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In this paper, the flow in a gas-turbine combustor with a novel air blast atomizer configuration is studied by URANS (unsteady Reynolds-averaged Navier-Stokes) calculations and compared to experimental data. The flow is characterized by a Reynolds number of 52000, a swirl number of 0.52 and an expansion ratio of 5. It is well known that at this high swirl level, flows exhibit a vortex breakdown which is characterized by a sudden axial deceleration in combination with a radial expansion and the formation of a stagnation point followed by a recirculation zone. At high Reynolds numbers, like in the present case, the vortex breakdown is either of a quasi-axisymmetric bubble type or of a precessing spiral type. Previous experimental studies confirmed the presence of a spiral type vortex breakdown for the configuration under concern. For the non-reacting flow, the structure as well as the frequency of the precessing vortex core is captured almost perfectly by the URANS predictions which is demonstrated by a direct comparison to LDV (Laser-Doppler Velocimetry) measurements. However, it was found that a suitable discretization as well as full three-dimensional computations are crucial in order to successfully predict the precessing spiral structure. In this context also the impact of two-equation and full transport Reynolds-stress turbulence models is discussed. After validation of the URANS method, it has been applied for developing improved designs which aim to suppress the unsteady flow pattern. The investigation of different design variants revealed that, if the mean axial velocity distribution of the flow upstream of the stagnation point is jet-like, the flow is more stable and less susceptible to the spiral type vortex breakdown. This fact which is known from laminar vortex breakdown investigations seems to be also valid for the turbulent mean flow and can be used as a design guideline for achieving a stable nearly symmetric bubble-like recirculation zone.

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