Abstract
本文主旨在探討前端開口Ｖ型駐焰器，尾流火焰之駐焰機構與反應流流場之特性。由前端開口進入之流体係以偏折噴流的形式存在尾流中，增強尾流中心區域的紊流強度，此外，此偏折噴流焰的作用，使反應剪流層向前延伸與擴展，提高近尾流場的溫度，故有利於燃燒反應。冷流時受Coanda效應作用若噴流偏折向下翼方向，將延緩下翼剪流層之渦漩捲動，但上、下剪流層渦漩捲動的型態與傳統駐焰器相同，由剪流層外側捲向尾流中心；但反應流中偏折噴流變為噴流火焰，流體受燃燒反應被加速，使得原來剪流層之分離渦漩，被噴流焰所導引之渦漩所抑制，整個迴流渦漩捲動變為沿著噴流火焰由尾流中心捲向上剪流層，迴流區並向上翼之偏移，形成所謂噴流主導型態之流場。由火焰暫態發展的觀測結果發現，傳統Ｖ型駐焰器對稱之剪流層火焰具有週期性的收縮現象，並破壞能量平衡狀態而導致火焰吹熄；前端開口之Ｖ型駐焰器其偏折焰抑制兩翼火焰層大尺度的捲曲，使尾流火焰穩定性較佳。但開口寬度過大時，噴流量增加造成過度之熱損，導致尾流焰局部淬熄。貧油時偏折火焰及其誘發之迴流渦漩，可提供充足的暫留時間，因此，尾流焰於局部吹熄後，新的火焰塊往往由偏折噴流焰處重新引燃並發展，所以貧油極限可視為由偏折噴流火焰所控制；而富油極限時偏折噴流誘發之渦漩，由尾流中心向上剪流層捲動，造成尾流中心區域與上剪流層之紊流強度增大，具有較佳的混合機構與條件，因此，新的火焰塊由上剪流層處重新引燃，所以，富油極限是由反應剪流層所控制。管道流(confined flow)中尾流火焰，因為不穩定之熱釋放與燃燒室或管道中壓力振盪的自然頻率發生偶合現象，造成燃燒不穩定的發生。而開口駐焰器因其偏折噴流與剪流層的交互作用，造成流場迴流區長度與渦漩流逸頻率的改變。受到偏折噴流焰的作用，使得原來發生於傳統駐焰器尾流場之壓力振盪主頻被抑制。The features of flame stabilization and the bl owoff processesof a v-shaped flame holder with as lit have been investigatedthrough measurements of temperature, velocity, turbulenceintensity and tr ansient behavior during flame development.Throug h the Coanda effect, the flow penetrating through theslit generated a diverted jet flame and then i nduced arecirculating vortexbubble. The vortex s hedding behind theslit v-gutter was suppressed be cause the velocity gradients ofthe lower shear la yer were decreased by the diverted jet. Themixin g was strong in the core region of the near wake a nd inthe upper shear layer because of great turbu lence intensity.From inspection of the flame developing proces s, the blowoffmechanism of the slit v-gutter was closely related to thediverted jet flame and jet- induced recirculating vortex. Theblowoff mechanis m was controlled in the lean limit by thediverted jet flame, but in the rich limit by the reacting shear layer behind the v-gutter. The combustion in ducedoscillation was associated with the unsteady heat release. Thepressure spectrum revealed that the dominant frequency of thecombustion induced p ressure oscillation for the regular v-gutter was c orresponding to a standing quarter-wave in the duct. However, the frequency ofthe slit v-gutter supp ressed thisfrequency of the standing quarter-wave, because the heattransfer was increased by the s trong recirculation of the jet-induced vortex and the diverted jet flame.