風洞中路塵排放係數之量測研究

Abstract

在擬定有效的揚塵控制策略時，所面臨的主要問題之一為評估揚塵排放係數時所產生的不確定性。本研究於實驗室中建立一風洞系統，以探討流場不同加速度及速度對揚塵排放係數及揚塵質量濃度的影響。本研究收集新竹市科學園區茂德半導體公司建廠工程整地階段之地面粉塵（測試粉塵1），以及新竹市南寮地區裸露地之地面粉塵（測試粉塵2），於風洞中進行揚塵實驗。實驗前，先利用美國325號標準篩網篩除粒徑大於44 mum的微粒。接著，將測試粉塵烘乾，放進一長5 cm、寬5 cm、高0.2 cm之鋁盒中，利用平板將測試粉塵表面抹平，以減少粉塵的邊界效應（表面不平整），再置入風洞內的工作平台上。工作平台下游處，裝設有10根薄壁採樣管，下接10個濾紙夾持器，用以量測揚塵質量濃度分布。此外，利用吸入效率公式校正揚塵質量濃度，以求得實際的揚塵質量濃度。另一方面，本研究亦探討邊界效應對揚塵排放係數及揚塵質量濃度的影響。 實驗結果顯示，粉塵堆中氣動直徑 > 10 mum的微粒，不論在流場加速度1.5 m/sec^2、1 m/sec^2或0.5 m/sec^2下，被揚起的比例極低，顯示大部分氣動直徑 > 10 mum的微粒仍舊留在粉塵堆中。當測試粉塵1不產生邊界效應時，揚塵現象的風速閥值為10~12 m/sec，不因流場的運動方式而改變。當測試粉塵2產生邊界效應時，揚塵現象的風速閥值降為9~10 m/sec。在相同流場變化下，若粉塵堆產生邊界效應，則揚塵濃度及揚塵排放係數皆比不產生邊界效應時高。對測試粉塵1而言，粉塵堆的邊界效應會使揚塵排放係數增加為20倍以上。對測試粉塵2而言，揚塵濃度對時間變化的揚塵濃度峰值，會因邊界效應而增高。當流場加速度增加時，邊界效應的影響越大。無邊界效應時，對測試粉塵1而言，當流場加速度由0.1 m/sec^2增加至1.5 m/sec^2時，揚塵排放係數會由1 x10^-4 kg/m^2-sec增加至7 x10^-4 kg/m^2-sec，且揚塵排放係數與流場加速度呈現正相關。無邊界效應時，對測試粉塵2而言，當流場加速度由0.25 m/sec^2增加至1.5 m/sec^2時，揚塵排放係數會由0.1 x10^-4 kg/m^2-sec增加至1 x10^-4 kg/m^2-sec，且揚塵排放係數與流場加速度亦呈現正相關。

One of the critical problems in making an effective strategy to control road dust emission is the uncertainty in estimating its emission factor. A wind tunnel was built in the laboratory to measure the emission factor of road dust under different wind speed and acceleration conditions. Test road dusts were collected from the construction site where the factory of the ProMOS Technologies was establishing in the Science Industrial Park of Hsin Chu ( test sample 1) and from an unpaved road at Nan Laio area of Hsin Chu ( test sample 2), respectively. The test samples were first sieved using a standard No. 325 mesh to remove particles greater than 44 mum. Thereafter, the test samples were dried and then contained within an aluminium cell of 5 cm (width) x 5 cm (length) x 2 mm (depth), which was embedded in a flat plate in the working section of the wind tunnel. The surface of test samples were kept flush with the plate before testing to avoid edge effect. Emission factor was calculated based on the mass concentration profile measured by ten open face filter holders which were installed downstream of the working platform in the wind tunnel. Aspiration efficiencies of the ten thin-wall samplers were calculated to derive the mass concentration profile of the reentrained dust in the free stream. On the other hand, the influence of edge effect (i.e. the surface is not smooth) to the emission factor and the reentrained dust concentration was also investigated. Results show that particles with diameter greater than 10 mum are hardly reentrained and remained in the aluminium cell even when air acceleration rate was varied from 0.5 to 1.5 m/sec^2. When the edge effect doesn't appear at the surface of the test sample, the threshold wind speed for reentrainment was found to be 10 ~ 12 m/sec. But the threshold wind speed is decreased to 9 ~ 10 m/sec when the edge effect appears. In the event of the edge effect, both the reentrained dust concentration and the emission factor will become much higher than those without the edge effect. For the test sample 1, the edge effect increases the emission factor by as much as 20 times. The influence of edge effect on the reentrained dust concentration increases with wind acceleration. As the flow acceleration is increased from 0.1 to 1.5 m/sec^2, the emission factor is increased linearly from 1.0 x 10^-4 kg/m^2-sec to 7.0 x 10^-4 kg/m^2-sec. For the test sample 2, the emission factor is also increased linearly from 0.1 x 10^-4 kg/m^2-sec to 1.0 x 10^-4 kg/m^2-sec as the flow acceleration is increased from 0.1 to 1.5 m/sec^2.