Numerical exploration and modelling of novel environmentally friendly combustion technique: droplet-laden MILD combustion

每千瓦发电的氮氧化物排放量的小幅减少将显著减少用于发电的燃烧对环境的影响。温和或强烈低氧稀释（MILD）燃烧技术提供了大幅减少排放的机会，同时提高了熔炉和锅炉的热效率。在燃气轮机中，尽管总的燃料-空气混合物是贫燃料的，并且MILD燃烧不直接适用，但是燃烧室的主区域中的富燃料区域表现出局部MILD区域，特别是对于液体燃料操作。这种新技术的物理和化学复杂性还没有得到很好的理解，因此确定了关键的问题，在宽范围的条件下使用该技术用于发电和工业过程的控制参数是具有挑战性的。该项目的目的是提供一个物理的理解和这种燃烧技术的建模，并确定关键的控制参数的一个步骤的变化。本文采用直接数值模拟（DNS）和大涡模拟（LES），对高热值气体和液体燃料的MILD燃烧进行了研究，并对燃烧过程的物理和化学过程进行了高保真的数学描述。对于液体燃料喷雾的液滴，将使用拉格朗日方法跟踪，而气相则使用欧拉方法进行模拟。液滴直径、当量比、（气体和液体燃料）、燃烧产物的稀释程度、挥发性（考虑不同燃料），湍流强度及其长度尺度对燃烧速率的影响，火焰结构（在化学反应途径分析和火焰和流动拓扑学方面）和污染物的形成将根据一个明智的参数分析进行分析，该分析基于三个-详细的化学DNS数据。在该项目中，从DNS数据中提取的基本物理理解将用于开发高保真模型，用于基于计算流体动力学（CFD）的工程模拟，以在根据可用的实验结果验证这些模型后使用LES识别关键控制参数。该项目将提供（1）用于MILD燃烧技术的鲁棒建模框架，这将是设计节能和清洁燃气轮机和工业炉的具有成本效益的可靠工具，以及（2）所确定的关键控制参数可以帮助设计改装的“更绿色”燃烧系统。

A small reduction in NOx emission per kilo-watt of generated power will have a significant reduction in environmental impact of combustion used for power generation. The MILD (Moderate or Intense Low-Oxygen Dilution) combustion technique offers an opportunity to drastically reduce emissions while improving thermal efficiency of furnaces and boil-ers. In gas turbines, though overall fuel-air mixture is fuel-lean and MILD combustion is not directly applicable, fuel-rich regions in the primary zone of the combustor exhibit localised MILD regimes, particularly for liquid fuel operation How-ever, the physical and chemical intricacies of this novel technique are not well understood and thus identifying key con-trol parameters for using this technique for power generation and industrial processes over wide range of conditions is challenging. This project aims to provide a step change in physical understanding and modelling of this combustion technique and to identify key control parameters. The aim is to investigate MILD combustion of high calorific value gaseous and liquid fuels for practical application using Direct Numerical Simulations (DNS) and Large Eddy Simula-tions (LES), with high-fidelity mathematical description for physical and chemical processes involved. The droplets of liquid fuel spray will be tracked using the Lagrangian approach while the gas phase is treated using the Eulerian ap-proach for the simulations. The effects of droplet diameter, equivalence ratio (both for gaseous and liquid fuels), extent of dilution by combustion products, volatility (by considering different fuels), turbulence intensity and its length scale on the burning rate, flame structure (in terms of chemical reaction pathways analysis and flame and flow topologies) and pollutants formation will be analysed based on a judicious parametric analysis based on three-dimensional detailed chemistry DNS data. In this project, the fundamental physical understanding extracted from DNS data will be utilised to develop high-fidelity models for engineering Computational Fluid Dynamics (CFD)-based simulations to identify key control parameters using LES after validating these models against the available experimental results. This project will provide (1) a ro-bust modelling framework for MILD combustion technique, which would be a cost-effective reliable tool for designing energy-efficient and clean gas turbines and industrial furnaces and (2) the key control parameters identified can help to design retro-fit "greener" combustion systems.