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Thermodynamics
constraints. Towards very low loads, i.e. high turn-down a
sudden strong increase of CO and UHC emissions occurs
whereas high NOx emissions limit the high-power end of
the range.
Low Load Operation of In-line Syngas Generation
To extend the turn-down the fuel can be converted to syn-
gas with a higher reactivity than natural gas. Theoretical
system analysis shows the feasibility and potential of the
process. Experimental investigations of the combination of
a fuel pre-processor which produces syngas with a hydro-
gen content of 30%, and two different generic gas turbine
combustors prove the technical feasibility. The lean limit
of premixed combustion in terms of flame temperature
for the two combustion concepts could be produced by
150-200K below the limit for natural gas. This corresponds
to a decrease of 15-20% thermal power without violating
CO emission limits.
Modeling of CO-Emissions for Gas Turbine
Combustors Operating at Part Load Conditions
Load decrease in gas turbines is limited by a sharp rise
of CO-emissions as the flame temperature decreases
and chemistry gets inhibited. The objective of this project
is a CFD-based model, which can predict CO in com-
bustion systems operating at part-load conditions. The
model supports the development of combustion systems
fulfilling future emissions legislation. So far, the model is
formulated and implemented. Furthermore, we conducted
experimental measurements at atmospheric conditions
for validation. In the following, validation using real engine
data will assess the performance at realistic conditions.
3. Explosion Research: Lean Hydrogen-Air Explosions
Severe accidents in nuclear power plants can be accom-
panied by the production of large amounts of hydrogen
and carbon monoxide. The formation of a flammable
mixture cloud is highly probable because of the wide igni-
tion limits of the fuel-air mixtures. The research focuses on
the hazardous deflagration-to-detonation transition (DDT),
which creates high pressure loads on the containing
structure, and on the important early stage of flame accel-
eration as well. In the early stage of flame propagation,
enlargement of the flame surface area is the main driver
for flame acceleration. In lean hydrogen-air mixtures,
flame front wrinkling caused by flame front instabilities
is a major cause of flame enlargement. Hence, these
effects need to be included in models for time averaged
reaction source term. Temporally high resolved optical
measurement techniques (OH-PLIF and shadowgraphy)
are employed to evaluate the flame front behavior in the
initial phase of flame propagation. This data is used for the
development and the validation of the model. Investiga-
tions were focused on the evaluation of microscopic flame
front curvature and showed a strong accelerating effect
that must be incorporated in future models.
Additionally, the existing experimental infrastructure of
the GraVent explosion channel is extended, allowing
the investigation of homogenous and inhomogeneous
H
2
-CO-air mixture distributions. The extension of the
existing numerical CFD framework in OpenFOAM aims
for large-scale detonation simulations with a wider fuel
flexibility and the possibility of further introduction of other
fuels. By applying the existing numerical H
2
-air framework
to smooth pipe accident scenarios of the chemicals
company BASF AG, it was shown that the large-scale
CFD framework can be adopted for the interests of the
chemical industry as well.
4. Internal Combustion Engines
Motivation and Objectives
Since dual-fuel combustion of natural gas with diesel
pilot ignition is a promising approach to address future
emission standards this topic is the subject of several
current studies at the Thermodynamics Institute. Charac-
terization and optimization of pilot ignition in the premixed
natural gas/air charge can lead to an increase in efficiency.
Another investigation tackles the formation of NO
2
under
these conditions, a toxic pollutant that is increasingly
emitted at certain loads. The third ongoing project aims
to reduce fuel slip caused by quenching effects in the
homogenously mixed charge by controlling the mixture
formation with high pressure direct injection of natural gas.
Experimental Investigations
The ignition and combustion processes in homogene-
ous charge methane/air mixtures were investigated in a
dynamically chargeable combustion cell under engine-like
conditions. It could be shown that ignition probability and
intensity are strongly influenced by the amount of pilot
fuel, pilot injection pressure, air-fuel ratio and the number
of injection holes. The investigations have revealed that in
most cases the pilot fuel suffers from too high dilution due
to its small quantity and long ignition delays. This results in
Shadowgraphy image of lean
(13 vol-%) hydrogen-air-flame
shortly after ignition