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 realising 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.
I. Combustion Emissions and Reliability
1. 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 stable and safe operation. A detailed understanding of the underlying physical mechanism as well as tools to predict the flashback limits are of great interest in the design of gas turbine burners.
Approach to Solution
Boundary layer flashback is numerically investigated with large eddy simulations. Combustion is modelled with finite rate chemistry including detailed diffusion modelling. This improves the insight into the mechanisms leading to flashback.
Key Results
The numerical model can reproduce the boundary layer flashback limits of flames confined in a duct. All relevant physical effects are thus accounted for by the chosen modelling approach. Furthermore, it was shown that the existence of average boundary layer separation is not a distinct criterion for the occurrence of confined boundary layer flashback. Instead, flashback is triggered if local flow separation zones at the flame bulges are large enough to locally promote flame propagation.
2. 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 limited 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.
2.1 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 investigations of the combination of a fuel pre-processor which produces syngas with a hydrogen 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.
2.2 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 combustion 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 accompanied 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 ignition 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 acceleration 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 behaviour in the initial phase of flame propagation. This data is used for the development and the validation of the model. Investigations 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 H2-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 H2-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. Characterisation and optimisation 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 fuel slip caused by quenching effects in the homogeneously 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, 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 a small number of ignited sprays and consequently leads to longer combustion durations. Furthermore, the experiments confirm that the natural gas of the background mixture influences the autoignition of the diesel pilot oil. 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 behaviour is governed by the interaction between the two jets. In both experiments, high speed imaging of flame luminescence and of shadowgraphy 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 which originate from flame quenching in the lean methane-air charge, which reacting with NO during the expansion stroke and in the exhaust system.
II. Combustion Instabilities and Noise
1. High-Frequency Transversal Thermoacoustics
Motivation and Objectives
High-frequency thermoacoustic instabilities in gas turbine combustion chambers result from constructive interferences between combustion heat release and acoustic oscillations. 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, for the development of prediction models and mitigation tools. Research activities in recent years were focused on swirl stabilised combustion systems and allowed a deep insight to be gained into the high-frequency thermoacoustic feedback mechanisms, both experimentally and numerically. To further develop a comprehensive understanding of physical flame response mechanisms to high-frequency pressure pulsations, the focus is extended towards reheat combustion systems, which represent a highly relevant technology for modern gas turbine systems for electrical power generation.
Methods and Approaches
A state-of-the-art test rig for investigation of reheat flame dynamics has been designed, commissioned and experiments on the reheat flame response were conducted. The experiment features a special design that promotes the first transverse acoustic resonant mode, while simultaneously allowing the establishment of a characteristic reheat flame with areas dominantly stabilised by auto-ignition processes. The institute’s acoustic prediction tools, know-how and methodologies were employed in an iterative design process to implement an experiment that allows for both in-depth academic studies by applying respective acoustic and flame diagnostic measurement techniques whilst reproducing characteristic flame features at lab scale. This experiment is the basis of future investigations to gain insight into distributed source terms that capture the underlying physics of high-frequency reheat flame response.
Key Results
The novel reheat combustor experiment represents realistic reheat flame features as found in industrial systems and allows for an extensive scope of investigations. The specific combustor design of the rig has been optimised simulating its thermomechanical and fluid dynamic characteristics. The acoustic design, sought to predominantly excite the first transverse mode, is developed by means of analytic approaches together with computational aero-acoustics in the frequency domain. In addition to this, a priori linear stability assessments are carried out to maximise the flame-acoustics constructive feedback. This rig design was successfully commissioned and is now used to conduct investigations to establish insights into the physics of reheat flame dynamics and to provide validation data for the development of analytical and numerical prediction models for thermoacoustic stability assessment tools as well.
2. 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 the 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 a validation database for a numerical methodology based on the linearised Euler equations to predict the stability margin of the rig quantitatively in a wide operating range, and to assess the influence of different damper configurations and fuel-stagings on the thermoacoustic stability to deduce basic guidelines for the application of passive damping concepts in annular combustors.
