Computational Simulation of Complex Turbulent Diffusion Flames

虽然描述湍流燃烧的原理和基本方程是已知的，但它们的直接数值解对于实际问题来说仍然是不可能的。通常的近似是时间平均或对基本方程进行空间滤波。第一种方法涉及全时间平均(雷诺平均的纳维-斯托克斯方程，或RAN)，而第二种方法只对较小的尺度进行滤波，允许在空间和时间上分辨较大的尺度(大涡模拟，或LES)。在任何一种情况下，与平均过程相关的半经验建模假设都是至关重要的，并一直是许多辩论的焦点。这些方法在许多火焰的模拟方面取得了重大成功，主要是由自由喷流和剪切层等边界层状流动以及不受局部熄灭干扰的燃烧所主导的火焰。然而，大多数实用的火焰是通过再循环或漩涡来稳定的，并表现出熄灭事件，例如，与燃烧器的对峙。这些现象没有被目前的模型很好地捕捉到。为了改进模型，必须考虑以下问题：强烈弯曲的流线、浮力、强烈变化的密度、热释放引起的膨胀及其对流动整体动力学的影响、以及高剪切率区域火焰化学的局部熄灭和重燃。本研究以悉尼大学钝体实验室火焰为原型，对这些现象进行了模拟研究。底部的钝体建立了稳定火焰的回流区。这项实验工作涵盖了快速化学和接近井喷行为之间的广泛条件。因此，这些实验展示了在更复杂、更实际的火焰中发生的许多具有挑战性的现象。然而，它们具有简单的轴对称结构，并且已经在实验中得到了很好的表征。因此，这些火焰是对复杂火焰行为建模的基础研究的极佳目标。拟开展下列研究活动：(A)为更好地了解(1)再环流稳定喷流火焰中的消光/重燃机制，以及(2)由大密度差、膨胀、弯曲流线和浮力主导的火焰的物理和稳定性，开展了大涡模拟和理论工作；(B)调查/评价LES次网格尺度模型，其中纳入了在消光/重燃和关于再循环火焰特有的流体力学问题上获得的新信息；(C)将大涡模拟计算与关于钝体稳定喷流火焰的实验室数据进行比较，以测试本项目开发的次网格尺度模型的性能。

Although the principles and fundamental equations that describe turbulent combustion are known, their direct numerical solution (DNS) remains impossible for practical problems. The usual approximations are to either time average or to spatially filter the fundamental equations. The first approach involves full time averaging (Reynolds averaged Navier-Stokes equations, or RANS), while the second filters only for the smaller scales, allowing the larger scales to be resolved in space and time (Large Eddy Simulation, or LES). In either case, the semi-empirical modeling assumptions associated with the averaging process are critical, and have been the focus of much debate. These approaches have had substantial successes in the modeling of many flames, principally those dominated by boundary-layer-like flows such as free jets and shear layers, and combustion that is uninterrupted by local extinction. Most practical flames are, however, stabilized by recirculation or swirl, and exhibit extinction events, e.g., standoff from the burner. These phenomena are not well captured by the present models. To improve the modeling one must consider issues such as strongly curved streamlines, buoyancy, strongly varying density, dilatation due to heat release and its influence on the overall dynamics of the flow, and local extinction and reignition of the flame chemistry in regions of high shear rates. In this study, the bluff-body laboratory flames of the University of Sydney are used as a prototype for the study of the modeling of these phenomena. A bluff body at the base establishes a recirculation zone that stabilizes the flame. This experimental work has covered a wide range of conditions between fast chemistry and close-to-blowout behavior. These experiments therefore exhibit many of the challenging phenomena occurring in more complex, practical flames. They are, however, of a simple axisymmetric configuration and have been experimentally well characterized. Thus, these flames are excellent targets for fundamental investigations into the modeling of complex flame behavior. The following research activities are proposed: (a) DNS and theoretical work towards better understanding of (1) the mechanisms of extinction/reignition in recirculation-stabilized jet flames, and (2) the physics and stability of flames dominated by large density differences, dilatation, curved streamlines, and buoyancy; (b) investigation/evaluation of LES subgrid-scale models that incorporate the new information gained on extinction/ reignition and on fluid-mechanics issues specific to recirculating flames; and (c) comparison of LES computations with laboratory data on bluff-body stabilized jet flames to test the performance of the subgrid-scale models developed in this project.