Our research guideline is the proposition that scientific research in an engineering school should be focused on problems with high technological relevance. A key to realizing our mission is the close cooperation with industry in general and in particular with partners who – developing their top-class global products at the leading edge of technology – have encountered barriers that might be overcome by fundamental research.
1. Combustion Emissions and Reliability
a. Boundary Layer Flashback in Premixed Combustion of Highly Reactive Fuels
Motivation and Objectives
If modern gas turbines are operated on highly reactive fuels such as hydrogen, flame flashback inside the burner’s wall boundary layer is a major issue which limits the stable and safe operation. A detailed understanding of the underlying physical mechanism as well as tools to predict the flashback limits are of high interest in the design of gas turbine burners.
Approach to Solution
Experimental studies including laser diagnostics have been performed to analyze the mechanism of boundary layer flashback. Based on this knowledge, semi-analytical models to predict the flow velocity at flashback have been developed for different flame configurations. In addition, first attempts to reproduce the experimental results in numerical simulations have been realized.
Key Results
Boundary layer flashback is mainly influenced by the velocity and turbulence field at the burner exit. Especially for hydrogen-rich fuels flame stretch significantly increases the flashback risk. Velocity oscillations due to thermoacoustic instabilities can cause the flame to periodically enter into the premixing section. This strongly increases the burner’s flashback propensity. The two semi-analytical models for the calculation of flashback limits developed in 2016 are a remarkable progress in flashback modelling
Related Projects
The combustion of hydrogen-rich fuels is further studied in the context of energy recovery from chemical hydrogen storage in micro gas turbines. For a high efficiency hydrogen combustion in a micro gas turbine, the exhaust gas heat needs to be recovered in a recuperator. This leads to high pre-combustion temperatures and therefore, a high susceptibility to flashback in the premixing zone.
b. Operational Flexibility of Gas Turbine Power Plants
Motivation and Objectives
To balance the increasing share of volatile power from renewable power sources highly flexible conventional power plants are needed. Gas turbine power plants have the potential to quickly adjust to changing power demand but their operating range is curbed by emission 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 syngas with a higher reactivity than natural gas. Theoretical system analysis shows the feasibility and potential of the process. Experimental investigation of the combination of a fuel pre-processor which produces syngas with a hydrogen content of 30% and a generic gas turbine combustor prove the technical feasibility. Here the lean limits of the premixed combustion flame temperature could be extended to 150K below the limit for natural gas. This corresponds to a decrease of 20% in terms of thermal power without violating CO emission limits.
Power Augmentation by Water Injection
Water injection in premixed gas turbine combustors reduces the flame temperature and increases the power output. In order to understand the effects occurring during premixed combustion with water injection experimental and numerical studies are performed. Low emission combustion with high water to fuel ratios was achieved and substantial understanding of CO formation has been gained by experimental and numerical studies. The results lead to design rules for water injection systems. CFD simulations in cooperation with another project on burn-out in staged combustion systems will consolidate the understanding of CO formation and validate the design rules.
Influence of Water Injection on Thermoacoustics of Premixed Flames
Combustion instabilities are of major concern in gas turbine combustors. Thus the impact of water injection on dynamic flame response, acoustic modes and its damping rates were investigated. Using a newly developed technique which analyzes combustion noise spectra the acoustic damping rate of the combustor eigenfrequencies could be expressed as a function of the water-to-fuel ratio. Based on this data it has been shown that stable operation of the tested combustion chamber can be achieved for water-to-fuel ratios of up to 2:1. For the thermal power load of the combustor this corresponds to an increase of more than 40% without raising flame temperature and pollutant emissions.
