DLR Stuttgart Research Articles

Coupling of flame geometry and combustion instabilities based on kilohertz formaldehyde PLIF measurements

In the past, flame geometry was found to play an important role in determining when strong pressure fluctuations associated with a combustion instability occur in a gas turbine model combustor. The goals of this study were to use flame surface area as a more accurate way to quantify the flame shape and the heat release rate, in lieu of chemiluminescence; to accurately resolve both the spatial structure and the time history of the heat release fluctuations, and to show that kilohertz formaldehyde planar laser-induced fluorescence provides a new way to achieve these goals. The dual-swirl burner, developed at DLR Stuttgart by Meier et al. was operated using dimethyl ether (DME) to study fluctuations in the flame surface area, flame brush, flame length, and heat release rate. To understand the instability, accurate measurements are needed of the correlation between heat release rate fluctuations and pressure fluctuations. Thus heat release rate must be recorded as a function of time and space. However conventional chemiluminescence offers only a line-of-sight measurement. High-speed CH2O PLIF was applied to study the motion of flame surfaces in response to the pressure oscillations. Results show that the flame surface density fluctuated at the acoustic frequency (typically 320Hz). The frequency of the combustion instability was found to increase as either the gas velocity or the flame speed was increased. These trends are consistent with previous n-τ theories that model the convective time delay. Strong pressure oscillations occur for conditions when the flame is nearly flat; the instability disappears when the flame becomes V-shaped. The explanation is that all points on a flatter flame have the same convective time delay but this is not true for the V-flame geometry.

Acoustic characterization of a partially-premixed gas turbine model combustor: Syngas and hydrocarbon fuel comparisons

In this work, the acoustic behavior of a combustion instability in a gas turbine model combustor was investigated as fuel properties, air flow rates, and burner geometry were varied. The dual-swirl burner, developed at DLR Stuttgart by Meier, was operated using syngas (H2/CO), ethylene, propane, and methane. The frequency of the instability was found to vary significantly from 250 to 480Hz. When the plenum volume and the exhaust pipe length and diameter were changed, the frequencies followed trends similar to a Helmholtz resonator. The variation of fuel type, flame speed, and air flow rate greatly altered the instability frequency and amplitude. These effects are not predicted by Helmholtz or organ tone acoustic theory. Higher frequencies were correlated with larger laminar burning velocities and higher air flow rates. The burner is a forced resonator, in which the flame oscillations couple with the flowfield to create convectively altered Helmholtz resonances. This suggests the need for an improved model of a forced Helmholtz resonator that includes flame properties. Alkane fuels displayed similar acoustic trends, but ethylene varied greatly from methane and propane. Syngas displayed different behavior than hydrocarbon fuels, even when the laminar flame speeds of the fuels were matched between ethylene and a syngas mixture. Flame characteristics such as anchoring, liftoff height, and shape appear to play a major role in the determination of instability strength and presence. With increasing hydrogen-content in the syngas-mixture, the flame transitions from a lifted to a fully anchored flame, resulting in a drastic decrease in the acoustic amplitude associated with non-resonating flames. Rayleigh indices show that flat flames create strong regions of thermo-acoustic coupling compared to axially extended V-shape flames. It is concluded that, in the current burner configuration, integrated-acoustics occur that involve a combination of Helmholtz and convective-mechanisms.

Selected papers about chemiluminescence of flames

Sensors that allow cheap, reliable and non-intrusive monitoring and subsequent active control of combustion processes are necessary for the development of combustors operating under potentially unstable operation conditions (e.g., for very lean or low-temperature combustion). Chemiluminescence in the UV-Vis region fulfills all of these requirements, and it has been shown that it can be used to determine flame parameters such as stoichiometry and heat release quantitatively. However, even though flame emissions have been studied for more than one century, important facts still remain unknown. Especially, reaction pathways leading to chemiluminescent species such as OH∗, CH∗, C∗2 and CO ∗ 2 are still under debate and cannot be modeled with standard codes for flame simulation. In several cases, even the source species of spectral features observed in flames are unknown. To investigate these processes in more detail, the cooperative research project “Chemiluminescence and Heat Release” has been started in 2006 with support of the Deutsche Forschungsgemeinschaft (DFG) under contract PAK 116. Seven individual projects, contributions from nine groups and institutions including the IWR and PCI at Heidelberg University, DLR Stuttgart, IVG of University of Duisburg– Essen, PC1 at Bielefeld University, EKT at TU Darmstadt, PTB Braunschweig, TCP at Karlsruhe Institute of Technology and LTD at TU Munchen. The project has been very

Market shares of various powertrains

Everyone is talking about electric vehicles, but nobody can foresee how the markets for battery-powered, fuel-cell and combustion-engine vehicles will develop. Which overall CO2 scenario will emerge from which mix of the various options between now and 2030? The Institute of Vehicle Concepts at the German Aerospace Center (DLR Stuttgart) has developed the simulation model “Vector 21”, which makes it possible to evaluate individual technologies in the overall context of a passenger car fleet and the technical, social and political framework conditions. It is thus possible to run through scenarios that can be modelled afresh when the parameters change or are updated. The DLR describes two particular scenarios in this article.

