Download Development of Novel Cycles and Combustors for Future

Development of Novel Cycles and Combustors for
Future Aircrafts
By
Bhupendra Khandelwal – Cranfield University
Discharging green house gases and particulates into the atmosphere has an
impact on global climate. Environmental concerns and depletion of fossil fuel resources
have become the driving force for research and development for decreasing the fuel
consumption, emissions and finding a fuel for future aviation. Additionally, emissions of
carbon dioxide, water vapour and oxides of nitrogen (as a consequence of fossil fuel
combustion) contribute to global warming.
Even though combustor technology is developing gradually, there is a need for
new technology and concepts to satisfy the emission norms to be laid by the ICAO. This
would also help in dealing with the fuel scarcity which is a big problem, arising for the
world now. Although the percentage of aircraft emissions is not a substantial amount as
compared to other counterparts which contribute to emissions. But aviation industry is
increasing at a substantially high rate as compared to other counterparts which puts
aviation industry under pressure to decrease the emissions. One of the options for
decreasing emissions and dealing with fuel scarcity is introducing hydrogen or other
fuels (nuclear) as a fuel for air transport. Different options of alternative fuel, combustors
with their implications are explored in this study.
One of the options to deal with uprising problems of emissions and fuel scarcity
is to use Hydrogen as a fuel. Detailed investigations are underway for checking the
feasibility of the same. Investigations till date suggest that a subsonic hydrogen fuelled
passenger aircraft will on average burn 16% less fuel then a similar conventional aircraft
and the advantage is 28% in case of supersonic aircraft. Using hydrogen has an
additional advantage of being safer as compared to presently used jet fuel. This is due
to the fact that, hydrogen is lighter gas and in case of emergency hydrogen escapes
into the atmosphere, whereas, kerosene remains there and poses a potential fire
hazard. Liquid hydrogen is stored in cryogenic tanks which are at substantially low
temperatures. This liquid hydrogen can also be used as a heat sink (coolant) for
combustor and other parts before introducing it in the combustion chamber as a fuel.
Control range of hydrogen is at leaner equivalence ratios, this suggests that hydrogen
could be burned at much lower equivalence ratios as compared to kerosene. This would
help in reducing the flame temperature thereby lowering the NO x emissions.
The problem which is being faced by Hydrogen is the risk of auto-ignition for
premixed systems and the problems of large scale hydrogen diffusion flames, the lean
non-premixing concept of micromix combustion, which is based on miniaturised
diffusive combustion, is suggested as a promising hydrogen combustor configuration.
The concept of miniaturised diffusive combustion is shown in Figure 1. As illustrated in
Figure 1(a), hydrogen is being injected radially through the holes into the main air flow,
which then burns as a diffusive combustion. An array of such miniaturised diffusive
combustor, in a ring shaped structure of the micromix hydrogen combustor is shown in
Figure 1(b). The proposed injector configurations in this paper are aimed to improve
these combustor designs for better performance.
Figure 1: Ring-shaped structure of the micromix hydrogen combustor (Air and hydrogen admission for
diffusive combustion in the micromix hydrogen combustor of third generation for the APU GTCP 36-300
engine).
A design concept of micromix concept based hydrogen combustor with high
mixing has been proposed in this study and its comparison with low mixing micromix
concept based combustor has been performed. The improved mixing allows the
combustion zone to operate at a much lower equivalence ratio than the conventional
kerosene based combustor and micromix concept based combustors (studied earlier).
The new design of Micromix concept based combustor is producing upto 60% less NO x
emissions as compared to work done by other researchers. The renewed model, not
only reduces NOx emissions, but also helps in decreasing the combustion chamber
length by almost half of the normal length, which will eventually reduce fuel
consumption also.
Details of the new concept discussed above could not be disclosed at this point
of time due to a patent application for the concept.
A new design of normal gas turbine combustor has been proposed in this study.
Investigations done on new design shows that fuel consumption will reduce
substantially. It also shows that the cooling air requirement, length of combustor,
emissions and complexity of combustor is far less as in new design as compared to
conventional gas turbine combustor. It also provides optimized temperature profile for
turbine blades, which leads to increase in life of turbine blade. Details of this concept
also could not be disclosed at this point of time due to patent application for the
concept. (Note: - Details about both patent applications could be provided on request.)
Some Results
Figure 2 is representing NOx emissions for different models of micromix concept hydrogen
combustors. It is observed that the model 3 which is proposed in this study is producing
substantially less emissions as compared to other cases. Several such designs have been
studied and designed. Details about designs could not be disclosed at this point of time.
Figure 2 Average NOx emissions at different cross sections along –z direction in the combustion
zone
In Figure 2, average NOx emissions have been calculated from different cross sections along –z
direction. The vertical axis represents the NOx emissions, and the horizontal axis means the
section coordinates along –z direction defined in the domain setup. The NOx emissions in all the
three models increase with the coordinate increase of horizontal axis. Comparing to Model 1
and 2, the NOx emissions in Model 3 are kept at a remarkable lower level. Although the NO x
emissions in Model 1 and 2 are almost the same at the outlet section, Model 2 has a sharper
increase in the first 10 millimetres of the combustion zone than Model 1, and the maximum NO x
emission level has been reached at the positions of 40mm and 30mm along –z axis,
respectively, in Model 1 and 2. The sharp increase of NOx emissions in Model 1 happened in
the zone between 20mm and 40mm.