The ignition characteristics of amorphous boron (B) particles in carbon dioxide (CO2) were experimentally investigated using a reflected shock tube over a wide temperature range from 2100 K to 3400 K. The shock tube was equipped with one spectrometer and two monochromators, enabling a comprehensive exploration of the influence of particle size, temperature, pressure and concentration of CO2 on the ignition delay time (tign). Notably, it was found that above 2850 K, tign became more dependent on the temperature, which might be attributed to the pyrolysis of CO2. Meanwhile, tign became more sensitive to the pressure with the decrease of temperature. Specifically, the temporal emission spectra of BO2 from B and pure B2O3 were detected at 2600 K in both CO2 diluted with argon (Ar) and pure Ar atmospheres. Different from the B oxidation in the pure Ar atmosphere, the normalized BO2 spectral signals and their first-order derivatives indicated two ignition stages of B particles in CO2. That is, the first ignition happens in the oxide layer, and the subsequent reaction is on the boron surface. These two stages involved three main reactions: (1) pyrolysis of B2O3 to BO2, (2) reaction of B2O3 and CO2, and (3) reaction of B and CO2. Finally, a theoretical analysis based on valence electron configurations and chemical bonding was performed to explain the bilateral diffusion in stage (1) at the atomic scale.

Nitrous oxide (N2O) and ethanol (C2H5OH) are green propellants that have great potential in propulsion systems. This study reports the experimental measurements of ignition delay times (IDTs) for N2O/C2H5OH mixtures utilizing a shock tube. The influences of the pressure, equivalence ratio, and temperature on IDTs are evaluated within the pressure of 2.5 to 7.5 atm, equivalence ratio of 0.5 to 2.0 and temperature of 1300 to 1914 K. The measured IDTs exhibit an Arrhenius-type temperature dependence and decrease with elevated pressures. Conversely, transitioning from fuel-lean to fuel-rich conditions significantly prolongs the IDTs, with the equivalence ratio exerting a more pronounced influence than pressure. Furthermore, these IDTs are substantially longer than those observed in conventional O2 atmospheres. Several established kinetic models are evaluated, culminating in the development and validation of a revised Shrestha mechanism that demonstrates substantially improved predictive accuracy. Subsequent sensitivity analyses reveal that the global reactivity is mainly driven by the highly endothermic unimolecular decomposition N2O (+ M) = N2 + O (+ M) and the reaction N2O + H = N2 + OH, while ethanol decomposition acts as principal ignition inhibitor. Rate of production and reaction pathway analyses further demonstrate that the drastically extended ignition delay stems from the absence of highly efficient chain-branching steps. Under fuel-rich conditions, intermediate fragments aggressively scavenge the limited radical pool, forcing a thermally-driven induction period. Furthermore, integrated reaction flux analysis elucidates the kinetic origins of high-pressure discrepancies, revealing that elevated pressures competitively dissipate the active radical flux. This work provides essential experimental data and a validated chemical kinetic model to support further engineering applications of N2O/C2H5OH mixtures.

This study successfully ignited 11 mu m amorphous boron (B) powder in a pure carbon dioxide (CO2) atmosphere using a shock tube ignition platform, providing important insights for CO2-B reaction which was generally neglected in propulsion system. The experiments were conducted at temperatures ranging from 2100 to 2900 K and 3.5 atm, focusing on the effects of temperature and CO2 concentration on the ignition characteristics. The results show that the ignition delay time decreases with increasing temperature. And the critical ignition temperature of boron was found to beyond 2100 K. The effects of CO2 concentration on the ignition delay time and critical ignition temperature were much complex. These findings provide new data on boron ignition characteristics in CO2 environments, offering valuable reference information for CO2 propulsion technologies in space exploration.

