基于数值计算的细颗粒物采样管路传输损失评估

细颗粒物(PM_(2.5))理化性质测量是研究大气PM_(2.5)污染来源及成因的重要手段之一,最大限度降低细颗粒物在采样输送过程中的损失对提高测量结果的准确性至关重要.为了评估常规测量体系中采样管路内PM_(2.5)的输送损失情况,并在此基础上探究合适的采样管路布置方案,本研究采用数值计算方法分析了管径、管长及弯管数目这3个主要参数在不同变化范围内对PM_(2.5)输送效率的影响.结果表明,流量为20.0 L·min~(-1)时,管径4 mm、管长1.0 m的竖直管路内PM_(2.5)质量浓度输送效率为89.6%,管径增至14 mm时输送效率升至98.3%.流量为1.0 L·min~(-1)时,管径4 mm、管长10.0 m的水平管路内PM_(2.5)质量浓度输送效率仅为86.7%,管长降至0.5 m时输送效率提高至99.2%.弯管弧度为90°时,流量20.0 L·min~(-1)、管径4 mm的湍流态弯管处,PM_(2.5)质量浓度输送效率低至85.2%.流量(L·min~(-1))与管径(mm)之比小于1.4使管内流态为层流时有利于降低颗粒物输送损失.为保证PM_(2.5)输送效率在97%以上,2.5、5.0和10.0 L·min~(-1)仪器建议选择管长在6.0 m以内的竖直采样管;流量为16.7 L·min~(-1)和20.0 L·min~(-1)仪器建议选择管径在12 mm以上的竖直采样管;水平管路管长由流量与管径之比确定;在湍流流态下,建议减少弯管的使用数量.

Transport Loss Estimation of Fine Particulate Matter in Sampling Tube Based on Numerical Computation

In-situ measurement of PM_2.5 physical and chemical properties is a substantial approach for the mechanism investigation of PM_2.5 pollution. Minimizing PM_2.5 transport loss in sampling tube is essential for ensuring the accuracy of the measurement result. In order to estimate the integrated PM_2.5 transport efficiency in sampling tube and optimize tube designs, the effects of different tube factors (length, bore size and bend number) on the PM_2.5 transport were analyzed based on numerical computation. The results showed that PM_2.5 mass concentration transport efficiency of vertical tube with flow rate at 20.0 L·min^-1, bore size at 4 mm, length at 1.0 m was 89.6%. However, the transport efficiency increased to 98.3% when the bore size increased to 14 mm. PM_2.5 mass concentration transport efficiency of horizontal tube with flow rate at 1.0 L·min^-1, bore size at 4 mm, length at 10.0 m was 86.7%, and increased to 99.2% with length at 0.5 m. Low transport efficiency of 85.2% for PM_2.5 mass concentration was estimated in bend with flow rate at 20.0 L·min^-1, bore size at 4 mm, curvature angle at 90°. Laminar flow of air in tube through keeping the ratio of flow rate (L·min^-1) and bore size (mm) below 1.4 was beneficial to decrease the PM_2.5 transport loss. For the target of PM_2.5 transport efficiency higher than 97%, it was advised to use vertical sampling tubes with length less than 6.0 m for the flow rates of 2.5, 5.0, 10.0 L·min^-1 and bore size larger than 12 mm for the flow rates of 16.7 or 20.0 L·min^-1. For horizontal sampling tubes, tube length was decided by the ratio of flow rate and bore size. Meanwhile, it was suggested to decrease the amount of the bends in tube of turbulent flow.