3. High-Frequency Dampers
In a similar project, the application of the damping devices in a can-combustor is investigated in high frequency regime. To avoid high-frequency vibrations, the increase in the acoustic damping of the combustion chamber is of central importance. This requires precise modelling tools which are not yet state of the art. Therefore, the purpose of this project is to develop a hybrid method for the damping calculation of can combustion chambers used in the high efficiency machines. This includes a combination of non-linear and linear field methods with network methods and the aim of creating a hybrid process that is as efficient as possible. In this context, two methods are introduced. The first approach is the CFD/LNSE method, where the modelling task is to calculate the acoustic processes in the combustion chamber (basket and transition) based on linearised Navier-Stokes equations (LNSE). In the other approach, perturbed non-linear non-conservative Euler (PENNE) equations will be implemented for computational aero acoustic (CAA) simulations. In general, the hybrid approach, separating the calculation of mean flow field and perturbation in time domain will help to identify the acoustic behaviour of combustion chambers and associated damping elements at comparatively low cost.
3. Combustion Instabilities in Rocket Engines
Motivation and Objectives
High frequency combustion instabilities arise from the interaction of field fluctuations due to the combustor acoustics with the heat release from the combustion. Consequences of unstable operation can reach up to destruction of the engine and thus mission failure. The prediction of these instabilities and the assessment of counter measures is the objective of this project, to support the reliable design of thermo-acoustically stable rocket engines.
Methods and Approaches
Numerical simulations are carried out in the frequency domain. Eigensolutions of linearised Euler equations are computed, describing modes and frequencies of the acoustic oscillations in the combustor. Absorbers are accounted for by impedance boundary conditions and a dome can be coupled to the chamber via a transfer matrix. Source terms in the energy equation account for flame feedback. Experimentally, studies are carried out on a cold-flow rocket engine configuration.
Key Results
For the stability predictions in rocket combustors, analysis has been extended to different propellant combinations. The modes in a hydrogen and a methane fuelled engine without flame feedback have been compared with each other. Computational and qualitative differences in the cut-on behaviour have been observed. In contrast to H2, for CH4 higher modes can be cut on in the front chamber part, before lower modes are cut on at the rear end. Further numerical and experimental studies have been continued examining the modification of chamber acoustics by the application of an absorber ring. The mode split observed previously has been studied with respect to the absorber design and a significant influence on the mode shape as well as eigenfrequencies has been found.
III. Transport Phenomena
1. Behavior of the Void Fraction in Subcooled Flow Boiling Close to Critical Heat Flux
Motivation and Objectives
In nuclear engineering the heat transfer in both reactor core and steam generator are of great interest regarding the safe operation of a nuclear power plant. Beyond a critical value of the heat flux (CHF), film boiling can occur. This boiling crisis and the departure from nucleate boiling must be avoided under all circumstances.
Approach to Solution
The experimental work conducted at TUM is aimed at providing detailed experimental data about CHF conditions for validation of CFD-based critical heat flux modelling. Special interest is placed on gathering data about the morphology of the two-phase flow close to CHF in the immediate vicinity of the heated wall. Measurements are carried out using a variety of measurement techniques, ranging from conventional thermocouples and pressure sensors to high-speed videometry and optical micro-fiber probes. The latter, shown in the figure representing a double fiber probe, provide a way of measuring the local void fraction, bubble velocity and bubble diameters inside a flow channel.
Key Results
It was found that inlet subcooling is the main influence parameter on void fraction. An analysis of the dynamic behaviour of the void along the entire boiling curve up to fully developed film boiling revealed a sudden peak in void fraction at the wall upon reaching critical heat flux in conjunction with a significant increase in bubble velocity close to the wall. The experimental data have been used to develop a calibration procedure of the classical CFD boiling model, as shown in the figure below.
2. Transport Phenomena in Desalination
Motivation and Objectives Water processing and its 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 saline water has therefore undergone a paradigm shift from a product-centered activity to a comprehensive interdisciplinary field.
Approach to Solution
Mainly two desalination techniques are currently investigated at the Institute: reverse osmosis (RO) and vacuum membrane distillation (VMD). The thermal 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 a necessary feature especially in brine treatment of desalination plants or waste water treatment up to the so-called zero liquid discharge (ZLD).
Key Results
Similar to heat exchangers, in the RO process the build-up of a concentration boundary layer decreases the performance of the system. Experimental and numerical studies on pulsatile flows showed that the water flux can be significantly increased due to the distortion of the boundary layer, depending on amplitude and frequency. Furthermore, optical studies of pulsating flows in channels including eddy promoters were performed to identify the main fluid dynamic phenomena. During the experimental investigations, a measurement technique was developed to measure highly dynamic volume flow rates in hydraulic systems. Concerning VMD, during the last years the institute has 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 behaviour and membrane wetting.
