The innovative setup developed during the project combines advanced measurement techniques driven by the expertise of the two participating centers and applied to an optically accessible vessel that replicates autoignition standard experimental conditions, enabling an in-depth experimental characterization of the ignition processes. The testing cell, shown in Figure 1, is a 0.45 L square cell with quartz windows. Temperature uniformity is achieved by insulating the cell with hot air contained in a larger square cell, aluminum foil, and glass wool. Particle Image Velocimetry (PIV) and small-gauge thermocouples are used to characterize the flow velocity and temperature field before, during, and after ignition. Fuel injection, vaporization, and dispersion are analyzed using Direct Imaging and Background-Oriented Schlieren (BOS) techniques. The ignition kernel(s) and the combustion processes are studied using chemiluminescence imaging and time-resolved spectrometry. All the different techniques are synchronized as well as possible to allow a time-resolved analysis. Figure 2 presents a sample of PIV analysis with titanium dioxide particles during ignition (ignition kernel top left) and BOS technique during injection (high concentration of fuel vapor represented by bright colors). The results, confronted with 2D and 3D models, highlight the critical role of the Leidenfrost effect and thermal convection in fuel dispersion. Ignition consistently occurs at the top of the cell, while exothermic reactions initiate at the hot plate. Notably, ignition can manifest as a weak, blue flame invisible to standard cameras rather than a bright flame.
In parallel, the ASTM-standardized apparatus constructed at EDL was improved to enhance our knowledge of AIT characterization, guide our research on the optically accessible setup, and propose new guidelines for better autoignition characterizations. Figure 3 shows the apparatus, which was upgraded with an automated injection system, synchronized high-speed temperature acquisition, and detailed analysis of ignition phenomena, considering the statistical behavior of the ignition, the fuel volume tested, and the type of ignition (flame brightness, temperature variation, opacity of the combustion products). Enhanced repeatability revealed that parameters such as the cell's internal temperature gradient and injection conditions (height, duration, and needle temperature) can strongly influence AIT measurements.
A synthetic paraffinic hydrocarbon (SPK), a potential candidate for sustainable aviation fuel (SAF), was investigated using the improved ASTM apparatus and compared with samples representative of the fuel currently used in the aviation fleet (Jet A). SPK exhibited a significantly lower AIT than Jet A, raising concerns about hot-surface ignition hazards in aircraft, as it does not meet FAA certification guidelines. Gas Chromatography-Mass Spectrometry (GC-MS) analysis revealed compositional differences between SPK and Jet A, particularly the absence of aromatic molecules in SPK. Further tests confirmed that fuels with aromatic content and highly branched isomers have higher AIT compared to single-component normal alkanes. These findings underscore the importance of aromatic content in enhancing fuel thermal stability and the need for a deeper understanding of AIT to advance the development of safe and reliable sustainable aviation fuels.
