永續航空燃料（sustainable aviation fuel, SAF）由於可與現有航空發動機相容，並具有降低碳排放的潛力，因此被視為最具前景的航空替代燃料之一。然而，目前針對 SAF 在高壓與高溫燃燒環境下之噴霧點火行為與煙灰生成特性的實驗數據仍相當有限。特別是，在具光學可視化條件之噴霧燃燒環境下，SAF 燃料組成、點火反應性、火焰穩定性與煙灰演化之間的關係仍尚未被充分釐清。本研究建立一套可視化定容燃燒室（visible constant-volume combustion chamber, VCVCC），用以探討純 HEFA-SAF 與傳統 Jet A-1 之噴霧點火與煙灰生成特性。此燃燒室內部體積為 4.3 L，可操作於最高 1300 K 與 100 bar 之環境條件。系統整合高壓共軌噴射系統與多種光學診斷技術。自然火焰光度影像用於評估光學點火延遲、火焰結構、空間積分自然火焰光度（spatially integrated natural luminosity, SINL）與時間積分自然火焰光度（time-integrated natural luminosity, TINL）。OH* 化學發光影像則用於分析反應區趨勢與火焰提升長度（lift-off length, LOL）。此外，雷射誘導白熾光與擴散背光技術進一步用於觀察煙灰分布、光學厚度與基於消光的煙灰質量估計值。實驗條件為環境溫度 773–873 K、噴射壓力 300–900 bar、氧氣濃度 15–21 vol.%，並固定燃燒室壓力為 30 bar。
研究結果顯示，與 Jet A-1 相比，SAF 具有較佳的點火特性。SAF 點火較早，具有較短的火焰提升長度，並形成較緊湊的火焰結構。平均而言，SAF 的光學點火延遲約比 Jet A-1 短 21.4%。SAF 的 OH* 化學發光強度約比 Jet A-1 高 23.8%，顯示其具有更高溫的反應區與較強的自由基生成能力。提高環境溫度與氧氣濃度可促進點火並增加 OH* 強度，而提高噴射壓力則可改善霧化並縮短火焰持續時間。在煙灰生成方面，自然火焰光度與 DBI 量測結果皆證實，在所有實驗條件下，SAF 生成的煙灰量低於 Jet A-1。雖然較高的環境溫度與氧氣濃度會增加自然火焰光度，但較高的噴射壓力可透過改善霧化與混合來抑制煙灰生成。KL 分布結果顯示，SAF 的峰值光學厚度約比 Jet A-1 低 0.51，且其煙灰氧化位置約較 Jet A-1 靠近噴嘴 0.56 cm。SAF 的煙灰質量亦低於 Jet A-1，依操作條件不同約可降低 5–19 μg。SAF 較低的煙灰生成趨勢也由替代燃料煙灰前驅物分析進一步支持。SAF 的計算煙灰生成指數約為 22，明顯低於 Jet A-1 的約 63。模擬結果亦顯示，SAF 中苯、萘、菲與芘的生成質量皆低於 Jet A-1，代表其 PAH 成長較弱，且煙灰前驅物生成受到抑制。上述結果主要歸因於 SAF 具有較高的 H/C 比與十六烷值、較低的汽化焓與黏度，以及顯著較低的芳香烴與環烷烴含量。本研究建立了一套高壓光學可視化燃燒平台，並針對 HEFA-SAF 與 Jet A-1 之噴霧燃燒特性進行系統性的實驗分析與機制探討。研究結果顯示，SAF 在維持良好點火性能的同時，可顯著降低煙灰生成，顯示其具有應用於未來低排放航空推進系統之潛力。

Sustainable aviation fuel (SAF) has been regarded as one of the most promising drop-in alternatives to conventional petroleum-derived jet fuels because of its compatibility with existing aircraft engines and its potential to reduce soot emissions. However, experimental data on the spray ignition behavior and soot formation characteristics of SAF under high-pressure and high-temperature combustion environments remain limited. In particular, the relationship between SAF fuel composition, ignition reactivity, flame stabilization, and soot evolution under optically accessible spray combustion conditions has not been fully clarified. In this study, a high-pressure visible constant-volume combustion chamber (VCVCC) was developed to investigate the spray ignition and soot formation characteristics of neat HEFA-SAF and conventional Jet A-1. The chamber was designed with an internal volume of 4.3 L and was capable of operating at temperatures up to 1300 K and pressures up to 100 bar. A high-pressure common-rail injection system and multiple optical diagnostic methods were integrated into the platform. Natural flame luminosity imaging was used to evaluate optical ignition delay, flame structure, spatially integrated natural luminosity (SINL), and time-integrated natural luminosity (TINL). OH* chemiluminescence imaging was employed to analyze reaction zones trend and lift-off length (LOL). Laser-induced incandescence and diffuse back-illumination techniques were further applied to examine soot distribution, optical thickness, and extinction-based soot-mass estimates. Experiments were conducted under ambient temperatures of 773–873 K, injection pressures of 300–900 bar, and oxygen concentrations of 15–21 vol.% at a constant chamber pressure of 30 bar.
The results show that SAF exhibited superior ignition characteristics compared with Jet A-1. SAF ignited earlier, had a shorter lift-off length, and produced a more compact flame structure. On average, the optical ignition delay of SAF was approximately 21.4% shorter than that of Jet A-1. The OH* chemiluminescence intensity of SAF was approximately 23.8% higher, indicating a more active high-temperature reaction zone and stronger radical formation. Increasing ambient temperature and oxygen concentration promoted ignition and increased OH* intensity, whereas increasing injection pressure enhanced atomization and shortened flame duration. For soot formation, natural flame luminosity and DBI-based measurements confirmed that SAF generated lower soot levels than Jet A-1 under all investigated conditions. Although higher ambient temperature and oxygen concentration increased soot-related luminosity, higher injection pressure suppressed soot formation through improved atomization and air–fuel mixing. The KL distribution showed that the peak optical thickness of SAF was approximately 0.51 lower than that of Jet A-1, and soot oxidation occurred about 0.56 cm closer to the injector. The estimated soot mass of SAF was consistently lower than that of Jet A-1, with reductions of approximately 5–19 μg depending on operating conditions. The lower soot formation tendency of SAF was further supported by surrogate-based soot precursor analysis. The calculated yield soot index of SAF was approximately 22, substantially lower than that of Jet A-1, which was approximately 63. The simulated masses of benzene, naphthalene, phenanthrene, and pyrene were also lower for SAF, indicating weaker PAH growth and reduced soot precursor formation. These results are mainly attributed to the higher H/C ratio, higher cetane number, lower enthalpy of vaporization, lower viscosity, and significantly lower aromatic and cycloalkane contents of SAF.

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