59844 - Thermoacoustic Behaviour of a Hydrogen Micromix Aviation Gas Turbine Combustor Under Typical Flight Conditions 

Hydrogen micromix is a candidate combustion technology for hydrogen aviation gas turbines. The introduction and development of new combustion technologies always carries the risk of suffering from damaging high amplitude thermoacoustic pressure oscillation. This was a particular problem with the introduction of lean premixed combustion systems to land based power generation gas turbines.

There is limited published information on the thermoacoustic behaviour of such hydrogen micromix combustors. Diffusion flames are less prone to flashback and autoignition problems than premixed flames and conventional diffusions flames are less prone to combustion dynamics issues. However, with the high laminar flame speed of hydrogen, lean fuel air ratio (FAR) and very compact flames, the risk of combustion dynamics for micromix flames should not be neglected and a comparison of the likely thermoacoustic behaviour of micromix combustors and conventional diffusion based aviation combustors would inform the early stage design of engine realistic micromix combustors.

This study develops a micromix combustor concept suitable for a modern three spool, high bypass ratio engine and derives the acoustic Flame Transfer Function (FTF) at typical engine operating conditions for top of climb, take-off, cruise, and end of runway. The FTF is derived using CFD and FTF models based on a characteristic flame delay. The relative thermoacoustic behaviour for the four conditions is assessed using a low order acoustic network code.  The comparisons suggest that the risk of thermoacoustic instabilities at low frequencies (below 1kHz) is small, but that instabilities could occur at higher frequencies. The sensitivity of the combustor thermoacoustic behaviour to key combustor dimensions and characteristic time delay is also investigated.

The characteristic time delay and thus FTF for a conventional kerosene combustor is derived from information in the literature and the thermoacoustic behaviour of the micromix combustor relative to that of the kerosene combustor is determined using the same low order modelling approach.  The comparison suggests that the micromix combustor is much less likely to produce thermoacoustic instabilities at low frequencies (below 1kHz), than the conventional combustor even though the risk in the conventional combustor is small. At higher frequencies, the likelihood of instabilities is similar for both combustors.

It is encouraging that this simple approach used in a preliminary design suggests that the micromix combustor has a similar thermoacoustic risk at high frequency and lower risk at low frequency than a conventional combustor. However, more detailed design, more rigorous thermoacoustic analysis and experimental validation are needed to confirm this. The paper suggests appropriate approaches to more detailed thermoacoustic analysis of micromix combustion systems.