Development of Solid Oxide Fuel Cells by Applying DC and RF Plasma Deposition Technologies

AbstractBased on advanced plasma deposition technology with both DC and RF plasmas DLR Stuttgart has developed a concept of a planar SOFC with consecutive deposition of all layers of a thin‐film cell onto a porous metallic substrate support. This concept is an alternative approach to conventionally used sintering techniques for SOFC fabrication without needing any sintering steps or other thermal post‐treatment. Furthermore, is has the potential to be developed into an automated continous production process. For both stationary and mobile applications, adequate stack designs and stack technologies have been developed. Future development work will focus on light‐weight stacks to be applied as an Auxillary Power Unit (APU) for on‐board electricity supply in passenger cars and airplanes.This paper describes the plasma deposition technologies used for cell fabrication and the DLR spray concept including the resulting stack designs. The current status of development and recent progress with respect to materials development and electrochemical characterization of single cells and short‐stacks is presented.

Electrochemical Characterisation of Vacuum Plasma Sprayed SOFCs on Different Porous Metallic Substrates

Metallic substrate supported thin-film SOFCs have been in development at DLR Stuttgart for nearly ten years. In the DLR SOFC concept, the entire cell is fabricated using an integrated multi-step vacuum plasma spray (VPS) process. The material and structure of the porous metallic substrates play an important role for the electrochemical performance of the sprayed cells. Therefore the paper concentrates on the investigation of different porous substrates, e.g. felts, foams and knitted wire structures, and on the electrochemical behaviour of the cells. The plasma sprayed cells with a size of 10 cm x 10 cm were characterised electrochemically by current-voltage measurements, impedance spectroscopy and by investigating their long term stability. In order to analyse possible degradation mechanisms, the cells were examined metallographically after operation. The different phases and elements were detected with the energy dispersive X-ray microanalysis (EDX) associated with the scanning electron microscope and the X-ray diffraction (XRD).

Ein vereinheitlichtes Modell für Vormischflammen als gasdynamische Diskontinuität

The recently established Sanderforschungsbereich 606: Nonstationary combustion: transport phenomena, chemical reactions, technical systems at the Universitat Karlsruhe in cooperation with the Forschungszentrum Karlsruhe, the Universitat Stuttgart, and the DLR Stuttgart aims to improve the fundamental understanding and to further develop advanced combustion concepts, where time dependant processes are relevant. In these concepts premixed combustion is being applied to an increasing extent. In the present habilitation thesis premixed flames are considered resolving their time and space structures, thus omitting both space and time averaging. Experiments show that premixed flames are frequently thin compared to the hydrodynamic scales and may be viewed as a gasdynamic discontinuity separating the fresh mixture from the lighter products. Darrieus (1938 und 1945) and Landau (1944) independently proposed a model of a flame as a gasdynamic discontinuity, which consists of the Euler equations for the inviscid flow on either side of the flame and an ansatz for the flame propagation. In particular they assumed that the flame propagates at a constant speed and that the flows on either side of the flame are related to each other by jump conditions conserving mass and momentum, Markstein (1951) proposed a model where variations of the flame speed occur due to the curvature of the flame. In contrast to the previous heuristic models Sivashinsky (1976) derived a flame speed relation applying strict asymptotic methods. The model is applicable if the Lewis number (ratio of the diffusivities of heat and limiting species mass) is bounded away from unity. The jump conditions of the models by Markstein-, Sivashinsky- and Darrieus-Landau are identical. Finally, Matalon & Matkowsky (1982) derived a flame speed relation, valid for Lewis numbers close to unity, where flame speed is a function of flame stretch. In addition jump conditions where derived, which no longer conserve mass and momentum and which represent perturbative corrections of the Darrieus -Landau jump conditions. In this habilitation thesis tensor algebra is applied to derive a unified model, consisting of a new flame speed relation and new jump conditions, which is applicable for arbitrary Lewis numbers. In appropriate limits the models of Sivashinsky and Matalon-Matkowsky are recovered. Moreover a new distinguished limit allows for larger Lewis number deviations from unity as compared to the Matalon-Matkowsky theory thus bridging the Matalon-Matkowsky and the Sivashinsky theory. Considering shortwave corrugations of the flame not considered previously, results in new terms coupling neighboring flame elements. These terms damp short wave perturbations of the flame surface, and thus in contrast to previous theories the unified theory is applicable when the flame is unstable with respects to shortwave pulsating instabilities. Remarkable is the fact, that the flame speed relation and jump conditions depend on the precise location of the discontinuity surface. When Gibbs (1879) studied the free interface problem of two adjacent immiscible fluids, he made a similar observation. Depending on the choice of the interface an excess surface mass must be assigned to the interface. In contrast to previous theories the surface is chosen to yield a vanishing excess surface mass. This results in substantially simplified jump conditions.

