59106 - Prediction and Mitigation Strategies for Compressor Instabilities Due to Large Pressurized Volumes in Micro Gas Turbine Systems 

Micro gas turbines are a versatile platform for advanced cycle concepts. In these novel cycles, basic micro gas turbine components – compressor, turbine, combustor and recuperator – are coupled with various other technologies to achieve higher efficiency and flexibility. Examples are hybrid power plants integrating pressurized fuel cells, solar receivers or thermal storages.

Characteristically, such complex cycles contain vast pressurized gas volumes between compressor and turbine, which are many times larger as these contained in conventional micro gas turbines. In some maneuvers the rotational speed of the compressor drops rapidly. However, the pressure decrease is delayed due to the large amount of gas contained in the volumes. Ultimately, this can lead to compressor flow instability or surge. To predict and mitigate such instabilities, not only the compressor surge limit must be known, but also the dynamic dependencies between shaft speed reduction, pressure and flow changes within the system. Since appropriate experiments may damage the system, investigations with numerical simulations are crucial.

In this work the DLR in-house simulation program TMTSyS (Transient Modular Turbo-System Simulator) is used to investigate the impact of transient maneuvers on a micro gas turbine test rig containing a large pressurized gas volume. The simulation system is validated against measurement data and is able to predict the occurrence of compressor instabilities induced by fast shaft speed reduction. It is also shown that the simulation tool enables these predictions using only measurement data of non-critical maneuvers. Subsequently, mitigation strategies based on adaptive use of compressor bleed air are investigated to allow for fast shaft speed changes without the risk of compressor instabilities.