Abstract
The low-temperature fluid flowing into the LNG unloading pipeline will cause violent heat exchange phenomenon, resulting in damage to the pipeline and affecting normal operation. Therefore, pre-cooling treatment is required in advance. Considering the fluid-solid heat transfer and fluid flow characteristics comprehensively, multi-physics coupling simulation study on the pre-cooling problem of the LNG discharge pipe with a π-shaped elbow is carried out. The analysis from BOG and LNG pre-cooling processes respectively shows that: the fluid with low temperature and high density is mainly concentrated at the bottom of the pipe and outside the bend, the wall temperature has a sudden drop at the bend, and there is a large temperature difference between the gas side and the liquid side wall surface. Mixed convection heat exchange mainly occurs in the tube. There will be a small area of natural convection-dominated heat exchange state in the elbow and the downstream pipe section.  
Highlights
In order to meet the increasing energy demand, look for fuels that can replace hydrocarbons such as diesel and gasoline [1], which produce little or no greenhouse gases. while natural gas meets the current needs, it is clean, renewable and mass production, etc
The numerical simulation method was used to analyze the flow and heat transfer of the whole process of liquefied natural gas (LNG) discharge pipe pre-cooling, and the following conclusions were drawn: (1) During the BOG pre-cooling process, the wall surface temperature rises along the direction of fluid flow, and there is a phenomenon of a sudden temperature drop of 3K at the bend
For the non-full flow stage, due to the low liquid phase temperature and high density, the temperature of the liquid sidewall surface is much lower than the temperature of the gas sidewall surface, and the temperature drop rate of the liquid sidewall surface is higher than that of the gas sidewall surface
Summary
In order to meet the increasing energy demand, look for fuels that can replace hydrocarbons such as diesel and gasoline [1], which produce little or no greenhouse gases. while natural gas meets the current needs, it is clean, renewable and mass production, etc. Chi and Vetere studied the flow and heat transfer conditions in the horizontal tube during low temperature cooling. Later [17].Jun Liao established a quasi-steady-state model to predict the temperature change process of the tube wall in the low-temperature fluid horizontal pipeline, and combined with the layered lowtemperature cooling flow structure to develop a new film boiling heat transfer model[18]. Yuan [20] scholar used experiments to conduct experiments on cryogenic fluid cooling equipment, and found that the data curves of the cryogenic cooling experiment and the boiling experiment have certain similarities, but the difference is that the test part did not supplement the external energy to cause the temperature to drop. Liquefied natural gas (LNG) is a low-temperature cryogenic medium of -162°C, and the design temperature range of its unloading pipeline is -170°C~60°C. LNG and BOG are complex mixtures, their methane content is above 90%, the main properties and physical properties are similar to methane, so in the simulation of pre-cooling research, methane is used instead of LNG for simulation; The material properties of the insulation layer are simplified to fixed parameters that do not change with temperature
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