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Dacolt scales up to LES to investigate PVC

Dacolt has been awarded 1M CPU-core hours on the Dutch national supercomputer Cartesius to further investigate the Precessing Vortex Core (PVC) phenomenon in a gas turbine combustor.

Modern gas turbines for power generation or aircraft propulsion are constraint by legislation to produce less NOx emissions. Such low NOx combustion systems have a very narrow operating window where the flame is burning in a stable way. Flame stability is achieved by introducing a strong swirling motion to the combustion air entering the combustion chamber. This swirling motion leads to strong recirculating flow patterns inside the combustion chamber, including a flow structure called the Precessing Vortex Core (PVC). This PVC interacts with the flame; such interactions in gas turbine combustors are a trending topic in current gas turbine research. It is believed that the PVC is a key phenomenon to understand in order to design stable low NOx combustion systems.

Dacolt has used Computational Fluid Dynamics (CFD) tools to study a model gas turbine combustor for which detailed experimental data is available. Both cold flow and reacting (hot) flow simulations have been performed. It was found that, at minimum, the unsteady Reynolds-Averaged Navier-Stokes (RANS) equations have to be solved using the Reynolds Stress Model (RSM) for the turbulent viscosity in order to capture the PVC. A next level of refinement was achieved by using Detached Eddy Simulation (DES), allowing capturing more of the turbulent velocity fluctuations. Still, significant deviation from experimental results in terms of velocity, mixing and species profiles is observed. DES has shown to predict better results than URANS RSM.

A next level of refinement is to use Large Eddy Simulation (LES) techniques, using OpenFOAM®. It is estimated that such advanced turbulence modelling should be able to fully describe the flow dynamics and hence PVC behavior. However, LES simulations are at least an order of magnitude more expensive in terms of computational effort as compared to URANS or DES. Such computational resources are far beyond the capacity of typical resources of engineering companies like Dacolt: a typical LES simulation on the burner studied to date is estimated to require approximately 50.000 CPU-core hours.

The aim of this project is to gain further insight in the PVC characteristics and its interaction with the flame. From such in-depth understanding an engineering methodology can be derived to do practical combustor simulations. Through the PRACE project, Dacolt has been awarded 1M CPU-core hours on the Dutch national supercomputer Cartesius at surfSARA. This work is realised in collaboration with Delft (Prof. Boersma) and Einhoven (Dr. van Oijen) Unviversities of Technology.

First results, fresh from Cartesius, illustrate the fuel / air mixing with iso-contours of the subgrid viscosity, showing typical flow structures. To be continued!