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標題: | 串列雙多孔性圓柱非預混焰之熱流場結構與熄滅極限 Thermal Flow Structure and Extinction Limit of Non-Premixed Flame on Dual Porous Cylinders Arranged in Tandem |
作者: | Shan-Jen Chen 陳善任 |
指導教授: | 潘國隆(Kuo-Long Pan) |
關鍵字: | 串列擺置,多孔性圓柱,紋影法,流場控制,熱交互作用,熄滅極限, Tandem arrangement,Porous cylindrical burner,Schlieren,Flow control,Thermal interaction,Extinction limit, |
出版年 : | 2018 |
學位: | 碩士 |
摘要: | 本研究藉串列雙圓柱做流場控制,以改進非預混燃燒之單多孔圓柱在接近熄滅時,尾焰受尾流區過量冷空氣與渦旋擾動,造成局部過於貧油或反應物滯留時間不足而熄滅之現象,並探討與單圓柱相比其能提升熄滅極限之能力。研究中以紋影法觀察圓柱暫態熱流場與火焰型態之關係,並藉化學螢光與沿尾流區中心線溫度之量測,以了解反應區條件;亦藉Fluent搭配SST k-ω紊流模型模擬協助分析不同距徑下之反應流場條件。
在流場結構方面,紋影像顯現串列擺置可抑制或推延尾流渦旋至更下游,使流場不穩定遠離反應區,雙圓柱間隙亦可使反應物躲避高速氣流並提供滯流與混和的空間;在熱交互作用方面,因圓柱間隙流場相對穩定,其可蓄存高溫未燃氣使雙圓柱之反應區進行燃燒,並藉熱交互作用以互相維持,故可觀察到火焰間歇性連接、再引燃等現象。以兩圓柱軸線間距與圓柱直徑之比值L/D為參數,設定液化石油燃氣噴出速度為Vw = 0.154 cm/s,串列雙圓柱之燃燒在L > 2D時因圓柱間隙過大引入渦旋,故上游圓柱熄滅後無法復燃;1.8 ≤ L ≤ 2D則會出現圓柱間隙內藉擾動將下游圓柱引燃之火苗傳遞至上游進行再引燃,使熄滅的上游圓柱復燃為尾焰;L < 1.8D兩圓柱之反應區不會完全分離,並會同時熄滅。 模擬結果顯示圓柱間隙紊流強度與流速隨間距提升而增加,燃氣濃度則因迴流結構增強而下降,使熄滅極限降低之原因:(1)距徑比過小,因空氣難藉渦旋引入圓柱間隙,反應區移出低速的間隙區域,(2)距徑比過大,因渦旋於間隙內增強,過多冷空氣捲入使熱交互作用式微,反應物滯留時間亦縮短。單與雙圓柱總燃料流率相同的條件下,單圓柱熄滅極限隨燃氣供給量增加而提升,但增長趨勢趨緩,串列組合間隙能藉流場控制與熱交互作用改善其熄滅極限增長趨緩的現象。 The objective of this research is to control the flow field by arranging dual porous cylinders in tandem, thus enhancing the ability of the cylinders from blow-out. The wake flame of a single cylinder near the extinction limit would endure excessive cold air and concomitant fierce turbulence. This will lead to locally fuel-lean condition and insufficient time to react. The capability for tandem array to extend extinction limit was investigated and the results were compared with the usage of a single cylinder. In the experiments, the thermal flow structure of non-premixed flame on dual porous circular cylinders arranged in tandem was visualized by schlieren imaging. The spacing to diameter ratio denoted by L/D is a major parameter in this research. The reaction conditions were studied by chemiluminescnece and temperature measurement along the center line of wake region. Furthermore, the software Fluent with SST k-ω turbulence model was used to analyze the reacting flow field with distinct spacing of cylinders. As to flow structure, not only the inhibition of vortices in wake region shown by schlieren imaging, but a shield for reactants in the gap and downstream cylinder from the high-speed airflow are achieved by tandem array. Also, the gap between the two cylinders created a confined space, which provided a region for fuel and air to stay and mix. As to the thermal interaction effect, reaction zones of the two cylinders could thermally support each other in the gap. The region was relatively stable in the whole flow field and was able to secure the hot unburnt gases. The phenomenon of intermittent connection of flames or re-ignition was thus observed. The fuel (LPG) ejection velocity Vw was set at 0.154 cm/s. When the two cylinders in tandem array are burning, the upstream one could not be reignited due to the fierce vortices in the gap with L > 2D. For 1.8 ≤ L/D ≤ 2, the upstream cylinder could be reignited by the flame kernel which is ignited by downstream cylinder and is moving from downstream to upstream. This process would make the upstream cylinder recover back to wake flame from extinction. For L < 1.8D, reaction zones of the two cylinders will not separate thoroughly and will be blown out simultaneously. Results of simulation show that the turbulence intensity and flow speed in the gap enhance with the increase of L/D. Causes for the decreased extinction limit are: (1) L/D is too small for the air to entrain into the gap by vortices. Therefore, the reaction zone moves outward from the gap where the flow is relatively stable. (2) L/D is too large and the turbulence intensity gets strong. Since excess air is introduced into the gap, the residence time for reactants decreases. The extinction limit of a single cylinder is enhanced with the increase in fuel flow rate, but the increasing trend would gradually slow down. With the same total fuel rate, the tandem array can elongate the residence time for reactants, thus improving the slowdown trend in extinction limit. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/72398 |
DOI: | 10.6342/NTU201803224 |
全文授權: | 有償授權 |
顯示於系所單位: | 機械工程學系 |
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