AI 中文总结
本文提出非漂移声学延迟特征边界条件（ADCBC）和平均比例积分线性松弛（APILR）方法，对热声不稳定预混火焰进行直接数值模拟，以降低计算成本并准确捕捉火焰动力学。
AI 中文摘要
预混火焰中的热声不稳定性源于火焰热释放（由燃烧参数决定）与周围声场（由燃烧室几何决定）之间的耦合。一次不稳定性导致火焰变平，同时内在火焰不稳定模态被稳定化。二次热声不稳定性导致参数性火焰不稳定性和声振幅的急剧增长。由于计算成本相对较高，这些现象的数值模拟仍然很少。本文对一端声学封闭、另一端开口的管道中热声不稳定的理想化预混火焰进行了直接数值模拟（DNS）。结果表明，随着火焰变平，一次不稳定性出现非线性饱和，同时出现非定常瑞利-泰勒效应特征的振荡火焰指状化。为降低计算成本，仅对火焰周围区域进行DNS。入口和出口处的声学采用纳维-斯托克斯特征边界条件（NSCBC）方法描述，以模拟其延迟重入计算域，该公式称为声学延迟特征边界条件（ADCBC）方法。还引入了一种新的平均比例积分线性松弛（APILR）方法，该方法对经典线性松弛（CLR）方法进行改进，以保持入口速度和出口压力的时间平均值。这里使用积分控制项来消除非零的平衡时间平均入口速度，该速度会影响对火焰位置的控制。这两种新方法在惰性和反向流火焰测试案例中展示了其能力。这些方法能够以显著降低的计算成本实现燃烧不稳定性的数值模拟。
英文摘要
Thermoacoustic instability in premixed flames results from the coupling between the flame's heat release, as determined by combustion parameters, and surrounding acoustics, as determined by combustor geometry. A primary instability results in flame flattening as intrinsic flame instability modes are stabilised. Secondary thermoacoustic instability results in a parametric flame instability and drastic growth of acoustic amplitudes. Due to their relative expense, numerical simulations of these phenomena remain scarce. In this work, Direct Numerical Simulations (DNS) of thermoacoustically unstable idealised premixed flames in a tube with acoustically closed upstream and open downstream ends are presented. Results herein demonstrate nonlinear saturation of the primary instability as the flame flattens as well as an oscillating flame fingering characteristic of the unsteady Rayleigh-Taylor effect. To reduce computational cost, we perform DNS only on the region surrounding the flame. Acoustics at in- and outflows are described using the Navier-Stokes Characteristic Boundary Condition (NSCBC) method to model their delayed reentry into the domain in a formulation referred to as the Acoustic Delay Characteristic Boundary Condition (ADCBC) method. A new Averaged Proportional and Integral Linear Relaxation (APILR) method is also introduced, which modifies the Classic Linear Relaxation (CLR) method to maintain time-averaged values of inflow velocity and outflow pressure. Here, an integral control term is used to remove non-zero equilibrium time-averaged inflow velocities which impinge control over flame position. Both new methods demonstrate their capability in inert and counterflow flames test cases. These methods enable the numerical simulation of combustion instabilities at significantly reduced computational expense.
