Clemens Naumann

Shock Tube Study of the Ignition of Lean CO/H2 Fuel Blends at Intermediate Temperatures and High Pressure

The study of the combustion characteristics of H2/CO fuel blends is of fundamental and technical interest because the H2/CO system is very important in the hierarchical structure of oxidation models of hydrocarbon fuels and in advanced combustion technologies such as Integrated Gasification Combined Cycle (IGCC) which are currently developed allowing a reduction of the emissions of the power generation using biogenic sources and coal as fuel. Therefore, ignition delay times of 6 different H2/CO/O2/Ar-mixtures (fuel: 5% H2/95% CO and 50% H2/50% CO, Φ = 0.5) were measured at pressures of about 16 bar and temperatures between 1020 and 1260 K. The data were compared to predictions of different literature mechanisms.

Shock Tube Study of the Influence of NOx on the Ignition Delay Times of Natural Gas at High Pressure

The ignition delay times of diluted reference gas/O2/NO2/Ar mixtures (Φ = 0.25, 0.5, and 1.0, dilution 1:2 and 1:5, [NO2] = 20–250 ppm) were determined in a high-pressure shock tube. The temperature range was 1000 K ≤ T ≤ 1700 K at pressures of about 16 bar. The addition of NO2 leads to a significant reduction of the ignition delay times. This reduction increases with decreasing equivalence ratio. The effect of NO2 is well predicted by the NOx chemistry of different published reaction mechanisms. The differences in the predictions of the ignition delay times using a common hydrocarbon reaction mechanism and NOx subsystems of four published reaction mechanisms are negligible. Supplemental materials are available for this article. Go to the publishers online edition of Combustion Science and Technology to view the free supplemental file.

Formation of H-atoms in the Pyrolysis of 1,3-butadiene and 2-butyne: A Shock Tube and Modelling Study

Abstract For investigating the pyrolysis of 1,3-butadiene (1,3-C4H6) and 2-butyne (2-C4H6), reactive gas mixtures highly diluted with argon as bath gas were prepared. The experiments were carried out in a high purity shock tube device over a temperature range of about 1500–1800 K at total pressures between 1.2 and 1.9 bar. The time-dependent formation of H-atoms was measured behind reflected shock waves by using the very sensitive method of atomic resonance absorption spectrometry (ARAS). A detailed chemical kinetic reaction mechanism consisting of 33 elementary reactions and 26 species was used to model the experimentally obtained H-atom profiles. From kinetic modelling, with help of sensitivity and reaction flux analysis, it was concluded that reaction R 1 2-C4H6 ↔ 2-C4H5 + H is crucial for the observed formation of H-atoms during the thermal decomposition of both investigated species and within the investigated range of temperatures and pressures. Moreover, at temperatures above about 1650 K, the decay of propargyl radicals (C3H3) turns out to contribute significantly to the amount of produced H-atoms. The following rate expressions were obtained for three reactions (R 1–R 3) – among them the isomerisation from 2-butyne to 1,3-butadiene – important with respect to the formation of H-atoms within the investigated parameter range. The uncertainties are estimated to be ±30%: 2-C4H6 ↔ 2-C4H5 + H (R 1) 2-C4H6 ↔ 1,2-C4H6 (R 2) 1,3-C4H6 ↔ C2H2 + C2H4 (R 3) k 1=3.8⋅1015exp(-44871K/T)s-1 k 2=6.9⋅1013exp(-32496K/T)s-1 k 3=7.0⋅1012exp(-33768K/T)s-1

Numerical and Experimental Investigation of a Semi-Technical Scale Burner Employing Model Synthetic Fuels

