高壓預混紊流球狀擴張火焰之自我相似性和其火焰速率於不同Lewis數(Le < 1, Le ≈ 1, Le >1);Self-similarity and flame speeds of premixed turbulent spherical expanding flames under elevated pressures at different Lewis numbers (Le < 1, Le ≈ 1, Le > 1)

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Title:	高壓預混紊流球狀擴張火焰之自我相似性和其火焰速率於不同Lewis數(Le < 1, Le ≈ 1, Le >1);Self-similarity and flame speeds of premixed turbulent spherical expanding flames under elevated pressures at different Lewis numbers (Le < 1, Le ≈ 1, Le > 1)
Authors:	陳立龍;Chen,Li-long
Contributors:	能源工程研究所
Keywords:	熱擴散不穩定;預混紊流球狀火焰;自我相似傳播;紊焰傳播速率;火焰紊流雷諾數;thermodiffusive instability;premixed turbulent spherical flame;self-similar propagation;turbulent flame speed;flame turbulent Reynolds number
Date:	2014-08-28
Issue Date:	2014-10-15 15:29:04 (UTC+8)
Publisher:	國立中央大學
Abstract:	本論文定量量測在高壓條件下(p = 1 ~ 5 atm)之預混紊焰的燃燒速率，主要目的，乃為探討熱擴散不穩定性(thermodiffusive instability)，是否會影響由中心引燃向外傳播之預混紊流球狀火焰的自我相似性傳播(self-similar propagation)。此一向外傳播預混紊流球狀火焰之自我相似性，首先由Chaudhri et al. (2012)所提出，他們使用Lewis數Le ≈ 1之甲烷-空氣預混燃氣(當量比 = 0.9)，在不同方均根紊流擾動速度u′和壓力p條件下之所有紊焰燃燒速率(d/dt)，均可用一正規化關係式來表示，即 (d/dt)/S_L^b ≈ 0.102ReT,flame0.54，其中為平均紊焰半徑，t為時間，S_L^b為未經過密度校正之層流燃燒速率，紊焰雷諾數ReT,flame = u′/DT，而DT為熱擴散係數。本實驗使用已建立之高壓雙腔體十字型風扇擾動預混紊流爆炸設施，其可產生近似等向性紊流，並使燃燒實驗可在固定p和u′條件下進行，我們分析三種與空氣預混之不同燃氣，其具有不同Lewis數，分別為Le ≈ 0.76 < 1之合成氣(35%H2/65%CO； = 0.5)、Le ≈ 1之甲烷( = 0.9；與Chaudhri et al.相同)和Le ≈ 1.62 > 1之丙烷( = 0.7)，每一種燃氣均涵蓋相當廣泛範圍之u′ ~ 6 m/s和p = 1 ~ 5 atm。結果顯示，Le數對紊焰燃燒速率有重要之影響，三種不同燃氣與ReT,flame之正規化關係式分別為：Le < 1合成氣為(d/dt)/S_L^b ≈ 0.190ReT,flame0.55；Le ≈ 1甲烷為(d/dt)/S_L^b ≈ 0.116ReT,flame0.54和Le > 1丙烷為(d/dt)/S_L^b ≈ 0.102ReT,flame0.51。分別與甲烷燃氣比較，合成氣正規化紊流燃燒速率(d/dt)/S_L^b值約為甲烷之1.64倍；而丙烷僅約為甲烷之0.88倍。這是由於Le < 1紊焰，除了受到天生存在的流力不穩定性之影響，還會額外受到熱擴散不穩定性之影響。而Le ≈ 1和Le > 1紊焰，僅受流力不穩定性之影響，其(d/dt)/S_L^b值在同ReT,flame值條件下，比Le < 1紊焰低很多。在此，我們提出一以Le數為修正之函數，即f(Le) = 2.15\|Le - 1\|，當Le ≠ 1；而當Le = 1時，f(Le) = 1，則原本相當分散之三條正規化關係式曲線，可合併成一正規化關係式：f(Le)(d/dt)/S_L^b ≈ 0.113ReT,flame0.54。此研究結果，對高壓預混紊流燃燒及其與車用和空用引擎之應用，應有所助益。;This thesis measures quantitatively the turbulent flame speed of premixed flames over an initial pressure range of p = 1 ~ 5 atm. The main objective is to investigate the effect of the thermodiffusive instability on the self-similar propagation of expanding spherical premixed flames. Such a self-similar propagation phenomenon was first found by Chaudhri et al. (2012). In it they measured the turbulent flame speed (d/dt) of unity Lewis number (Le) methane-air mixtures at the equivalence ratio  = 0.9, such that all d/dt data measured at various values of the root-mean-square turbulent fluctuation velocity (u′) and pressures (p) can be represented by a normalized relationship: (d/dt)/S_L^b ≈ 0.102ReT,flame0.54. is the average flame radius, t is time, S_L^b is the laminar burning velocity before density correlation, and flame turbulent Reynolds number ReT,flame= u′/DT where DT is the thermal diffusivity of unburned mixtures. All present experiments are carried out in a recently-built high-pressure, double-chamber, cruciform fan-stirred premixed turbulent explosion facility, capable of generating intense near-isotropic turbulence and making combustion experiments conducted at fixed p and u′ conditions possible. Three different gas fuels/air mixtures with different values of Le are measured, respectively (i) syngas (35%H2/65%CO) at  = 0.5 having Le ≈ 0.76 < 1, (ii) methane CH4 at  = 0.9 with Le ≈ 1 (same as Chaudhri et al. for comparison), and (iii) propane C3H8 at  = 0.7 having Le ≈ 1.62 > 1. Each case covers a wide range of u′ = 1.4 ~ 6 m/s and p = 1 ~ 5 atm. Results show that the effect of Le has an important impact on the turbulent flame speed. The corresponding normalized relationships for the aforesaid three different mixtures were: (d/dt)/S_L^b ≈ 0.190ReT,flame0.55 for Le < 1 syngas flames, d/dt)/S_L^b ≈ 0.116ReT,flame0.54 for Le ≈ 1 methane flames, and (d/dt)/S_L^b ≈ 0.102ReT,flame0.51 for Le > 1 propane flames. In comparison with methane flames, values of d/dt)/S_L^b of syngas and propane flames are 1.64 times higher and 0.88 times lower, respectively. This is because Le < 1 turbulent flames are not only influence by the inherent hydrodynamics instability, but also strongly affected by the thermaldiffusive instability, while Le ≈ 1 and Le > 1 turbulent flames are only influenced by the hydrodynamics instability, resulting in lower values of (d/dt)/S_L^b than that of Le < 1 turbulent flames at the same ReT,flame. Here we propose a correction function f(Le) = 2.15\|Le - 1\| based on the Lewis number for non-unity Lewis number turbulent flames and f(Le) = 1 if Le ≈ 1, such that the above-mentioned three different normalized relationship curves can be collapsed onto one single normalized relationship curve, f(Le)[(d/dt)/S_L^b] ≈ 0.113ReT,flame0.54. These results should be useful to our understanding of high-pressure premixed turbulent combustion and applicable to automobile and aviation internal combustion engines.
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