应用CFD方法研究结构对管道内气体混合均匀性影响

Computational fluid dynamics analysis of influence of different pipe structures on gas mixing uniformity

目的研究管道结构对核电厂烟囱气载流出物混合均匀性影响。方法利用计算流体力学(computational fluid dynamica,CFD)方法,仿真截面分别为方形和圆形的长直管(I型)、90°单弯管(L型)、90°双弯管(S型和U型)内的速度分布及气体混合情况。结果对于长直管,由于缺乏结构变化产生的流场扰动,无法带来良好的混合效果,当管道截面为圆形,达到相关标准要求的混合均匀性指标,可能需要20倍水力直径的混合距离,当管道截面为方形,所需距离可能更长;对于单弯管,方形管内速度均匀性经弯管后有更大幅度提高,示踪气体也在11倍水力直径处先于圆形管满足了混合均匀性;对于S型双弯管,方形管内的示踪气体在6倍水力直径后呈现均匀混合,圆形管则要到下游7倍水力直径;对于U型双弯管,方形管道内的气体也在下游更短距离内实现了混合均匀,且气流在遇弯管时表现出更大的扰动效果。结论CFD方法可以对不同管道结构的气体混合均匀性变化规律做出准确的预测,能够取代部分物理实验,来研究核电厂烟囱气载流出物混合均匀性影响因素。

Objective Tostudy the influence of pipe structures on the mixing uniformity of airborne effluents from nuclear power plant chimneys.Methods We used the computational fluid dynamics(CFD)method to simulate the velocity distribution and gas mixing in long straight pipes(I type)with square section and circular section,90°single-bend pipes(L type)with square section and circular section,and 90°double-bend pipes(S type and U type)with square section and circular section.Results For the long straight pipe,due to the lack of flow disturbance caused by structural changes,the mixing effect was not good;when the pipe section was circular,it might take mixing distance 20 times the hydraulic diameter to achieve the uniformity index required by the relevant standard;for the square pipe,the distance might be longer.In the single bend pipe with square section,the velocity uniformity was improved more greatly after the bend,and the tracer gas met the mixing uniformity at a shorter distance(11 times the hydraulic diameter),as compared with the single bend pipe with circular section.For the S-type double-bend pipe,the tracer gas appeared uniformly mixed after a distance 6 times the hydraulic diameter in the square pipe,and 7 times the hydraulic diameter in the circular pipe.For the U-type double-bend pipe,the gas in the square pipe also achieved uniform mixing ata shorter distance downstream,and the airflow showed greater disturbance when passing through the bend.Conclusion The CFD method can make an accurate prediction for the change patterns of gas mixing uniformity in pipes with different structures,and can partially replace physical experiments to study the factors affecting the mixing uniformity of airborne effluents from the chimney of nuclear power plants.