Le biodiesel est un biocarburant, composé d'un mélange de mono-esters d'acide gras saturés et insaturés avec une longue chaîne carbonée. Ce travail de thèse présente les données de la littérature sur l'origine du biodiesel et son procédé de fabrication ; sont présentées aussi les performances et les émissions des moteurs diesel fonctionnant au biodiesel et la cinétique d'oxydation du biodiesel. Des efforts ont été faits pour mettre en évidence les principales différences entre les esters méthyliques et éthyliques tout en montrant que d'autres recherches sont encore à développer. Pour ces raisons, les délais d'auto-inflammation de cinq esters méthyliques et éthyliques ont été mesurés dans un tube à onde de choc : l'acrylate d'éthyle, l'acrylate de méthyle, le crotonate d'éthyle, le crotonate de méthyle et le butanoate d'éthyle. Les mécanismes cinétiques détaillés d'oxydation des cinq esters étudiés ont été générés automatiquement en utilisant le logiciel EXGAS. L'oxydation du butanoate d'éthyle, molécule modèle d'esters éthyliques d'huiles végétales (EEHV) a été étudiée dans un réacteur piston à pression atmosphérique pour une gamme de température allant de 500 à 1200 K. Les résultats représentent les profils de concentration des réactifs, les intermédiaires stables et les produits finaux. Le modèle cinétique a été validé de façon satisfaisante par une comparaison entre les résultats simulés et expérimentaux / An increasingly popular biofuel is biodiesel, composed of a mixture of saturated and unsaturated fatty acid methyl or ethyl esters, with a long aliphatic main chain. This PhD dissertation provides a literature review concerning the origin of biodiesel, its manufacturing process, performance and emissions of diesel engines fueled with biodiesel, and the kinetics of oxidation of biodiesel. Efforts were made to highlight the main differences between methyl and ethyl esters while showing where further research needs to be developed or pursued. For these reasons, the autoignition of five esters were measured behind reflected shock tube: ethyl acrylate, methyl acrylate, ethyl crotonate, methyl crotonate, and ethyl butanoate. Detailed mechanisms for the oxidation of the five studied esters were automatically generated using the version of EXGAS software. In addition, the oxidation of ethyl butanoate as a model compound for Fatty Acid Ethyl Esters (FAEE) has been investigated in tubular plug flow reactor at atmospheric pressure over wide range of temperature (500-1200 K). The results consist of concentration profiles of the reactants, stable intermediates, and final products. The model was again validated satisfactorily by comparison between computed results and the generated experimental data

<p dir="ltr">Laser absorption spectroscopy (LAS) is a widely used technique to acquire path-integrated measurements of gas properties such as temperature and mole fraction. Although extremely useful, the application of LAS to study heterogeneous combustion environments can be challenging. For example, beam steering can be one such challenge that arises during measurements in heterogeneous combustion environments such as metallized propellant flames or measurements at high-pressure conditions. The ability to only obtain path integrated measurements has been a major challenge of conventional LAS techniques, especially in characterizing combustion environments with a non-uniform thermo-chemical distribution along the line of sight (LOS). Additionally, simultaneous measurements of multiple species using LAS with narrow-bandwidth lasers often necessitates employing multiple light sources. Aerospace applications, such as characterizing hypersonic flows may require ultrashort time resolution to study fast-evolving chemistry. Similarly, atmospheric entry most often requires measurements of atoms and molecules that absorb at wavelengths ranging from ultraviolet to mid-infrared. The availability of appropriate light sources for such measurements has been limited. In the past, several researchers have come up with diagnostic techniques to overcome the above-mentioned challenges to a certain extent. Most often, these solutions have been need-based while compromising on other diagnostic capabilities. Therefore, LAS diagnostics capable of acquiring broadband measurements with ultrafast time resolution and the ability to acquire measurements at wavelengths in ultraviolet through mid-infrared is required to study advanced combustion systems and for the development of advanced aerospace systems for future space missions. Ultrafast laser absorption spectroscopy is one such technique that provides broadband measurements, enabling simultaneous multi-species and high-pressure measurements. The light source utilized for ULAS provides the ultrafast time resolution necessary for resolving fast-occurring chemistry and more importantly the ability to acquire measurements at a wide range of wavelengths ranging from ultraviolet to far-infrared. The development and application of ULAS for characterizing propellant flames