NON-REACTING SPRAY CHARACTERISTICS OF ALTERNATIVE AVIATION FUELS AT GAS TURBINE ENGINE CONDITIONS
thesisposted on 06.04.2021, 15:47 authored by Dongyun ShinDongyun Shin
The aviation industry is continuously growing amid tight restrictions on global emission
reductions. Alternative aviation fuels have gained attention and developed to replace the
conventional petroleum-derived aviation fuels. The replacement of conventional fuels with
alternative fuels, which are composed solely of hydrocarbons (non-petroleum), can mitigate
impacts on the environment and diversify the energy supply, potentially reducing fuel costs.
To ensure the performance of alternative fuels, extensive laboratory and full-scale engine
testings are required, thereby a lengthy and expensive process. The National Jet Fuel Combustion
Program (NJFCP) proposed a plan to reduce this certification process time and
the cost dramatically by implementing a computational model in the process, which can be
replaced with some of the testings. This requires an understanding of the influence of chemical/
physical properties of alternative fuels on combustion performance. The main objective
of this work is to investigate the spray characteristics of alternative aviation fuels compared
to that of conventional aviation fuels, which have been characterized by different physical
liquid properties at different gas turbine-relevant conditions.
The experimental work focuses on the spray characteristics of standard and alternative
aviation fuels at three operating conditions such as near lean blowout (LBO), cold engine
start, and high ambient pressure conditions. The spray generated by a hybrid pressureswirl
airblast atomizer was investigated by measuring the drop size and drop velocity at
a different axial distance downstream of the injector using a phase Doppler anemometry
(PDA) measurement system. This provided an approximate trajectory of the largest droplet
as it traveled down from the injector. At LBO conditions, the trend of decreasing drop size
and increasing drop velocity with an increase in gas pressure drop was observed for both
conventional (A-2) and alternative aviation fuels (C-1, C-5, C-7, and C-8), while the effect of
fuel injection pressure on the mean drop size and drop velocity was observed to be limited.
Moreover, the high-speed shadowgraph images were also taken to investigate the effect of
the pressure drop and fuel injection pressures on the cone angles. Their effects were found
to be limited on the cone angle.
The spray characteristics of standard (A-2 and A-3) and alternative (C-3) fuels were
investigated at engine cold-start conditions. At such a crucial condition, sufficient atomization
needs to be maintained to operate the engine properly. The effect of fuel properties,
especially the viscosity, was investigated on spray drop size and drop velocity using both
conventional and alternative aviation fuels. The effect of fuel viscosity was found to be minimal
and dominated by the effect of the surface tension, even though it showed a weak trend
of increasing drop size with increasing surface tension. The higher swirler pressure drop
reduced the drop size and increased drop velocity due to greater inertial force of the gas for
both conventional and alternative aviation fuels at the cold start condition. However, the
effect of pressure drop was observed to be reduced at cold start condition compared to the
results from the LBO condition.
The final aspect of experimental work focuses on the effect of ambient pressures on the
spray characteristics for both conventional (A-2) and alternative (C-5) aviation fuels. Advanced
aviation technology, especially in turbomachinery, has resulted in a greater pressure
ratio in the compressor; therefore, greater pressure in combustors for better thermal efficiency.
The effect of ambient pressure on drop size, drop velocity, and spray cone angle was
investigated using the PDA system and simultaneous Planar Laser-Induced Fluorescence
(PLIF) and Mie scattering measurement. A significant reduction in mean drop size was
observed with increasing ambient pressure, up to 5 bar. However, the reduction in the mean
drop size was found to be limited with a further increase in the ambient pressure. The effect
of the pressure drop across the swirler was observed to be significant at ambient pressure of
5 bar. The spray cone angle estimation at near the swirler exit and at 25.4 mm downstream
from the swirler exit plane using instantaneous Mie images was found to be independent of
ambient pressure. However, the cone angle at measurement plane of 18 mm in the spray
was observed to increase with increasing ambient pressure due to entrainment of smaller
droplets at higher ambient pressure. Furthermore, the fuel droplet and vapor distribution in
the spray were imaged and identified by comparing instantaneous PLIF and Mie images.
Lastly, a semi-empirical model was also developed using a phenomenological three-step
approach for the atomization process of the hybrid pressure-swirl airblast atomizer. This
model includes three sub-models: pressure-swirl spray droplet formation, droplet impingement, and film formation, and aerodynamic breakup. The model predicted drop sizes as a
function of ALR, atomizing gas velocity, surface tension, density, and ligament length and
diameter and successfully demonstrated the drop size trend observed with fuel viscosity,
surface tension, pressure drop, and ambient pressure. The model provided insights into the
effect of fuel properties and engine operating parameters on the drop size. More experimental
work is required to validate the model over a wider range of operating conditions and
physical fuel properties.
Overall, this work provides valuable information to increase understanding of the spray
characteristics of conventional and alternative aviation fuels at various engine operating
conditions. This work can provide valuable data for developing an advanced computational
combustor model, ultimately expediting the certification of new alternative aviation fuels.