On the path towards climate-neutral future mobility, the usage of synthetic fuels derived from renewable power sources, so-called e-fuels, will be necessary. Oxygenated e-fuels, which contain oxygen in their chemical structure, not only have the potential to realize a climate-neutral powertrain, but also to burn more cleanly in terms of soot formation. Polyoxymethylene dimethyl ethers (PODE or OMEs) are a frequently discussed representative of such combustibles. However, to operate compression ignition engines with these fuels achieving maximum efficiency and minimum emissions, the physical-chemical behavior of OMEs needs to be understood and quantified. Especially the detailed characterization of physical and chemical properties of the spray is of utmost importance for the optimization of the injection and the mixture formation process. The presented work aimed to develop a comprehensive CFD model to specify the differences between OMEs and dodecane, which served as a reference diesel-like fuel, with regards to spray atomization, mixing and auto-ignition for single- and multi-injection patterns. The simulation results were validated against experimental data from a high-temperature and high-pressure combustion vessel. The sprays’ liquid and vapor phase penetration were measured with Mie-scattering and schlieren-imaging as well as diffuse back illumination and Rayleigh-scattering for both fuels. To characterize the ignition process and the flame propagation, measurements of the OH* chemiluminescence of the flame were carried out. Significant differences in the ignition behavior between OMEs and dodecane could be identified in both experiments and CFD simulations. Liquid penetration as well as flame lift-off length are shown to be consistently longer for OMEs. Zones of high reaction activity differ substantially for the two fuels: Along the spray center axis for OMEs and at the shear boundary layers of fuel and ambient air for dodecane. Additionally, the transient behavior of high temperature reactions for OME is predicted to be much faster.
Imaging using THz waves has been a promising option for penetrative measurements in environments that are opaque to visible wavelengths. However, available THz imaging systems have been limited to relatively low frame rates and cannot be applied to study fast dynamics. This work explores the use of upconversion imaging techniques based on nonlinear optics to enable wavelength-flexible high frame rate THz imaging. UpConversion Imaging (UCI) uses nonlinear conversion techniques to shift the THz wavelengths carrying a target image to shorter visible or near-IR wavelengths that can be detected by available high-speed cameras. This report describes the analysis methodology used to design a prototype high-rate THz UCI system and gives a detailed explanations of the design choices that were made. The design uses a high-rate pulse-burst laser system to pump both THz generation and THz upconversion detection, allowing for scaling to acquisition rates in excess of 10 kHz. The design of the prototype system described in this report has been completed and all necessary materials have been procured. Assembly and characterization testing is on-going at the submission of this report. This report proposes future directions for work on high-rate THz UCI and potential applications of future systems.
Time-resolved soot and PAH formation from gasoline and diesel spray pyrolysis are visualized and quantified using diffuse back illumination (DBI) and laser induced fluorescence (LIF) at 355 nm, respectively, in a constant-volume vessel at 60 bar from 1400 to 1700 K for up to 30 ms. The delay, maximum formation rate, and yield of soot and PAHs are compared across fuels and temperatures and correlated with the yield sooting indices on either the mass or mole basis. The delays generally decrease with increasing temperature, and the formation rates of both PAHs and soot generally increase with temperature. The apparent PAH-LIF yield may decrease with temperature due to PAH growth and conversion into larger species, signal trapping, and thermal quneching. Soot yield generally increases with temperature. The mass-based YSI correlates reasonably well with soot delay, but YSI does not correlate well with soot yield. The mass-based YSI is a more appropriate predictor of sooting propensity than the mole-based YSI.
This work describes the diagnostic implementation and image processing methods to quantitatively measure diesel spray mixing injected into a high-pressure, high-temperature environment. We used a high-repetition-rate pulse-burst laser developed in-house, a high-speed CMOS camera, and optimized the optical configuration to capture Rayleigh scattering images of the vaporized fuel jets inside a constant volume chamber. The experimental installation was modified to reduce reflections and flare levels to maximize the images’ signal-to-noise ratios by anti-reflection coatings on windows and surfaces, as well as series of optical baffles. Because of the specificities of the high-speed system, several image processing techniques had to be developed and implemented to provide quantitative fuel concentration measurements. These methods involve various correction procedures such as camera linearity, laser intensity fluctuation, dynamic background flare, as well as beam-steering effects. Image inpainting was also applied to correct the Rayleigh scattering signal from large scatterers (e.g. particulates). The experiments demonstrate that applying planar laser Rayleigh scattering at high repetition rate to quantitatively resolve the mixing of fuel and ambient gases in diesel jets is challenging, but possible. The thorough analysis of the experimental uncertainty and comparisons to past data prove that such measurements can be accurate, whilst providing valuable information about the mixing processes of high-pressure diesel jets.
