燃料射流-空气协流湍流混合过程中的自点火机理研究

Autoignition of fuel-air mixture in the premixing duct is a serious concern for the development of next generation low emissions gas turbine engines that employ lean premixed combustion technology. Autoignition-induced flashback may burn the premixing duct or even the fuel nozzle. Autoignition in the premixing duct is essentially the autoignition of inhomogeneous mixtures during the turbulent mixing process. It differs from the traditional kinetics-controlled autoignition process of homogeneous mixtures in that both the mixture fraction field and the temperature field are impacted by the turbulence, and hence the autoignition delay time is also impacted by the turbulence. In particular, when the turbulent mixing time scale is comparable to the chemical time scale, the ignition delay time is determined by the strong coupling between turbulence and chemistry. The proposed project aims to study the mechanisms that govern the autoignition and subsequent flame propagation during the turbulent mixing of fuel jet and preheated air coflow, using a flow reactor. The study will elucidate the coupled effects of turbulent transport and NTC chemistry on the autoignition process, and clarify the mechanism of turbulence-chemistry coupling in the process. A model will be developed to predict the autoignition delay time in the presence of turbulence-chemistry interaction. Methods will be proposed to avoid autoignition and flashback associated with the turbulent mixing process to assist the development of lean premixed/prevaporized combustor.

新一代燃气涡轮航空发动机贫预混低排放燃烧室的开发受到燃料空气预混段中自点火问题的困扰：自点火可发展为回火，烧蚀预混段甚至燃料喷嘴。该问题实质上是湍流混合过程中非均匀混合物的自点火问题，与传统意义上均匀混合条件下的自点火有很大区别：其组分场与温度场受湍流输送影响，导致点火延迟时间受湍流影响，而不是仅由化学动力学决定。尤其是当湍流时间尺度与化学时间尺度接近时，其自点火过程受湍流-化学强烈耦合作用。本项目将使用湍流流动反应器，研究燃料射流与预热空气协流湍流混合过程中的自点火机理，以及后续的火焰传播机理，分析湍流输送与化学反应耦合（在负温度系数区间）对该过程的影响，阐明其中的湍流-化学反应耦合机制和作用规律。进一步建立数学模型，以较为准确地预测湍流-化学耦合作用下的自点火延迟时间。并据此提出避免湍流混合过程中发生自点火及回火的方法，为贫预混（预挥发）燃烧室的设计提供指导。

燃气轮机贫预混燃烧室中燃料与空气湍流混合过程中发生自点火并引起回火，烧毁燃料喷嘴，是发展新一代高参数燃机亟需解决的关键问题。针对这一问题，本课题以燃料射流-空气协流为研究对象，搭建了射流火焰实验平台，开展燃料射流与空气协流混合过程的自点火研究。实验研究燃料-空气混合过程中湍流-化学耦合对自点火的影响，以及自点火发生后的火焰稳定机理，建立模型，预测自点火发生的时间（火焰抬升高度），并判断其稳定机理。.实验中观测到三类自点火现象：稳定的抬升火焰、随机出现的间歇点火和附着火焰。基于二元逻辑回归建立了机器学习模型以预测自点火的发生和类型。结果显示，机器学习方法能快速、准确地预测混合过程中自点火的发生和火焰类型，并揭示其关键影响因素。.进一步地，通过调节空气协流中的氧含量、及空气协流速度、燃料射流速度和燃料稀释浓度，获得了自点火后形成的射流火焰抬升高度的变化规律。结果显示，随着空气中氧含量的提高，射流火焰抬升高度降低，吹熄极限速度增加；随着燃料射流速增加，自点火后形成的火焰类型发生转变，火焰抬升高度会发生突增。通过将实验数据与现有抬升高度预测模型对比，得出了影响自点火射流火焰稳定的主要因素，并以此为基础发展了新的模型，能够较为准确的预测火焰的抬升高度和着火时间，并判断其主要稳定机理。