news / 2016-12-14

Better understanding thermo-acoustical phenomena in gas turbines

The new SCARLET test rig went into operation at the end of 2016 (source: DLR (CC_By 3.0))

Gas turbines used in power plants and for the propulsion of airplanes should emit as few pollutants as possible. To achieve this important development goal, the engineers need to better understand the combustion processes within the combustion chamber of the turbine. On a newly developed test rig, researchers from the German Aerospace Center (DLR) and Rolls Royce Deutschland have jointly conducted an initial series of experiments. The project is called SCARLET and went into operation at the end of 2016.

What happens in the combustion chamber?

SCARLET was developed on the high-pressure combustion chamber test rig (HBK) 3 at the DLR Institute of Propulsion Technology. It is specifically designed for acoustic measurements (source: DLR (CC_By 3.0))

Until now it has been difficult to accurately predict the processes within engine combustion chambers that lead to thermo-acoustical instabilities with lean fuel compositions. This is due to the multi-layered interactions within complex aero gas turbines. There is still a lack of knowledge as to how flames physically react to acoustic excitation. To fill these knowledge gaps, the researchers are developing model descriptions, numerical simulations and, in particular, measurements as part of validation tests on a test rig. These correspond to the conditions in a real engine.

 

This is where the new SCARLET (Scaled Acoustic Rig for Low Emission Technology) thermo-acoustic test facility comes into play. SCARLET was developed at the high-pressure combustion chamber test rig (HBK) 3 belonging to the DLR Institute of Propulsion Technology as part of the AG-Turbo alliance, which is funded by the German Federal Ministry for Economic Affairs and Energy. With SCARLET, this is the first time a test rig has been developed that was designed primarily for investigating thermo-acoustic tasks. Up to now it had not been possible to optimally investigate the phenomena decisive for the complex acoustics of air-turbine combustion chambers.

 

SCARLET has two acoustic measuring sections located upstream and downstream of the combustion chamber in the flow direction. By means of several dynamic pressure sensors, pressure fluctuations can be measured in both measuring sections in order to determine the acoustic field in front of and behind the combustion chamber. In addition, around 300 static sensors measure pressures, temperatures and mass flows.

 

The air can be preheated to about 700 °C before entering the measuring range and reaches a maximum operating pressure of 32 bar, which corresponds to the real conditions at the combustion chamber inlet. Particular emphasis was placed on avoiding acoustic reflections at the connection points of the measurement sections as these would falsify the measuring results. At the exit of the test carrier, an exhaust gas sensor measures the emission values of nitrogen oxides, carbon monoxide and dioxide and hydrocarbons, as well as the particle concentration of the exhaust gases.

Rich or lean?

SCARLET has two acoustic measuring sections located upstream and downstream of the combustion chamber. By means of several dynamic pressure sensors, pressure fluctuations can be measured in both measuring sections in order to determine the acoustic field in front of and behind the combustion chamber. In addition, around 300 static sensors measure pressures, temperatures and mass flows. (Source: DLR (CC_By 3.0))

The air/fuel mixing ratio is one of the keys to producing less pollutant emissions from gas turbines. With “lean combustion”, surplus air is available to the fuel in the primary zone of the combustion chamber. As a result, the nitrogen oxide output is lower than with “rich combustion”, which operates with surplus fuel in the primary zone of the combustion chamber. Lean combustion makes it possible to significantly reduce the thermally induced formation of nitric oxide by lowering the combustion temperature below 1,600 °C.

 

However, this decisive advantage offered by the lean combustion concept is not without serious problems. For example, the concept tends to produce periodic pressure fluctuations – so-called thermo-acoustical instabilities. In extreme cases these can even damage the combustion chamber or the fuel injection.

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