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Abstract:Due to their extremely low vapor pressure, low toxicity and high thermal stability, hypergolic ionic liquids are considered to be the most potentially green fuels to replace hydrazine and its derivatives as a new generation of propellant. A better understanding of hypergolic combustion and ignition process will promote the practical application of ionic liquids-based propellant systems. In this paper, recent advances in ignition and combustion mechanism of hypergolic ionic liquids are reviewed, which include three aspects: reaction routes and mechanism, ignition and combustion process, and theoretical prediction of hypergolicity. The reaction mechanism of dicyanamide anion and nitric acid, several stages and phenomena of ignition and combustion of hypergolic ionic liquid, several methods for predicting hypergolicity are introduced and summarized. The future researches should include the reaction mechanism of other anions, the development of green oxidizer to replace those toxic ones, normalization of the ignition device and method, and the accurate prediction of ignition performance.

Abstract:High-density and high-thermal-stability alkyl diamondoid fuels have drawn tremendous attention since they can provide more propulsion energy and remove excess heat for aerospace vehicles. Recent progress on synthesis of alkyl diamondoid fuels via rearrangement and alkylation is summarized in this review. Proposed rearrangement pathway is put forward by density functional theory calculation and reaction results. Also, regioselective synthesis of alkyl diamondoids with specific alkyl group (methyl, ethyl, propyl, butyl) is conducted via alkylation of adamantane derivatives. The performance of alkyl diamondoid fuels is highlighted including density, low-temperature performance and especially thermal stability, and correlated with the hydrocarbon structure. The compact structure with short alkyl substituents and more rings of carbon leads to a high density, while the low-temperature performance is dependent on the symmetry of molecular structure. The thermal stability is mainly related to the type of carbon contained in the molecule structure, in the order of quaternary carbon > primary carbon > secondary carbon > tertiary carbon. Through the regioselective and high throughput synthesis method which is expected to be applied in the future, the alkyl group can be orientatedly substituted on the tertiary carbon atom of the adamantane core, and then converted to the quaternary carbon atom, improving thermal stability while maintaining higher density of alky diamondoid.

Abstract:Fuel cooling ability is highly critical to advanced aircraft, so it is of great urgency to develop high-performance endothermic fuel and related reaction technology. Based on the characteristics of alcohol promoting the pyrolysis of heavy oil and improving the selectivity of low carbon olefins, we propose to couple the catalytic cracking of hydrocarbon fuel with ethanol transformation to improve the heat-absorbing capability. A variety of coating catalysts were prepared on a high-temperature alloy tube using a slurry coating method. Endothermic reaction performances were evaluated on a self-made electric heating reaction evaluation apparatus. The research found that, under low and medium temperatures, the Ni/ZSM-5 coating can significantly promote the dehydration of ethanol to produce ethylene, and the heat sink of mixed fuel is increased by about 20% at 510 ℃. At higher temperatures, the water removed from ethanol participates in the steam reforming reaction to promote fuel heat absorption. A homogeneous additive can also work synergistically with the coating catalyst to show a good coking inhibition performance via improving the stable operating temperature and give a maximum heat sink of 3.71 MJ·kg^-1 at 791 ℃. These results provide a strong reference for the future design of the actively cooling scheme of aircraft.

Abstract:The ignition delay times (IDTs) of two different certified gasoline and diesel fuel blends are reported. These measurements were performed in a shock tube and in a rapid compression machine over a wide range of experimental conditions(φ= 0.5-2.0, T=700-1400 K and p=10-20 bar) relevant to internal combustion engine operation. In addition, the measured IDTs were compared with two relevant gasoline fuels: Coryton gasoline and Haltermann gasoline systematically under the same experimental conditions. Two different gasoline surrogates a primary reference fuel (PRF) and toluene PRF (TPRF) were formulated, and two different gasoline surrogate models were employed to simulate the experiments. Typical pressure and equivalence ratio effects were obtained, and the reactivity of the four different fuels diverge in the negative temperature coefficient (NTC) regime (700-900 K). Particularly at 750 K, the discrepancy is about a factor of 1.5-2.0. For the high Research Octane Number (RON) and high-octane sensitivity fuel, the simulation results obtained using the TPRF surrogate was found to be unreasonably slow compared to experimental results, due to the large quantity of toluene (77.6% by volume) present. Further investigation including reactants′concentration profile, flux and sensitivity analyses were simultaneously carried out, from which, toluene chemistry and its interaction with alkane (n-heptane and iso-octane) chemistry were explained in detail.

Abstract:Endothermic hydrocarbon fuels undergo thermal cracking before entering the combustion chamber and can produce a mixture of unreacted fuels and pyrolysis products (i.e. cracked fuels). The objective of this work is to investigate the effects of pyrolysis conversions, pyrolysis pressures, ignition pressures and free radicals on ignition characteristics of cracked n-decane over temperature of 1300-1800 K, pressure of 0.1-3.0 MPa and equivalence ratio of 1.0. Components of the thermally cracked n-decane at 3.0 and 5.0 MPa in a flow reactor were calculated theoretically using an accurately combined mechanism, which are in good agreement with the experimental results in literature. The results showed the conversion rates of n-decane cracking at 3 and 5 MPa are 46.2% and 58.8%, respectively. The distribution of cracking products is consistent, but the ethylene content decreases with the increase of pressure, while the alkane content increases with the increase of pressure. Meanwhile, the content of free radicals at 3 MPa is slightly higher than that at 5 MPa, but the content of free radicals is very low. Ignition delay time increases with the decreasing of n-decane conversion and pyrolysis pressure, while higher ignition pressure can shorten it significantly. Furthermore, the presence of free radicals in cracked n-decane could accelerate the ignition process with ignition delay time shortening more than 15% when the conversion was less than 40%, compared with that of cracked n-decane without radicals.

