應用粒子圖像測速於增強康達效應之流場觀測

Abstract

本研究欲透過貼壁噴嘴之康達效應機制，縮小氣流擴散並減緩速度衰減，為了增強康達效應機制，本研究將分析討論不同風口仰角和風口與壁面間距之關係，並在出風口處設置導流板，透過導流板之康達效應可維持更多氣流往壁面流動，增強整體氣流之流動特性，找尋最佳化之設計參數。實驗利用PIV技術針對噴口流場型態進行觀測，同時分析整體流場之速度分佈，透過中心速度衰減常數此無因次參數進行分析比較。結果顯示貼壁噴嘴可使氣流沿著壁面向前流動，並縮小擴散和維持中心速度，其結果之K值亦比自由噴嘴較大1.1倍，同時發現當風管仰角為5°且風口與壁面間距為2 cm時，與當風管仰角為10°且風口與壁面間距為4 cm時，整體結果其中心速度衰減常數有最大值產生，且均比0°仰角之最佳結果較佳，其K值比自由噴嘴較大1.2~1.4倍。顯示透過增加風管仰角並取得最適當之風口與壁面間距時，可使氣流直接往壁面方向流動，增強康達效應機制，使整體氣流有更佳的流動特性表現。分析設置導流板之情形，從PIV流場速度分析可明顯發現受到康達效應影響，氣流具有往板面擠壓的現象，大幅降低氣流向外擴散，減緩中心速度衰減。

This thesis studies the difference of flow pattern in free jet and wall jet. In the wall jet, the airflow will entrain air from surrounding and a lower pressure on the wall side will deflect the jet flow pattern. The effect will limit the spreading angle and slow down the velocity decay, which is known as the Coanda effect. In this study, some designs such as elevation angle of tube and a baffle between jet and wall for enhanced Coanda effect are examined. The centerline velocity decay coefficient (K–value) is presented using an analysis of the flow pattern via particle image velocimetry. The result shows that the K-value of the wall jet can be estimated by multiplying 1.1 relative to that of a free jet. When the elevation angle is 5° with the gap between tube and wall is 2 cm and the elevation angle is 10° with the gap is 4 cm, the K-value shows the maximum and can be estimated by multiplying 1.2~1.4 with that of a free jet. When setting up a baffle between jet and wall, the results show a beneficial outcome for the spreading angle from is appreciably reduced from the flow pattern observation. In summary, the wall jet outperforms free jet as far as spreading flow pattern is concerned. In addition, there also has an opportune gap for each elevation angle of the jet.