and hypersonic flows under non-equilibrium conditions by overcoming the above-mentioned challenges is presented here. </p><p>This work describes the development of a single-shot ultrafast laser absorption spectroscopy (ULAS) diagnostic for simultaneous measurements of temperature and concentrations of CO, NO, and H₂O in flames and aluminized fireballs of HMX (C₄H₈N₈O₈). Ultrashort (55 fs) pulses from a Ti:Sapphire oscillator emitting near 800 nm were amplified and converted into the mid-infrared through optical parametric amplification (OPA) at a repetition rate of 5 kHz. Ultimately, pulses with a spectral bandwidth of ≈600 cm^-1 centered near 4.9 µm were utilized in combination with a mid-infrared spectrograph to measure absorbance spectra of CO, NO, and H₂O across a 30 nm bandwidth with a spectral resolution of 0.3 nm. The gas temperature and species concentrations were determined by least-squares fitting simulated absorbance spectra to measured absorbance spectra. Measurements of temperature, CO, NO, and H₂O were acquired in an HMX flame burning in air at atmospheric pressure and the measurements agree well with previously published results. Measurements were also acquired in fireballs of HMX with and without 16.7 wt% H-5 micro-aluminum. Time histories of temperature and column densities are reported with a 1-σ precision of 0.4% for temperature and 0.3% (CO), 0.6% (NO), and 0.5% (H₂O), and 95% confidence intervals (C.I.) of 2.5% for temperature and 2.5% (CO), 11% (NO), and 7% (H₂O), thereby demonstrating the ability of ULAS to provide high-fidelity, multi-parameter measurements in harsh combustion environments. The results indicate that the addition of the micron-aluminum increases the fireball peak temperature by ≈100 K and leads to larger concentrations of CO. The addition of aluminum also increases the duration fireballs remain at elevated temperatures above 2000 K.</p><p dir="ltr">Next, the application of ULAS for dual-zone temperature and multi-species (CO, NO, H₂O, CO₂, HCl, and HF) measurements in solid-propellant flames is presented. ULAS measurements were acquired at three different central wavelengths (5.121 µm, 4.18 µm, and 3.044 µm) for simultaneous measurements of temperature and: 1) CO, NO, and H₂O, 2) CO₂ and HCl, and 3) HF and H₂O. Absorption measurements with a spectral resolution of 0.35 nm and bandwidth of 7 cm^-1, 18 cm^-1, and 35 cm^-1, respectively were acquired. In some cases, a dual-zone absorption spectroscopy model was implemented to accurately determine the gas temperature in the hot flame core and cold flame boundary layer via broadband absorption measurements of CO₂, thereby overcoming the impact of line-of-sight non-uniformities. Results illustrate that the hot-zone temperature of CO₂ agrees well with the equilibrium flame temperature and single-zone thermometry of CO, the latter of which is insensitive to the cold boundary layer due to the corresponding oxidation of CO to CO₂.</p><p dir="ltr">The initial development and implementation of an ultraviolet and broadband ultrafast-laser-absorption-imaging (UV-ULAI) diagnostic for one dimensional (1D) imaging of temperature and CN via its <i>B</i>²Σ⁺←<i>X</i>²Σ⁺absorption bands near 385 nm. The diagnostic was demonstrated by acquiring single-shot measurements of 1D temperature and CN profiles in HMX flames at a repetition rate of 25 Hz. Ultrashort pulses (55 fs) at 800 nm were generated using a Ti:Sapphire oscillator and then amplification and wavelength conversion to the ultraviolet was carried out utilizing an optical parametric amplifier and frequency doubling crystals. The broadband pulses were spectrally resolved using a 1200 l/mm grating and imaged on an EMCCD camera to obtain CN absorbance spectra with a resolution of ≈0.065 nm and a bandwidth of ≈4 nm (i.e. 260 cm^-1). Simulated absorbance spectra of CN were fit to the measured absorbance spectra using non-linear curve fitting to determine the gas properties. The spatial evolution of gas temperature and CN concentration near the burning surface of an HMX flame was measured with a spatial resolution of ≈10 µm. 1D profiles of temperature and CN concentration were obtained with a 1-σ spatial precision of 49.3 K and 4 ppm. This work demonstrates the ability of UV-ULAI to acquire high-precision, spatially resolved absorption measurements with unprecedented temporal and spatial resolution. Further, this work lays the foundation for ultraviolet imaging of numerous atomic and molecular species with ultrafast time resolution.