| 摘要: | 在1995, Bédat & Cheng (B & C)提出一弱漩渦燃燒器，它具有增加火焰穩定性、降低NOx生成及增加熱效率等優點。本研究以B & C之設計為參考範本，實作一弱漩渦噴流燃燒器(low swirl jet burner, LSJB)，量測其紊流燃燒速度(turbulent burning velocities, ST)並首度探討其加氫效應。實驗使用貧油甲烷添加不同體積比例之氫氣(0 ~ 30%)為預混燃料，以高速質點影像測速儀(particle imaging velocimetry, PIV)，定量量測LSJB沿噴嘴出口到弱漩渦流場所形成之碗狀預混火焰最低點之平均速度(U)和紊流強度(turbulent intensity, u')以在穩定碗狀火焰之最底部處的U值定義為ST值，並確認LSJB在不同漩渦數(swirl number, S =方位角方向與軸向動量之軸通量比)和噴嘴出口雷諾數(Rej)之火焰跳脫與吹熄的操作範圍。我們首次運用小波轉換(wavelet transform, WT)分析LSJB速度場資料，以獲取重要之時空物理尺度，並使用氣體分析儀量測燃燒後生成物之[NOx]與[CO]，以瞭解掌握貧油預混燃燒加氫之技術。實驗結果顯示，隨Rej增加，會使LSJB火焰能穩定操作之S值範圍變窄。由未反應與反應之冷熱流場PIV量測，發現在碗狀火焰底部處之冷流場U值約為熱流場之兩倍，但u'值約略相同。由熱流場之機率密度函數與能量頻譜分析，顯示此一化學反應流場並非等向性紊流場，此點與B & C所得結果不同。並由WT分析碗狀火焰底部處區域之流場軸向速度(u)與徑向速度(v)的資料，獲取特徵長度與時間尺度，分析噴嘴中心往下游方向之u與v發現在所得之泰勒長度尺度與火焰平均傳遞變數(mean progress variable)厚度尺度(即火焰震盪範圍)相同，約0.8 cm左右。有關特徵時間尺度，軸向與徑向約略相同。在ST值量測方面，發現ST值及u'值會隨著Rej增加而增加，至少在Rej = 4700 ~ 11000的範圍內。但在任一固定Rej值下，ST與u'值有不小的誤差，尤其是當S值在吹熄極限附近，所以LSJB並非ㄧ理想量測ST之裝置。有關加氫效應，隨著加氫量少量的增加，[NOx]有些微增加的趨勢，而[CO]值則有劇減的趨勢，且ST值均較未加氫時高，例如加氫量由0%增加到30%，ST增加幅度可達約30%，但u'值增加幅度均小於10%，[NOx]則從0 ppm增加到1.7 ppm，以及[CO]從5500 ppm劇減至2500 ppm。因此本研究之LSJB並非ㄧ準確量測ST之標準設備，但是LSJB確實為一極佳之低氮氧化物燃燒器，可廣泛的實際應用，如氣渦輪機發電系統。 In 1995, Bédat & Cheng (B & C) proposed a low swirl burner which can enhance flame stability, reduce NOx formation and increase thermal efficiency. In this study we apply the concept of B & C to design and make a low swirl jet burner (LSJB) for measurements of turbulent burning velocities (ST) and investigation of the effect of hydrogen addition for the first time. Lean methane doping with 0 ~ 30% hydrogen by volume as a fuel is used. Quantitative measurements of velocity fields including both average velocities (U) and r.m.s turbulent intensities (u') from the nozzle exit to the stabilized bowl-shape flame and above are obtained using high-speed particle imaging velocimetry (PIV). The value of ST is chosen as the value of U just at the bottom position of the stabilized bowl-shape flame. Identification of the stable operation ranges between flashback and blowoff limits of the LSJB is also made over a range of a swirling number (S) defined as the ratio of the axial flux of the angular momentum divided by the radius of the burner exit to the axial flux of the linear momentum, and the jet Reynolds number (Rej). We apply the wavelet transform (WT) to analyze spatiotemporal scales of the LSJB using these PIV time sequent data for the first time. [NOx] and [CO] in the products are measured by the gas analyzer. Thus, the knowledge of hydrogen addition on lean premixed combustion can be learned. The results show that the stable combustion ranges of S for the LSJB becomes narrower with increasing Rej. PIV measurements indicate that at the same lowermost point of the bowl-shaped flame, U of nonreacting cold flow is twice more in magnitude than that of reacting hot flow with about the same u'. Probability density functions and energy spectra of reacting hot flows indicate that the present flow is not isotropic and this result differs with that found by B & C. The spatial characteristic length scales, approximate to the Taylor length scale, determined from the axial and radial velocity data along the nozzle axis using WT are found to be approximately the same as the thicknesses of a mean progress variable (flame brush thickness), about 0.8 cm. Temporal characteristic scales are also identified by the WT analysis and both values are roughly the same in both axial and radial directions. The measured values of ST and u' are found to increase with increasing Rej, at least in the range of Rej = 4,700 ~ 11,000, where errors of values of ST and u' at a fixed Rej cannot be neglected especially when S approaching blowoff limits. Moreover, by adding a small amount of hydrogen [CO] can be significantly reduced with only a slightly increase of [NOx]. As the hydrogen addition increases from 0% to 30%, values of ST can be increased 30% more the increases of u' smaller than 10%, and [NOx] and [CO] vary from 0 to 1.7 ppm and from 5,500 to 2,500 ppm, respectively. Hence, it is concluded that the present LSJB cannot be as a benchmark device for accurate measurements of ST, but it is indeed an excellent low-NOx burner which can be need in many practical applications such as gas turbines for electricity generation. |
