NON-REACTING SPRAY CHARACTERISTICS OF ALTERNATIVE AVIATION FUELS AT GAS TURBINE ENGINE CONDITIONS

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.