Research Of Capillary Tube Throttling Characteristic Based On Lattice Boltzmann Method

With the economic development,the consumption of energy is surging. Energy saving and environmental advocates begin to get a response in the world. At the same time, people’s living standards are increasing, leading to a sharp increase in the number of home appliances, with very good prospects for energy conservation. As a common throttling component in refrigerators and air conditioning refrigeration cycle system, capillary tube’s throttle characteristic has a great impact on the refrigeration efficiency in refrigeration system, making the study of capillary tube’s throttle characteristic of important significance.The mesoscopic lattice Boltzmann method was used to simulate the flow in a rectangular capillary tube before bubble generating, adopting a free energy model for two-phase flows with large density differences. The present thesis firstly reviewed the background and development process of capillary tube’s throttle characteristic, and then introduced the model and application of lattice Boltzmann method in the simulation of multiphase flow. To verify the correctness of the model selection and programming, the process of two bubbles motion and fusion is simulated, and the simulation results coincided with the reference.Choosing R600a as the working fluid, the flow of metastable liquid segment in square capillary tube is simulated. It is found that the density keep the same during the simulation time. Analyses about this phenomena give birth to a conclusion that simulating the refrigerant flow in capillary tube by cutting it into a set of short ones are unavailable.The whole tube simulation was then achieved by compressing the length of the capillary tube without changing the other parameters. Through a large number of calculations and debugging, the process of phase change in capillary tube has been simulated with different initial and boundary condition setting.The density, pressure, streamlines of capillary tube throttling process are get.The flow under different length compression ratio was discussed to verify the simulation results.It is concluded that a small amount of vapor firstly appeared on the wall surface. Since then the low-density zone expanded along the circumferential as well as axial direction. Besides, the low-density fluid nearing the wall was coerced to move toward the tube center by the internal high-speed fluid and eventually converged to form a large area of low density in the center, providing the basis for generating a large number of bubbles.Hereby, the capillary tube throttling process is considered to start on the wall, then expand circumferentially and forwardly, gathering together to the center.