Flow Structure of Lifted Swirling Jet Flames

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Swirling combustion is widely applied in various applications such as gas turbines, utility boilers or waste incinerators. This article contributes to the ongoing research by providing experimental data that are gathered in the mixing zone of a lifted swirling premixed natural gas flame. The objective of this paper is fivefold: (1) to introduce the lifted swirling flame featuring low NOx emissions (2) to provide experimental data such as major species distributions, temperature and streamlines of the flow pattern, (3) to report on velocity bias in probability density function (PDF) distributions and to present PDF sequences of velocities in medium scale swirling flows, (4) to make an assessment on the local small-scale turbulence that is present in the swirling mixing layer and (5) to provide new experimental data for model verification and development.

The PDFs are corrected in order to compensate for the velocity bias phenomenon, which is typical for randomly sampled LDA data. Sequences of axial PDF data are presented and measurement locations of interest are selected to look at the PDF characteristics of the internal and external recirculation zones, the mixing layer and the onset of the reacting flow into detail. The mixing layer PDFs covered a wide velocity range and revealed bimodality; even the concept of multi-modality is suggested and explored. Analysis showed that a sum of two Gaussian distributions can accurately envelop the experimental PDFs. The reason for this broadband turbulence behavior is to be found in combination of precessing and flapping motion of the flow structures, and also in combustion generated instabilities of the lifted flame. As a result, the flame brush is wide (large scale motion) and the mixing (small-scale turbulence) flattens any high temperatures in the combustion process.

The multi-scale turbulence concept is subsequently used to make an assessment of the local turbulence characteristics in the mixing layer. The idea is that the PDFs capture both contributions of the flow-inherent fine grain turbulence (u′l) which is superposed on slow large scale fluctuating structures. It is this u′l that will be of interest in continued research on the classification of the lifted flame into a combustion regime diagram (e.g. Borghi diagram). Finally, the bimodality character in reacting flows and the prediction of large-scale structures may be a challenge for LES researchers.

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