High-fidelity simulations of turbulent combustion relevant to gas turbines and compression ignition engines

燃气轮机 (GT) 和压缩点火 (CI) 发动机不太可能在未来几十年内从交通和能源领域被取代，因为它们提供无与伦比的能量和功率密度水平，并且非常适合补充间歇性可再生能源（例如风力涡轮机、太阳能）。尽管不断改进，GT 和 CI 发动机仍受到颗粒物 (PM)、氮氧化物、二氧化碳和未燃烧碳氢化合物 (UHC) 排放的影响。在当前的环境、经济和政治背景下，发动机制造商必须大幅减少这些不良污染物，同时提高燃料灵活性和效率，但这项艰巨的任务因我们对这些发动机所涉及的高度复杂的燃烧过程了解甚少而受到限制。从历史上看，由于环境极具挑战性（光学通道、高压等），实验研究仅提供 GT 和 CI 发动机条件下的定性结果。因此，尽管控制燃烧过程和污染物形成的机制很重要，但人们对它们的了解仍然肤浅。直接数值模拟 (DNS) 是一种高保真模拟方法，可解析湍流燃烧过程的所有相关时间和长度尺度，直到最近才作为解决这些复杂机制的替代方案出现。在拟议的研究中，DNS 将用于探索目前限制 GT 和 CI 发动机策略减少污染物排放潜力的关键科学问题。该计划的第一部分将重点关注 GT 燃烧。为了减少二氧化碳排放并从不断增长的替代燃料市场中受益，氢气被用作燃料添加剂。然而，氢的高反应性完全改变了燃烧动力学。目前还不清楚强烈的湍流（与 GT 运行条件相关）对这些动力学的影响，特别是在高压和高温条件下，这方面的研究很少。在此背景下，将进行一系列 DNS 分析，以揭示燃料成分、热化学条件和湍流强度对 GT 条件下湍流火焰的结构/稳定性和 NOx 形成的影响。第二部分将讨论 CI 发动机条件下喷射火焰的稳定机制。这些火焰在距喷油器一定距离处点燃、发展并稳定，喷油器控制燃烧前的燃油-空气混合量，从而控制产生的污染物（PM 和 UHC）水平。由于人们对复杂的稳定机制知之甚少，CI 发动机受到燃料灵活性的影响。拟议的 DNS 将首先探索 CI 喷射火焰的稳定机制对重要缸内控制参数（例如温度和废气稀释）的敏感性，并将研究燃料影响，重点是生物柴油等替代燃料。

It is unlikely that gas turbines (GTs) and compression ignition (CI) engines will be displaced from the transportation and energy sectors within decades to come as they provide unmatched levels of energy and power density and are highly suitable to complement intermittent renewables (e.g., wind turbines, solar). Despite continuous improvements, GTs and CI engines suffer from particulate matter (PM), NOx, CO2, and unburnt hydrocarbon (UHC) emissions. In the current environmental, economical and political context, it is essential for engine manufacturers to drastically reduce these undesirable pollutants, while increasing fuel flexibility and efficiency, but this challenging task is constrained by our poor understanding of the highly complex combustion processes involved in these engines. Historically, experimental studies have only provided qualitative results at GT and CI engine conditions due to the extremely challenging environments (optical access, high pressure, etc.). As a result, despite their importance, the mechanisms that control the combustion process and pollutant formation remain superficially understood. Direct numerical simulation (DNS), a high-fidelity simulation approach that resolves all relevant time and length scales of the turbulent combustion process, has only recently emerged as an alternative to unravel these complex mechanisms. In the proposed research, DNS will be used to explore key scientific questions that currently limit the potential of GT and CI engine strategies to reduce pollutant emissions. A first portion of the program will focus on GT combustion. To reduce CO2 emissions and to benefit from the growing alternative fuel market, hydrogen is being used as a fuel additive. However, the high reactivity of hydrogen changes completely the combustion dynamics. It is unclear what role intense turbulence (relevant to GT operating conditions) plays on these dynamics, especially at high pressure and temperature conditions, which has been little explored fundamentally. In this context, a series of DNS will be conducted to expose the effects of fuel composition, thermochemical conditions and turbulence intensity on the structure/stabilization and NOx formation of turbulent flames at GT conditions. The second portion will address the stabilization mechanism of jet flames at CI engine conditions. These flames ignite, develop, and stabilize at a distance away from the fuel injector, which controls the amount of fuel-air mixing prior to combustion and, as a result, the level of pollutants produced (PM and UHC). As little is understood about the complex stabilization mechanism, CI engines suffer from fuel flexibility. The proposed DNS will first explore the sensitivity of the stabilization mechanism of CI jet flames to important in-cylinder controlling parameters (e.g., temperature and exhaust gas dilution) and fuel effects will be investigated with a focus on alternative fuels such as bio-diesel.