航空发动机湍流不稳定燃烧内生共振耦合的热声振荡机理研究

While premixed combustion can effectively reduce NOx emission, turbulent flow in premixed combustor may cause unsteady heat release, which brings about combustor thermoacoustic instability by interacting with combustor cavity modes. Recently, it is found that unsteady heat release may leads to intrinsic thermoacoustic modes in premixed flame through theoretical, numerical as well as experimental studies. Furthermore, these flame intrinsic modes, which need more seriously investigate, are independent on the combustor cavity modes. Present project tries to address the flame intrinsic mode through studying the interaction between unsteady heat release and perturbations of acoustics, entropy, and vorticity based on the modeling of mechanisms of linear and nonlinear unsteady heat release affected by disturbances from pressure, temperature, velocity and equivalence ratio in different kinds of premixed flames (triangle, cone as well as wedge flame geometries). Meanwhile, special attentions are placed on the stability criteria in theoretical, numerical and experimental studies. Furthermore, interactions between intrinsic modes and combustor thermoacoustic modes are modeled under the effects of turbulent damping, non-uniform mean flow and temperature, as well as complex boundary conditions of the combustor’s inlet and outlet. Parametrical analysis is imposed on its coupled features. Present study can provide theoretical support on thermoacoustic instabilities in combustors of aircraft engine.

贫油预混燃烧可以有效降低NOx排放，同时气流湍流流动引起不稳定热释放，与燃烧室声学特征耦合可以形成热声振荡。理论、数值与实验研究表明不稳定热释放能够引发火焰内生模式的不稳定振荡，且不受燃烧室声学特征的影响；然而当前对其机理的理解还不够完善。本项目拟对贫油预混湍流燃烧中火焰内生模式机理与特征开展研究，并深入分析该模式与燃烧室热声振荡的线性与非线性耦合特征。项目将在建模不同火焰面（锥形、三角形、楔形）中速度、温度、压强、燃料空气混合比等扰动线性以及非线性作用下不稳定热释放的基础上，构建声、涡与熵扰动与湍流随机脉动作用下的火焰内生模型动力学模型，从理论、计算与实验三方面分析火焰内生模式特征以及稳定性判据。构建湍流漩涡耗散、非均匀流场和温度场、燃烧室出入口复杂边界条件影响下的内生模式与热声振荡耦合动力学模型，参数化分析不同工况下耦合特征。研究成果为航空发动机燃烧室热声振荡抑制提供理论基础。

贫油预混燃烧可以有效降低NOx排放，同时气流湍流流动引起不稳定热释放，与燃烧室声学特征耦合可以形成热声振荡。理论、数值与实验研究表明不稳定热释放能够引发火焰内生模式的不稳定振荡，且不受燃烧室声学特征的影响。从流体动力学方程出发，在忽略热粘耗散的情况下，本项目推导了燃烧室中声波、涡波以及熵波的动力学模型，构建了三类典型燃烧室（纵向、旋转以及环形）结构中火焰内生模式与腔体振荡模式的耦合模型。利用模型分析了相应的动力学特征，研究了不同火焰的不稳定热释放模型对内生模态以及腔体模态的影响，给出了内生模式稳定性的解析判决条件，深入分析了不同工况下内生与腔体模式到燃烧室热声振荡模态的演化过程，研究了基于声阻抗（反射系数）的抑制所有燃烧室热声振荡的可行性问题。通过构建基于OpenFOAM的燃烧室热声振荡DNS仿真平台以及试验平台，深入分析了声与火焰的耦合机理。研究表明燃烧室中热声振荡由内生模式与腔体声波模式组成，在线性情况下，两类模式不发生耦合。旋流燃烧室中旋流位置将显著影响内生模式的稳定性，对腔体声学模式的影响具备周期性特征。环形燃烧室中燃烧腔的增加显著增加了内生模式数量，呈现聚类特征。内生模式到热声振荡模式的演化过程存在交叉的现象。此外，通过改变声阻抗/反射系数无法保证同时抑制内生模式与腔体声学模式引发的热声振荡。研究成果为航空发动机燃烧室热声振荡抑制提供理论基础。