One of the major aims of research in gas turbine combustor systems is the minimisation of non-desirable emissions. The primary method of reducing pollutants such as soot and NOx has been to run the combustion primary zone lean. Unfortunately, this causes problems when the combustor is run under idle and relight conditions as the primary zone air fuel ratio (AFR) can exceed the flammability limit.
Altering this AFR directly affects the primary zone aerodynamics through changes in the spray profile. One method of determining the influence of changes in AFR upon the primary zone is to use Computational Fluid Dynamic (CFD) models. However, to model the flow through an air-blast fuel injector and accurately predict the resulting primary zone aerodynamics requires hundreds of thousands, if not millions, of cells. Therefore, with current computer capabilities simplifications need to be made.
One simplification is to model the primary zone as a 2-D case. This reduces the number of cells to a computationally solvable level. However, by reducing the problem to 2-D the ability to accurately model air-blast fuel injectors is lost as they are intrinsically 3-D devices. Therefore, it is necessary to define boundary conditions for the fuel injector.
Currently, due to difficulties in obtaining experimental measurements inside a air-blast fuel injector, these boundary conditions are often derived using semi-empirical methods. This paper presents and compares two such models; the model proposed by Crocker et al. in 1996 and one developed at DERA specifically for modelling air-blast fuel injectors.
The work also highlights the importance of the often neglected radial component upon the primary zone aerodynamics.