Computational and experimental study of a multi-stream swirler

The fuel injector in a modern gas turbine encompasses a multi-stream swirler, the shear layers from which are used to atomise the liquid fuel. The aerodynamic characteristics of the swirler are known to affect the placement of the fuel directly, and, ultimately, the emissions produced. A full appreciation of the aerodynamics potentially enables improved injector design and hence lower emissions. A rig was designed to study the flow resulting from three axial, co-rotating swirler passages, separated by shrouds, with the downstream flow field being confined in a duct. The swirler module was three times full size and has a 450 repeatable sector. A detailed survey of the downstream flow field has been carried out using a five hole pressure probe and a three component laser doppler anemometry (LDA) system. A gearing mechanism was employed to rotate the swirler within the rig casing such that the extent of any three dimensionality in the flow field could be assessed. The central recirculation caused by the highly swirling flow was found to extend beyond the final measurement plane, prompting the moderately loaded exhaust nozzle to be replaced by a cylinder positioned centrally within the rig. LDA measurements were taken at thirty downstream planes, providing sufficient detail for validation of a computational model. The three dimensionality of the flow field was found to be minimal, which has direct implications for the requirements of computational modelling. A three-dimensional computational fluid dynamics (CFD) code, encompassing both k - E and Reynolds Stress Transport (RST) turbulence models was employed to model the flow. Test cases from the open literature were utilised to validate the physical models within the code on simple geometries, with the results comparing favourably to those previously published. A solid model of the experimental geometry was created using a CAD package, which was extracted and used as direct input to the grid generator. A structured grid was employed, with the calculation including both flow through the swirler passages and in the downstream mixing duct. The experimental results were used to validate the computational model. Calculations starting downstream of the swirler exit plane, utilising experimental measurements as inlet boundary conditions, indicated that the improved physical description of the RST model provided enhanced results over the simpler k - E model. Calculations performed through the swirler vane passages using the k - E model indicated that the results were improved significantly by moving the inlet boundary condition to an upstream location, possibly due to the estimation of the turbulence dissipation.