Through building the pipeline model, giving the boundary conditions and setting up the solver in Fluent software. analyze the character of mixture gas using species transport model. Through the analysis result know that the shape of the mixture gas changes from the shape of straight line to the shape of bullet as the inlet velocity increases. The concentration of nitrogen decreases with line regular pattern in the axis direction, as lime goes, the diffusion range is spreading: the nitrogen replacement velocity increases with the increasing of inlet velocity, at the beginning of the replacement, the increasing rate of the mixture length is faster than latter. The replacement length increases as time goes, the faster the replacement velocity the higher increasing ratio of the replacement length, and the replacement efficiency is increasing as the inlet velocity increases, hut the ratio is decreasing.%通过建立管道模型,给定边界条件,在Fluent软件中进行求解器设置后,运用组分输运模型模拟分析了氮气置换过程中混气段的特性.由分析得到:入口处混气头形状随入口速度增大由“直线”形状过渡到“子弹头”形状；在速度方向上,混气段氮气体积分数沿管道轴线呈线性规律递减,随着时间延长,扩散范围不断变大；氮气置换速度随着入口流速的增加而增大,置换初期,混气段长度增长速度快,大于置换后期；管道置换长度随时间增加而增大,置换速度越快,管道置换长度增长越快,管道置换效率随着流速的增加而增大,但是增长率呈下降趋势.
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