news / 2013-10-18

Turbine concept: Compact design for decentralised gas turbines

Drive concept is a rotating combustion chamber

The simulation shows the speed of the fluid in the compressor. Source: RWTH Aachen University

The core of the new drive concept is a rotating combustion chamber. The swirling flow remains between the compressor and turbine. This allows the compressor's stationary blade in front of the combustion chamber to be dispensed with. This increases efficiency because in a centrifugal compressor two-thirds of losses occur in the stationary blade. Also, in the new design, the turbine's stationary blade after the combustion chamber is superfluous. The first turbine stationary blade is traditionally subjected to the highest temperatures, so it has to be cooled significantly. This cooling air is now redundant. The removal of the stationary blades also reduces weight and costs.

 

Combustion takes place in a swirl-afflicted current within the rotating combustion chamber. The flame speed increases due to the strong centrifugal force and density-driven mixing effects. As a result of the findings from theoretical studies, the length of the combustion chamber can be significantly reduced. By shortening the combustion chamber, weight, costs and friction losses are all reduced. In addition, harmful NOx emissions can also be reduced.

 

Comparatively large radial clearances between the blades and the vertical housing may occur in conventionally manufactured small gas turbines. This reduces their efficiency. The rotating housing prevents radial clearances which increases efficiency by approximately five per cent. Scientists calculated this value using computer simulations in which the pressure ratio and geometry of the compressor were varied.

 

High turbine inlet temperature in rotating housing

Since the blades are carried by the rotating housing, ceramic materials may be used. This allows the turbine inlet temperature, one of the most important parameters for efficiency, to be increased. The new material allows a temperature increase up to 1,230° C. If the blades are placed on the rotor, only high-temperature steels can be used for stability reasons. Due to the small size of the blades in a small gas turbine, the use of an air cooling system in and on the blade is too complicated to produce.

 

The installation of a diagonal compressor with a thick trailing edge enables a pressure ratio of 4.1. To achieve the highest possible pressure ratios, a centrifugal compressor would be desirable. But since the combustion chamber extends in an axial direction, where the radius remains constant, it is necessary to divert the flow from the radial direction into an axial direction. Thus a lower pressure ratio can be achieved with a diagonal compressor than with a radial compressor, however, the flow goes into the combustion chamber in an axial direction and the pressure ratio is greater than in an axial-flow compressor.
If all the ideas to reduce efficiency losses in small gas turbines can be implemented, the research partners estimate that a thermal efficiency of over 40 per cent can be achieved. To date, standard small gas turbines have an efficiency of below 30 per cent.