Outwardly Expanding Premixed Flames in Turbulent Media

化石燃料的燃烧仍然是发电厂、汽车发动机、炉子和锅炉的主要能源。这是一个复杂的过程，涉及大量化学物种的相互扩散和化学反应产生的大量热释放。在大多数应用中，燃烧发生在湍流中，这给系统增加了波动的、依赖时间的、三维的方面。这个项目的目标是开发能够解释相关物理和化学的预测工具。重点是从一个小的点火源开始的向外扩展的火焰的演变。火焰最初看起来是光滑的和球形的，但由于燃烧过程和湍流固有的不稳定性，变得高度波纹。火焰表面积的增加导致燃料消耗和传播速度的提高，这一过程可能会转变为爆炸和/或爆轰。对造成这些复杂火焰特性的机理的基本了解将提高燃烧系统的可预测性，并提高其安全和有效的运行。拟议的研究还具有重要的教学价值；倡导的以物理为基础的方法将用于课堂和旨在扩大科学和技术人力资源基础的外联方案。我们将在流体力学理论的框架内，利用多尺度渐近方法从一般守恒定律出发，系统地研究由火焰不稳定性和湍流引起的球形扩张火焰的复杂动力学。火焰被视为密度不连续的表面，根据获得的火焰速度关系传播到新鲜混合物中，该关系与火焰前沿的条件相结合，模拟了火焰区内发生的扩散和化学反应的影响。该模型没有常用的湍流燃烧模型中的参数调整和特殊的子网格模型。它的数值实现使用嵌入流形方法，其中火焰表面隐式表示为标量场的第零水平集。由于火焰表面是明确确定的，所有与其传播相关的信息将直接包含在火焰拓扑和同一位置的流场中。所获得的关于火焰不稳定性和火焰-湍流相互作用的知识将有助于指导这一领域的实验研究并改进大规模的计算工作，通过使用基金会的智力优势和更广泛的影响审查标准来替换特殊的集中参数和基于物理第一原则的原型火焰配置。该奖项反映了NSF的法定使命，并被认为值得支持。

Combustion of fossil fuels remains the primary energy source in power generating plants, vehicle engines, furnaces, and boilers. It is a complex process that involves the inter-diffusion of a large number of chemical species and substantial heat release generated from the chemical reactions. In most applications the burning takes place within a turbulent flow that adds a fluctuating, time-dependent, three-dimensional aspect to the system. The objective of this project is to develop predictive tools that account for the relevant physics and chemistry. The focus is on the evolution of an outwardly expanding flame initiated from a small ignition source. The flame, which appears initially smooth and spherical, becomes highly corrugated due to instabilities inherent to the combustion process and to the turbulence. The increase in flame surface area results in enhanced fuel consumption and propagation speed, a process that may potentially transition to an explosion and/or detonation. Fundamental understanding of the mechanisms responsible for these complex flame properties will improve predictability of combustion systems and increase their safe and efficient operation. The proposed research has also a significant pedagogical value; the advocated physics-based approach will be used in the classroom and in outreach programs intended to extend the human-resources base of science and technology. The complex dynamics of spherically expanding flames resulting from flame instabilities and turbulence will be investigated within the framework of the hydrodynamic theory, derived systematically from the general conservation laws using a multi-scale asymptotic approach. The flame, treated as a surface of density discontinuity, propagates into the fresh mixture in accordance to the procured flame speed relation which, in conjunction with the conditions across the flame front, mimic the influences of diffusion and chemical reaction occurring within the flame zone. The model is free of tuning parameters and ad-hoc sub-grid models that plague commonly used turbulent combustion models. Its numerical implementation uses an embedded manifold approach, wherein the flame surface is represented implicitly as the zeroth level set of a scalar field. Since the flame surface is determined unambiguously, all pertinent information to its propagation will be directly contained in the flame topology and in the flow field at the same location. The acquired knowledge on flame instabilities and flame-turbulence interactions will serve to guide the experimental studies in this area and improve large-scale computational efforts, by replacing ad-hoc lumped parameters with prototypical flame configurations that are fundamentally sound and based on physical first principles.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.