层流预混火焰燃烧特性的研究对降低发动机污染物排放与稳定燃烧有重要意义。本论文采用球形火焰，进行了层流预混火焰传播特性与不稳定的实验研究。本研究的目标是：提高层流火焰速度测量精确度并探究其对火焰化学的敏感性；揭示火焰锋面不稳定与火焰加速全局振荡的物理控制机制。为进一步深入研究火焰传播与不稳定性特性指明方向。首先，采用氢气/空气和丙烷/空气火焰，研究了降低层流火焰速度测量中外推模型不确定度的方法。结果表明，外推模型的不确定度包括模型误差与随机性误差。模型误差可通过减小|Lb/Rf|来降低。固定工况下，减小|Lb/Rf|可通过增大火焰半径上界或下界来实现。根据实验数据定义了一个经验型参数Rf,new来表征整个半径范围，以便预测模型误差。同时，随机性误差由实验点的数目控制。其次，采用乙烯/空气火焰，研究了压力与当量比对火焰化学的影响。测量了宽压力与当量比范围下乙烯/空气火焰的层流火焰速度，并量化了不确定度。敏感性结果表明，压力越高，层流火焰速度对化学反应越敏感；主要反应的相对重要性受当量比影响显著，而对压力不敏感。之后，采用H2/O2/N2火焰，通过改变N2含量控制已燃火焰温度，研究了流体力学不稳定性、热质扩散不稳定性各自以及两者耦合对不稳定火焰形态与临界半径的影响。结果表明，大尺寸胞状结构受流体力学不稳定性控制，微小结构受热质扩散不稳定性控制；临界半径随压力升高、刘易斯数减小而显著减小，但对火焰温度不敏感。无量纲临界半径随刘易斯数减小而显著减小，对压力和火焰温度不敏感；此外，无量纲临界拉伸率随马克斯坦数增加而单调递减。最后，对火焰加速现象的研究发现，火焰传播分为三个阶段，即：平滑、转捩以及全局振荡阶段。全局振荡的频率随火焰锋面不稳定性增强而变大，这与假设中的全局振荡由胞状结构的生长与分裂引起相一致。在流体力学不稳定性与热质扩散不稳定性耦合作用下，氢气/空气火焰的全局振荡频率无量纲化之后对压力不敏感，但随刘易斯数减小而单调增加。这表明无量纲化消除了压力和火焰温度的影响。同时计算了转捩阶段与全局振荡阶段的加速因子，后者小于自湍流的预测值1.5。
Understanding the properties of laminar premixed flames is of essential importance in the pollutant reduction and stability control of the flames in engines. In this dissertation, the outwardly expanding spherical flame is adopted in an experimental study of the propagation and instability of premixed flames. Specifically, this dissertation aims at improving the accuracy of laminar flame speed measurements, revealing the response of the laminar flame speed to flame chemistry, and providing physical insights into the flame-front instability as well as global pulsation in flame acceleration.In this dissertation, the extrapolation uncertainty in laminar flame speed measurements was first quantified and reduced, using hydrogen/air and propane/air flames. Results show that the extrapolation uncertainty consists of model error and random error. A small value of the parameter, |Lb/Rf|, allows for the neglect of the model error by increasing the upper and the lower bounds of the flame radius range. A new empirical parameter, Rf,new, was defined according to the experimental results to represent the entire flame radius range. The random error is mainly affected by the number of points in the extrapolation. Second, the pressure and equivalence ratio effects on the flame chemistry were investigated using ethylene/air flames. The laminar flame speeds, with quantified uncertainties, were measured over a wide range of pressures and equivalence ratios. Sensitivity analysis shows that the laminar flame speed becomes more sensitive to flame chemistry at higher pressures, while the relative importance of chemical reactions is notably affected by the equivalence ratio and insensitive to pressure. Third, H2/O2/N2 flames were used to reveal the separate and coupling effects of hydrodynamic and diffusional-thermal instabilities on the morphologies and critical flame radii of unstable flames, whose burned flame temperatures are controlled through manipulating the amount of N2 in the air. It is shown that large cellular structures are dominated by the hydrodynamic instability while the smaller ones by the diffusional-thermal instability. The critical flame radius decreases significantly with increasing pressure or decreasing Lewis number, and is insensitive to the burned flame temperature. The normalized critical flame radius decreases as the Lewis number decreases and is insensitive to pressure and burned flame temperature. Meanwhile, the normalized critical flame stretch monotonously decreases as the Markstein number increases.Finally, three stages were observed in the self-acceleration of unstable flames, i.e. smooth expansion, transition and global pulsation stages. It is shown that the global pulsation frequency increases with increasing intensity of the flame-front instability, which is consistent with predictions from the hypothesis that the global pulsation behavior arises from the continuous cell growth and splitting during the flame acceleration. The normalized global pulsation frequencies of H2/air flames, subjected to the coupling of hydrodynamic and diffusional-thermal instabilities, collapse under different pressures and decrease with increasing equivalence ratio. This indicates that the pressure and flame temperature effects are properly scaled out through the normalization. The acceleration exponents of the transition stage and global pulsation stage were also determined, with the latter slightly smaller than the critical value of 1.5 suggested for self-turbulization.
