Research on mixing law of natural gas pipeline replaced by nitrogen

Before the natural gas pipeline was put into operation, nitrogen is usually used to replace the air in the natural gas pipeline. In order to prevent air and natural gas from forming explosive mixtures, it is necessary to study the mixing law in the displacement process. In this article, based on computational fluid dynamics basic theory and turbulence model theory, the mathematical model and boundary conditions of nitrogen replacement were determined. The nitrogen replacement process was simulated by Fluent software. The following conclusions are drawn from the analysis: (1) when the flow is laminar, the mixing length is longer than that of the turbulent state, so try to keep the gas in the turbulent state; (2) with an increase in the flow velocity, pipeline diameter, and pipeline length, mixing length increases; (3) with an increase in the back pressure, gas mixing length increases gradually, so try to avoid the presence of back pressure venting; (4) with an increase in the temperature, the mixing length increases gradually; (5) the formula for calculating the mixing length obtained by fitting is L m / D = ( 1 . 95 ~ 2 . 55 ) v 0 . 39 ( L / D ) 0 . 53 .