| 摘要: | 本研究自製一小型超焓燃燒器(二維瑞士捲燃燒器，最大直徑7.5cm~21 cm，高度5.0 cm)，作流場觀測、溫度量測和燃燒後污染物排放成份濃度量測，並嚐試應用所獲之知識以研發小體積之省能熱水器。超焓燃燒器是應用熱再循環和熱釋放之熱力學轉換原理，試將熱損失減到最小、提高燃燒效能，達到超貧油燃燒之目的。實驗採用丙烷/空氣預混燃氣，因丙烷為液態比氣態燃氣密度高上千倍，故可大大縮小燃料儲存所須空間。預混燃氣循瑞士捲層層捲道，進入中心燃燒室，引燃燃燒後，高溫之生成物則循另一捲道流出，因反應物捲道與生成物捲道乃環環相繞，故可將大量的生成物熱經熱傳導和熱輻射來加熱反應物，使燃燒可呈超焓狀態。 研究重點是針對瑞士捲燃燒器的幾何形狀和操作參數變化，對於燃燒穩定性、貧油可燃極限(lean flammability limit)和瑞士捲內流道溫度分佈之影響，作系統化分析。驗結果顯示，燃燒器在當量比(equivalence ratio)φ »0.10時仍可持續在燃燒室內燃燒，証明超焓燃燒器可大大延伸貧油可燃極限(傳統丙烷之貧油可燃極限為φ＝0.57)。我們藉由燃燒室液態冷流場模擬和實際燃燒流場的觀測，找到了一較佳的燃燒室設計，使火焰能穩定駐留在燃燒室內燃燒。分別改變化學物理參數(如φ)、流場參數(如燃氣流速Vf、雷諾數Ref 等)和幾何參數(如捲數N、流道間距D)條件，並使用熱再循環率(HR)來作為燃燒器的設計參考依據，結果發現瑞士捲燃燒器之燃燒室內平均最高溫度(Tm)分佈，會隨著加熱時間、N和f等增加而提高，例如φ 從0.30增加到0.50，Tm會從1054oC提高到1282oC，在固定N =3.0，D =1.0cm和加熱時間(t=10 min)條件下。燃燒室火焰穩定性與Ref等參數相關，將詳加討論。在φ =0.40~0.50時，於廢氣出口處量測污染物濃度，發現NOx值均低於10 ppm以下，而CO值約在於40~60 ppm之間，但在臨界貧油當量比(critical equivalence ratio, φc<0.30)時，CO濃度則有明顯增加的趨勢，因此可知在φ = φc之燃燒是不完全和不穩定的。當N值增加及D值減小時， HR值可有效提升，故可進行更貧油燃燒(fc值更小)。 有關省能熱水器之初步測試，是利用水管循瑞士捲燃燒器之生成物捲道上方，捲繞到燃燒室中心區，並採用熱傳導方式加熱水溫，發現加熱效果並不理想。因此，須改變加熱方式，未來擬將仍有300oC左右之燃燒後氣體直接注入水箱中，應可研發出一高省能、小體積(20 cm×20 cm×5 cm)之熱水器。 This thesis investigates experimentally combustion characteristics of a small premixed excess enthalpy burner, a two dimension Swiss-roll burner whit the maximum diameter varying from 7.5~21 cm and with a height of 5.0 cm. We visualize variations of the flow fields the combustion zone of Swiss-roll burners with six different designs. Temperature distributions and pollutant emissions in these excess enthalpy burners are measured quantitatively for the first time to develop a compact, very low fuel consumed water heater. The excess enthalpy burners apply the principle of heat recirculating and the conversion of thermodynamics to minimize heat losses, increase burning efficiency, and make extra lean premixed burning possible. Propane/air mixtures are used, because propane is a liquid, with a density nearly one thousand times greater than that of gaseous fuels such that the space for the fuel storage can be significantly reduced. It is found that the present Swiss-roll burner can be operated at extra lean conditions, where the equivalence ratio f is slightly smaller than 0.1. This φ»0.1 is much much less than the common lean flammability limit of C3H8/air mixtures in which φ»0.57. We also found a better design of the combustion zone of the Swiss-roll burner in which flames can be stabilized in the combustion zone. Using the heat recirculation rate (HR) as a criterion for the performance of the burners, the optimal design of the burners was examined in terms of φ, the fluid velocity Vf, the flow Reynolds number Ref , the number of rolls of the burner N, and the interval channel width of the Swiss-roll burner D. Results show that the mean temperature (Tm) inside the combustion zone is strongly influenced by the operating (heating) time period t, N andf. For examples, when φincreases from 0.30 to 0.50, Tm increases from 1054oC to 1282oC at fixed N=3.0, D=1.0 cm, and t =10 min conditions. Emission measurements on the outlet of the burner show that the concentrations of [NOx] are less than 10 ppm and [CO] »40~60 ppm for any value of φ between 0.4 and 0.5, when φ<0.3, [CO] increases largely, indicating that burning is incomplete and unstable near the critical equivalence ratio (φc). HR increases with N but decreases with D, so that extra lean combustion can be performed at larger N and smaller D. Concerning the preliminary application of the energy saving water heater, we use a long water tube wrapped along the upper product channel of the Swiss roll burner so that the water can be heated via the heat conduction from the high temperature product channel. It is found that this heating arrangement is inefficient. Alternative heating methods have to be considered, such as the direct injection of the exhausted gas (~300oC) into a water tank for developing a compact (20 cm×20 cm×5 cm), high efficiency bath water heater. |
