In this study, experimental methods are used to investigate the effects of different vibration and pressure parameters on heat transfer performance are analyzed. The results show that at a subcritical pressure (0.1 MPa), the external vibration begins to affect the heat transfer when the fuel passes the phase-change point and becomes gaseous. At a near-critical pressure (3 MPa), the external vibration deteriorates the heat transfer of fuel across the critical-temperature zone. At the supercritical pressure (5 MPa), the external vibration intensifies the heat transfer in the hot end of the channel only when the fuel is below the critical temperature and the internal wall is above the critical pressure. Combined with data analysis, it can be seen that the external vibration mainly acts on the temperature boundary layer of the fuel oil to affect the wall temperature and heat transfer coefficient.
The cooling channel of a scramjet is the fundamental structure of the active thermal protection for an engine. Till now, studies have focused mainly on the steady-state flow and heat transfer process in the cooling channel. However, the vibration intensity of an engine increases sharply as the flight speed increases, because of which, the flow and heat exchange mechanisms based on the cooling channel under stable conditions cannot be applied under vibration. In this study, experimental methods are used to study the characteristics of the forced vibration of a cooling channel on the flow and heat transfer of hydrocarbon fuel at supercritical pressure. In addition, the influences of different vibration frequencies and vibration amplitudes on the flow and heat transfer are analyzed. The research results show that at supercritical pressure, when the fuel temperature is below the critical temperature and the inner wall temperature is above the critical temperature, external vibrations would enhance the heat transfer characteristics of the cooling channel. However, when the pressure and temperature are unstable, the forced vibration of the cooling channel would suppress the instability of temperature and pressure while strengthening the heat exchange.
Hydrocarbon fuel is used as coolant to cool scramjet by flowing through cooling channels at atmospheric pressure and quasi-critical pressure conditions. The instability of the heat transfer will occur in this process. However, the effect of scramjet vibration on the heat transfer instability is unclear. In order to study the effect of cooling channel forced vibration on the unstable heat transfer performance at trans-crtical pressure, cooling channel heat transfer characteristics under different vibration condition are analyzed. Experimental results show that at atmospheric pressure, cooling channel vibration causes a drastic change in the temperature of the inner wall during unstable heat transfer process, but vibration will not change the fuel bulk temperature oscillation process. As a result, forced vibration can lead to heat transfer deterioration in the gas-liquid two-phase flow. Under the condition of quasi-critical pressure, cooling channel vibration not only change the inner wall temperature, but also influence the fuel bulk temperature. The forced vibration can lead to heat transfer enhancement. High frequency vibration can effectively suppress heat transfer instability and reduces heat transfer fluctuations.
Adding an initiator is an effective method of promoting hydrocarbon pyrolysis and improving the heat sink of fuels. Nitropropane was proposed as an initiator with good performance, owing to its lower reaction activation energy for C–N bond cleavage. To study the effects of this initiator on hydrocarbon pyrolysis, a miniature tube reactor that can simulate a real heating procedure in an aeroengine was used to investigate the n-decane pyrolysis with and without nitropropane under experimental supercritical conditions. The results demonstrate that the nitropropane initiator promotes the pyrolysis of fuel as it flows through a tube with a large length–diameter ratio within a certain temperature range. The initial decomposition temperature of n-decane is reduced by approximately 100 K, and the increase in the conversion leads to a higher heat sink for n-decane, which can result in decreases in the fuel and reactor temperatures under the same heating condition and within the effective temperature range. A stronger promoting effect can be achieved by increasing the concentration of the nitropropane initiator. The variation laws for the n-decane pyrolysis reaction rate along the flow reactor are changed by the initiator, the presence of nitropropane greatly accelerates the pyrolysis reaction of fuel at a lower temperature, and the opposite tendency appears as the fuel temperature increases, which is caused by the consumption of the initiator. In addition, the selectivity of methane, propane, and alkenes, especially ethylene, increases because of the propyl radical generated by the C–N dissociation of nitropropane before the initiator is consumed.
Endothermic hydrocarbon fuel has many advantages as a cooling medium in hypersonic aircraft; however substantial application has been greatly hindered due to easy coking, uncontrollable gas products, and low heat sink issues. Catalytic cracking of hydrocarbon fuel has been regarded as one of the most effective ways to improve the heat sink via modulating the cracking pathway. The incipient-wetness impregnation method was selected to load Co salts in commercial ZSM-5 in a large-scale manner; subsequent calcination in Ar gas resulted in Co3O4 nanosheet wrapped HZSM-5 composites. Salinization treatment was adopted to facilitate the dispersion of catalyst in a nonpolar solvent. The ratio of Brönsted/Lewis acidity decreased from 8.00 to 2.79 after modification by Co3O4 nanosheets. The synergistically catalytic effect between Co3O4 nanosheets and ZSM-5 was beneficial to the generation of a larger gas production rate with higher content of alkene, and thus resulting in a higher heat sink than benchmarked fuels. Catalytic cracking of n-decane (C10) in the presence of 0.1 wt % Co3O4 nanosheets@ZSM-5 could yield a heat sink as high as 4.64 MJ/kg at 758 °C, much higher than those of bare ZSM-5 (2.99 MJ/kg at 687 °C) and thermal cracking of C10 (3.77 MJ/kg at 728 °C). Meanwhile, the smart combination of Co3O4 nanosheets and commercial ZSM-5 could effectively suppress the coke deposition on the external surface of composites, thus resulting in efficient catalytic cracking at elevated temperatures for obtaining a higher heat sink.
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