創新開槽設計對突張室非定常流場與固態燃料燃燒之調控效應

Modulation Effects of Slot-Liner Design on Unsteady Flow and Solid-Fuel Combustion in a Sudden-Expansion Chamber

張明哲(Ming-Che Chang)

指導教授 ： 陳理定 ； 楊鏡堂

國立清華大學/工學院/動力機械工程學系/碩士(2010年)

摘要

本文提出創新突張室開槽設計，可有效調控突張室內非定常流場結構及固態燃料之燃燒機制。於不同的開槽設置條件下，本文利用粒子影像測速儀量測突張室內非定常流場結構，並同時以高速攝影機紀錄分析固態燃料於開槽突張室內之燃燒情形。研究結果顯示：在無燃燒的常溫流場中，改變開槽位置可有效調控主迴流區長度及角落渦流(corner eddy)尺度；主迴流區上緣剪流層渦流泡因受到溝槽之牽引，致使再接觸點(reattachment point)的擺盪頻率大幅降低；槽內氣流沿下壁面回流至上游階梯角落，形成一股回饋噴流(feedback-jet)，導致角落渦流與主迴流區之強度增加，並使氣流在主迴流區的滯留時間增長，迴流量因而增加。
本文以高速高溫氣流(流速U0 = 31 m/s, 溫度T0 = 800-850 oC, 氧濃度[O2] ~ 11.7 % )流經背向階梯下游之固態燃料板(PMMA)，解析固態燃料在燃燒室中的非穩態引燃過程與火焰傳播模式。在無開槽的情況下，引燃點位於流場再接觸區之下游，會產生逆向火焰傳播，由於逆向火焰傳播承受了較大的對流熱損失，故火焰發展時間較順向火焰傳播為晚。相較之下，本文所提出之創新開槽突張室設計，能夠有效調控流場主迴流區之結構，並使燃氣在主迴流區的滯留時間增長，提升燃氣與空氣混合程度，增加引燃點出現於迴流區內的機率，有效調控火焰傳播機制並減少引燃延遲時間，固態燃料的燃燒效能因而獲得提升。
本文之研究結果，證明並凸顯此創新開槽設計之調控功能與潛在多樣性，將有益於發展固態燃料不穩定燃燒之控制技術，亦能對未來能源領域固態廢棄物衍生燃料之燃燒研究建構基礎，在學術及工程上皆有所貢獻。

關鍵字

突張室 ； 開槽設置 ； 非定常流場 ； 固態燃料(聚甲基丙烯酸甲酯-壓克力) ； 固態燃料衝壓引擎 ； 粒子影像速度 ； 非預混火焰 ； 引燃 ； 火焰傳播 ； 火焰與渦流交互作用 ； 渦流結構 ； 回饋噴流

並列摘要

In this thesis, a novel design associated with a slot-liner in a sudden-expansion chamber is introduced. The slot-liner design effectively modulates the unsteady flow structures and combustion mechanisms of solid-fuel within a sudden-expansion chamber. For distinct slot-liner design, unsteady flow fields within the slotted sudden-expansion chamber were experimentally quantified via digital particle image velocimetry; a high speed camera was concurrently deployed to photograph flame structures, facilitating combustion status diagnosis. The experimental results reveal that for an uncombusted sudden-expansion chamber, alteration of the slot location is able to modulate the length of the main recirculation zone as well as the dimension of the corner eddy. Due to attraction and restriction effects of the slot on the vortex bubbles distributed along the shear layer immediately above the main recirculation zone, the vibration frequency of the reattachment point is significantly reduced. Air stream drawn into the slot was found to flow upstream and be ejected from the step corner, forming a feedback-jet. This feedback-jet augments the strength of the corner eddy and the main recirculation zone, lengthening the residual time of the air stream within the main recirculation zone, and consequently enlarging the amount of recirculating air.
High-speed and high-temperature air stream (velocity U0=31 m/s, temperature T0 =800-850 oC, oxygen concentration [O2] ~ 11.7 %) was forced to convect over a solid-fuel (PMMA) plate placed downstream of a backward facing step, so as to examine unsteady ignition transients of the solid-fuel as well as the flame propagation modes in the combustion chamber. For cases without the slot arrangement, we found the flame-ignition point is situated downstream of the reattachment zone, incurring a counter-stream flame propagation. Since the counter-stream flame propagation is subject to a stronger heat loss due to flow convection, the counter-stream flame propagation occurs chronologically later than the co-stream flame propagation. By contrast, the slot-liner design devised in this work is capable of effectively modulating the flow structure of the main recirculation zone. The residual time of fuel gas within the main recirculation zone is elongated; accordingly, the extent of mixing of air and the fuel gas is enhanced. The possibility for the flame ignition to occur within the recirculation zone is increased as well. Flame-propagation dynamics associated with solid-fuel combustion is hence well controllable. The flame ignition delay is diminished and the combustion efficiency of the solid fuel is significantly improved.
Research results in this work verify and demonstrate the modulation capability and potential diversity of the slot-liner design, which is beneficial for the development of control techniques associated with unsteady combustion of solid fuels. This work also provides a research foundation for future studies on solid fuels derived from wastes. To conclude, this work has contributions from both academic and engineering perspectives.

並列關鍵字

Sudden-expansion chamber ； Slot arrangement ； Unsteady flow ； Solid fuel (Polymethylmethacrylate, PMMA) ； Solid fuel ramjet ； Digital particle image velocimetry(DPIV) ； Non-premixed combustion ； Ignition ； Flame spread ； Flame-vortex interactions ； Vortical structure ； Feedback-jet

參考文獻

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