A NUMERICAL STUDY ON THE EFFECTS OF ACOUSTIC FORCING ON FUEL SPRAY DYNAMICS IN GAS TURBINES

In the case of aircraft engines, the fuel is injected as a liquid spray which may play a role in thermoacoustic instabilities through creating changes to the mixture fraction inside the combustion chamber. This study uses two-phase incompressible non-reacting large eddy simulation with Lagrangian particle tracking to show how spray droplets of different sizes can be affected by large scale hydrodynamic structures and acoustic forcing. The forcing is applied at the inlets of a truncated computational domain that only includes the geometry downstream of the fuel injector using the newly developed PODFS (proper orthogonal decomposition Fourier series) method. The PODFS is a model that can reproduce the effects of acoustic forcing by extracting planes of data from an auxiliary acoustically forced compressible unsteady Reynolds averaged Navier-Stokes simulation. A proper orthogonal decomposition analysis shows that fuel droplets of a typical size seen in jet engines are more sensitive to acoustic and hydrodynamic structures than droplets with an order of magnitude larger or smaller diameter, consistent with their Stokes number. Phase and azimuthally averaged results show that fluctuations of the spray mixture fraction represented by large droplets affect the total spray mixture fraction much more than fluctuations of the small droplets. An additional intermittent spray dispersion mechanism was identified that is due to intermittent vorticity being generated between the two outer injector flow passages. An injector design modification has been suggested that will reduce the prevalence of this mechanism.