</p><p dir="ltr">Ultraviolet ULAS was applied to characterize the temporal evolution of non-Boltzmann CN (<i>X</i>²Σ⁺) formed behind strong shock waves in N₂-CH₄ mixtures at conditions relevant to entry into Titan's atmosphere. An ultrafast (femtosecond) light source was utilized to produce 55 fs pulses near 385 nm at a repetition rate of 5 kHz and a spectrometer with a 2400 lines/mm grating was utilized to spectrally resolve the pulses after passing through the Purdue High-Pressure Shock Tube. This enabled broadband single-shot absorption measurements of CN to be acquired with a spectral resolution and bandwidth of ≈0.02 nm and ≈6 nm (≈402 cm^-1 at these wavelengths), respectively. A line-by-line absorption spectroscopy model for the <i>B</i>²Σ⁺←<i>X</i>²Σ⁺ system of CN was developed and utilized to determine six internal temperatures (two vibrational temperatures, four rotational) of CN from the (0,0), (1,1), (2,2) and (3,3) absorption bands. Measurements were acquired behind reflected shock waves in 5.65% CH₄ and 94.35% N₂ with an initial pressure of 1.56 mbar and incident shock speed of ≈2.1 km/s. For this test condition, the chemically and vibrationally frozen temperature of the mixture behind the reflected shock was 5000 K and the pressure was 0.6 atm. The high repeatability of the shock-tube experiments (0.3% variation in shock speed across tests) enabled multi-shock time histories of CN mole fraction and six internal temperatures to be acquired with a single-shot time resolution of less than 1 ns. The measurements revealed that CN <i>X</i>²Σ⁺ is non-Boltzmann rotationally and vibrationally for greater than 200 µs, thereby strongly suggesting that chemical reactions are responsible for the non-Boltzmann population distributions. </p><p><br></p>

The re-entry space vehicles encounter high temperatures when they enter the earth atmosphere and the high temperature air in the shock layer around the body undergoes partial dissociation. Also, the gas molecules injected into the shock layer from the ablative thermal protection system (TPS) undergo pyrolysis which helps in reducing the net heat flux to the vehicle surface. The chemical species due to the pyrolysis add complexity to the stagnation flow chemistry (52 chemical reactions) models which include species like NOx, CO and hydrocarbons (HCs). Although the ablative TPS is responsible for the safety of re-entry space vehicle, the induced chemical species result in variety of adverse effects on environment such as global warming, acid rain, green house effect etc. The well known three-way-catalyst (TWC) involves simultaneous removal of all the three gases (i.e, NOx, CO, Hydrocarbons) present in the shock layer. Interaction of such three-way-catalyst on the heat shield materials or on the wall of the re-entry space vehicle is to reduce the heat flux and to remove the gases in the shock layer, which is an important issue. For the re-entry vehicle the maximum aerodynamic heating occurs at an altitude ranging about 68 to 45 km during which the vehicle is surrounded by high temperature dissociated air. Then the simplest real gas model of air is the five species model which is based on N2, O2, O, NO and N. This five species model assumes no ionization and no pyrolysis gases are emitted from the heat shield materials. The experimental research work presented in this thesis is directed towards the understanding of catalytic and non-catalytic surface reactions on high temperature materials in presence of strong shock heated test gas. We have also explored the possibility of using shock tube as a high enthalpy device for synthesis of new materials. In the first Chapter, we have presented an overview of re-entry space vehicles, thermal protection system (TPS) and importance of real gas effects in the shock layer. Literature survey on TPS, ablative materials and aerothermochemistry at the stagnation point of reentry capsule, in addition to catalytic and non-catalytic surface reactions between the wall and dissociated air in the shock layer are presented. In Chapters 2 and 3, we present the experimental techniques used to study surface reactions on high temperature materials. A brief description of HST2 shock tunnel is presented and this shock tunnel is capable of generating flow stagnation enthalpies ranging from 0.7 to 5 MJ/kg and has an effective test time of ~ 800 µs. High speed data acquisition system (National Instruments and Yokogawa) used to acquire data from shock tube experiments. The experimental methods like X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), Raman and FTIR spectroscopy have been used to characterize the shock-exposed materials. Preliminary research work on surface nitridation of pure metals with shock heated nitrogen gas is discussed in Chapter 2. Surface nitridation of pure Al thin film with shock heated N2 is presented in Chapter 3. An XPS study shows that Al 2p peak at 74.2 eV is due to the formation AlN on the surface of Al thin film due to heterogeneous non-catalytic surface reaction. SEM results show changes in surface morphology of AlN film due to shock wave interaction. Thickness of AlN film on the surface increased with the increase in temperature of the shock heated nitrogen gas. However, HST2 did not produce sufficient temperature and pressure to carry out real conditions of re-entry. Therefore design and development of a new high enthalpy shock tunnel was taken up. In Chapter 4, we present the details of design and fabrication of free piston driven shock tunnel (FPST) to generate high enthalpy test gas along with the development