New results of PEFC electrodes produced by the DLR dry preparation technique

At DLR Stuttgart a dry production technique has been developed for the preparation of electrodes and membrane electrode assemblies (MEA) for hydrogen PEFC as well as for direct methanol PEFC. Different spray strategies and different mixtures of electrodes and catalysts were tested to improve cell performance and reproducibility. The main advantage of the dry production technique [J. Power Sources 86 (2000) 352, Fuel Cell Bull. 15 (1999) 8] is the solvent-free coating of the electrodes, which allows a continuous production of MEA. In addition, thin layers (<5 μm) can be produced saving expensive catalyst material.

Current Status of Metallic Substrate Supported Thin–Film SOFC at DLR Stuttgart

Current Status of Metallic Substrate Supported Thin–Film SOFC at DLR Stuttgart

Integrated Fabrication Processes for Solid-Oxide Fuel Cells Using Thermal Plasma Spray Technology

In the manufacture of solid-oxide fuel cells (SOFCs), thermal-plasma technology, especially plasma spraying, offers several advantages, the most important of which is the ability to achieve considerably higher deposition rates than those that can be obtained with such approaches as physical or chemical vapor deposition (PVD or CVD). This article describes an international collaborative research project partially funded by NEDO (New Energy and Industrial Technology Development Organization, Japan) and carried out at five research laboratories around the world: at the University of Tokyo and National Institute for Research in Inorganic Materials (NIRIM) in Japan, the University of Minnesota in the United States, the University of Sherbrooke in Canada, and DLR—Stuttgart in Germany. The objective of the project was to conduct a comparative study on the possible use of four thermal-plasma spraying techniques for the integrated fabrication of SOFCs.

Quantitative measurements of OH concentration fields by two-dimensional laser-induced fluorescence

Two-dimensional laser-induced fluorescence measurements (2-D LIF) of the hydroxyl radical distribution in a laminar, premixed methane/air flat flame at various pressures were performed. Instantaneous and time averaged concentration fields were determined for three different pressures (1, 5, 20 bar ). The 2-D LIF data were set to an absolute scale by a calibration with a novel, single pulse one-dimensional UV absorption measurement made simultaneously with only one intensified CCD camera. To compensate effects of beam steering a spherical lens was inserted into the sheet path after the flame. The measurements are compared with results from numerical calculations. PACS: 07.65; 42.30.Va; 82.40.Py Combustion processes are characterized by the complex interaction between different transport processes and numerous chemical reactions. To study the various processes and to validate numerical calculations, non-intrusive in-situ measurements of concentration and temperature are necessary. For this purpose several laser-based spectroscopic methods have been developed and successfully applied in recent years [1–3]. As most technical combustion systems are turbulent, multidimensional single-pulse diagnostics is required. Two-dimensional laser-induced fluorescence (2-D LIF) is a powerful technique to measure many of the chemically important species, and temperature in a flame, with high spatial and temporal resolution with single pulse capability [4]. For 2-D LIF a laser beam is formed into a light sheet by several lenses. The light sheet intersects the flow field in a twodimensional plane and excites the molecules or atoms. The occurring fluorescence is collected with a two-dimensional detector [5, 6] such as an image intensified CCD camera [7]. Concentration fields of OH radicals are of special interest owing to its important role in combustion chemistry and for comparison with results from reduced reaction schemes. Although qualitative results are sometimes valuable, often quantitative data are necessary. However, quantification is not a trivial problem, especially at higher pressures. Usually other spectroscopic methods, e.g. absorption spectroscopy [8–12] have to be used to calibrate LIF data. In the present paper we propose to combine planar LIF measurements with absorption measurements at any height above the burner surface (1-D absorption) on a single pulse basis. The calibration of the LIF signals at any axis in the plane by a separate determination of the absorption along that axis reduces strongly the influence of an unknown quenching rate, since absorption measurements are less influenced by quenching. One could consider the proposed method to be just a 1-D absorption measurement. However, the LIF measurements are needed not only to determine the length of the absorption axis but also the distribution of the OH radicals along the absorption path. 1 High pressure burner The flame under study was a methane/air flat flame stabilized on a water cooled sinter plate of 20 mm diameter. This burner, constructed following drawings of O.N.E.R.A. allows steady combustion within a pressure range between 1 and 40 bar . Gas velocity and mixture composition were controlled by mass flow controllers. Thereby mixing ratios λ (air to fuel ratio) between 0.5 and 1.3 are possible though most measurements were done for λ= 1 and λ= 1.1. Optical access is realized by four suprasil windows orthogonal to each other. Some major advantages of this type of burner are the very high stability of the flame, which allows time integrated measurements, and the fact that similar devices are available at several other groups like O.N.E.R.A. and DLR Stuttgart. So data of these research groups can be compared directly. 2 Laser and detection system To detect OH radicals using 2-D LIF several detection schemes have been published. Advantages and disadvantages of these schemes are discussed by Ketterle et al. [13]. The absorption of the (3–0) band in the A2Σ–X2Π system [14] is too weak to be measured with pulsed laser systems. Excitation