c. Explosion Research: Deflagration to Detonation Transition
Using the open-source library OpenFOAM, a new CFD combustion solver has been developed at the institute in the framework of two research projects funded by the German Federal Ministry of Economic Affairs and Energy (BMWi). It supports the analysis of large-scale hydrogen explosions as seen in the Fukushima-Daiichi reactor accident. A focus is placed on the hazardous Deflagration-to-Detonation Transition (DDT) which creates high pressure loads on the containing structure. The method is supposed to further advance the state-of-the-art in nuclear safety analysis which is currently based on empirical combustion regime transition criteria. A topic of particular interest is the influence of mixture inhomogeneity on DDT. For code validation and further detailed investigations the institute established the laboratory-scale GraVent explosion channel providing access for high-speed conventional and optical measurement techniques. Both numerical and experimental studies revealed that inhomogeneous mixtures can promote flame acceleration and ultimately DDT in comparison with homogeneous mixtures of the same average fuel-oxidator ratio. Using the SuperMUC high performance cluster of the Leibniz Supercomputing Center the explosion analysis in the containment of a Konvoi-type pressurized water reactor demonstrated the capabilities of the developed solver in a full-scale application. In a joint project with Korea Electric Power Company Engineering & Construction, the manufacturer of South Korea’s new pressurized water reactor APR1400, the method has recently been introduced in nuclear industry. Future experimental and numerical research is directed towards the influence of carbon monoxide – another flammable gas that may be produced in significant amounts after the failure of the reactor pressure vessel in core-meltdown accidents.
d. 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 subject of several current studies at the Thermodynamics Institute. Characterization 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 NO2 under these conditions, a toxic pollutant that is increasingly emitted at certain loads. The third ongoing project aims to reduce the 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 homogeneous 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 and the air-fuel ratio. The investigation of natural gas high pressure direct injection combustion with Diesel spray piloting was performed on a rapid compression machine. The variation of spatial and temporal overlap of the pilot spray and gas jet shows how the ignition behavior is governed by the interaction between them. In both experiments high speed imaging of flame luminescense and of shadography were applied.
Numerical Investigations
The effect of fuel substitution on the ignition probability of the resulting fuel blend was studied using detailed reaction mechanisms. An auto-ignition model capable of handling mixtures of two fuel types with significantly different reactivity was developed and successfully implemented in a commercial CFD software package. With these tools ignition and heat release in dual-fuel diesel engines are investigated for the two cases of homogeneous charge and high pressure direct injection of gaseous fuel. Detailed kinetics simulations in homogenous reactors revealed the thermodynamic conditions responsible for the significant NO-NO2 conversion observed in these engines. High emissions of NO2 were shown to be caused by small amounts of unburned hydrocarbons (due to flame quenching in the lean methane-air charge) reacting with NO during the expansion stroke and in the exhaust system.
Related Projects
- Optimization of Diesel Pilot Ignition in Dual-Fuel Engines with High Mean Effective Pressures
- Investigation of Direct Injection Dual-Fuel Combustion with Flexible Fuel Combinations
- Numerical Simulation of NO to NO2 Conversion in Dual Fuel Engines
2. Combustion Instabilities and Noise
a. High-Frequency Transversal Thermoacoustics
Motivation and Objectives
High-frequency thermoacoustic instabilities result from constructive interferences between combustion heat release and acoustic oscillations. Multidimensional modes govern these instabilities and are of equal concern in both gas turbine and rocket motor combustion chambers. They physically manifest themselves as high-amplitude, self-sustained pressure pulsations in the combustion chamber. Potential consequences range from hardware damage, increased pollutant production to system failure. Avoiding these instabilities requires a thorough understanding of the physical mechanisms, the development of prediction models and mitigation tools.
Methods and Approaches
Experiments were conducted using a model gas turbine combustor that exhibits self-sustained thermoacoustic pulsations at frequencies of 3000 Hz connected to the first transversal mode. The dynamic interaction between flame and acoustics in the chamber can be generalized by formulating adequate thermoacoustic source terms. They are obtained by employing advanced experimental measurement, post-processing and modeling approaches.
Reduced Order Models for simulating the time domain system dynamics are developed and cross-validated against experimental data to obtain insight into the limit cycle behavior of high-frequency thermoacoustic oscillations. Beyond this, the time domain models are utilized to develop and verify methods for system identification, which yield essential information needed to design mitigation methodologies.
Numerical simulations in frequency domain based on the Linearized Navier Stokes Equations are carried out to yield high-fidelity solutions of multidimensional thermoacoustic mode shapes in both gas turbine and rocket combustors. Furthermore, linear stability assessments can be conducted by prescribing the source terms in the governing equations with either analytically or numerically obtained transfer functions.
