Modelling the generation of temperature inhomogeneities by a premixed flame

What is it about?

In premixed combustors (gas turbines), inhomogeneities in the fuel-to-air ratio arising, for example, as a result of imperfect mixing, lead to the occurrence of hot and cold spots inside the burned flow. When those spots are accelerated, acoustic perturbations are generated which is usually unwanted. In this paper we propose modelling strategies for (1) the process of how mixing inhomogeneities convert into temperature changes during the combustion process (2) the transport processes of temperature and mixing inhomogeneities which allows for simple predictions of temperature fluctuations downstream the combustion chamber.

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Why is it important?

The so-called entropy feedback loop, which is connected to the generation and transport of hot and cold spots, might lead to combustion instability or to combustion noise. One finding of this study, which was not expected by the authors, was that the assumption of a constant specific heat capacity of the burned fluid leads to a significant overestimation of the magnitude of the generated temperature inhomogeneities. Also the formulation of the transport problem in terms of mixture fraction, which is usually used only for diffusion flames, simplified the analysis significantly: The transport of fuel-air mixing and temperature inhomogeneities can thereby be described by only one single equation.

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