抽氣閉口在側風下形成的捕集區──風洞實驗

Capture Envelope of a Hood Opening under Crosswind Condition--A Wind Tunnel Experiment

吸氣開口於均勻側風中會在開口前方形成一捕集區，所有位於捕集區內的流線均會導入開口：所有位於捕集區外的流線則導入側風下游無限遠處。因此氣罩在側風中的捕集能力可由所形成捕集區涵蓋範圍決定。一氣罩對特定位置污染源欲達到有效捕集，其抽氣風量所造成的捕集區至少需能涵蓋該污染源。在本研究中，以一低速風洞所產生的穩定均勻風速模擬側風，風洞測試段中之圓形抽氣開口則用以模擬附突綠的外裝型氣罩。經檢視由雷射都←勒測速儀所測得的流速向量分布，即可獲得捕集區的涵蓋範圍。經多次測試後，發現捕集區的幾何尺寸對開口直徑的比值與抽氣開口平均風速對側風速度比相關，並獲得二比值間的經驗關系。此外，上述實驗結果與依勢流理論計算結果相當接近，其中又以高抽氣風速時最為接近。

The flow induced by an exhaust opening in the presence of a uniform crosswind will form a capture envelope in front of the opening. All streamlines within the envelope will lead to the opening; those outside the envelope then will lead to infinity downstream. Hence, capture performance of an exterior hood can be evaluated by the extent of the cap- ture envelope under a given uniform crosswind. To contain a contaminant source inside the capture envelope is a necessary condition for accomplishing an effective capture by the hood. In this study, the flow velocity vector measured by the laser-Doppler velocimetry re- vealed the capture envelope. The flow field was generated by a flanged circular opening in a low speed wind tunnel, which provided a uniform crosswind. The dimensions of the cap- ture envelope normalized to the opening size were found to be a function of the ratio of av-erage exhaust velocity at the opening V f to the crosswind velocity Vc . The geometric form based on the classical “Rankine's nose' problem was used to describe the capture en- velope associated with the finite opening once the dimensions were determined. The capture envelopes observed by the experiment were also compared with those computed by the potential flow theory. The computational results were found to agree with the experimental results in most cases. The best agreement between the computation and the experiment was found at high exhausted airflow at the opening.