Research Of Capillary Tube Throttling Phase Transition Characteristic Based On Lattice Boltzmann Method

Capillary is an important throttle element for small refrigeration and air-conditioning equipment.At present,it is widely used in small refrigerating devices with refrigerating capacity less than lOkW.The structure of capillary is simple,but the two phase flow in the tube is quite complex.Although a lot of relevant experience has been accumulated in the previous researches,the details of the phase transition of the capillary are rarely involved due to the limitations of the experimental conditions.There is no comprehensive understanding of the two phase flow in the tube,so there is still much work to be done for studying the characteristics of the capillary phase transition.Based on this background,the lattice Boltzmann method(LBM)is used as a numerical simulation method to simulate the phase change process of capillary flow.In this research,a new non isothermal lattice Boltzmann model for gas-liquid phase transformation is established by combining free energy model describing the gas-liquid two phases with lattice Boltzmann thermal model,and according to the mathematical model,the C++ calculation program is written.In the simulation,taking the existing R600a experimental data as a reference,by setting different computing conditions,including the setting of mathematical parameters,pressure distribution,velocity distribution and boundary treatment format,we get the optimal simulation conditions.This paper simulates the three-dimensional rectangular capillary of two different diameter ratios with the computing domain of 0.692mmx0.692mmx69.2mmand 0.692mmx0.692mmx 103.8mm.The distribution of physical parameters such as density,temperature,pressure and velocity in the process of flow is simulated,and the phase transition process is studied from a more subtle angle.The simulation results show that:the pressure drop in the tube is an important factor to drive the capillary flow phase transition.The greater the pressure drops in the tube,the more intensively the phase change.Before the phase change,there is a large superheat because of the high temperature at the center of the tube,where liquid nucleation is easy to happen.So area of low density is formed firstly at the center of the tube,where phase change starts to occur.After the phase change,the initial small mass of gas developed into a long gas column in the middle of the pipe with the gas increasing rapidly.Finally,the annular flow pattern appeared,and this flow pattern was found in the capillary visualization experiment of the small refrigerating device.The temperature at the interface between gas and liquid is high,indicating that the low density liquid closing to the gas can absorb heat from the interface continually in the process of phase change until it evaporates into gas completely The simulation results show that the velocity of the gas in the central area is much higher than that of the liquid on both sides,which is consistent with the fact that the central area of the capillary flow is a high speed area.At the same time,the velocity of gas in the simulation is close to the actual value,which proves the validity of the simulation.