| 摘要: | 摘 要 本研究目標之一是建構一個以瑞士捲燃燒器為基礎的小型省能熱水器，藉由燃燒器幾何形狀與水管排列方式的不同，針對熱水器的出口水溫變化與其熱效率進行量測與分析。瑞士捲燃燒器是應用熱再循環原理，嚐試將熱損失減到最小以提高燃燒效率，並達到超焓燃燒之目的。本實驗採用丙烷和空氣預混燃氣作為燃料，預混燃氣沿著瑞士捲燃燒器的進氣流道進入燃燒室中心，引燃後所產生的高溫生成物沿排氣流道流出。水管管路則是沿著燃燒器內部的排氣流道上方捲繞至燃燒室中心，並採用熱傳導的方式加熱水溫。本研究另一目標則是結合熱再循環與觸媒燃燒兩大技術，利用觸媒燃燒屬於低溫表面反應的特性，建立一個能夠長時間提供穩定熱源的小型超焓燃燒器。 有關省能熱水器的研究，在不同的水流量Qw、燃氣流道雷諾數Ref = VfD/ν、水管層數(lay)等不同操作條件，針對水溫與熱效率的影響進行分析討論，Vf為燃氣流道流速，D為進氣流道寬度和ν為流體的運動粘滯係數。實驗結果顯示，在固定的預熱時間與當量比(equivalence ratio, ψ)之下，熱水器的出口水溫Tw-out與熱效率h會隨著管路水流量的增加而減低，而燃氣流道雷諾數的提高，亦會有相同的趨勢，例如在lay = 1、Qw = 2.0 L/min的操作條件下，流道流速從Ref = 503提升到Ref = 754，熱效率會從h ＝ 80.1%降低到h ＝ 37.2%。此外在相同的操作條件下，增加水管層數能夠增加出口水溫，對於熱效率也有所提升，在Ref = 503、Qw = 2.0L/min時，雙層水管的熱效率會比單層水管高出10%以上。 有關觸媒燃燒實驗，我們改變不同實驗條件，來探討其對觸媒溫度與污染物排放的影響。結果證實觸媒段的出口溫度Tc-out會隨著流道圈數N與觸媒反應總表面積的增加而上升，若是提高Ref、觸媒段長度Lc，則出口溫度會有下降的情形發生。在使用鈀觸媒的情形下，觸媒燃燒器的臨界貧油當量比在ψc =0.06時仍然可以很穩定地產生放熱反應，大幅地延伸了傳統的貧油可燃極限(丙烷的貧油可燃極限為ψ= 0.57)。實驗中針對不同種類與幾何形式的觸媒進行比較，發現到不論是在出口溫度、貧油可燃極限、污染物排放等量測結果，蜂巢式鈀觸媒表現皆優於白金觸媒，而顆粒狀白金觸媒因反應表面積較少，故表現結果最不理想。整個觸媒實驗過程大多屬於低溫燃燒反應(<800℃)，因此幾乎量測不到NOx的濃度，而CO的排放濃度在ψ= 0.4以下皆低於20 ppm(ψ= 0.2時CO僅有1 ppm)。在ψ= 0.5、Ref = 377的條件下進行觸媒的耐久性測試，發現到隨著反應時間的增加，觸媒段出口溫度會有下降的趨勢。利用掃描式電子顯微鏡(SEM)拍攝觸媒基材表面顆粒的分佈情形，觀察到觸媒燒結的部份會隨操作時間的增加而增加，在t=120min、ψ= 0.2?0.4的條件下，觸媒基材表面的顆粒因受溫度的影響，而會有燒結現象，在白金觸媒部份甚至出現整塊燒結的情形，而在t=240min、ψ= 0.2?0.4的條件下，兩種觸媒都有整片燒結的情形發生，圖片中的龜裂現象，研判是觸媒在實驗結束後因冷卻收縮不均而產生的。 This thesis includes two parts. The first part is to design a small energy-saving geyser using the concept of the Swiss-roll burner. We measure the outlet water temperature and analyze the thermal efficiency of geyser by changing the geometry of the burner as well as varying the arrangement of the water tube. The Swiss-roll burner employs the principle of heat recirculation to minimize heat losses, to increase burning efficiency, and thus to make excess enthalpy burning possible. The premixed fuels used in the experiments are propane and air mixtures. 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. The second part of the thesis is to fabricate a small excess enthalpy burner that can supply a stable heat source using catalytic combustion, which is attempted to generate electricity in the future. Concerning the study of the energy-saving geyser, we varied the water tube layer number (lay), the water volume flow rate (Qw), and the flow Reynolds number (Ref =VfD/ν) to evaluate the influence of these parameters on the water outlet temperature (Tw-out) and the corresponding thermal efficiency (η) of the geyser, where Vf is the fuel mixture’s mean velocity, D is the reactant channel’s width, andνis the reactant kinematic viscosity. It is found that Tw-out and η are decreased when Qw and/or Ref are increased. For examples, when the value of Ref increases from 503 to 754, η reduces from 80.1% to 37.2% for the conditions of lay = 1 and Qw = 2.0 liter/min. When lay increases from 1 to 2, η can increase up to 10% at Ref = 503 and Qw = 2.0 liter/min. For the catalytic burner, we varied the operating conditions to measure temperature distributions and the emissions. The results indicate that the catalytic outlet temperature Tc-out (just outside the honeycomb catalyst) is increasing when the number of rolls of the burner N is increasing. If we increase Ref and the length of catalyst Lc, Tc-out will decrease. We also found that the Pd catalytic burner can be operated at a critical equivalence ratio c = 0.06, which is much less than the common lean flammability limit of C3H8/air mixtures where = 0.57. The honeycomb catalytic burn is much better than that using Pt pellets. For emission measurements, the NOx emission is below 1 ppm because the catalytic experiment is operated at low combustion temperature (< 800℃). The concentrations of CO are less than 20 ppm at < 0.4. If = 0.2, the concentrations of CO are no more than 1 ppm. For the test of durable ability of the catalytic burner, the results show that Tc-out is decreased by increasing the operating time at fixed = 0.5 and Ref = 377 conditions. Using the scanning electron microscope (SEM) to take the particle surface of catalytic substrate, we found that the scope of catalytic sinter is increased by increasing the operating time. The particle of catalytic substrate surface is obviously sintered by high temperature (T up to 1000℃) after 120min operations at = 0.2 ~ 0.4 conditions. On the surface of Pt catalytic, we found that the lump sinter is produced. Both Pt and Pd catalytic surface are forming great range of sinter when the operation time is up to 4 hours at = 0.2 ~ 0.4 for highest temperature up to 1000℃. Furthermore, we found that the chap phenomenon on the catalytic surface may be formed by the unbalanced cooling shrink after experiment. In the future, we shall build a small catalytic burner together with thermoelectrical materials to generate electricity. |
