微重力环境消除了由重力引起的自然对流, 在近极限火焰的现象研究方面有着较大的优势。该文以二甲醚层流射流扩散火焰为研究对象, 采用清华大学自由落体设施以及相应的射流火焰装置, 探究了在常重力和微重力环境中火焰推举高度的控制因素。结果表明, 随着氮气稀释比例的增大, 火焰的推举会由最初的浮力主控转变为由惯性力主导; 随着二甲醚体积分数的增加, 火焰推举高度上下限之间的距离逐渐缩小, 可能存在某一推举火焰的临界体积分数。火焰进入微重力状态后, 在推举高度不变的情况下, 对应水平线与化学当量比轮廓交点处的流速增加, 火焰温度随着其比表面积的增大而降低, 展现出较小的火焰传播速度, 导致火焰的推举高度增加。该文通过对比常重力和微重力环境下二甲醚层流射流扩散火焰的推举高度变化, 揭示了浮力、惯性力以及化学当量比对火焰行为的控制机制, 深化了对近极限火焰动力学特性的理解, 为微重力环境下燃烧过程的优化提供了理论依据。

Objective: Studying near-limit flames under microgravity conditions offers significant advantages because the elimination of gravity-driven natural convection removes buoyancy effects, which simplifies the analysis of flame dynamics. Key objectives include examining the influence of nitrogen dilution and gravity on flame lift-off height and its mechanisms governing flame stability. Methods: This research investigates the controlling factors in lifted laminar diffusion flames of dimethyl ether (DME) under both normal gravity and microgravity environments, utilizing the Tsinghua University Freefall Facility and its dedicated jet flame apparatus. Results: The experimental results reveal that increased nitrogen dilution ratio shifts flame lift-off behavior from buoyancy-dominated to inertia-dominated. This shift is attributed to reduced chemical heat release and diminished buoyancy-driven convection. Additionally, as the DME volume fraction increases, the range between the upper and lower limits of lift-off height narrows, suggesting a critical DME concentration. Beyond this critical point, the flame struggles to stabilize, indicating fundamental constraints in flame behavior under certain fuel concentrations.Under microgravity, flames behave differently compared to normal gravity. At the same lift-off height, flames display higher flow velocity at the intersection of the stoichiometric contour and horizontal lift-off plane. This outcome is attributed to reduced momentum dissipation in the absence of gravity, leading to a concentrated and faster flow field. Furthermore, the flame's relative surface area increases, leading to enhanced heat loss and a subsequent reduction in flame temperature. Consequently, the flame propagation speed decreases, causing the flame to lift higher to maintain stability.The study also explores the theoretical aspects of flame stability by balancing flow velocity with flame propagation speed along the stoichiometric contour. In normal gravity, buoyancy acts as a key stabilizing force. However, in microgravity, where buoyancy is negligible, stability relies on chemical reactions, radiation, and forced convection. Theoretical models support these findings, showing how flow fields and flame propagation dynamics differ across gravity environments.The relative time scales of buoyancy, chemical reactions, and heat transfer are critical in determining flame behavior. Under normal gravity, the time scale of buoyancy-induced convection aligns with that of chemical reactions, making buoyancy a major factor in flame stability. However, in microgravity, the buoyancy time scale becomes much larger, rendering buoyancy effects negligible and allowing other factors, such as chemical kinetics and heat loss, to dominate.The findings of this study have broader implications for combustion science, particularly in space exploration and propulsion system design. Understanding flame behavior in microgravity is crucial for designing safe and efficient combustion systems for spacecraft, where fire safety and fuel efficiency are paramount. Additionally, these insights have terrestrial applications, contributing to cleaner and more efficient engine designs. Conclusions: This research sheds light on the transition from buoyancy-dominated to inertia-dominated flame lift-off behavior with increasing nitrogen dilution, highlighting the role of a critical DME volume fraction that limits flame lift-off behavior. The study enhances our understanding of DME flame dynamics under microgravity, with potential applications in spacecraft fire safety, combustion optimization, and propulsion systems. The results underscore the critical role of gravity in designing and analyzing combustion systems, particularly for space-related applications. Future work could extend these findings to other fuels and more complex combustion scenarios, further advancing our understanding of combustion in extreme environments.