This work evaluated the iX Cameras iSpeed 727, a commercial CMOS-based continuous- recording high-speed camera. Various parameters of importance in the scheme of accurate time-resolved measurements and photonic quantification have been measured under controlled conditions on the bench, using state-of-the-art instrumentation. We will detail the procedures and results of the tests laid out to measure sensor sensitivity, linearity, signal-to-noise ratio and image lag. We also looked into the electronic shutter performance and accuracy, as exposure time is of particular interest to high-speed imaging. The results of the tests show that this camera matches or exceeds the performance of competing units in most aspects, but that, as is the case for other high-speed camera systems, corrections are necessary to make full use of the image data from a quantitative perspective.
Mitigating particulate matter (PM) emissions while simultaneously controlling nitrogen oxide and hydrocarbon emissions is critical for both gasoline and diesel engines. The problem is especially critical during cold-start cycles where aftertreatment devices are less effective. Understanding how liquid sprays and films form PM and designing to change the outcome requires advanced combustion concepts developed through joint experimental and computational efforts. However, existing spray and soot computational models are oversimplified and non-physical, and are therefore unable to reliably capture quantitative or even qualitative trends over a wide range of engine operating conditions. This task involves the development and application of advanced optical diagnostics and high-pressure gas and particle sampling/analysis in unique high-temperature, high-pressure vessels to investigate spray dynamics and soot formation with the objective of providing fundamental understanding about soot processes under relevant engine conditions to aid the development of improved soot models for commercial CFD codes
In this LDRD project, we developed a capability for quantitative high - speed imaging measurements of high - pressure fuel injection dynamics to advance understanding of turbulent mixing in transcritical flows, ignition, and flame stabilization mechanisms, and to provide e ssential validation data for developing predictive tools for engine combustion simulations. Advanced, fuel - efficient engine technologies rely on fuel injection into a high - pressure, high - temperature environment for mixture preparation and com bustion. Howe ver, the dynamics of fuel injection are not well understood and pose significant experimental and modeling challenges. To address the need for quantitative high - speed measurements, we developed a Nd:YAG laser that provides a 5ms burst of pulses at 100 kHz o n a robust mobile platform . Using this laser, we demonstrated s patially and temporally resolved Rayleigh scattering imaging and particle image velocimetry measurements of turbulent mixing in high - pressure gas - phase flows and vaporizing sprays . Quantitativ e interpretation of high - pressure measurements was advanced by reducing and correcting interferences and imaging artifacts.
We investigate the mixing, penetration, and ignition characteristics of high-pressure n-dodecane sprays having a split injection schedule (0.5/0.5 dwell/0.5 ms) in a pre-burn combustion vessel at ambient temperatures of 750 K, 800 K and 900 K. High-speed imaging techniques provide a time-resolved measure of vapor penetration and the timing and progression of the first- and second-stage ignition events. Simultaneous single-shot planar laser-induced fluorescence (PLIF) imaging identifies the timing and location where formaldehyde (CH2O) is produced from first-stage ignition and consumed following second-stage ignition. At the 900-K condition, the second injection penetrates into high-temperature combustion products remaining in the near-nozzle region from the first injection. Consequently, the ignition delay for the second injection is shorter than that of the first injection (by a factor of two) and the second injection ignites at a more upstream location near the liquid length. At the 750 K and 800 K conditions, high-temperature ignition does not occur in the near-nozzle region after the end of the first injection, though formaldehyde remains from first-stage reactions. Under these conditions, the second injection penetrates into cool-flame products that are slightly elevated in temperature (∼100 K) relative to the ambient. This modest temperature increase and the availability of reactive cool-flame products reduces the first- and second-stage ignition delay of the second injection by a factor of approximately two relative to the first injection. At the 750-K ambient condition, high-temperature ignition of the first injection does not occur until the second injection enriches the very fuel-lean downstream regions.
The ignition and flame stabilization characteristics of two synthetic fuels, having significantly different cetane numbers, are investigated in a constant volume combustion vessel over a range of ambient conditions representative of a compression ignition engine operating at variable loads. The synthetic fuel with a cetane number of 63 (S-1) is characterized by ignition delays that are only moderately longer than n-dodecane (cetane number of 87) over a range of ambient conditions. By comparison, the synthetic fuel with a cetane number of 17 (S-2) requires temperatures approximately 300 K higher to achieve the same ignition delays. The much different ignition characteristics and operating temperature range present a scenario where the lift-off stabilization may be substantially different. At temperatures below 1000 K, the S-2 fuel undergoes a long transient stabilization phase during which the lift-off location moves as much as 15 mm upstream (i.e., toward the injector orifice) after the ignition of the first flame kernel. This behavior is much different than S-1, n-dodecane, or with conventional diesel, in which past research shows that the lift-off location stabilizes very close to the ignition location shortly after the premixed burn. The longer ignition delays for S-2 frequently result in fuel-lean mixtures at the ignition location where the spray becomes over-mixed (i.e., too fuel-lean) and the high-temperature ignition event is noticeably less robust (i.e., smaller and less intense ignition kernels) as observed by high-speed chemiluminescence imaging. High-speed chemiluminescence imaging and pressure measurements show strong evidence of cool-flame (i.e., first-stage or low-temperature) reactions prior to high-temperature ignition for S-1 while they are less evident for S-2.
