Effects of Turbulence on Droplet Evaporation and Combustion: Droplet-Resolved Direct Numerical Simulation

在大多数用于汽车和飞机的内燃机中，燃料是以液体的形式喷射的。液体燃料液滴的蒸发以及随后与周围空气的混合决定了燃料如何被点燃和燃烧。内燃机中的湍流运动提高了蒸发、混合和燃烧的速率。更好地理解这些过程将导致更高效的引擎。在本项目中，湍流对燃料液滴蒸发和燃烧的影响将采用一种新的计算方法进行研究。数值模拟将解决液滴周围和内部的所有相关过程，以促进对发动机相关条件下液滴-湍流-化学相互作用的基本理解。先进的基本认识将指导工程模型的改进，并加速内燃机的发展，以满足未来的燃油效率和污染物排放标准。教育部分包括对本科生和研究生进行计算燃烧研究方面的培训，并通过学院内的既定项目进行K-12外展。该项目是表征湍流对液滴蒸发速率、液滴间标量混合、点火和火焰结构的影响，当液滴的大小与最小湍流长度尺度Kolmogorov尺度相当或更大时。将开发一种新的计算方法，能够在发动机相关条件下精确稳定地计算湍流中液滴的蒸发和燃烧。采用本文提出的方法，对均匀各向同性湍流和瞬时发展混合层中单组分和双组分燃料液滴的蒸发和燃烧进行直接数值模拟。将对DNS数据进行分析，以描述现有液滴蒸发速率的层流理论可以使用的条件。分析的重点还将放在确定表征湍流对蒸发速率影响的关键参数上。将分析蒸发和燃烧液滴附近的小尺度混合统计数据，以帮助改进喷雾燃烧建模。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

In most combustion engines used in automobiles and airplanes, fuel is injected in the form of the liquid. The evaporation of liquid fuel droplets and subsequent mixing with the surrounding air determine how fuel is ignited and burned. Turbulent fluid motions in the combustion engines enhance the rates of evaporation, mixing and burning. Better understanding of these processes will lead to more efficient engines. In this project, the effects of turbulence on the evaporation and combustion of fuel droplets will be investigated using a new computational method. Numerical simulations that resolve all relevant processes around and inside the droplets will be performed to advance the fundamental understanding of the droplet-turbulence-chemistry interactions under engine-relevant conditions. The advanced fundamental understanding will guide the improvement of engineering models and accelerate the development of combustion engines that meet future standards for fuel efficiency and pollutant emissions. The educational component includes the training of undergraduate and graduate students in computational combustion research and the K-12 outreach through an established program in the college.The project is to characterize the effects of turbulence on droplet evaporation rates, inter-droplet scalar mixing, ignition, and flame structures, when the size of droplets is comparable to or larger than the Kolmogorov scale, the smallest turbulence length scale. A new computational method, which can enable the accurate and stable computation of droplet evaporation and combustion in turbulent flows under engine-relevant conditions, will be developed. Direct numerical simulation (DNS) of the evaporation and combustion of single-component and bi-component fuel droplets in homogeneous isotropic turbulence and in temporally developing mixing layers will be performed using the developed method. The DNS data will be analyzed to delineate the conditions under which the existing laminar theory for the droplet evaporation rates can be used. The emphasis of the analysis will also be placed on identifying the key parameters that characterize the turbulence effects on the evaporation rates. The small-scale mixing statistics in the vicinity of evaporating and burning droplets will be analyzed to help improve spray combustion modeling.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.