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LNG(液化天然气)进入常温卸料管道前进行预冷,在低温氮气预冷过程中,会出现管道顶底温差较大现象,过大的顶底温差会造成管道拱起。利用Fluent软件,建立LNG卸料管道氮气预冷三维模型,采用阶段降低氮气入口温度的预冷方式,模拟氮气预冷卸料管道温度分布规律,探究卸料管道顶底温差产生原因及影响因素。结果表明:在预冷过程中,管道近管壁处温度梯度大,管道内部温度梯度较小;在预冷过程中,管道内自然对流作用不可忽略,与管道换热后的氮气温度升高、密度减小,在浮升力作用下向上运动,从而导致顶部温度高于底部温度;影响顶底温差的大小的因素有预冷时间、质量流量、氮气温度等;顶底温差随时间先增大后减小,质量流量越大,氮气入口温度越低,管道顶底温差越大。为避免管道顶底温差过大和预冷速度不超过10 K/h,采用阶梯式预冷,逐渐将温度降至123 K左右。
LNG (liquefied natural gas) enters the room temperature unloading pipeline for pre-cooling. During the pre-cooling of low-temperature nitrogen, there will be a large temperature difference between the top and bottom of the pipeline. An over-heated top-bottom temperature difference will cause the pipeline to arched. The Fluent software is used to establish a three-dimensional nitrogen precooling model for LNG discharge pipelines. Precooling mode is adopted to reduce the nitrogen inlet temperature. The temperature distribution of nitrogen precooling and unloading pipelines is simulated to find out the causes and influencing factors. The results show that during the precooling process, the temperature gradient near the pipe wall is large and the temperature gradient inside the pipe is small. During the precooling process, the natural convection in the pipe can not be neglected. When the temperature of the pipe is increased, The density decreases and moves upward under the buoyancy force, resulting in the temperature at the top being higher than the temperature at the bottom. The factors influencing the temperature difference between the top and bottom are the precooling time, mass flow rate, nitrogen temperature, etc. The top and bottom temperature difference first increases with time Reduce, the greater the mass flow, the lower the nitrogen inlet temperature, the greater the temperature difference between the top and bottom of the pipe. In order to avoid excessive temperature difference between the top and bottom of the pipe and the precooling rate of not more than 10 K / h, the temperature should be gradually reduced to about 123 K by using the stepwise precooling.