Abstract
The development of better laser-based experimental methods and the fast rise in computer power has created an unprecedented shift in turbulent combustion research. The range of species and quantities measured and the advent of kHz-level planar diagnostics are now providing great insights in important phenomena and applications such as local and global extinction, pollutants, and spray combustion that were hitherto unavailable. In simulations, the shift to LES allows better representation of the turbulent flow in complex geometries, but despite the fact that the grid size is smaller than in RANS, the push towards realistic conditions and the need to include more detailed chemistry that includes very fast species and thin reaction zones emphasize the necessity of a sub-grid turbulent combustion model. The paper discusses examples from current research with experiments and modelling that focus on flame transients (self-excited oscillations, local extinction), sprays, soot emissions, and on practical applications. These demonstrate how current models are being validated by experimental data and the concerted efforts the community is taking to promote the modelling tools to industry. In addition, the various coordinated International Workshops on non-premixed, premixed, and spray flames, and on soot are discussed and some of their target flames are explored. These comprise flames that are relatively simple to describe from a fluid mechanics perspective but contain difficult-to-model combustion problems such as extinction, pollutants and multi-mode reaction zones. Recently, swirl spray flames, which are more representative of industrial devices, have been added to the target flames. Typically, good agreement is found with LES and some combustion models such as the progress variable - mixture fraction flamelet model, the Conditional Moment Closure, and the Transported PDF method, but predicting soot emissions and the condition of complete extinction in complex geometries is still elusive.
Highlights
1.1 Why study turbulent combustion?Turbulent combustion is the phenomenon at the heart of modern propulsion and energy and, even in a future low-carbon energy system, it is likely that there will be significant use of fuels for aviation, shipping, and manufacturing processes
The idea was this: establish a flame that is simple from the perspective of fluid mechanics, but bring it to the limit in terms of combustion science
The online solution of the chemistry is achieved in various turbulent combustion models, but mainly they are based on the transport of the probability density function [21, 22] of scalars, the Conditional Moment Closure [28], the Thickened Flame Model [26], and the Eddy Dissipation Concept [32]
Summary
Turbulent combustion is the phenomenon at the heart of modern propulsion and energy and, even in a future low-carbon energy system, it is likely that there will be significant use of fuels for aviation, shipping, and manufacturing processes. Combustor development is still largely by cut-and-try testing in experimental rigs and in prototype and in-service engines and power plants” This was written in 2011, and perhaps takes a somewhat conservative perspective on the use of Computational Fluid Dynamics (CFD) for combustor design. This paper is by no means an exhaustive review of current trends or of the literature in experimental, theoretical, and modelling research in turbulent combustion It contains only a selection of material from the authors’ experience, with the aim to present to the reader a personal view of the key areas of current activity and the limits of combustion CFD today. The paper is mostly aimed at a starting PhD student who wishes to consult some entries to the vast literature on turbulent combustion
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