Rotating detonation engine (RDE) is one of the engine in which detonation waves propagate circumferentially in annular or cylindrical combustor. On July 27th 2021, a detonation engine system (DES) utilizing gaseous methane and gaseous oxygen was launched using JAXA/ISAS sounding rocket S-520 No. 31. As a result, the operation of RDE for 6 s was successfully demonstrated in space. However, using liquid propellant is essential to achieve high delta V. Thus, the demonstration of liquid RDE can elevate the technology readiness level of RDE as a propulsion system. On November 14th 2024, DES2 with liquid-ethanol liquid-nitro-oxide RDE was launched from Uchinoura Space Center using S-520 No.34 and the combustion for approximately 7 s was confirmed. According to the signal of the vibration sensor mounted on the RDE combustor, it was found that double-wave and single-wave mode of the RDE were sustained. The time-averaged thrust and propellant-based specific impulse during the RDE operation was estimated at 415±52 N and 244±28 s, respectively.

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This paper describes introduction on fundamentals of supersonic simulation for combustion technology for super/hyper sonic propulsion system. First, fundamental equations of compressible flow are shown with explanation of terms in the equations and numerical treatment for simulations. Second, boundary treatments are described based on the characteristic velocities of compressible flow. For compressible flow, all the information propagates at characteristic velocities as local velocity, and local velocity plus and minus sound speed. Treatments of inflow and outflow of compressible flow are explained with some typical conditions. Finally, two kinds of simulation results are shown, supersonic crossflow with air and fuel injection and rotating detonation engines.

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Experimental investigations have been conducted to investigate hydrocarbon-fueled supersonic combustion mechanisms using a model scramjet combustor equipped with a cavity flameholder, connected to a high-enthalpy wind tunnel at the University of Tokyo. Due to the high thermal load, cooling of the combustor walls is essential, necessitating the use of endothermic reactions of hydrocarbon fuels, specifically thermal cracking within the cooling channel. This process primarily produces hydrogen, methane, ethane, ethylene, and propane, and the impact of these components on supersonic combustion has been thoroughly examined. The inclusion of hydrogen in the thermally cracked components showed limited enhancement in combustion performance. Conversely, ethylene emerged as the most reactive component among the cracked hydrocarbons, significantly improving combustion performance upon addition. Detailed investigations into supersonic combustion modes were conducted for fuel mixtures of methane and ethylene, identifying combustion mode characteristics across various stagnation temperatures. Additionally, combustion instability in ethylene supersonic combustion was explored using data-driven methodologies, including proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD). The sparsity-promoting dynamic mode decomposition (SP-DMD) elucidated the combustion instability mechanism between supersonic jet wake-stabilized combustion and ram combustion. Non-linear dimensionality reduction techniques, such as the Gaussian process latent variable model (GPLVM) and the Gaussian process dynamic model (GPDM), were employed to assess the influence of fuel penetration height on combustion modes. These advanced data-driven approaches provided a detailed understanding of the mechanisms underlying supersonic combustion instabilities.

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Propulsion systems for supersonic to hypersonic speed regime, especially for hypersonic flight were overviewed, and domestic R&D activities were surveyed with emphasis on combustion problems. Both ramjet and scramjet engines were the prime concern, and several types of combined cycle engines were picked up. In addition, a unique hypersonic flight test was introduced.

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Using hydrogen is expected to be one of the optimal choices for a Carbon Neutral Society because it is a CO₂-free energy source and serves both as an energy carrier for renewable energy and a raw material for chemical products. This paper outlines the results of our research on more than 250 hydrogen-hydrocarbon-dilution gas-oxygen mixtures to clarify the importance of the preferential diffusion effect in premixed turbulent combustion characteristics. The preferential diffusion effect in turbulent combustion influences the local burning velocity of turbulent flames due to the molecular diffusivity of the reactants. Therefore, the turbulent combustion of hydrogen, which has a higher molecular diffusivity because of its lowest molecular weight, is strongly affected by the preferential diffusion effect. First, to clarify the turbulent burning velocity characteristics of fully hydrogen, it is shown that the equivalence ratios and fuel types affect the turbulent burning velocity and the local burning velocity characteristics by using special mixtures with the same laminar burning velocity. Then, a model is proposed to predict the turbulent burning velocity and the quenching limit by using the local burning velocity as a reference instead of the original laminar burning velocity. Furthermore, the influences of dilution gases and hydrocarbons on turbulent burning velocity characteristics are presented. Finally, for the development of a comprehensive turbulent combustion control method, we will examine the relationship between the local burning velocity, an important factor in determining turbulent burning velocity characteristics, and the Lewis number or the Markstein number, which can be determined from physical properties and laminar flames.

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During the operation of gas water heaters, a self-excited oscillation phenomenon known as combustion oscillation may occur, where the combustion reaction excites acoustic oscillations of the gas at the system's eigenfrequencies. Combustion oscillation can generate noise and compromise safety and reliability. Even gas water heaters tested to avoid combustion oscillation may still experience it if the exhausting duct shape is altered to meet practical requirements. During the development phase, repeated tests are necessary to ensure stability across various duct configurations, which can be time-consuming. To address this issue, we propose a method of diagnosing combustion oscillation in gas water heaters with only minimal experimentation. We experimentally determine the frequency dependence of the acoustic impedance of a household gas water heater, which is then used to estimate the natural oscillation frequency when an exhausting duct of arbitrary length is attached. Additionally, the energy balance between the acoustic power supplied by the gas water heater and the acoustic power dissipated in the duct is evaluated using the measured acoustic impedance. Verification experiments confirmed the occurrence of combustion oscillations at the predicted duct lengths and frequencies.

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