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Project ID: G4RD-CT-2002-00644
Finanziato nell'ambito di: FP5-GROWTH
Paese: Italy

4.4 Fluctuation analysis of single (multiple) jet in under/super critical regimes

The efficiency and pollutant formation as well as the combustion stability in a gas turbine engine operated in lean premix conditions, is strongly dependent on the atomization and mixing processes. An efficient atomization process, producing a spray with a small mean drop size, will achieve a greater degree of vaporization in a given premix time than a spray of larger mean drop size and, therefore, achieve a higher degree of premix in a given fuel injector. Similarly, the degree of uniformity of the spray, depending on the liquid droplets dispersion in the air-stream, may have an impact on temperature uniformity. While both of these features influence pollutant formation, reaction temperature uniformity has also an impact on flame stability that could result in, or suppress, resonant pressure fluctuations via Rayleigh coupling.
Liquid jets in cross flows are often used as a means of introducing fuel into premix ducts. The dynamic behaviour of a liquid jet yields stripping/shedding mechanisms, which may be the inception of sub-harmonic fluctuation in downstream premixed combustion. A more detailed understanding of the phenomena occurring can clarify the role of different mechanisms in promoting or suppressing the oscillating behaviour of combustion and is a mandatory step to further evolve gas-turbine engines. This task aimed to study the injection of a round liquid jet of fuel in a premixing channel where a high pressure, high temperature airflow flows across the injection direction.

A better understanding of the unsteady phenomena associated with jet break-up and droplet formation will also improve the accuracy of CFD calculations. It will help to improve the specification of boundary conditions through a better quantification of initial droplet conditions and will also lead to improved modelling capabilities through a better understanding of the underlying physics.

In this framework the main achievements of this part of the project were
- The region close to the nozzle tip presents a dramatic reduction of the instability with respect to the surrounding, allowing identifying an extremely stable (virtual) core of the liquid jet.

- Liquid properties play a significant role in the onset of oscillating behaviour of the jet. The computation of a fluctuation index showed that the lower surface tension of jet A-1 is responsible of a more extensive destabilization of the spray plume. This is an important result to be taken in account when using experimental results obtained on the round of experiments made on jet of other liquids (typically water).

- The orientation of the jet axis with respect to the airflow direction affects the liquid atomization and dispersion in the airflow. Experiments suggested that the inclination of the spray axis in a partially opposed direction with respect to the airflow is beneficial to the liquid dispersion in the air and can be useful in achieving a faster and more uniform mixing. Further studies are required to identify the optimum geometry.

- Atomization and deformation do not significantly affect the penetration of the kerosene jet along the liquid injection direction; the distance of the breakdown position depends mainly on the momentum ratio and only weakly on the Weber number. Only a weak dependence on the air viscosity has been found.

- A significant correction on the extension of the liquid entrainment along gas streamwise direction has to be performed in respect to the low temperature condition.
More specifically the breakdown position along this direction is dependent on aerodynamic Weber number calculated on the liquid jet velocity. This affects the deformation and consequently the drag entrainment of the kerosene column as well as of liquid fragments of the primary atomization.

- The generalized profile equation can be successfully scaled with only two parameters, which are the coordinates of the breakdown position.

- The breakdown process occurs downstream of the break-up and contact lengths. Its average position can be determined reliably in non-reacting environment with automatic and objective procedures.

- The spray evaporation does not affect significantly these atomization aspects, even though it affects other characteristics.


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