Enhancement and Control of Turbulent Reactive Flows via Electrical Fields - A Mesoscopic Perspective

根据英国和世界可预见未来的能源结构，需要新的燃烧概念和先进的发动机技术来大幅提高能源效率和减少排放，以便对气候和人类健康产生最直接和最显著的有益影响。电场辅助燃烧是控制火焰的可行选择，可改善火焰稳定性，提高效率并减少污染物排放。火焰在弱电离等离子体状态下，因为在反应区通过化学电离和随后的离子化学产生带电粒子。一项有前途的技术是利用电场作为执行器来调节火焰，以实现最佳燃烧和最小排放。电场辅助燃烧是一个多物理、多尺度、非平衡的过程。电场对带电粒子（阳离子、阴离子和电子）的直接作用发生在原子尺度上。再往上，就会产生阳离子的漂移或离子风。在宏观尺度上，可以观察到火焰的传播、结构以及在某些情况下的不稳定性。湍流具有从微观到宏观的广泛范围，在电场、离子风、湍流和火焰之间发生了大量的中尺度相互作用。现有的研究主要集中在宏观现象上，很少有实验或数值方法研究原子事件与宏观现象之间的重要联系。此外，与上述现象相关的时间尺度范围很广，这导致了水动力、热力学和化学的不平衡。非平衡效应很少被量化，如果有研究的话。随着ARCHER等国家HEC平台的可用性，现在可以及时地解决电场、燃料化学、离子化学、火焰和湍流之间复杂的相互作用，以进一步了解潜在的机制。此外，电场对湍流火焰的影响将通过先进的模拟技术进行量化。在这个项目中，先进的数值模拟将进一步发展，并用于阐明电场-火焰相互作用的关键物理机制，最终导致利用电场控制和优化燃烧的技术。在大量内部专业知识和成功的初步研究的基础上，直接数值模拟（DNS）将进一步发展，以结合现实离子化学来研究宏观行为，如湍流火焰结构、动力学和外部电场存在下的不稳定性。此外，我们新开发的介观模拟方法，即能够模拟非平衡燃烧的离散玻尔兹曼方法（DBM），将用于揭示电场与火焰在介尺度上的关键相互作用。这项研究将回答电场“神奇”效应背后许多悬而未决的基本问题。例如，施加的电场如何影响化学途径？湍流如何影响电场和火焰之间的动量和能量传递？火焰的宏观性质如何受到介观和原子事件的影响？回答这些问题将有助于我们制定燃烧控制策略，从而降低排放，提高能源利用效率。

Based on the UK and the world's energy structures for the foreseeable future, new combustion concepts and advanced engine technologies are required to drastically increase energy efficiency and reduce emissions in order to have the most direct and significant beneficial impact on the climate and human health. Electric field assisted combustion can be a viable option in the control of flames, leading to improved flame stability, higher efficiency and reduced pollutant emissions. Flames are under weakly ionized plasma states, since charged particles are generated in the reaction zones through chemi-ionization and subsequent ion chemistry. One promising technology is to utilize an electric field as an actuator for modulating a flame in order to achieve optimal burning and minimal emissions.Electric field assisted combustion is a multi-physical, multiscale, and nonequilibrium process. The direct action of the electric field is on charged particles (cations, anions and electrons), which happens at the atomic scale. Further up the scale, a drift of cations or an ionic wind is generated. At macroscales, flame propagation, structure and, in some cases, instability are observed. As turbulence spans a wide range from micro- to macroscales, numerous mesoscale interactions occur among the electric field, ionic wind, turbulence and flame. Existing studies have been focused on macro-phenomena, while crucial links between the atomic events and the macro-phenomena have rarely been investigated by either experimental or numerical methods. In addition, there is a wide range of time scales associated with the above phenomena, which causes hydrodynamic, thermodynamic and chemical nonequilibrium. Nonequilibrium effects have rarely been quantified if studied at all. With the availability of the national HEC platform such as ARCHER, it is now feasible and timely to tackle the complex interactions among the electric field, fuel chemistry, ion chemistry, flame and turbulence in order to further our understanding of the underlying mechanisms. Moreover, effects of the electric field on turbulent flames will be quantified by advanced simulation techniques.In this project, advanced numerical simulations will be further developed and employed to clarify the key physical mechanisms responsible for the electric field - flame interactions, ultimately leading to technologies for control and optimization of combustion using electric fields. Building on substantial in-house expertise and successful preliminary studies, direct numerical simulation (DNS) will be further developed to incorporate realistic ion chemistry to study the macro-behaviours such as turbulent flame structure, dynamics and instability in the presence of an externally applied electric field. In addition, our newly developed mesoscopic simulation approach, the discrete Boltzmann method (DBM) capable of simulating nonequilibrium combustion, will be applied to revealing the crucial interactions between the electric field and the flame at mesoscales. The study will answer many unanswered fundamental questions behind the "magic" effect of the electric field. For example, how are chemical pathways affected by the imposed electric field? How does turbulence affect momentum and energy transfer between the electric field and the flame? How are macro-properties of flames affected by mesoscopic and atomistic events? Answering these questions will help us to develop strategies for combustion control, leading to lower emissions and more efficient energy utilization.