Analysis of the premixed methane/air flames near to lean blowout in plasma assisted swirl combustor

Lean fuel burning is one of the most prominent methods to achieve low-temperature combustion
which reduces a substantial amount of nitrogen oxides (NOx) emissions. However, as a result, strong
flame instabilities and very lean operating conditions are associated with the high risk of a lean blowout
(LBO). This experimental work investigates the behavior of premixed methane/air flames close to LBO
limits at different operating conditions using a plasma-assisted swirl combustor. The combustor was
equipped with ring-needle plasma actuator power by a sinusoidal high voltage generator at the maximum
frequency of 20 kHz directly coupled with the flame near the nozzle exit. The electrical characterization
of plasma-assisted flame was performed at two different fueling conditions by powering the electrode
with different voltage amplitudes of the sinusoidal generator. The comparative analysis of LBO stability
of baseline case (without plasma) and with different plasma actuation conditions has been performed.
The flame behavior was characterized using a high-speed chemiluminescence diagnostic to reveal the
spatially and temporally distribution of the flame front and the primary combustion zone. The
CH*emissions, which were acquired by an intensified camera equipped with a narrow bandpass filter,
were used to analyze the effects of equivalence ratio and plasma actuation on the flame. Proper
Orthogonal Decomposition of CH* chemiluminescence images permitted the identification of the
dominant flame structures and frequency ranges of the flame fluctuations. A significant improvement in
the flame behavior close to the LBO limits was underlined.