Numerical Simulation and Aerodynamics of a Fuel Flexible Injector for Micro-Gas Turbine Engines

Abstract The present work focuses on developing a novel injector concept for a fuel-flexible micro gas turbine engine that is sized for 1 kN thrust requirements. The lean burn fuel injection design is adopted from the available modern aero-engine combustors. The injector hardware comprises an assembly of pilot and main swirlers, which are arranged in clockwise and counter-clockwise directions having multiple fuel passages in between. The pilot consists of two axial swirlers arranged in clockwise and counterclockwise direction and main consists of one axial and two radial swirlers arranged in clockwise and counter-clockwise direction. As the name suggests both the liquid and gaseous fuel can be used in the assembly either separately or simultaneously. Both the fuels can be alternatively used depending on the thrust or required power operating conditions. The injector is designed using based on the area mass flow split ratio. The mass flow split of 24:76 is distributed between the pilot and main geometry. A simplex nozzle is used for atomizing the liquid fuel with an annular channel having plurality of swirling vanes around it for gaseous fuel makes up the pilot. The main injector has annular toroidal passageways for both liquid and gaseous fuel after an air resist path, which prevents the impingement of main liquid fuel on the wall of pilot. By iterating through several swirler geometrical parameters, numerical studies are made to optimize the recirculation zone structure that can atomize the liquid phase. The CFD formulation is based on the Reynolds-Averaged Navier-Stokes (RANS) equations for accounting turbulence with 0.075 kg/s mass flow rate having a pressure drop of 4% with the atmospheric pressure inside the injector as operating conditions. Standard k-ε model is practiced in computing and optimizing the results. The computational domain of diameter 100 mm, length 80 mm for the inlet and 200 mm, 160 mm for the outlet is used for capturing the aerodynamics of pilot. For main, the computational domain of diameter 100 mm, length 120 mm for the inlet and 200 mm, 200 mm for the outlet is used. The grid independence study for the injector assembly is also performed with varying number of elements from 1.7 million to 6.6 million and employing hybrid meshing method in order to get a suitable mesh for all the iterations. Recirculation zones are captured pilot, main and injector individually. After optimization and variation through various parameters mentioned below, the pilot and main is combined which formed a central toroidal recirculation zone (CTRZ) having a forward and a backward stagnation point with a counter-rotating shear layer at the injector exit. The crucial parameters for the formation of CTRZ’s optimization is the distance that acts as a mixing length between the exit pilot flare and main venturi exits and flow split between the pilot and main injector swirlers.