| 摘要: | 本研究利用熱再循環燃燒技術，發展一低氮氧化物燃燒器，並首次搭配CFD-RC 數值分析軟體進行瑞士捲燃燒器數值模型的建立，結合丙烷氣化學反應機制進行燃燒反應模擬。在實驗量測上，沿著瑞士捲進氣流道到燃燒室中心區，再到出口流道，共擺放了9 支R 型和2 支K 型熱電偶，以掌握1.5 圈瑞士捲燃燒器內溫度變化之情形並評估其熱再循環率(HR)，HR 定義為循環熱與燃料化學熱加上循環熱之比。實驗結果顯示，在當量比φ＝0.5，燃氣雷諾數(Re)為60 ~ 880 範圍內，捲式燃燒器內部所有熱電偶之溫度值會隨著Re 增加而增加，且最大溫度量測值均發生於特別設計之燃燒室中心，這表示預混丙烷焰可被穩定地駐留於燃燒室中心。我們進一步分析Re 對HR 之影響，發現HR 會隨著Re 增加而增加，當Re 從60 增加到 880 時，HR 則從11%增加到25%。藉由熱傳導的作用，燃燒器的壁面溫度也會上升，同時單位時間內有更多的預混燃氣在常溫狀態下進入燃燒器，因此壁面與燃氣間的溫度梯度越大，造成對流效應的影響越大，對於預熱未反應燃氣的效果越好，故HR 越大。在生成物排放量測上，當φ＝0.5 時，於Re = 380 ~ 750 範圍內，所量得NOx 之濃度均在10 ppm 以下，而CO 濃度則會隨著Re 增加而有增加的趨勢，判斷原因有二：一為Re 越大，使得CO 生成物滯留在燃燒器內部的時間就越短，無法有充分的時間進行轉化成CO2 的過程，二是在燃燒室維持高溫的情形下，有少部分CO2 會轉為CO(吸熱反應)。數值分析方面，針對在Re= 630 和φ＝0.5條件下，進行2-D 數值模擬，並與實驗結果作比較。有關數值模擬考慮絕熱和非絕熱兩種情況，其結果與實驗結果，雖然在定性上溫度分佈之趨勢是相同的，但在定量方面則仍有不小的差異，顯示2-D 的數值模型無法完全的模擬出實際的燃燒特性。再者，非結構性網格模型的採用和化學反應步驟的過於簡化，也會影響模擬結果之準確性，這些均是未來數值模擬應努力改善的重點。在應用方面，結合1.5 圈捲式燃燒器(約8×８cm2 大小)和多片熱電材料，成功地建構一小型熱電產生器，在串聯兩片熱電材料下(每片尺寸: 3×3 cm2)，可輕易獲得1.6W 的穩定輸出電功率，可使數個小功率馬達或燈泡長時間運轉和發光。 This study applies heat-recirculation technology to develop a low NOx burner. A commercial numerical program (CFD-RC) is used as a design tool to model chemical reacting flow using propane/air mixtures in a Swiss-roll burner (SRB). Measurements of the temperature distributions, heat-recirculation rate, and concentrations of exhaust gas in the SRB are carried. We employ 2 K-type and 9 R-type thermocouples to measure temperature distributions of the SRB and estimate the heat-recirculation rate (HR=Qr×100%/(Qi +Qr)), where Qr is the recirculation heat per unit time and Qi is the chemical heat of the fuel per unit time. The experimental result shows, φ =0.5 and the Reynolds number (Re) ranging from 60 to 880, values of the temperature distribution in SRB increase as Re increases. The maximum temperature occurs in the center of combustor, proving that C3H8 /air premixed flames can be stabilized in the combustion zone. We analyze the effect of Re on HR, for which HR increases as Re increases. As Re increases from 60 to 880, the percentage of HR increases from 11% to 25%. As Re increases, the wall temperature increases and more reactants with ambient temperature enter into the SRB. Therefore, the temperature gradient between the mixture and the walls is enhanced with increasing Re. This promotes the convective heat transfer and thus increasing HR. Measurements of emissions show that NOx concentrations are less 10 ppm at φ =0.5 for Re = 60~880. On the other hand, the concentrations of CO increase as Re increases. This is probably because CO has insufficient time to react to CO2 due to large Re, or the backward reaction step for CO2 to CO occurs during high temperature in the combustion zone. For numerical simulations, only 2-D simulations are carried out to compare with experimental result for Re = 630 at φ =0.5. The comparison shows that the temperature distribution between numerical and experimental results has similar trend, but there are quantitative differences due to 2-D numerical simulation, the unstructured grid model, and the overly simplified chemical reaction steps. Finally, as the goal of the present work, we combine a series of Bismuth Telluride thermoelectric (TE) materials with the aforementioned SRB to successfully establish a small TE power generator. The result indicates that only two TE chips, each with 3×3 cm2, can easily produce more than 1.6 watts for lighting several small bulbs constantly. |