Due to the incurred damages to the combustors, large-amplitude self-sustained thermoacoustic oscillations are unwanted in many propulsion systems, such as liquid/solid rocket motors and aero-engines. To suppress these thermoacoustic oscillations efficiently, the mechanism of thermoacoustic instability needs to be clarified. Following the previous experimental work, the transitions to instability in a Rijke-type thermoacoustic system with an axially distributed heat source are studied numerically in this paper. The URANS numerical method is utilized and verified by means of a mesh sensitivity analysis. The influences of the axially distributed heater length, the heater location, and the mean flow velocity on the nonlinear dynamic behaviors of thermoacoustic oscillations are evaluated. To explore the corresponding mechanism behind these influences, the principle of acoustic energy conservation has been applied. The acoustic energy gains from the thermal-acoustic coupling are quantified via Rayleigh's integral, and their phase differences are calculated by the cross-correlation function. The acoustic damping induced by the vortex dissipation is qualitatively analyzed by the characteristics of the flow fields in the Rijke tube. Finally, as the heater length, the heater location, or the mean flow velocity is varied, three mechanisms of the transitions to instability in a Rijke-type thermoacoustic system are identified.

氧化亚氮基单组元复合物(nitrous oxide fuel blend,NOFBX)推进剂具有绿色无毒、能量高、可深度节流以及自增压等优点,是一种有前途的新型推进剂.总结了NOFBX推进剂相较于传统推进剂的优势和不足,介绍了国内外在NOFBX推进剂研制、不同量级的发动机设计、发动机热试车、防回火研究和冷却研究等方面的发展现状,提出了NOFBX推进剂发展的关键问题.

In this paper, the effects of the geometry of a submerged nozzle on the nozzle damping characteristics are studied numerically. Firstly, the numerical method is verified by the previous experimental data. Then, the mesh sensitivity analysis and the monitor position independence analysis are carried out. Thirdly, the effects of nozzle geometry on nozzle damping are systematically studied, and focuses are placed on the cavity size, convergent angle and divergent angle. The pulse decay method is utilized to evaluate the nozzle decay coefficient. Several important results are obtained: the submerged cavity with large volume leads to low frequency acoustic oscillations in the combustion chamber and corresponds to a small nozzle decay coefficient; then, as the nozzle convergent angle is decreased, the nozzle decay coefficient is increased. In addition, the nozzle divergent angle has a trivial effect on the nozzle decay coefficient; and lastly, the effects of the temperature on the nozzle damping capability are conducted. The results show that an increase of the working temperature leads to an increase of the nozzle decay coefficient; therefore, the damping force is increased.

Thermoacoustic oscillations are favorable in thermoacoustic engines, but unwanted in many propulsion systems, due to the induced detrimental outcomes to the combustors. To prevent or eliminate the onset of large-amplitude thermoacoustic oscillations, a better understanding of its mechanism is necessary. In the present work, the effects of two different types of background colored noises on the stability of an open-ended standing wave thermoacoustic system are studied theoretically and numerically. A multiplicative and an additive colored noises are considered, which act as a frequency disturbance and a combustion-related disturbance to acoustic waves, respectively. First, a noisy nonlinear thermoacoustic system with a confined premixed flame is modeled. The unsteady heat release rate of the premixed flame to acoustic pressure is described by a fifth-order polynomial. Then an expansion of Galerkin series is applied to decouple the acoustic waves in the spatial and time domains. Stochastic averaging is utilized, and analytical stationary probability density functions (PDFs) of acoustic pressure amplitudes are obtained. Through qualitative changes of PDFs, the stochastic P-bifurcations are discussed. To validate the theoretical predictions of this thermoacoustic system driven by the multiplicative and additive colored noises, a Monte Carlo simulation is conducted. By comparison, the analytical PDFs agree well with the numerical PDFs. Several important results have been obtained: (1) for the two types of colored noises, the noise intensity D and the correlation time τ have significant but opposite effects on the bimodal width and stability range. Furthermore, as the correlation time τ increases, the width of the bimodal region for the noise intensity is increased, and a larger noise intensity is needed to destabilize the thermoacoustic system; (2) compared with the multiplicative colored noise, the additive colored noise impacts the stability of the system in a limited range of operating conditions; (3) by varying the parameters of colored noise, two different types of stochastic P-bifurcations have been observed.