The Dry Low NOx (DLN) silo combustor of the Nuovo Pignone PGT10B gas turbine is being redesigned to meet Dual-Fuel capability. A prototype with specially designed fuel injectors, placed on airfoil-shaped elements, was tested at cold conditions (using water instead of Diesel fuel) to map the spray mass distribution at the premixer exit. The resulting profile showed high concentrations of liquid near the premixer centerline and on the premixer wall.
Parallel to this test, a small-scale experimental and numerical study was made of a single atomizer of the fuel system, placed in cross flow position. This small-scale study was launched in order to gain insight in the behavior of the spray, as well as to assess the relative importance of spray modeling parameters. The PDPA experiments and 2D CFD simulations of these experiments showed fair agreement on the average drop size distribution and drop size-velocity correlation. The flow visualization also revealed liquid film formation on the surface of the airfoil, behind the injector, due to the low atomization pressure differential at cold conditions.
Using this modeling experience, the spray patternation test with the prototype combustor has been modeled using an existing 3D CFD model of the premixer. The model also showed high liquid concentration on the wall, but not near the centerline. From the results of the small-scale study it is concluded that the measured high concentration near the premixer centerline is not a result of the flow field. It is assumed that in the complete assembly the liquid film from the injector vanes accumulates on the center body, resulting in a high liquid concentration downstream on the premixer centerline.
Overall, the application of CFD analyses on the tests performed proved to be a very useful tool to evaluate the test results. The modeling experience identified the important factors in modeling the fuel spray in a gas turbine environment, but further evolution of computer resources is required before large-scale test results will be reproducible with CFD models.