基于动态分区概念的高超声速燃烧大涡模拟

LARGE EDDY SIMULATION OF HYPERSONIC COMBUSTION BASED ON DYNAMIC ZONE CONCEPT

本文基于动态分区概念开展了亿级网格的高马赫数全尺寸超燃冲压发动机内外流耦合一体化改进延迟分离涡(IDDES)模拟研究.研究建立了包括动态分区火焰面湍流燃烧模型(DZFM)、分区自适应化学(ZDAC)和分区并行自适应建表(Z-ISAT)的完整动态分区燃烧模拟框架,并通过1.15亿网格的马赫数12 REST标准高超声速燃烧室模型初步验证了分区模拟框架的保真性.DZFM通过分区解耦的思想既准确表征了当地湍流化学交互作用关系,又有效提升了整场湍流燃烧的计算效率.Z-DAC和Z-ISAT通过在分区框架内对化学反应机理进行动态实时简化和建表查询,可进一步提升当前分区内化学反应的求解效率.基于1.25和1.4亿网格动态分区框架对比分析了马赫数10条件下中心支板(strut)和壁面撑挡型(pylon)两类构型氢气高超声速燃烧室特性.支板或撑挡结构均诱发了明显的边界层分离和头部回流区,由此两种燃烧室均出现了较长区域的喷注点前部燃烧现象.基于Borghi图的数值分析表明当前氢气高超声速燃烧室中广泛存在扩散控制为主的火焰面模式,效率提升的瓶颈在于高效增混.壁面撑挡燃烧室具有较高的穿透深度和近场混合效率,因而燃烧效率高于净推力准则80%,相应的比冲1234 s也远高于中心支板燃烧室的437 s.分区自适应化学方法在将近一半的计算域上降低了反应求解计算代价,特别是在无燃料区反应机理的简化幅度更加明显.相比与传统的有限速率PaSR模型,DZFM模型实现了高达11倍的加速比.

Based on the concept of dynamic zone partition, improved delayed detached eddy simulation(IDDES)modeling of high-Ma full-scale scramjets with more than 100 million cells was conducted for the integrated internal and external flow fields. A complete dynamic zonal combustion modeling framework was established, including dynamic zone flamelet model(DZFM), zonal dynamic adaptive chemistry(Z-DAC), and zonal in situ adaptive tabulation(ZISAT). The fidelity of the zonal modeling framework is preliminarily verified by the 115-million-cell modeling of a benchmark hypersonic combustor named REST, which was designed to operate at Mach 12. Through the idea of local flow-chemistry decoupling within each zone, DZFM not only accurately represents the local turbulence-chemistry interaction but also effectively improves the computational efficiency of turbulent combustion in the whole field. Z-DAC and Z-ISAT can further improve the resolving efficiency of chemical reactions in each zone by dynamically reducing the chemical mechanism and tabulating the thermochemical states. Then based on 125 and 140 million cells, respectively, the characteristics of hydrogen-fueled strut and pylon hypersonic combustors were comparatively analyzed for Mach 10.Both the pylon and strut structures induce obvious boundary layer separation and fore-body recirculation zone, resulting in long pre-combustion regions in front of the injection point in both combustors. Numerical analysis based on the Borghi diagram shows that the diffusion-dominated flame mode widely exists in the current hydrogen-fueled hypersonic combustor, and the bottleneck of efficiency improvement lies in efficient mixing. The pylon combustor has higher jet penetration depth and better near-field mixing, and thus the combustion efficiency of 80% is above the criterion of achieving net thrust. The specific impulse of 1234 s in the pylon combustor is also much higher than the 437 s in the strut combustor. Z-DAC reduces the computational cost of reaction systems in nearly half of the computational domain,especially in the fuel-free regions. Compared with the traditional finite-rate PaSR model, the DZFM model achieves an acceleration ratio of up to 11.