Flame dynamics

Perfurbed Flame Dynamics and Thermo-Acoushc Insfabilities.80... 

This chapter considers the first group of instabilities and introduces the analysis of processes implying an interaction with external flow-field perturbahons. This is exemplified by investigations of coupling between pressure waves and plane flames and also between an external acceleration field and flame fronts. The coupling between flow perturbations and flames giving rise to heat release unsteadiness and coupling with acoushc modes is considered in Chapter 5.2, which deals with the relationship between perturbed flame dynamics and radiated acoustic field, a fundamental process of thermo-acoustic instabilities. 

Flames submitted to convective disturbances experience geometrical variations, which can in turn give rise to heat release unsteadiness. This process can be examined by considering different types of interactions between incident velocity or equivalence ratio modulations and combustion. The flame dynamics resulting from these interactions give rise to sound radiation and... 

Perturbed Flame Dynamics and Thermo-Acoustic Instabilities... 

Flame dynamics is intimately related to combustion instability and noise radiation. In this chapter, relationships between these different processes are described by making use of systematic experiments in which laminar flames respond to incident perturbations. The response to incoming disturbances is examined and expressions of the radiated pressure are compared with the measurements of heat release rate in the flame. The data indicate that flame dynamics determines the radiation of sound from flames. Links between combustion noise and combustion instabilities are drawn on this basis. These two aspects, usually treated separately, appear as manifestations of the same dynamical process. 

The analysis of combustion dynamics is then intimately linked to an understanding of perturbed flame dynamics, the subsequent generation of unsteady rates of heat release, and the associated radiation of sound and resulting acoustic feedback. In practical configurations, the resonance loop involves the flow, the combustion process, and the acoustic modes of the system as represented schematically in Figure 5.2.2. 

The qualitative analysis developed in the previous section is now complemented by examining correlations between the sound field and the flame dynamics. This is accomplished by comparing time traces of pressure fluctuation p and light emission I signals recorded during the various experiments. Results are displayed in Figures 5.2.11 and 5.2.12, respectively, for the various... 

The first item is easily treated by considering the eigen-modes of the system and expanding the pressure field on a basis formed by these modes. The second item is less well documented but is clearly important. The presence of boundaries not only modifies the structure of the mean flow but also influences the flame dynamics. This is demonstrated in a set of recent experiments in which the lateral confinement was varied systematically [45]. 

S. Candel, A.-L. Birbaud, E. Richecoeur, S. Ducruix, and C. Nottin. Computational flame dynamics (invited lecture). In Second ECCOMAS Thematic Conference on Computational Combustion, Delft, The Netherlands, July 2007. 

In Chapter 5.2, S. Candel, D. Durox, and T. Schuller consider certain aspects of perturbed flame dynamics. The relation between combustion instability and noise generation is described by reference to systematic experiments. The data indicate that acoustic emission is determined by flame dynamics. On this basis, combustion noise can be linked with combustion instability. 

Aldredge, R. C., and F. A. Williams. 1991. Influence of wrinkled premixed-flame dynamics on large-scale, low-intensity turbulent flow. J. Fluid Mechanics 228 487-511. 

Buoyancy is a major effect in most diffusion flames and even in premixed systems it can be responsible for much of the flame dynamics. The computational problem introduced here is one of... 

In the context of LES, a new modeling issue appears for two-phase flow simulations, either in the EL or EE formulation, and is linked to the subgrid scale model for the turbulent droplet dispersion. This problem has already been addressed in [278 279[ but is still an open question. However in the case of reacting flows, turbulent droplet dispersion occurs in a very limited zone between the atomizer and the flame and it is greatly influenced by the flame dynamics, therefore limiting the impact of the subgrid scale model. 

Recently some progress has been made along these lines (N. Peters and F. A. Williams, Effects of Chemical Equilibrium on the Structure and Extinction of Laminar Diffusion Flames, Dynamics of Flames and Reactive Systems, J. R. Bowen, N, Manson, A. K. Oppenheim and R. 1. Soloukhin, eds. vol. 95 of Progress in Astronautics and Aeronautics, New York American Institute of Aeronautics and Astronautics, 1984, 37-60). 

Pizza G, MantzarasJ, Prouzakis CE Flame dynamics in catalytic and non-catalytic mesoscale microreactors, Catal Today 155 123—130, 2010b. 

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