| 摘要: | 本論文於先前已建立之大型可產生近似等向性紊流場的十字型風扇擾動雙腔體燃燒器中進行實驗，使用體積比45%氫氣/55%氨氣和空氣預混燃氣並操作於當量比 = 0.8，研究目的在於探討固定流場雷諾數(ReT,flow= uLI/ , u為均方根紊流擾動速度、LI為紊流積分尺度、為運動黏滯係數)條件下，壓力變化對層流及紊流火焰速度的影響。由於先前本實驗室已對此問題進行過甲烷/空氣、合成氣/空氣混合燃氣的探討，且已知甲烷/空氣在 = 0.8時層流火焰速度(SL)約為26 cm/s，故盡量挑選在相同當量比SL與其相近的氫/氨/空氣混合比例(SL ≈ 29 cm/s)來進行比較。實驗結果顯示，在1-5 atm範圍內，所有測試的流場雷諾數條件(ReT,flow = 6700、9100、11600、14200)，紊流與層流火焰速度皆隨壓力升高而下降，其中層流火焰速度與壓力的關係為SL ~ p-0.4，而紊流火焰速度(ST)隨不同雷諾數則有不同下降幅度(ST ~ p-n, n = 0.48~0.61)。此ST ~ p-n的結果，與一般在固定u時，ST會隨壓力上升而上升相反，後者是因壓力上升，運動黏滯係數下降造成ReT,flow上升，並且火焰厚度會隨壓力增加而變薄，導致火焰不穩定性增強。另一方面，ST隨壓力下降的幅度均大於SL，因此ST/SL亦隨壓力升高而呈下降的趨勢。此趨勢在1-5 atm範圍內與甲烷/空氣混合燃氣略有差異，但與氫/一氧化碳/空氣混合燃氣則相符。針對此現象，本論文推測由於壓力升高時H自由基的三體終止反應H + O2 (+M) → HO2 (+M)會大幅加劇，加上紊流場的影響，因此當H自由基受壓力抑制而導致局部反應區活性下降，整體紊流火焰速度將同步受限，這使得在高壓條件下，紊流火焰速度的衰減程度大於層流火焰速度的變化。最後，本論文結合本實驗室先前相同混合燃氣已有數據，透過以下四個不同團隊所提出之一般通式進行正規化分析: (1) Chaudhuri et al. (2012)； ST,c̅=0.5/SL = 0.438(ReT,flame)0.5(R2 = 0.77)；(2) Shy et al. (2019)； ST,c̅=0.5/u′ = 0.37(DaLe-1)0.5(R2 = 0.99)；(3) Wang et al. (2020)； ST,c̅=0.5/ SL−1 = 0.178(ReT,flame Le-2)0.56(R2 = 0.77)；(4) Lhuillier et al. (2021)；(ST,c̅=0.5/SL)(1/Da) = 2.27Ka0.91(R2 = 0.97)，其中c̅為火焰平均傳遞變數(mean progress variable)，相關無因次參數定義如下: ReT,flame = (urms/SL)(〈R〉/L)、Da = (SL/urms)(LI/L)、Ka = (urms/SL)1.5(LI/L)-0.5， R2為coefficient of determination，而R2大於0.7，表示這些通式皆具有不錯的正規化擬合程度，其中又以Shy及Lhuillier團隊提出的通式有最佳的擬合程度。;In this study, experiments are conducted in a previously established large dual-chamber fan-stirred cruciform burner capable of generating a near-isotropic turbulence field. The mixture consists of 45% H₂/55% NH₃/air in volume percentages at an equivalence ratio of ϕ = 0.8. The objective is to investigate the effect of pressure on laminar and turbulent flame speeds under constant flow Reynolds number conditions (ReT,flow= u′LI/ν, where u′ is the root-mean-square turbulent velocity fluctuation, LI is the integral length scale of turbulence, and ν is the kinematic viscosity of reactants). Since previous studies in our laboratory have examined lean CH4/air and syngas/air mixtures, where the laminar flame speed (SL) of CH₄/air at ϕ = 0.8 is approximately 26 cm/s, the present study selects lean premixed 45% H₂/55% NH₃/air flame with a similar SL ≈ 29 cm/s for comparison. The results show that within 1-5 atm, both laminar and turbulent flame speeds decrease with increasing pressure for all tested ReT,flow values of 6700, 9100, 11600, 14200. The pressure dependences follow SL ~ p-0.4 and ST ~ p-n, where n ranges from 0.48 to 0.61 depending on ReT,flow. Such results are different from the conventional observations under constant u′ conditions, where ST increases with pressure due to the enhancement of flame instabilities through thinner flames and the concurrent increase in ReT,flow resulting from reduced kinematic viscosity. Moreover, the decrease in ST with pressure is more pronounced than that in SL, resulting in a decreasing ST/SL ratio as pressure increases. This trend differs slightly from that of CH₄/air but agrees well with H₂/CO/air mixtures. To account for this phenomenon, this study proposes that under high-pressure conditions, the enhanced three-body termination reaction H+ O2 (+M) → HO2 (+M) significantly reduces H-radical concentrations. This effect, when coupled with turbulence that limits the local reaction-zone activity, leads to a stronger decrease in ST than in SL. By incorporating our previous experimental data, these ST/SL data can be merged onto the following four general correlations proposed by (1) Chaudhuri et al. (2012), ST,c̅=0.5/SL = 0.438(ReT,flame)0.5 (R2 = 0.77) (2) Shy et al. (2019), ST,c̅=0.5/u′ = 0.37(DaLe-1)0.5(R2 = 0.99) (3) Wang et al. (2020), ST,c̅=0.5/ SL−1 = 0.178(ReT,flame Le-2)0.56(R2 = 0.77) (4) Lhuillier et al. (2021), (ST,c̅=0.5/SL)(1/Da) = 2.27Ka0.91(R2 = 0.97), where ReT,flame = (urms/SL)(〈R〉/δL), Da = (SL/urms)(LI/δL), and Ka = (urms/SL)1.5(LI/δL)-0.5. All correlations yield the coefficients of determination (R2) above 0.7, indicating reasonable fitting accuracy, where the correlations proposed by Shy et al. and Lhuillier et al. have the highest R2 values. |
