The effect of unsteady boundary conditions on simulations of prefiliming airblast atomisation

Liquid fuelled gas turbines are likely to remain a dominant force in aviation propulsion for the foreseeable future, and therefore understanding the atomisation process is key to meeting future emissions and performance legislation. To make experiments and simulations possible, simplified geometry and boundary conditions are often used, for example, simulations of primary atomisation often use a fixed film height and velocity. This paper aims to quantify the effect of a fully developed unsteady film on the atomisation process. A custom Coupled Level Set & Volume of Fluid (CLSVOF) solver with adaptive meshing built in OpenFOAM v9 is used. Two CLSVOF simulations are presented. The first is of the film development, and the second of the atomisation process. The second uses data from the first as inlet boundary conditions. These results are compared to previous CLSVOF simulations from Wetherell et al. (2020) using traditional boundary conditions. The unsteady film has doubled the modal ligament length and widened the distribution. The SMD has been increased by 58%. The ligament length distribution and SMD are now closer to the experimental results. Comparisons of the time-history of the inlet film mass flow and spray properties has shown a clear correlation between the film mass flow and spray. The delay between inlet and spray has been quantified and lies within expected values. The drop size distribution has not changed significantly, however the volume weighted distribution shows that a much greater proportion of the spray is contained in larger droplets, and the non-weighted distribution is biased by the large number of small droplets. Further work is required to understand how large-scale temporal variations of the type seen in this paper would affect real combustion system performance.<p></p>