Thermo-acoustic instabilities are problematic in the design of continuous-combustion propulsion systems such as gas turbine engines, rocket motors, jet engine afterburners, and ramjets. Conceptually, the coupling between acoustics and flame dynamics can be divided into two categories: flame area fluctuations and changes in the local flame speed. The latter can be caused by the thermodynamic fluctuations that accompany an acoustic wave. This coupling is the focus of the present work. In this paper, we are concerned with the dynamics of laminar premixed flames involving large hydrocarbon species. Through high-fidelity numerical simulations, we investigate the flame response for a wide range of fuels and acoustic frequencies. The combustion of hydrogen and methane is considered for verification purposes and as baseline cases for comparison with two large hydrocarbon fuels, n-heptane and n-dodecane. We extract the phase and gain of the unsteady heat release response, which are directly related to the Rayleigh criterion and thus the stability of the system. For all fuels, we observe a local peak in the heat release gain. At high frequencies, we find that the fluctuations of the different species mass fractions decrease with the inverse of the acoustic frequency, leading to chemistry being “frozen” in the high-frequency limit. This allows us to predict the flame behavior directly from the steady-state solution.

在连续燃烧推进系统的设计中，例如燃气涡轮发动机，火箭发动机，喷气发动机加力燃烧室和冲压喷气发动机，热声不稳定性是有问题的。从概念上讲，声学和火焰动力学之间的耦合可以分为两类：火焰面积波动和局部火焰速度的变化。后者可能是由伴随声波的热力学波动引起的。这种耦合是当前工作的重点。在本文中，我们关注涉及大型烃类的层流预混火焰的动力学。通过高保真数值模拟，我们研究了多种燃料和声频的火焰响应。氢和甲烷的燃烧被认为是用于验证的目的，并且是与两种大型烃燃料（正庚烷和正十二烷）进行比较的基准案例。我们提取了不稳定热释放响应的相位和增益，这与瑞利准则以及系统的稳定性直接相关。对于所有燃料，我们都观察到了放热增益的局部峰值。在高频下，我们发现，不同物种质量分数的波动随着声频的倒数而减小，从而导致化学物质在高频范围内被“冻结”。这使我们能够直接从稳态解中预测火焰行为。与瑞利准则直接相关，从而与系统的稳定性直接相关。对于所有燃料，我们都观察到了放热增益的局部峰值。在高频下，我们发现，不同物种质量分数的波动随着声频的倒数而减小，从而导致化学物质在高频范围内被“冻结”。这使我们可以直接从稳态解中预测火焰行为。与瑞利准则直接相关，从而与系统的稳定性直接相关。对于所有燃料，我们都观察到了放热增益的局部峰值。在高频下，我们发现，不同物种质量分数的波动随着声频的倒数而减小，从而导致化学物质在高频范围内被“冻结”。这使我们可以直接从稳态解中预测火焰行为。