Air-film coupling in prefilming airblast atomisers and the implications for subsequent atomisation

Prefilming airblast atomisers are commonly used in gas turbine combustion system fuel injectors. As the film propagates across the prefilmer it interacts with the high velocity gas stream above it. In this paper a numerical investigation into this interaction is presented. A Coupled Level Set & Volume of Fluid method is used to simulate the development of the film along the KIT-ITS planar prefilmer (Gepperth et al., in: 23rd European conference on liquid atomization and spray systems (ILASS-Europe 2010), Brno, Czech Republic, September, 2010). Initial results showed the importance of correctly specifying the contact angle as too high a value leads to the formation of rivulets instead of a continuous film. An analysis of the film and air showed two-way coupling. The presence of the film increases the growth rate of the gas phase boundary layer, and the strength and size of the turbulent structures within it. Surface waves form in the film, initially driven by the turbulent fluctuations, but developing into transverse waves. These waves are shown to be independent, stochastic events instead of a periodic wave system. At the trailing edge of the prefilmer the increased turbulence level in the air, the variations in the film thickness and the associated change in fuel mass flow and momentum will have large implications for the atomisation process and subsequent fuel spray. These will also impact simulation of the atomisation, as the boundary condition complexity is much greater than commonly used, and the variations will require larger domains and longer simulation times to obtain fully converged atomisation statistics.