The mixing field of sprays injected into high temperature and pressure environments has been observed to be tightly connected to spreading angle, therefore linking vaporization and combustion processes to the angular dispersion of the spray. Visualization of the Engine Combustion Network three-hole, Spray B diesel injector shows substantial variation in near-field spreading angle with respect to time compared to past measurements of the single-hole, Spray A injector. The source of these variations originating inside the nozzle, and the implications on mixing, evaporation, and combustion of the diesel plume, need to be understood. In this study, we characterize the ECN-target plume for a Spray B injector (Serial # 211201), which already benefits from extensive and detailed internal measurements of nozzle geometry and needle movement, while comparing to the single-hole Spray A with the same type of detailed geometry and understanding. We measure the spreading angle, liquid penetration, and vapor penetration with respect to time of the spray of interest using standardized diagnostics in a high-temperature, high-pressure capable optically accessible combustion chamber. High-speed Mie scattering and diffused back-illumination imaging (DBI) are applied for liquid penetration, and schlieren imaging, for vapor penetration. The measurements show that the near-field spreading angle is wide for the first 300 μs after the start of injection before dropping rapidly during a quasi-steady period and then increasing well before the end of injection. Changes in spreading angle are not coincident with needle motion throttling, suggesting more complicated internal flow transients. With DBI long-distance microscopy, a partially transparent region indicates that an intact liquid core at the nozzle exit occurs frequently in quasi-steady period, which is coincident with a narrow spreading angle. The liquid penetration measured by DBI is comparable to that of Mie-scattering using criteria and standardization already established by the ECN community for Spray A. The Spray B liquid and vapor penetration rates are slower than that of Spray A, showing responses connected to the transient spreading angle.
We applied simultaneous schlieren and formaldehyde (CH2O) planar laser-induced fluorescence (PLIF) imaging to investigate the low- and high-temperature auto-ignition events in a high-pressure (60 bar) spray of n-dodecane. High-speed (150 kHz) schlieren imaging allowed visualization of the temporal progression of the fuel vapor penetration as well as the low- and high-temperature ignition events, while formaldehyde fluorescence was induced by a pulsed (7-ns), 355-nm planar laser sheet at a select time during the same injection. Fluorescence from polycyclic aromatic hydrocarbons (PAH) was also observed and was distinguished from formaldehyde PLIF both temporally and spatially. A characteristic feature previously recorded in schlieren images of similar flames, in which refractive index gradients significantly diminish, has been confirmed to be coincident with large formaldehyde fluorescence signal during low-temperature ignition. Low-temperature reactions initiate near the radial periphery of the spray on the injector side of the spray head. Formaldehyde persists on the injector side of the lift-off length and forms rapidly near the injector following the end of injection. The consumption of formaldehyde coincides with the position and timing of high-temperature ignition and low-density zones that are clearly evident in the schlieren imaging. After the end of injection, the formaldehyde that formed on the injector side of the lift-off length is consumed as a high-temperature ignition front propagates back toward the injector tip.
The development of accurate predictive engine simulations requires experimental data to both inform and validate the models, but very limited information is presently available about the chemical structure of high pressure spray flames under engine- relevant conditions. Probing such flames for chemical information using non- intrusive optical methods or intrusive sampling techniques, however, is challenging because of the physical and optical harshness of the environment. This work details two new diagnostics that have been developed and deployed to obtain quantitative species concentrations and soot volume fractions from a high-pressure combusting spray. A high-speed, high-pressure sampling system was developed to extract gaseous species (including soot precursor species) from within the flame for offline analysis by time-of-flight mass spectrometry. A high-speed multi-wavelength optical extinction diagnostic was also developed to quantify transient and quasi-steady soot processes. High-pressure sampling and offline characterization of gas-phase species formed following the pre-burn event was accomplished as well as characterization of gas-phase species present in the lift-off region of a high-pressure n-dodecane spray flame. For the initial samples discussed in this work several species were identified, including polycyclic aromatic hydrocarbons (PAH); however, quantitative mole fractions were not determined. Nevertheless, the diagnostic developed here does have this capability. Quantitative, time-resolved measurements of soot extinction were also accomplished and the novel use of multiple incident wavelengths proved valuable toward characterizing changes in soot optical properties within different regions of the spray flame.