| 摘要: | 本論文針對兩近似辛烷值(Research Octane Number RON)之汽油替代燃料，分別為異辛烷90%與正庚烷和甲苯皆為5%之甲苯參考燃料(Toluene Reference Fuel, TRF96)，以及異辛烷88.5%，正庚烷6.5% 與甲苯為5% 之TRF95，於近似汽油引擎運作之高溫高壓條件下，量測其與空氣預混之層流和紊流燃燒速度(SL和ST)。兩TRF燃氣之當量比皆為1.0，Lewis 數(Le)也皆為1.45。實驗在一高溫高壓雙腔體三維十字型燃燒設備執行，其三維十字型燃燒室中心區可產生一近似等向性紊流場，與近似恆常環境之溫度(T)、壓力(p)、方均根紊流擾動速度(u′) 與紊流場雷諾數 (ReT,flow= u′LI/v，其中LI為紊流長度積分尺度，v為運動黏滯係數)。將燃氣於中心引燃後，可利用高速紋影法得到自中央向外傳遞之球狀火焰影像，以獲得其半徑之時序資料R(t)，來估算其燃燒速度。本研究的焦點在於等u′或等ReT,flow情況下，壓力效應對SL與ST之影響。TRF96燃氣在T = 373 K、p = 1 ~ 5 atm、 u′= 0 ~ 4.2m/s 之實驗條件下進行實驗; 而TRF95燃氣在T = 358 K、p = 1 ~ 5 atm、ReT,flow= 0 ~ 11,600 之實驗件下進行實驗。量測結果可顯示兩重點:(1) 在固定u′下，ST會隨p之增加而增加;(2) 然而在固定 ReT,flow下，ST卻隨p之增加而下降，且其趨勢與SL隨p呈指數性下降變化相似。前者的結果主要是當p上升而密度上升，導致 v下降，造成 ReT,flow上升因此ST上升(主要影響因素)，這與傳統認知有點不同，也就是高壓條件下火焰厚度變薄，導致流力不穩定性加劇因此ST上升之機制，此應為次要因素。因此，實驗結果顯示ST與ReT,flow間應存在重大關係。接著，本研究將會統整異辛烷、主要參考燃料(Primarily Reference Fuel, PRF)、以及本研究之TRFs之ST資料，以ST一般通式進行資料分析並考量Le數，以尋找其自我相似性。分析後發現，Le > 1 之所有燃氣的正規化 ST 值小於 Le < 1之正規化 ST 值，且考量Le數後，可發現所有資料能擬合成一條曲線，顯示紊流球狀火焰具有自我相似性。最後，本研究對於瞭解汽車引擎內部之高溫高壓條件下之預混紊流燃燒現像應有所幫助。;This thesis aims to obtain a better understanding on laminar and turbulent burning velocities (SL and ST) for centrally-ignited, spherically-propagating toluene reference fuel (TRF)/air flames at high-temperature and high-pressure conditions over a wide range of the r.m.s. turbulent fluctuation velocity (u’) relevant to gasoline engine conditions. We measure values of SL and ST for two TRFs with slightly different research octane number, i.e. TRF95: 88.5% i-C8H18 + 6.5% C7H16+ 5% C7H8 and TRF96: 90% i-C8H18 + 5% C7H16+ 5% C7H8, both having an effective Lewis number Le ? 1.45. Experiments were conducted in an already-established double-chamber, 3D cruciform fan-stirred explosion facility, capable of generating near-isotropic turbulence field in the central uniform-temperature region of the cruciform bomb. High-speed Schlieren imaging technique was used to record the time evolutions of flame radii of the TRF/air spherical expanding flames, so that SL and ST can be estimated. The focus is placed on the pressure effect to SL and ST under either at the same u’ or at the constant turbulent flow Reynolds number situations (ReT,flow= u′LI/v) where LI represents the integral length scale of turbulence and v is kinematic viscosity. For the TRF96/air mixture, the experimental conditions are T = 373 K, p = 1 ~ 5 atm, and u’ = 0 ~ 4.2 m/s. As to the TRF95/air mixture, the experimental conditions are T = 358 K, p = 1 ~ 5 atm, and ReT,flow = 0 ~ 11600. Results show two points: (1) ST increases with increasing pressure at all fixed u’ conditions; (2) under the constant ReT,flow conditions, it is found that ST decreases with increasing pressure, similar to SL, showing a global response of burning velocities at elevated pressures. The former is mainly because the increase of pressure results in an increase of density (ρ) and thus the kinematic viscosity (ν) is decreased due to p ~ ρ ~ ν-1 leading to the increase of ReT,flow, while the commonly-held view of ST increment through the thinned flame thickness induced by hydrodynamic instability at high-pressure conditions may play a secondary role. Finally, some general correlations of the ST present data along with previous ST data of isooctane and primary reference fuels are discussed with the consideration of Le in attempt to find the self-similarity of these ST data. It is found that without the consideration of Le, the normalized ST data for Le < 1 flames have higher values than that for Le > 1 flames. |
