Experimental investigations of flame interaction inmulti-burner arrayswith lean-lifted spray flames

Lean burn in gas turbine technology has drawn a lot of interest recently as
a way to lower emissions from combustion systems. With lean combustion,
a larger amount of air is fed into the combustion chamber compared to
the amount of fuel, resulting in a leaner fuel-air mixture. This enables
to control of the combustion temperature and delimits the nitric oxides
(NOx). These advantages may result in less fuel use, lower operational costs,
and a lesser environmental footprint. However, lean combustion can also
pose challenges, such as unstable flame behavior or periodic variations in
pressure and temperature within the combustion chamber, which can cause
damage to the gas turbine. These challengesmust be carefully considered and
addressed in the design and operation of gas turbines using lean combustion
technology to realize the full potential of this technology. A novel combustor
concept that integrates lean combustion, low swirl, lifted flame, and a helical
burner arrangement is presented in this research. The combination of these
techniques offers potential improvements in terms ofmechanical and thermal
performance. The main objective of current study is to provide experimental results on the
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baseline configuration of the innovative CHAIRLIFT combustor concept,
which can be used to identify/understand the relevant phenomena i.e. related
to the stability limits, emission performance and flame structure. Lean lifted
spray flames and their very low nitrogen oxides emissions are combined
with an inclination of burners in annular combustor leading to a more compact
combustor. CHAIRLIFT combustor concept is based on “low swirl”
lean lifted spray flames, which features a high degree of premixing and
consequently significantly reduced nitrogen oxides emissions and flashback
risk compared to conventional swirl stabilized flames. In the CHAIRLIFT
combustor concept, the lifted flames are combined with Short Helical Combustors
(SHC) arrangement to attain stable combustion by tilting the axis
of the flames relative to the axis of the turbine to enhance the interaction
of adjacent flames in a circumferential direction. A series of experimental
tests were conducted at a multi-burner array test rig consisting of up to five
modular burners at different burner inclination angles, equivalence ratios (ϕ),
different relative air pressure drop across the nozzle (Δp
p ), and varied air inlet
temperature TAir at ambient pressure. For all investigated configurations,
good stability limits for non-piloted burners was observed. The inline and
inclined have better stability range compared moderate swirl by previous
investigations. A slightly higher stability was observed for the inline arrangement
compared to the inclined arrangement due to local flow exchange
at higher temperature level. The unwanted flow deflection of highly swirled
flames in Short Helical Combustors arrangement could be avoided with the
investigated low swirl lifted flames.Moreover, the flame chemiluminescence
(OH*) measurements were used to provide a qualitative characterization of
the flame topology. Moreover, exhaust gas measurements at the inline and
the inclined multi-burner configuration was performed, showing that the
nitric oxides emissions (NOx) are at the expected low range for such lifted
flames compared to previous investigations and the inclined arrangement
has a better emissions performance for most operating conditions due to
higher lift off height. The ion probe measurement system was installed at the
inclined multi-burner array to investigate if conditions near to LBO can be
detected by certain parameters. The increase of timescale of the ion current
fluctuations was identified as suitable indicator.
The results show that a remarkable high stability of the low swirl and lifted
flames was observed compared to moderate swirl flames with larger inner
recirculation zone. The study highlights the significant influence of fuel
droplet evaporation time on flame stability near the flame root under low
air inlet temperatures. Moreover, the complex flame stability mechanism
in inclined configurations is found to be controlled by various parameters,
such as outer recirculation zone size, air inlet temperature, and additional
vortices generated by pressure differences near the wall. A slightly higher
stability was observed for the inline arrangement compared to the inclined
arrangement and different stabilization mechanism and flame structure at
different air inlet temperature was explained in details. Exhaust gas analyses
were performed for the multi-burner arrangement emissions. It was observed
that, these lifted flames provide very low NOx emissions over a wide range
of operating conditions for all in- vestigated configurations. These findings
have important implications for the future development and optimization of
lean combustion technology for aircraft engines.