Residence time measurement of an isothermal combustor flow field |
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Authors: | Liangta?Cheng Email author" target="_blank">Adrian?SpencerEmail author |
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Institution: | (1) Department of Aero & Auto Engineering, Loughborough University, Loughborough, UK; |
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Abstract: | Residence times of combustors have commonly been used to help understand NOx emissions and flame blowout. Both the time mean velocity and turbulence fields are important to the residence time, but determining
the residence time via analysis of a measured velocity field is difficult due to the inherent unsteadiness and the three-dimensional
nature of a high-Re swirling flow. A more direct approach to measure residence time is reported here that examines the dynamic
response of fuel concentration to a sudden cutoff in the fuel injection. Residence time measurement was mainly taken using
a time-resolved planar laser-induced fluorescence (PLIF) technique, but a second camera for particle image velocimetry (PIV)
was added to check that the step change does not alter the velocity field and the spectral content of the coherent structures.
Characteristic timescales evaluated from the measurements are referred to as convection and half-life times: The former describes
the time delay from a fuel injector exit reference point to a downstream point of interest, and the latter describes the rate
of decay once the effect of the reduced scalar concentration at the injection source has been transported to the point of
interest. Residence time is often defined as the time taken for a conserved scalar to reduce to half its initial value after
injection is stopped: this equivalent to the sum of the convection time and the half-life values. The technique was applied
to a high-swirl fuel injector typical of that found in combustor applications. Two test cases have been studied: with central
jet (with-jet) and without central jet (no-jet). It was found that the relatively unstable central recirculation zone of the
no-jet case resulted in increased transport of fuel into the central region that is dominated by a precessing vortex core,
where long half-life times are also found. Based on this, it was inferred that the no-jet case may be more prone to NOx production. The technique is described here for a single-phase isothermal flow field, but with consideration, it could be
extended to studying reacting flows to provide more insight into important mixing phenomena and relevant timescales. |
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