Key Results
For the first time distributed source terms for non-compact (multidimensional) gas turbine thermoacoustics were identified, modeled and cross validated. The employment of these source terms in the developed high-frequency thermoacoustic analysis tools allowed the simulation of linear stability and limit cycle dynamics of the model gas turbine combustor. These studies provided fundamental understanding on transversal mode dynamics. For rocket engines source terms are obtained by simulations of the reactive flow field including real gas effects. With these linear stability limits of different experimental benchmarks can be reproduced. In addition to this acoustic propagation in three-dimensional space comprising nozzle, chamber, propellant domes and absorber rings are computed with frequency domain field methods.
b. Annular Combustor Damping
Motivation and Objectives
A major concern in modern industrial gas turbines is the occurrence of combustion instabilities. Annular combustors burning under lean conditions are susceptible to self-sustained azimuthal oscillations. A widely used countermeasure is the use of passive damping devices to suppress high amplitude pressure pulsations. Efficient dissipation of acoustic energy by such resonators and hence the disruption of the thermoacoustic feedback cycle, requires appropriate dimensioning and an effective placement strategy especially in case of annular combustors.
Experimental Approach
Different damper configurations with respect to the number of dampers, their spatial distribution and the amount of purge air are investigated and compared to the baseline case without dampers. To assess the stability quantitatively three methods for damping rate computation from dynamic pressure data have been developed: The first method is based on the analysis of the decay of the pulsating pressures after sudden shut-down of sirens providing single frequency acoustic excitation. The second method employs so-called Lorentzian fitting to the pressure spectra resulting from turbulent combustion noise and the third method consists of the analysis of the autocorrelation of the acoustic pressures.
Numerical Approach
The measured damping rates serve as validation database for a numerical methodology based on the Linearized Euler Equations to quantitatively predictthe influence of dampers on the stability of the rig. The contribution of different combustor components to the acoustic damping of the entire system is also investigated based on this numerical approach and the effect of variations in the number of implemented resonators on predicted damping rates.
c. Combustion Noise
The reduction of noise emissions of modern aero engines demands for an improvement in the understanding of the sources of combustion noise as well as the development of accurate predictive tools. Considerable progress on this topic has been made in the framework of the European project RECORD (Research On Core Noise Reduction). A statistical noise model postprocessor was implemented in a hybrid CFD/CAA-method in order to determine the combustion noise spectra of two laboratory scale combustors. The approach was validated by experiments and numerical computations with different levels of complexity. Very good agreement with experimental data was obtained in terms of pressure spectra in the frequency range of interest. Furthermore, the influences of the source distribution, of mean flow convection and of the boundary conditions on combustion noise spectra were studied. The accuracy and efficiency of the noise model postprocessor was evaluated by a comparison with sources obtained from LES simulations. In this way the capability of the method for fast and efficient combustion noise prediction was demonstrated.
3. Transport Phenomena
a. Transport Phenomena in Desalination
Motivation and Objectives
Water processing and the related power consumption within the constraints of an ecologically sustainable use of globally essential resources has become one of the major challenges of the 21st century. During the last decade research on the recovery of potable water from of sea- and saline waste water has therefore undergone a paradigm shift from a product centered activity to a comprehensive interdisciplinary field.
Membrane desalination technologies draw increasing attention due to the lower power requirement as compared to thermal systems. Facing the high power consumption of desalination processes the major research fields of the institute are the operation of Reverse Osmosis (RO) systems powered by renewable sources and the investigation of Membrane Distillation (MD). Drawing on the scientific heritage from heat and mass transfer phenomena in single- and multiphase flows, as well as on supply and process engineering the research goals lie in the knowledge based improvement of the processes.
Approach to Solution
The institute collaborates with the Dead Sea and Arava Science Center in Israel, OWT GmbH, Millenium electric, Gore and memsys GmbH in two research projects funded by the Federal Ministry of Education and Research (BMBF). The PV/T-RO project focuses on the process operation due to the fluctuating power supply of solar systems (Photovoltaic/ Thermal, PV/T) and smallest energy storages. The project SPACE considers Vacuum Membrane Distillation (VMD) processes to treat saline solutions up to saturation for different applications in Air Conditioning and Zero Liquid Discharge (ZLD).
The thermally driven process of VMD can operate at salinities that go beyond the limits of RO, which has to overcome the osmotic pressure of saline solutions. This is an necessary feature especially in brine treatment of desalination plants or waste water treatment up to the so called ZLD. Its operability at low temperatures (up to 80 °C) allows to utilize waste heat from industrial processes or the coupling to renewable sources.
