. The numerical results exhibited correct parametric dependencies of the bubble departure diameter and bubble release frequency.A new three-dimensional lattice Boltzmann model for phase change is proposed. The processes of bubbles growth, coalescences and departure on horizontal wall are simulated by this new model. The simulated results show the dynamics characteristics of vapor bubble during nucleate boiling accurately and clearly. The results confirm the applicability and feasibility of the model for numerical simulation of boiling phenomenon.Finally, the process of bubble growth on enhanced surface and heat/mass transfer mechanism on enhanced surface are investigated due to terseness advantage of lattice Boltzmann method in deal with complex boundary. The effect of bubble equivalent diameter, release frequence and Nu on nucleate boiling are analysed detailly. Simulated results present that the T-shaped surface possesses the best heat transfer performance, while the plain surface possesses the lowest heat transfer performance. Based on the numerical results, the reasons for high heat transfer performance of enhanced surfaces are analyzed.