of platinum (Pt) and thermocouple sensors for heat transfer measurement. A free piston driven shock tunnel consists of a high pressure gas reservoir, compression tube, shock tube, nozzle, test section and dump tank connected to a vacuum pumping system. Compression tube has a provision to fill helium gas and four ports, used to mount optical sensors to monitor the piston speed and pressure transducer to record pressure at the end of the compression tube when the piston is launched. Piston can attain a maximum speed of 150 m/s and compress the gas inside the compression tube. The compressed gas bursts the metal diaphragm and generates strong shock wave in the shock tube. This tunnel produces total pressure of about 300 bar and temperature of about 6000 K and is capable of producing a stagnation enthalpy up to 45 MJ/kg. The calibration of nozzle was carried out by measuring the pitot tube pressure in the dump tank. Experimentally recorded P5 pressure at end of the shock tube is compared with Numerical codes. Calibrated pressure P5 values are used to calculate the temperature T5 of the reflected shock waves. This high pressure and high temperature shock heated test gas interacts with the surface of the high temperature test materials. For the measurement of heat transfer rate, platinum thin film sensors are developed using DC magnetron sputtering unit. Hard protective layer of aluminum nitride (AlN) on Pt thin film was deposited by reactive DC magnetron sputtering to measure heat transfer rate in high enthalpy tunnel. After the calibration studies, FPST is used to study the heat transfer rate and to investigate catalytic/non-catalytic surface reaction on high temperature materials. In Chapter 5, an experimental investigation of non-catalytic surface reactions on pure carbon material is presented. The pure carbon C60 films and conducting carbon films are deposited on Macor substrate in the laboratory to perform shock tube experiments. These carbon films were exposed to strong shock heated N2 gas in the shock tube portion of the FPST tunnel. The typical shock Mach number obtained is about 7 with the corresponding pressure and temperature jumps of about 110 bar and 5400 K after reflection at end of the shock tube. Shock exposed carbon films were examined by different experimental techniques. XPS spectra of C(1s) peak at 285.8 eV is attributed to sp2 (C=N) and 287.3 eV peak is attributed to sp3 (C-N) bond in CNx due to carbon nitride. Similarly, N(1s) core level peak at 398.6 eV and 400.1 eV observed are attributed to sp3-C-N and sp2-C=N of carbon nitride, respectively. SEM study shows the formation of carbon nitride crystals. Carbon C60 had melted and undergone non-catalytic surface reaction with N2 while forming carbon nitride. Similar observations were made with conducting carbon films but the crystals were spherical in shape. Micro Raman and FTIR study gave further evidence on the formation of carbon nitride film. This experimental investigation confirms the formation of carbon nitride in presence of shock-heated nitrogen gas by non-catalytic surface reaction. In Chapters 6 and 7, we present a novel method to understand fully catalytic surface reactions after exposure to shock heated N2, O2 and Ar test gas with high temperature materials. We have employed nano ZrO2 and nano Ce0.5Zr0.5O2 ceramic high temperature materials to investigate surface catalytic reactions in presence of shock heated test gases. These nano crystalline oxides are synthesized by a single step solution combustion method. Catalytic reaction was confirmed for both powder and film samples of ZrO2. As per the theoretical model, it is known that the catalytic recombination reaction produces maximum heating on the surface of re-entry space vehicles. This was demonstrated in this experiment when a metastable cubic ZrO2 changed to stable monoclinic ZrO2 phase after exposure to shock waves. The change of crystal structure was seen using XRD studies and needle type monoclinic crystal growth with aspect ratio (L/D) more than 15 was confirmed by SEM studies. XPS of Zr(3d) core level spectra show no change in binding energy before and after exposure to shock waves, confirming that ZrO2 does not change its chemical nature, which is the signature of catalytic surface reaction. When a shock heated argon gas interacted with Ce0.5Zr0.5O2 compound, there was a change in colour from pale yellow to black due to reduction of the compound, which is the effect of heat transfer from the shock wave to the compound in presence of argon gas. The reduction reaction shows the release of oxygen from the compound due to high temperature interaction. The XPS of Ce(3d) and Zr(3d) spectra confirm the reduction of both Ce and Zr to lower valent states. The oxygen storage and release capacity of the Ce0.5Zr0.5O2 compound was confirmed by analyzing the reduction of Ce4+ and Zr4+ with high temperature gas interaction. When Ce0.5Zr0.5O2 (which is same as Ce2Zr2O8) in cubic fluorite structure was subjected to strong shock, it changed to pyrochlore (Ce2Zr2O7) structure by releasing oxygen and on further heating it changed to Ce2Zr2O6.3 which is also crystallized in pyrochlore structure by further releasing oxygen. If this heating is carried out in presence of argon test gas, fluorite structure can easily change to pyrochlore Ce2Zr2O6.3 structure, which is a good electrical conductor. Due to its oxygen storage capability (OSC) and redox (Ce4+/Ce3+) properties, Ce0.5Zr0.5O2 had been used as oxygen storage material in three-way-catalyst. Importance of these reactions is that the O2 gas released from the compound will react with gas released from the heat shield materials, like NOx, CO and hydrocarbon (HCs) species which results in reduction of temperature in the shock layer of the re-entry space vehicle. The compound Ce0.5Zr0.5O2 changes its crystal structure from fluorite to pyrochlore phase in presence of shock heated test gas. The results presented in these two Chapters are first of their kind, which demonstrates the surface catalytic reactions. In Chapter 8, we present preliminary results of the oxygen recombination on the surface of heat shield material procured from Indian Space Research Organization (ISRO) used as TPS in re-entry space capsule (Space capsule Recovery Experiment SRE-1) and on thin film SiO2 deposited on silicon substrate. The formation of SiO between the junctions of SiO2/Si was confirmed using XPS study when shock exposed oxygen reacted on these materials. The surface morphology of the ablated SiO2 film was studied using SEM. The damage induced due to impact of shock wave in presence of oxygen gas was analyzed using Focused Ion Beam (FIB) microscope. The results reveal the damage on the surface of SiO2 film and also in the cross-section of the film. We are further investigating use of FIB, particularly related to residual stress developed on thin films due to high pressure and high temperature shock wave interaction. In Chapter 9, conclusions on the performance of FPST, synthesis of high temperature materials, catalytic and non-catalytic surface reactions on the high temperature material due to shock-heated test gases are presented. Possible scope for future studies is also addressed in this Chapter.

En raison de la complexité de leur composition, l’étude de l’oxydation des gazoles et des carburants biodiesel nécessite de choisir des molécules modèles représentant ces mélanges. Dans ce contexte nous avons sélectionné deux molécules pouvant représenter les gazoles : le n-décane, souvent considéré comme molécule modèle des paraffines contenues dans les gazoles, et le n-hexadécane, molécule de référence pour l’estimation de l’indice cétane, ainsi que deux molécules représentant les carburants biodiesel : le palmitate de méthyle (C17H34O2, ester méthylique saturé) et l’oléate de méthyle (C19H36O2, ester méthylique insaturé). L’étude de l’oxydation de ces molécules a été menée en réacteur auto-agité par jets gazeux, à une richesse de 1, des températures comprises entre 550 et 1100 K, à pression atmosphérique et à un temps de passage constant de 1,5 s. La formation d’un nombre important d’espèces a été observée parmi lesquelles figurent des oléfines, des diènes, des esters méthyliques insaturés, des éthers cycliques avec différentes tailles de cycle, des cétones et des aldéhydes. Grâce à deux nouvelles versions du logiciel EXGAS, des mécanismes cinétiques détaillés de l’oxydation des molécules étudiées ont été générés et validés par comparaison avec les résultats expérimentaux. Enfin, une comparaison de la réactivité du n-décane, du n-hexadécane, du palmitate de méthyle et de l’oléate de méthyle et des quantités de produits formées (dont certains polluants) a été effectuée / Because of the complexity of their compositions, the study of the oxidation of diesel and biodiesel fuels requires choosing model molecules (surrogates) representing the real mixtures. In this context, we have selected two molecules to represent the diesel fuel: n-decane, usually considered as model molecule of paraffin contained in diesel fuel, and n-hexadecane, molecule of reference for the estimation of the cetane number, and two molecules representing biodiesel fuel: methyl palmitate (C17H34O2, a saturated methyl ester) and methyl oleate (C19H36O2, an unsaturated methyl ester). The study of oxidation of these molecules has been conducted in a jet-stirred reactor, with an equivalence ratio of 1, temperatures between 550 and 1100 K, at atmospheric pressure and for a constant residence time of 1.5 sec. The formation of a large number of species has been observed which includes olefins, dienes, unsaturated methyl esters, cyclic ethers with different size of ring, ketones and aldehydes. Using two new versions of EXGAS software, detailed kinetic mechanisms for the oxidation of the studied molecules were generated and validated by comparaison with experiemental results. Finally, a comparison of the reactivity of n-decane, n-hexadecane, methyl palmitate and methyl oleate and amounts of formed products (including some pollutants) has been performed