Reliable simulation models require validation on accurate data of the dominant physical processes at the membrane under the relevant operating conditions. Combining Interferometry and Laser-Schlieren optics with Refractive Field simulation aims at the non-invasive, optical measurement of membrane boundary layers with strong refractive-index-gradients at RO operating pressures up to 60 bar.
Key Results
This year a solar driven RO pilot plant was installed in Israel and experiments were successfully conducted. Based on these results and on a thermodynamic analysis of solar powered desalination systems, design guidelines were developed. Transferring these results to other membrane processes, the performance of a new combination of Electrodialysis (ED) and PV/T collectors was numerically analyzed and presented at the EDS conference in Rome.
Prediction and modeling of the performance of RO systems need highly accurate experimental data. Independent experimental data for a boundary layer at the same operating conditions allow not only quantitative validation of each measurement result, but make an increase in measurement accuracy possible, especially in presence of strong concentration- and thus refractive-index-gradients. Starting from the investigation of dynamic phenomena in RO due to fluctuating energy supply, it was found that pulsatile flows can lead to a significant performance increase. A CFD simulation study showed that the water flux can be increased up to 18 % dependent on geometry, amplitude and frequency of the pulsation. The results were presented at the EDS conference in Rome.
Concerning VMD, during the last years, the institute built up a research infrastructure to investigate the technology from Multi-effect industrial systems down to heat and mass transfer phenomena in the membrane channels with respect to scaling behavior and membrane wetting. A two-year operation study of a multi-effect VMD system together with memsys GmbH for regeneration of liquid desiccants has been successfully completed and published.
b. Dynamic Coupling of TFM and VOF Approaches to Simulate Horizontal Two-Phase Flow with Strong Scale Variations
Motivation and Objectives
The modeling of the accident-induced transient thermo-hydraulic behavior of nuclear facilities is a major part within the framework of reactor safety research. A particular phenomenon hereby is the occurrence of gas-liquid-two-phase flow, possible as a result of loss-of-coolant-accidents (LOCA).
Due to gravitational forces, a separation of the phases in horizontal coolant pipes could arise. This may lead to stratified flow, where certain flow conditions could indicate the transition to plug flow or slug flow. Such highly intermittent flow regimes are responsible for pressure pulsations which can stress structure materials inadmissibly. The entrainment of the gaseous phase at the front of propagating slugs seems to have crucial influence on the pressure drop along the flow. To predict the behavior of nuclear facilities in post-accidental situations, modeling of the presented flow regimes is required. The related project is funded by the Federal Ministry for Economic Affairs and Energy (BMWi) and the Gesellschaft für Anlagen- und Reaktorsicherheit (GRS).
Approach to Solution
Since the phenomena occurring in the flow regimes mentioned are distributed over a wide range of temporal and spatial scales which, in themselves, cover a wide variety of orders of magnitudes, modeling and prediction using numerical methods emerges as pretty complex. Starting from a scale averaging model based on an Eulerian framework, the two-fluid model (TFM) is conditioned in order to allow partial scale resolving features. The model solves one set of partial differential equations for each flow involved fluid phase. Special treatment for the momentum exchange based on non-resolved interfacial morphology was developed and implemented in an open source CFD toolbox (OpenFOAM). Additional model extensions to simulate the transport of interfacial area concentration allow the dynamic detection of dispersed and stratified flow regions. The model was validated using experimental data from an optical test rig at the Institute which allows the investigation of the two-phase flow patterns.
Key Results
Multiphase-flow patterns with strong variations in scales are placing particular demands on numerical modeling approaches, with respect to storage capacity, computational coasts and flexibility. To meet these requirements, a numerical method was developed which combines the advantages of the scale averaging Euler-Euler two-fluid model in terms of computational effort with the ability of the volume-of-fluid model to simulate stratified flow regions in a quantitatively correct manner. The new method is characterized by a high degree of flexibility considering dynamic transition processes between disperses and stratified flow regions. The algorithm to distinguish the involved flow regions utilizes interfacial curvature as a scalar field variable, which is dynamically transported to incorporate the effects of interface structure interaction on phase exchange processes.