Periodic Reporting for period 1 - Hephaestus (Investigation of the ignition of a wall-impinging jet on a hot surface: fuel jet and liquid gas jet)
Reporting period: 2022-10-01 to 2024-09-30
Ignition of flammable gases, combustible liquids, or powders by hot surfaces is a critical safety concern across industries, including power generation, petroleum, automotive, chemical, and aerospace. The autoignition temperature (AIT), i.e. the minimum temperature for self-sustaining combustion without an external ignition source like a spark or flame, is regularly used to evaluate these risks. The most common methods for determining AIT are the ASTM-E659 standard [1] and the international standard ISO/IEC [2], consisting of injecting a small quantity of liquid fuel (50 to 300 µL) into a vessel heated at a constant temperature in a furnace and observing the presence or absence of a visible flame.
However, numerous studies have highlighted significant limitations of the standardized tests from which AIT values are derived. It has been shown that AIT results significantly vary according to factors other than fuel characteristics, such as the material, cleanliness, shape, and volume of the vessel, the ratio of the combustible substance to air, and the ambient temperature and pressure. Moreover, the standard AIT test does not explicitly examine hot surface ignition. Instead, it involves a hot flammable atmosphere surrounded by a hot surface in a confined geometry. While this setup is valuable for evaluating hazards related to self-heating or thermal runaway, many industrial hazards involve hot surfaces in a cold atmosphere and open, unconfined conditions. Under these conditions, ignition threshold temperatures for surfaces can be much higher (500-600°C) than standard AIT values.
Hephaestus project combines the expertise of two renowned research centers: the von Karman Institute for Fluid Dynamics (VKI) in Belgium under the supervision of Professor Laboureur and the Explosion Dynamics Laboratory (EDL) at the California Institute of Technology (Caltech) in the United States under the supervision of Professor Shepherd. The ambition of the project is to significantly reduce the occurrence of accidents due to accidental ignition via more robust safety assessments, contribute to better-controlled ignition processes yielding reduced pollutant emissions, and support the transition to greener energies. The aim is to provide fundamental scientific data and analyses on autoignition in a hot atmosphere and over a hot surface. The expertise acquired during the project’s first phase at Caltech will be transferred to Europe by supporting the development of a laboratory for ignition characterization of liquid gas (biogas and hydrogen).
Hephaestus represents a unique chance to go beyond the state-of-the-art, achieve an in-depth understanding of the ignition processes, and update safety guidelines.
[1] ASTM-E659, 2005. Standard test method for autoignition temperature of liquid chemicals. American Society for Testing and Materials.
[2] ISO/IEC, 2017. ISO/IEC 80079-20-1: Explosive Atmospheres - Part 20-1: Material Characteristics for Gas and Vapor Classification-Test Methods and Data. Technical Report. International Organization for Standardization.
However, numerous studies have highlighted significant limitations of the standardized tests from which AIT values are derived. It has been shown that AIT results significantly vary according to factors other than fuel characteristics, such as the material, cleanliness, shape, and volume of the vessel, the ratio of the combustible substance to air, and the ambient temperature and pressure. Moreover, the standard AIT test does not explicitly examine hot surface ignition. Instead, it involves a hot flammable atmosphere surrounded by a hot surface in a confined geometry. While this setup is valuable for evaluating hazards related to self-heating or thermal runaway, many industrial hazards involve hot surfaces in a cold atmosphere and open, unconfined conditions. Under these conditions, ignition threshold temperatures for surfaces can be much higher (500-600°C) than standard AIT values.
Hephaestus project combines the expertise of two renowned research centers: the von Karman Institute for Fluid Dynamics (VKI) in Belgium under the supervision of Professor Laboureur and the Explosion Dynamics Laboratory (EDL) at the California Institute of Technology (Caltech) in the United States under the supervision of Professor Shepherd. The ambition of the project is to significantly reduce the occurrence of accidents due to accidental ignition via more robust safety assessments, contribute to better-controlled ignition processes yielding reduced pollutant emissions, and support the transition to greener energies. The aim is to provide fundamental scientific data and analyses on autoignition in a hot atmosphere and over a hot surface. The expertise acquired during the project’s first phase at Caltech will be transferred to Europe by supporting the development of a laboratory for ignition characterization of liquid gas (biogas and hydrogen).
Hephaestus represents a unique chance to go beyond the state-of-the-art, achieve an in-depth understanding of the ignition processes, and update safety guidelines.
[1] ASTM-E659, 2005. Standard test method for autoignition temperature of liquid chemicals. American Society for Testing and Materials.
[2] ISO/IEC, 2017. ISO/IEC 80079-20-1: Explosive Atmospheres - Part 20-1: Material Characteristics for Gas and Vapor Classification-Test Methods and Data. Technical Report. International Organization for Standardization.
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.
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.
The first phase of the Hephaestus project has delivered groundbreaking insights into ignition phenomena. First, the project highlighted limitations in the widely used ASTM apparatus for determining autoignition temperature (AIT). These limitations, such as simplified assumptions and the dependence on environmental and operational conditions, were addressed through enhancements that include a more reliable setup and a more detailed statistical analysis of ignition events, opening the way for more robust safety assessments and updated guidelines.
Second, the optically accessible experimental setup developed during the project allows for a high level of detail in characterizing the ignition process. The integration of synchronized advanced diagnostics has revealed the intricate interplay between injection, vaporization, dispersion, and chemical reactions leading to ignition and offers new avenues for understanding ignition beyond standard models, which often oversimplify the process.
By advancing the understanding of ignition processes, the project contributes to safer industrial operations, particularly in sectors like aerospace and energy, where ignition risks pose a critical challenge. The findings also support the development of sustainable aviation fuels (SAFs) by identifying key molecular characteristics that enhance thermal stability.
Second, the optically accessible experimental setup developed during the project allows for a high level of detail in characterizing the ignition process. The integration of synchronized advanced diagnostics has revealed the intricate interplay between injection, vaporization, dispersion, and chemical reactions leading to ignition and offers new avenues for understanding ignition beyond standard models, which often oversimplify the process.
By advancing the understanding of ignition processes, the project contributes to safer industrial operations, particularly in sectors like aerospace and energy, where ignition risks pose a critical challenge. The findings also support the development of sustainable aviation fuels (SAFs) by identifying key molecular characteristics that enhance thermal stability.