Study On The Effects Of N₂ On CO₂ Physical Properties During The Pipeline Leakage Process

Impurities contained in CO₂ not only have a great influence on the efficiency of pipeline transportation,but also are factors mainly considered in the pipeline leakage and safety assessment.Domestic studies on the impacts of impurities on transportation and leakage process are still at the stage of analogue simulation;therefore,few data tested through experiments can be referenced.This paper mainly adopted small scale pipeline experiment to study the characteristics of the pure CO₂ leakage process,and demonstrated the phase transformation and the heat transfer law of the leakage process.On the basis of pure CO₂leakage experiment,the paper carried out the experiment of CO₂ pipeline leakage with impurity N₂.It used state equation to calculate the results,aiming to find the influence of impurity N₂ content on the change of CO₂ physical properties.The main contents and conclusions are as follows:?1?Designed and established a small-scale CO₂ leakage experimental device.The experiment studied the variation characteristics of pressure and temperature in the pipe during the pure CO₂ leakage process.The result showed that the pressure response of the medium in the pipe was decreased by negative exponent when gas-phase CO₂ leaked;the pressure response of the medium in the pipe during supercritical and dense-phase CO₂ leakage experienced two stages:rapid decrease and negative exponential decay.During the gas-phase,supercritical,and dense-phase CO₂ leakage,the temperature drop range of the medium in the pipe at the far end of the leak hole was larger than that at the near end,and the phenomenon of?near-end hole hot and far-end hole cold?appeared.Under the same size of leakage orifice diameter,the temperature drop range of the dense-phase medium was the largest,decreased by 76?,that of supercritical medium was followed,and that of gas-phase medium was the smallest.Hence,it can be concluded that the larger the leakage orifice diameter,the greater the temperature drop range.?2?The experiment studied the variation characteristics of phase state in the pipe during the CO₂ pipeline leakage process.The phase change of the medium in the pipe would not occur in the d=1 mm gas-phase CO₂ leakage process,and the phase change of the full orifice diameter was?gas phase–gas-liquid phase–liquid phase–gas-liquid phase–gas phase?.The phase change of d=1 mm supercritical CO₂ leakage process was?supercritical phase–gas-liquid phase–gas phase?,and the phase change process of the full orifice diameter leakage was?supercritical phase–gas-liquid phase–liquid phase–gas-liquid phase–gas phase?;the phase change of different orifice diameters dense-phase CO₂ leakage process was?dense phase–liquid phase–gas-liquid phase–gas phase?,the lager the leakage orifice diameter,the longer the two phase duration.The lowest temperature at the far end of d=10.8mm dense-phase CO₂ leakage reached to-51?,and then dry ice may occur.?3?The experiment studied the heat transfer rule during the CO₂ pipeline leakage process.Choked flow strength at the leak hole and heat exchange strength between the medium along the pipeline and pipe wall were measured by Nusselt number.In the supercritical leakage experiment,the Nusselt number at the leak hole rapidly decreased at the initial stage of the leakage.After passing through the critical point,the Nusselt number was steady between 0and 1 as the leaked liquid was in stable flow.The effect of chocked flow was constantly enhanced as the size of leak outlet increased.When it reached to a certain size,it was no longer strengthened.As for the heat transfer rule along the pipe in the leakage experiment with the same size of leak orifice diameter,the Nusselt number in the dense-phase leakage process was the largest,that in the supercritical-phase leakage phase took the second,and that in gas-phase the third;the Nusselt number at the far end of leak hole was the largest,that at the leak outlet the second,that in the middle of the pipe followed;at the same initial phase state and with the same size of leak orifice diameter,the Nusselt number reached to the largest at the initial stage of leakage.?4?GERG-2008 state equation was chosen as a calculation tool to analyze skewing rule of N₂ content to CO₂ saturation curve and the effect of impurity content on physical properties within the quasi-critical temperature range.The result showed that N₂ content lowered the critical temperature of CO₂ and increased its critical pressure,and the saturation curve formed into two phrase region;as the N₂ content increased within the quasi-critical temperature range,the density decreased by 49%,the viscosity decreased by 59.2%,and the peak value of specific heat at constant pressure decreased by 36.47%,which reduced the transportation capacity of pipeline.?5?Based on this device,CO₂ leakage experiments with impurities were conducted so as to study the influence rule of N₂ content on the physical property parameters in the CO₂leakage process.The result showed that the more N₂ content,the shorter the leakage time,which increased the average pressure drop rate and made the temperature-pressure curve shift to the saturation curve.In the leakage process and at the same pressure level,the higher the N₂content,the lower the values of density,heat capacity and viscosity;within the quasi-critical region,as pressure reduced,the influence of N₂ concentration on the physical property parameters became smaller and smaller.