Development and testing of hydrogen fuelled combustion chambers for the possible use in an ultra micro gas turbine

This PhD thesis presents the scientific evaluation and development history of different combustion chamber designs based upon the “PowerMEMS” design parameters. With hydrogen as chosen fuel, the non-premixed diffusive “micromix” concept was selected as combustion principle. Originally designed for full scale gas turbine applications in two different variants, consequently the microcombustor development had to start with the downscaling of these two principles towards ì-scale. Both principles have the advantage to be inherently safe against flashback, due to the non-premixed concept, which is an important issue even in this small scale application when burning hydrogen. By means of water analogy and CFD simulations the hydrogen injection system and the chamber geometry could be validated and optimized. Besides the specific design topics that emerged during the downscaling process of the chosen combustion concepts, the general difficulties of microcombustor design like e.g. high power density, low Reynolds numbers, short residence time, and manufacturing restrictions had to be tackled as well.

As full scale experimental test campaigns are still mandatory in the field of combustion research, extensive experimental testing of the different prototypes was performed. All test campaigns were conducted with a newly designed test rig in a combustion lab modified for microcombustion investigations, allowing testing of miniaturized combustors according to full engine requirements with regard to mass flow, inlet temperature, and chamber pressure. The main results regarding efficiency, equivalence ratio, and combustion temperature were obtained by evaluating the measured exhaust gas composition. Together with the performed ignition and extinction trials, the evaluation and analysis of the obtained test results leads to a full characterization of each tested prototype and delivered vital information about the possible operating regime in a later UMGT application. In addition to the stability and efficiency characteristics, another critical parameter in combustor research, the NOx emissions, was investigated and analyzed for the different combustor prototypes.

As an advancement of the initial downscaled micromix prototypes, the following microcombustor prototype was not only a combustion demonstrator any more, but already aimed for easy module integration into the real UMGT. With a further optimized combustion efficiency, it also featured an innovative recuperative cooling of the chamber walls and thus allowing an cost effective all stainless steel design.

Finally, a statement about the pros and cons of the different micromix combustion concepts and their correspondent combustor designs towards a possible ì-scale application could be given.