圓形開口凸緣氣罩控制風速的理論探討

A Theoretical Study on the Control Velocity of Flanged Circular Hoods

氣罩捕集性能取決於氣罩風量及其所產生的風速。氣罩必須在有害物發生源造成足夠的風速方能克服外來氣流的干擾，使氣罩對有害物粒子發揮有效的捕集 作用。根據勢流理論，假設有害物粒子的擴散與慣性可忽略，在給定側風條件 下，可使用數值計算求得氣罩流場內的粒子軌跡。氣罩的有效捕集範圍，或者是捕集區則可由極限軌跡求得。據此可進一步計算不同發生源位置所應具備的氣罩風量與控制風速。本文即根據上述概念探討圓形開口外裝型凸緣氣罩的捕集性能，提出可應用於實際設計使用的設計圖表；同時提出各種狀況（如側風方向無法預知時、發生源位於氣罩中軸時、發生源離開氣罩相當距離時）下的設計要求，以簡化計算結果的應用：並對理論的合理性（如忽略沈降、慣性與擴散效應）進行檢討。

The capture performance of a hood depends on its flow rate and velocity. To capture contaminant particles effectively, the hood-generated velocity at a contaminant source must be large enough to overcome the external wind. Under a given cross-draft, the trajectory of a particle can be calculated numerically by using hood flow field. The theoretical hood flow field is made available from the potential flow theory. The capture zone, within which the particles can be captured by the hood, can then be obtained from the limiting trajectory by neglecting diffusion and dispersion of particles. Subsequently, the hood flow rate and control velocity required to capture the particles released from a contamination at a given position are calculated. Based on above methodology, this paper presents the capture characteristics of external flanged hoods with circular opening. The calculation results are presented in chart and formulas for design use. The design requirement is further simplified under certain situations such as unknown direction of cross-draft or particle lateral initial velocity, location of the contaminant source at hood center axis, distance between the contaminant source and the hood Assumptions of the theory are also discussed.