Abstract:Aviation kerosene is a typical endothermic hydrocarbon fuel, and its pyrolysis gas absorbs heat before entering combustion chamber, which plays an important role in the heat protection of high-speed aircraft. Auto-ignition delay time of hydrocarbon fuel and its pyrolysis gas is one of main parameters for ramjet design, and is important data to validate the combustion reaction mechanism. In this work, the reflected shock wave was used to ignite fuels. Aviation kerosene and its pyrolysis gas in a chemical shock wave tube was studied. Ignition delay time was defined as the time interval between the arrival of reflected wave indicated by the jump of pressure signal and the onset of CH* emission signal. Auto-ignition delay times of RP-3kerosene，pyrolysis gas, hydrogen, methane, ethylene and ethane were measured in the temperature range of 900-1820 K, at pressure of 1.01×10⁵ Pa and equivalence ratio of 1.0. Experimental results demonstrate that ignition delay time decreases with the increase of temperature. In the same condition, ignition delay time of methane is the longest and that of hydrogen is the shortest, and the ignition delay of pyrolysis gas is slightly longer than that of aviation kerosene. The activation energy of pyrolysis gas is very close to that of aviation kerosene, around 180 kJ·mol^-1.The lowest ignition activation energy of all single component is hydrogen, which is 127.8 kJ·mol^-1. The experimental results were compared with the simulation results of combustion kinetic mechanism, which can predict the influence of temperature on ignition delay time. Furthermore, sensitivity analysis of the mechanism was carried out, and the main elementary reactions affecting fuel ignition were obtained.

Abstract:To increase the energy of liquid fuels and solve the problem that the nanoparticles cannot be stably dispersed, here a low-molecular weight gellant (LMWG) was used to synthesize JP-10 gel containing nano-sized aluminum particles (Al/JP-10). The critical gellant concentration and the transition temperature of JP-10 gels were determined, and the basic physical properties such as density, viscosity, volumetric heat and physical stability of metallized JP-10 gels were studied. Their rheological properties were also investigated by shear-thinning tests, thixotropic tests, strain sweep tests and frequency-sweep tests. Results show that aluminum nanoparticles can be stably dispersed in the LMWG/JP-10 gels, and the gels can be converted into liquid state under the condition of shearing or heating. The addition of LMWG and nano-sized aluminum nanoparticles significantly increases the density, volumetric heat value and viscosity of gels. When the content of aluminum is 25%, the density, viscosity and volumetric heat value of 2% LMWG/JP-10 is 1.156 g·mL^-1, 840 mPa·s, 45.8 MJ·L^-1, respectively. When the aluminum particles content is less than 25%, the stability of the gel system is decreased. While when the aluminum particles content reaches 25%, the stability of gelled fuel is better than that of gelled fuel without nanoparticles. Although the presence of metal nanoparticles significantly enhances the mechanical strength and structural stability of the gel system, the gels still maintain good shear-thinning characteristics and have less gel recovery after shearing.

Abstract:In this study, the suitable process conditions for upgrading coal-derived oil for aerospace application have been explored. Shenhua coal direct liquefaction oil was used as the raw material. Due to the characteristics of high sulfur content, high oxygen content, and large unsaturation, a process of “alkali washing-hydrodesulfurization-hydrogenation saturation” was employed. Using the self-developed catalysts (NiMoW/Al₂O₃ and Pd/Al₂O₃), the optimal reaction conditions were determined to be 5 MPa and 300 ℃ for hydrodesulfurization, and 4 MPa and 210 ℃ for hydrosaturation. The as-prepared product oil possessed high density and high net heat value. The as-prepared fuel also exhibited high withstanding temperature of 550 ℃ and good thermal oxidation stability.

Abstract:In order to enhance the combustion performance of high-density hydrocarbon fuel exo-tetrahydrodicyclopentadiene (JP-10), JP-10 based Pt nanofluid fuels stabilized by oil soluble hyperbranched polyglycerol (HPG) and hyperbranched polyethyleneimine (HPEI) were designed and synthesized. Fuel-dispersible Pt nanoparticles Pt@HPG and Pt@HPEI were synthesized through the phase-transfer method and characterized by transmission electron microscopy (TEM), X-ray photoelectron spectrometry (XPS), X-ray diffraction (XRD) and thermogravimetric analysis (TG). The average sizes of Pt@HPG and Pt@HPEI were 1.3 nm and 2.4 nm. There were slight differences in density and viscosity between Pt@HPG/JP-10 and JP-10. Compared to JP-10, Pt@HPEI/JP-10 had no significant difference in density, while the viscosity decreased obviously, which can reduce the flow resistance. Oxygen bomb calorimetric experiments were carried out to investigate the combustion performance of nanofluids Pt@HPG/JP-10 and Pt@HPEI/JP-10. The apparent values of combustion heat of various nanofluids increased markedly in comparison with that of JP-10. The apparent combustion heat of Pt@HPG/JP-10 and Pt@HPEI/JP-10 increased by about 7.8% and 7.6%, respectively. JP-10 based Pt nanofluids stabilized by hyperbranched polymers are supposed to be novel kinds of potential high-energy fuels.

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