The department has years of experience in the development, characterisation and optimisation of combustion systems for MGTs – from concept through to successful integration and operation in MGT test rigs and power plants.
Research activities in combustion systems include the minimisation of pollutants, improvement of reliability and fuel flexibility with regard to conventional and alternative liquid and gaseous fuels.
Currently, combustion systems for:
and also for:
are being developed and tested. The thermal output range of the systems extends from one to 350 kilowatts. Combustion chamber pressures range from one to 4.5 bar at combustion chamber inlet air temperatures of up to 850 degrees Celsius.
Special focus is placed on the development of combustion chambers based on the FLameless OXidation (FLOX®) principle. In comparison to conventional burner systems, this concept enables increased efficiency, whilst simultaneously reducing pollutant emissions. A further advantage is the low risk of flashback, meaning that a high level of fuel flexibility can be achieved and it is possible to use fuels containing hydrogen.
FLOX® combustor designs developed for use in gas turbines consists of circular, equidistantly arranged fuel and air nozzles. The fuel is injected coaxially and premixed with air in a mixing section before entering the main combustion chamber. The combustion characteristics are essentially those for typical MGT operation, from ignition up to full load. Usually, under full load, a jet stabilised single flame is produced, but under partial load, due to the leaning of the fuel/air mixture, a single spatially distributed heat release zone is produced.
In addition, the use of new materials and manufacture methods are being tested. For example, the DLR Institute of Materials Research is working on the use of ceramics (e.g. Wound HIghly Porous OXide composite; WHIPOX®). In particular, during prototype production, and for complex geometries, Selective Laser Melting (SLM) is used for high-temperature materials.
The design of a new combustor and combustion chamber is based on an initial concept development using numerical simulation of reactive flows (Computational Fluid Dynamics; CFD). For this, commercially available programs are used to generate computational grids. Numerical flow simulations are conducted using the DLR-developed ‘Turbulent Heat Release Extension of the TAU Code’ (THETA) software and extended using computer simulation models developed by the department Computer Simulation. In addition, the commercially available ANSYS CFX / Fluent CFD software is employed. The numerical models used to simulate turbulence and combustion reactions vary according to development progress and the prescribed requirements. The range of combustion models used goes from simple combustion models (for example, Eddy Dissipation Model/Finite Rate Chemistry (EDM/FRC)) with comprehensive reaction mechanisms, to complex models that provide chemical reaction mechanisms in detail. Complex combustion models are particularly used for the combustion of synthesis gases and for external exhaust gas recirculation, as well as for lean FLOX® operations, since here, chemical-kinetic effects play a dominant role. In-depth study of steady and non-steady combustion processes with Reynolds-Averaged Navier-Stokes (RANS) equation modelling is also numerically supported by the use of hybrid Large Eddy Simulation (LES)/RANS methods.
Mobile, modular and optically accessible, atmospheric single burner test rigs with air preheating of up to 850 degree Celsius are available via the Atmospheric Burner Laboratories (ATM) for testing and characterisation of numerically designed combustion systems. Areas of priority for the characterisation of combustion systems are the provision of a stable operating range taking into account pressure loss and exhaust emissions, and the development of control and operating concepts.
With the results, the combustion systems are optimised in accordance with their field of application for microturbines. Subsequently, integration is carried out using the existing test rigs at the Institute (Test Centre) using MGT test rigs based on the Turbec T100 and the large-scale Research Power Plant based on the MTT EnerTwin and the Garrett GTCP 36-28 Auxiliary Power Unit (APU).