Continuing the work of the last years a new model of a multi-effect VMD system has been successfully completed. This tool developed can be used to optimise plant design of multi-effect VMD systems and decrease the energy consumption with respect to high concentrated saline solutions and possible scaling effects.
3. Solar Cooling by Coupling of PV and Compression Chillers
Motivation and Objectives
Thermal conditioning of residential buildings plays a major role in the energy consumption of countries in a temperate climate as well as in the hot and humid climate of countries in the Gulf region. The market share of HVAC devices powered by renewable energy is low for both regions. This issue is addressed in the PVCool project of the Institute of Thermodynamics at TUM and the Hamad Bin Khalifa University (HBKU) in Doha, Qatar, started in March 2017.
Approach to Solution
Compression chillers (CC) directly coupled with photovoltaic (PV) systems (PV-CC) have gained increased attention, mainly due to the decreasing costs of PV systems and the low investment costs of conventional compression chillers, which makes them more and more economically feasible. However, one of the main challenges in PV-CC is their unsteady operation and need for low part load capability under fluctuating solar irradiation. Therefore, a swash-plate compressor is integrated into a CC for residential buildings and tested under desert and temperate climate boundary conditions.
Key Results
In a first step, experimental comparisons between swash plate compressor and scroll compressor, that is commonly used in HVAC applications, were executed under steady state conditions. It could be shown, that the part load capability of the swash-plate compressor is substantially lower than for the scroll compressor. The coefficient of performance of the swash plate compressor is slightly lower than the COP of the scroll compressor. These promising first results lead to further investigations comparing the dynamic operation of the two compressor types.
4. Energy Efficient Heat Source Management During the Initial Heat-up Period of Vehicle Cabin
Motivation and Objectives
Even in current, combustion-engine powered vehicles, cabin-heating in winter conditions is mainly provided by the heat rejection of the drivetrain. Consequently, development goals to achieve higher effective efficiency and lower fuel consumption entail decreasing waste heat production. As a result, in particular diesel engine-powered vehicles have to deal with insufficient heat supply to the vehicle cabin under low ambient temperatures.
Approach to Solution
Present investigations are focused on the influence of different engine operation modes of a state-of-the-art automotive six-cylinder diesel engine on its energy balance and its emissions especially under low-load conditions. Primarily, the parameters affecting the combustion process and the intake system are investigated. Additionally, load point shifting due to electrical loads is examined. As an evaluation criterion, a new kind of efficiency factor is introduced.
Key Results
Cabin heating has a crucial influence on the thermal- and emission behavior of the engine. Therefore, the amount of heat or enthalpy transferred to the environment must be reduced significantly. This can be achieved most effectively by increasing the EGR rate and the charge air temperature. Indirect charge air cooling systems, for example, are advantageous for both measures.
5. Low Dimensional Modelling of Flow and Mixing in Automotive HVAC Units Using Proper Orthogonal Decomposition
Motivation and Objectives
Passenger comfort has become a major aspect in modern vehicle air conditioning concepts. Therefore, the temperatures at the outlets of the HVAC units are controlled and measured using temperature sensors. These sensors provide the most important value for the automatic climate control (ACC). To predict the temperatures at the outlets a novel proper orthogonal decomposition (POD) based approach is investigated to avoid costly sensors and enable model-based control of the HVAC unit.
Approach to Solution
The POD method is widely used to extract dominant flow features from spatio-temporal flow observations. By using only the most dominant features, or so-called POD modes, a low dimensional model of the flow field can be formulated. In this work, the POD is applied to the mixing process in the HVAC unit to find a correlation between input and output parameters, in terms of volume flow rates and enthalpy flow rates, respectively. By coupling the POD model with traditional modelling techniques, a low dimensional description of the HVAC unit is obtained.
Key Results
The approach was applied and evaluated on a real HVAC unit of a sport utility vehicle. A fluid resistance network was established to calculate the volume flow rates at the outlets. By combining the fluid resistance network with the POD approach the outlet temperatures can be computed. As a major outcome it could be shown that the accuracy of the model is comparable to the measurement uncertainty of the sensors (+/- 2K). Furthermore, the application of the model for model-based control was demonstrated.