實驗室氣櫃氣動力效應與污染物洩漏評估及性能改良技術之開發

Aerodynamics and Assessment of Contaminant Leakage and Development of Improvement Technique of Laboratory Fume Hoods

曾麗靜(Li-Ching Tseng)

指導教授 ： 陳志傑

國立臺灣大學/公共衛生學院/職業醫學與工業衛生研究所/博士(2007年)

https://doi.org/10.6342/NTU.2007.00083

摘要

實驗室氣櫃係屬一可調整開口大小的五面密閉似的局部排氣裝置，以捕捉、限制及排除氣櫃內產生的危害燻煙等。近十幾年來，諸多研究建議，由於實驗室氣櫃的幾何、設備參數、抽氣率、側風等等因素，以傳統測量面速度方法來決定氣櫃之性能，並不足以有效防範有害化學燻煙等流入實驗室人員之呼吸區域。
本研究建立一標準實驗操作條件，進行傳統化學性氣櫃研究，以面速度及其變異性量測及流場可視化方法，結合追蹤氣體量測探究氣櫃於靜態(不受外界干擾氣流影響)與動態(櫃門迅速開關及有側風情況)，探討氣櫃內部及氣櫃介面之流場結構影響與污染源洩漏之關聯性。綜合9間大學9間實驗室現場氣櫃及3種不同廠商設計型式之氣櫃，實驗結果發現污染源洩漏相當嚴重的位置，集中於櫃門側邊及門檻附近區域及操作人員身體產生尾流區域附近，與所觀察到的大循環式迴流區具重要之關聯性，推論污染源洩漏機制係由於氣流通過櫃門側邊及門檻附近區域所產生的速度擾動及與時間息息相關的非平衡性的渦流之交互影響，藉由動量及能量交換，引起相當不穩定且混亂的擾流型態，污染源透過此種擾流擴散方式運動比僅透過分子擴散或布朗分子擴散具重要意義。另以假人模擬氣櫃操作人員真實狀況，由於氣流經過假人身體的邊界層形成的尾流區，易使得污染源以迴流方式流入假人呼吸區域，且與氣櫃內部與介面的污染源進行氣體混合交換，而形成複雜紊流三維結構流場，明顯增加污染源暴露濃度，此上述兩種效應對於評估氣櫃操作人員真實污染源暴露狀況，具相當重要意義。研究結論評估實驗室氣櫃性能的良劣，應全面性評估氣櫃櫃面每一局部位置的濃度分佈情形結合假人尾流效應，結合探究氣櫃之流場分佈，對於實驗室操作人員暴露風險具重要意義。以僅量測模擬操作人員的假人呼吸區域污染源濃度並不是一評估氣櫃整體暴露風險的最佳方式。本研究開發改良式測試方法結合局部區域與假人身體效應作為評估氣櫃整體暴露風險之建議方法。
櫃門迅速開關及有側流時，大量的紊流渦漩伴隨噴流式氣流明顯的將櫃內污染物攜出至外界。此紊流式混合延伸及擴張大量的旋轉式紊流渦漩的範圍及強度，於瞬間即引起嚴重櫃內污染源之洩露。此流場觀察結果與追蹤氣體測試呈現一致的關聯性。
本研究參酌上述結果以氣動力學觀點檢視氣櫃的設計對性能的影響，並發展新式改良型噴流阻截式氣櫃，利用於氣櫃內部與氣櫃外面介面增設新鮮空氣供氣機制，引用吹吸式技術，包圍及阻斷非屬理想的傳統型氣櫃於櫃門側邊及門檻附近區域及操作人員身體尾流區所產生的渦流，就不同的吹吸速度決定流場模態分區，做為包覆渦流式氣櫃操作最佳化的參考指標。在此種操作模態下，原來傳統性氣櫃由於複雜、不穩定的紊流型態形成三維複雜的大迴流區域，已明顯被包覆，大幅度改善傳統型氣櫃於流體力學上之缺點。

關鍵字

實驗室氣櫃 ； 面速度 ； 氣櫃性能 ； 流場可視化 ； 追蹤氣體量測 ； 氣動學

並列摘要

A laboratory fume hood is a three-sided enclosure with an adjustable front opening. It is designed to capture, contain and exhaust the fumes generated inside its enclosure. Traditionally, the ability of the laboratory fume hood to capture and exhaust contaminants is often equated to its face velocity. However, many reports suggest that maintaining a specific face velocity does not guarantee that a fume hood will contain harmful chemical fumes. Actually, no correlation between face velocity and containment was observed. Important parameters, such as geometry of the hood, interior equipment parameters, airflow rates, air currents, frequent sash opening and closing, cross-drafts, affect containment ability of laboratory fume hoods.
In order to speculate the physical mechanisms of the contaminant dispersion and leakage during the ventilation process through a laboratory fume hood, the complicated three-dimensional flow patterns and the real-time tracer gas leakage are studied via the laser-assisted flow visualization method and the standard/special gas sampling technique, respectively. The results of the experiment establish the fact that the large-scale vortex structures occurring near the perimeter edge could induce strong turbulence and therefore enhance dispersion of the recirculated contaminant. In the vicinity of the near-wake region of the manikin, large recirculation zones and wavy structures are also identified. These phenomena are in agreement with the high degree of contaminant leakage. The recirculation zone could be attributed to the complex interaction of the various large vortices that consume large amounts of mechanical energy and create fluctuations in velocities. The interaction between time-dependent nonequilibrium vortex and velocity fluctuations is an essential mechanism to create and maintain chaotic and unstable turbulence, this interaction may easily draw contaminants from elsewhere into the vortices. This effect of turbulent dispersion on the contaminant transport strongly relies on the mixing behavior of the complex flow and the concentration of diffusing species that is much greater than the molecular diffusion.
It is argued that an effective method for evaluating the laboratory fume hoods should be based on the aerodynamic features and multi-point leakage detections instead of the breathing-zone sampling alone. A modified test method combined with the region-by-region approach at the presence of the manikin showed substantially different results of the containment.
Large-scale turbulent eddies accompanied with the jet-like currents obviously bring large amount of in-hood smoke out to the atmosphere during the sash movement and the walk-bys. The turbulent mixing extends the size and the strength of the large-scale eddy circulations, and predominantly contributes to the mechanism that causes the severe spread of contaminant leakage in few seconds. The tracer gas tests show consistent containment results with the flow visualization findings. The temporally evolving large-scale turbulent eddies induced by the sash movement and the walk-bys cause substantially high contaminant leakage to the environment and the breathing zone of the operator.
By employing the concept of push-pull isolation, a vortex-isolated type of fume hood is constructed and studied by using the laser-light-sheet-assisted smoke flow visualization method. Results of the flow patterns reveal that the up-blow type fume hood has a much improved flow patterns when compared with the traditional one. No global recirculations are found in the hood. No apparent traces of the in-hood released smoke particles are entrained out of the hood and go into these local recirculation bubbles. Diagnostics of the flow patterns and the experiments of the tracer-gas concentration measurements present extra-ordinarily satisfactory results.

並列關鍵字

Laboratory fume hood ； Face velocity ； Performance ； Cross- Draft ； Flow visualization ； Tracer gas technique ； Vortex ； Wake

參考文獻

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51. Huang, R. F., Lin, S. Y., Jan, S. Y., Hsieh, R. H., Chen, Y. K., Chen, C. W., Yeh, W. Y., Chang, C. P., Shih, T. S., and Chen, C. C., “Aerodynamic Characteristics and Design Guidelines of Push-Pull Ventilation Systems ,” Annals of Occupational Hygiene, 49(1):1-15, 2005.

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