| Endothermic hydrocarbon fuels (EHFs), as a kind of flammable coolants, circulate through the cooling passages located on the hot wall surface to absorb the waste heat before they are injected into the combustion chamber. The heat sink of EHF comes from the physical heat (enthalpy change and heat of phase transition) and chemical heat (ring-opening, chain breaking and dehydrogenation reactions). When working in an engine, an EHF served as both propellant and coolant is under supercritical condition. Therefore, the characteristics concerning thermal cracking, coking and heat transfer under supercritical conditions have been attracting much attention. In this article, the determination of heat sink, the cracking kinetics and the application of nanoparticles in EHFs are studied in detail. The results are expected to provide experimental and theoretical information for the engieneering application of EHFs in hypersonic aircrafts.The main contents and results are summarized as follows:An electrically heated tube reactor is built by simulating a single heat-exchanger passage in the engine to determine the heat sink of hydrocarbon fuels. Nitrogen, cyclohexane and p-xylene are used to calibrate the apparatus, and the instrument constant is obtained. As a standard substance, the heat sink of toluene is determined at 300?750? and 3.5 MPa with the mass flow rate of 1 g·s-1. Compared with the theoretically calculated values, the relative errors of the experimental values are less than 3%, which can satisfy the requirement for engineering applications. The impacts of temperature, pressure and flow rate on the heat sink of a kerosene-based fuel are investigated. The results show that the heat sink increases with increasing the temperature or decreasing the flow rate. The pressure affects not only the conversion, but also the selectivity of alkene of the kerosene-based fuel. So, the impact of pressure on heat sink of fuel is much complex.n-Nonane, n-decane, n-undecane, n-dodecane and n-tridecane are chosen as model fuels for normal paraffins, and the impacts of temperature and flow rate on the conversions and heat sinks of the five normal paraffins are investigated under supercritical conditions (450?750?,3.5 MPa). It follows that the conversion and heat sink increase significantly with increasing the temperature or with decreasing the flow rate. Under the same conditions, the normal paraffin with larger carbon number performs cracking more easily and has a higher chemical heat sink. At the low temperature region (< 550?,3.5 MPa,1 g·s-1), the cracking conversion of normal paraffin is very small, and the total heat sink only depends on the physical part with the chemical part being ignored. Thus, the heat sink of the longer chain paraffin is smaller than that of the shorter chain paraffin. At the high temperature region (> 600?,3.5 MPa,1 g·s-1), the cracking conversion becomes larger. The chemical heat sink occupies a larger proportion of the total heat sink. The largest conversion is 93% and the largest chemical heat sink occupies 38% of the total heat sink. Thus, the heat sink of the longer chain paraffin is larger than that of the shorter chain paraffin. The coking behaviors of the five normal paraffins are also investigated at 710? and 750?. It indicates that more coke precursors are produced during the cracking process of the longer chain paraffin, which leads to more coke deposit.Thermal cracking kinetics of a model fuel, 1,1'-bicyclohexyl, has been performed in a batch type reactor to investigate its thermal stability. The Arrhenius parameters are determined with the pre-exponential factor A=6.22×1020 h-1 and the activation energy Ea=293 kJ·mol-1. Compared with four typical hydrocarbon compounds, the thermal stability trend is observed in the order:decalin>n-propylcyclohexane> bicyclohexyl>1,3,5-triisopropylcyclohexane>n-dodecane. On the basis of the product distribution, it is found that bicyclohexyl decomposes into cyclohexane and cyclohexene equivalently at the beginning of the reaction. Cyclohexene is consumed to form aromatics and small molecules due to the secondary reactions. Cyclohexane is not consumed due to its high thermal stability. A probable mechanism on the basis of density functional theory (DFT) calculation and GC-MS analyses for the decomposition of bicyclohexyl is proposed to explain the product distribution. Three different kinds of lipophilicity palladium nanoparticles modified by octadecanethiol, octadecylamine, and the mixture of them have been prepared, which are simply marked as Pd@S, Pd@N and Pd@S&N in turn. The average diameters for the nanoparticles are 1-3 nm. The cracking performances of the nanofluids composed of Pd nanoparticles and decalin/kerosene are studied. It follows that Pd nanoparticles can catalyze the cracking of decalin/kerosene effectively, and the catalytic ability of Pd@N is better than those of Pd@S and Pd@S&N.The fundamental properties and cracking performances of seven model fuels and five practical fuels have been investigated, and are correlated with their hydrogen to carbon ratios (H/C) and molecular weight (M). It follows that the hydrocarbon fuel with a higher H/C has a higher mass energy content and lower volumetric energy content. The densities and viscosities decrease with the increase of (H/C)/M0.1 and (H/C)/M0.5. The flash points show a decreasing trend with the increase of (H/C)/M2. As cracking in an electrically heated tube reactor, the hydrocarbon fuel with a higher H/C performs cracking more easily, absorbs more heat and forms fewer cokes.The impacts of temperature, flow rate and pressure on the heat transfer security of a naphthene-based fuel (Fuel 1 containing additives, H/C=1.89) and a paraffin-based fuel (Fuel 5, H/C=2.10) have been investigated under the conditions:700-770?, 0.4-1.0 g·s-1 and 2.5-5.5 MPa. The results show that the run time of heat transfer test would be shortened with increasing the temperature, decreasing the flow rate and raising the pressure. It also indicates that the higher temperature and the lower flow rate are conducive to the release of heat sink, while the higher pressure limits the release of heat sink. Under the same condition, Fuel 1 possesses a longer run time of heat transfer test and a smaller heat sink than Fuel 5. The value of the density of Fuel 1 is 6.8% larger than that of Fuel 5, and the heat sink of Fuel 1 is 4.7-5.8% smaller than that of Fuel 5. In consideration of the volumetric heat sink, Fuel 1 is more preferable than Fuel 5. It means that Fuel 1 can perform a longer time of heat transfer test in an engine at a higher working temperature. |
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