Air travel growth is predicted to continue at five percent per year and therefore the future rate of gains in fuel efficiency is expected to be outpaced by the projected growth in air traffic [1
]. Thus, the aircraft industry will need an increasing amount of fuel and as a consequence, the aviation industry is interested in alternative energy sources and alternative fuels in particular, to assure security of supply. Besides, candidate fuels are expected to positively affect global warming, environmental protection and diversity and sustainability. New fuels should be compatible with the current fleet and thus be “drop in” fuels similar to current Jet A-1 fuel, ideally with a lower CO2
The increasing demand for alternative, sustainable fuels in the transport sector is linked to the availability of alternative fuels for gas turbines [3
]. The Fischer–Tropsch (FT) process offers a solution to this issue, providing synthetic middle distillate fuel components. The FT method or the “anything-To-Liquid” process (xTL) offers a versatile pathway to create synthetic fuels, converting carbon- and energy-containing feedstock to high quality fuels. The feedstock can be either natural gas (Gas-To-Liquid, GTL), biomass (Biomass-To-Liquid, BTL) or coal (Coal-To-Liquid, CTL). During the gasification step, the connection to the starting material is lost, so FT liquids from any starting material will be essentially the same [6
Focusing on the GTL procedure, FT kerosene can provide advantages as an alternative aviation fuel, as it is physically similar to kerosene and thus compatible with current fuel storage and handling facilities. Moreover, the FT kerosene is a product virtually free from the sulfur-, oxygen- and (occasionally) nitrogen-containing compounds found in conventional jet fuel; it emits fewer particulates than conventional jet fuel and it is a sulfur-free fuel leading to the complete elimination of SOx
emissions. There are no sulfur dioxide (SO2
) or sulfuric acid (H2
) aerosol emissions, which also form contrails. Furthermore, the synthetic FT kerosene produces lower budgets of other pollutants such as particulate matter, and hydrocarbon emissions [3
On the other hand, the FT synthetic fuel present disadvantages related to the low aromatics content. FT kerosene will have a density value lower than the minimum requirement, whereas switching from conventional jet fuel to aromatic-free FT synthetic fuel, will cause some of the elastomers (used in aircraft fuel systems to swell) to shrink, which may lead to fuel leaks [3
]. In this view, there is concern in the industry regarding switching from conventional jet fuel to aromatic-free synthetic fuel. The effects of aromatics on elastomers is an area of active research and a possible solution could be an additive that would ensure that elastomers swell even in the absence of aromatics. The disadvantages of the low aromatics content disappear if the FT synthetic fuel is blended with conventional jet fuel, although the advantage of lower emissions is reduced. However, it is envisaged that usage of synthetic jet fuels will improve air quality around airports, which will be particularly advantageous at inner city airports.
The use of alternative fuels in aviation aero engines requires a thorough understanding of their atomization and combustion properties. These processes are expected to be strongly affected by the physical properties and chemical composition of a particular blend. In addition, transfer phenomena (turbulent heat, mass and momentum transfer) as well as chemical kinetics are involved in the interaction between the oxidizer flow field and the fuel droplets. Each of these phenomena is extremely complicated in its own right, and when coupled together in a real flow situation, their study becomes even more difficult.
The performance of a combustor depends on the droplet size produced by the nozzle atomizer and the way in which the air mixes with the droplets [8
]. Spray formation at a nozzle is dominated by different mechanisms, depending on the relative velocity and the properties of the liquid and the ambient gas [13
]. These dependencies can be identified using appropriate non-dimensional numbers. The Reynolds number, Re
, characterizes the balance between the inertial and viscosity forces (u
is the exit velocity, D
the diameter of the nozzle, ρ
the gas density, and μ
the dynamic viscosity). The Weber number, We
, is the main quantity controlling droplet breakup mechanisms and the resulting formation of droplets, relating the surface tension force to the aerodynamic force exerted by the ambient gas (σ
is the surface tension of the liquid). Finally, the Ohnesorge number,
takes into account viscosity dissipation in relation to the surface tension energy. Low Ohnesorge numbers are associated with weak friction losses and most of the inserted energy is converted to surface tension energy whereas, at high Ohnesorge number values, internal viscous dissipation becomes dominant. Reitz and Lefebvre [15
] have identified four regimes for spray formation in relation to Re
numbers, namely the Rayleigh regime, the first and the second wind-induced regimes and the atomization regime. Farther downstream, spray break up may continue due to aerodynamic forces.
Experimental data are often required for the determination of the droplet size and velocity distributions. Consequently, the atomization characteristics of candidate alternative fuels have to be assessed through detailed experiments of the spray field including local measurements of droplet Sauter mean diameter (SMD) distributions as well as axial and radial velocity profiles [17
]. Moreover, the characteristics of liquid fuel atomization and vaporization are vital, because the atomization and evaporation of the fuel spray directly affect the engine performance and emission characteristics [18
]. Furthermore, the sensitivity of the fuel droplet velocity field to the physical properties, such as density, viscosity and surface tension, is of great importance in relation to the atomization process. These effects are considered essential nowadays, when the implementation of advanced, staged combustion schemes, which require precise control of the flow pattern, are under consideration on both sides of the Atlantic (e.g., ASCENT and Clean Sky projects).
The present work is in line with several contemporary efforts to assess the influence of composition and physical properties on the atomization characteristics of alternative fuels [19
]. The work was part of a larger effort to investigate the applicability of fuels that can be processed to look like kerosene, as practical alternatives to fossil fuels by the Virtual Fuel Centre of the Environmentally Compatible Air Transport System project (ECATS) [24
]. Aiming to contribute to the assessment of compatibility of the spray characteristics of alternative fuel blends, the objective of the present work was to evaluate the relevant physical properties including surface tension, kinematic viscosity and density, and investigate their effect on the atomization process, comparing the resulting spray properties with those of the reference Jet A-1 fuel spray. To this end, a generic experiment was conducted in the Lab of Applied Thermodynamics at University of Patras (LAT/UP), to assess the spray characteristics of specified fuel blends, evaluating the spray quality.
The spray characteristics of alternative fuel blends provided by Shell Global Solutions have been assessed experimentally in relation to currently used Jet A-1 fuel. The physical properties of the tested blends were within 10% of the reference fuel properties. The droplet mean axial velocity measurements indicate that, under the conditions used in the present study, all tested blends produce flow fields of a similar pattern, although velocity magnitude, especially in the near field, presented differences as large as 20% of the peak values, depending on the blend composition. These differences diminished downstream and at the last stations were of the order of the uncertainties. Larger differences, persisting downstream, were observed in the measurements of the axial velocity rms values, indicating small differences in the turbulence and mixing for the different blends. The composition of the blends has a distinct effect on the Sauter mean diameter of the resulting sprays. Mean diameters are clearly correlated with the blend composition. It is hard to distinguish the effect of different blends on the volumetric flux measurements, although a difference in the spray pattern due to the increase of the injection pressure can be surmised. In general, it can be said that all blends have comparable physical properties and produce similar sprays to the reference Jet A-1 fuel and as drop in fuels, may be expected to behave like the reference fuel, as far as spraying characteristics are concerned. For this purpose, the P–N–Ar blend (Paraffins, 50%–Naphthenes, 30%–Aromatics, 20%) seems to be the best candidate. It has a density and a surface tension very close to those of the reference fuel and produces a similar spray pattern, with matching droplet velocity and Sauter mean diameter distributions. It has to be noted that observed differences may have an effect on the implementation of staged combustion schemes which require precise control of the flow pattern.