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

在未来几十年内，燃气轮机（GT）和压缩点火（CI）发动机不太可能从运输和能源部门中被取代，因为它们提供了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，但GT和CI发动机仍遭受颗粒物（PM）、NOx、CO2和未燃碳氢化合物（UHC）排放的困扰。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受欢迎的污染物，同时提高燃料灵活性和效率，但这一具有挑战性的任务受到我们对这些发动机所涉及的高度复杂的燃烧过程的理解不足的限制。历史上，由于极端具有挑战性的环境（光学通路、高压等），实验研究仅在GT和CI发动机条件下提供定性结果。因此，尽管它们的重要性，控制燃烧过程和污染物形成的机制仍然是肤浅的理解。直接数值模拟（DNS），解决所有相关的时间和长度尺度的湍流燃烧过程的高保真模拟方法，最近才出现作为一种替代方案来解开这些复杂的机制。在拟议的研究中，DNS将用于探索目前限制GT和CI发动机战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集中在燃气轮机燃烧。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（与燃气轮机运行条件相关）对这些动力学的作用，特别是在高压和高温条件下，这几乎没有从根本上探讨。在这种情况下，一系列的DNS将进行暴露的影响，燃料成分，热化学条件和湍流强度的结构/稳定和NOx的形成湍流火焰在GT条件。第二部分将讨论CI发动机条件下射流火焰的稳定机理。这些火焰在远离燃料喷射器的距离处点燃、发展并稳定，燃料喷射器控制燃烧前燃料-空气混合的量，并因此控制产生的污染物（PM和UHC）的水平。由于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。建议的DNS将首先探索CI射流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）和燃料效应，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.