Numerical Study On Coupling Of Two-phase Flow And Heat Transfer In The Chilldown Process Of Cryogenic Fluid Transfer Lines

Before the cryogenic fluid transmitted through the pipe network system,the pipe network system should be fully precooled to avoid the occurrence of gas-liquid two phases in the transportation process of the cryogenic fluid.Flow film boiling plays a dominant role in cryogenic chilldown process,which involves complicated heat transfer and flow regime transition.Therefore,it is necessary to clarify the coupling mechanism of gas-liquid flow and heat transfer in the cryogenic chilldown process.In this paper,liquid nitrogen is used as the research working medium to do the following work on the simulation research of the cryogenic fluid transmission pipeline chilldown process:Aiming at the film boiling in pipeline precooling process,a computational Fluid Dynamics(CFD)model based on a wall heat flux partition algorithm is built.The AIAD framework implemented in the two-fluid model is employed to appropriately calculate the drag force on the liquid-vapor interfaces.The CFD model is validated by the satisfactory coincidence between the simulated heat fluxes and experimental data in literature.On this basis,the influence of two-phase interaction on flow pattern and heat transfer is further studied,and the flow pattern and pressure drop characteristics are analyzed.The results show that the gas-liquid interface interaction has a significant effect on the flow pattern,and the flow patterns in horizontal and vertical pipes are stratified flow and reverse annular flow respectively.When the inlet mass flow rate is low in vertical tube chilldown process,the film thickness and the fluctuation of the gas-liquid interface play a dominant role in the wall heat flux.With the increase of the inlet mass flow rate,axial convection heat transfer dominates.In addition,FFT analysis of pressure drop time series shows that the dominant frequency of pressure drop time series in horizontal single tube chilldown process has weak correlation with superheat and inlet flow rate.For the vertical single tube precooling process,the superheat mainly affects the amplitude of pressure drop pulsation,while the inlet mass flow rate mainly affects the main frequency of pressure drop pulsation.According to the simulation research of single tube chilldown process,the fluctuation of gas-liquid interface with stratified flow will directly affect the stability of pipeline,therefore,it is necessary to investigate the chilldown process of the cryogenic pipeline system and the pressure fluctuation caused by flow boiling.Within the calculation range,with the increase of inlet pressure,the pressure fluctuation in the pipeline chilldown process will increase and the chilldown efficiency will gradually decrease,so the inlet pressure can be appropriately reduced to improve the stability of the pipeline system and the chilldown efficiency when meeting the requirements of fluid transportation in the actual precooling process.Increasing the degree of subcooling can effectively reduce the consumption of liquid nitrogen in the pipe network system,and the pre-cooling efficiency of the entire pipe network is significantly improved.However,the pressure drop of the pipeline fluctuates more drastically,which leads to greater flow fluctuations.The precooling efficiency of pulse flow increases first and then tends to be stable with the increase of pulse frequency,and decreases with the increase of pulse ratio.The frequency obtained from single-tube chilldown simulation can provide reference for pipeline system precooling,but it is not suitable for direct application to pipeline system.