In this paper the development and the application of a numerical code suited for the simulation of gas-turbine combustion chambers is presented. In order to obtain an accurate and flexible framework, a finite-rate chemistry model is implemented, and transport equations for all species and enthalpy are solved. An assumed PDF approach takes effects of temperature and species turbulent fluctuations on the chemistry source term into account. In order to increase code stability and to overcome numerical stiffness due to the large-varying chemical kinetics timescales, an implicit and fully-coupled treatment of the species transport equations is chosen. Low-Mach number flow equations and k-e turbulence model complete the framework, and make the code able to describe the most important physical phenomena which take place in gas-turbine combustion chambers. In order to validate the numerical simulations, experimental measurements are carried out on a generic non-premixed swirl-flame combustor, fuelled with syngas-air mixtures and studied using optical diagnostic techniques. The combustor is operated at atmospheric and high-pressure conditions with simulated syngas mixtures consisting of H2, N2, CH4, CO. The combustor is housed in an optically-accessible combustion chamber to facilitate the application of chemiluminescence imaging of OH* and planar laser-induced fluorescence (PLIF) of the OH-radical. To investigate the velocity field, particle image velocimetry (PIV) is used. The OH* chemiluminescence imaging is used to visualise the shape of the flame zone and the region of heat release. The OH-PLIF is used to identify reaction zones and regions of burnt gas. The fuel composition is modelled after a hydrogen-rich synthesis gas, which can result after gasification of lignite followed by a CO shift reaction and a sequestration of CO2. Actual gas compositions and boundary conditions are chosen so that it is possible to outline differences and similarities among fuels, and at the same time conclusions about flame stability and combustion efficiency can be drawn. A comparison between experimental and numerical data is presented, and main strengths and deficiencies of the numerical modelling are discussed.Copyright

A Single Pulse Shock Tube Study on the Pyrolysis of 2,5-Dimethylfuran

Abstract The pyrolysis of 2,5-dimethylfuran has been studied in a single pulse shock tube equipped with fast probing device at temperatures between 1175 K and 1450 K and pressures of 8.0±0.5 bar. The initial concentration of 2,5-dimethylfuran diluted in argon (500 ppm) was much lower than in previous studies reported in the literature. Sixteen different product species were quantified by gas chromatography. The product distribution pattern was compared with the prediction of two comprehensive chemical kinetic reaction mechanisms taken from the literature. In general, the predictions of the mechanisms fit the results of the experiments; however, the comparison reveals some differences between the two mechanisms as well as between simulations and experiments.

Alternative Fuels Based on Biomass: An Experimental and Modeling Study of Ethanol Co-Firing to Natural Gas

In response to the limited resources of fossil fuels as well as to their combustion contributing to global warming through CO2 emissions, it is currently discussed to which extent future energy demands can be satisfied by using biomass and biogenic by-products, e.g. by co-firing. However, new concepts and new unconventional fuels for electric power generation require a re-investigation of at least the gas turbine burner if not the gas turbine itself to ensure a safe operation and a maximum range in tolerating fuel variations and combustion conditions.Within this context, alcohols, in particular ethanol, are of high interest as alternative fuel. Presently, the use of ethanol for power generation — in decentralized (micro gas turbines) or centralized gas turbine units, neat, or co-fired with gaseous fuels like natural gas and biogas — is discussed, besides its role within the transport sector.Chemical kinetic modeling has become an important tool for interpreting and understanding the combustion phenomena observed; for example, focusing on heat release (burning velocities) and reactivity (ignition delay times). Furthermore, a chemical kinetic reaction model validated by relevant experiments performed within a large parameter range allows a more sophisticated computer assisted design of burners as well as of combustion chambers, when used within CFD (computational fluid dynamics) codes.Therefore, a detailed experimental and modeling study of ethanol co-firing to natural gas will be presented focusing on two major combustion properties within a relevant parameter range: (i) ignition delay times measured in a shock tube device, at ambient (p = 1 bar) and elevated (p = 4 bar) pressures, for lean (φ = 0.5) and stoichiometric fuel-air mixtures, and (ii) laminar flame speed data at several preheat temperatures, also for ambient and elevated pressure, gathered from literature. Chemical kinetic modeling will be used for an in-depth characterization of ignition delays and flame speeds at technical relevant conditions.An extensive database will be presented identifying the characteristic differences of the combustion properties of natural gas, ethanol, and ethanol co-fired to natural gas.Copyright

Numerical and experimental investigation of syngas combustion in a semi-technical scale burner

A semi-technical scale burner burning a syngas mixture is presented and deeply investigated by means numerical tools and experimental campaigns. The combustor is operated at atmospheric and high-pressure conditions with simulated syngas mixtures consisting of H2, N2, CH4, CO. Actual gas compositions and operating conditions are chosen so that a sensitivity analysis with respect to fuel composition and combustion stability can be carried out. To investigate the velocity field, particle image velocimetry (PIV) is used. The DLR CFD code THETA is used to simulate the turbulent reacting flow. A finite-rate chemistry approach is used to retain the whole spectrum of the chemistry timescales and to allow the investigation of single reactions on the combustion efficiency. Turbulent reaction rate terms are closed by employing an assumed-PDF approach, which is able to describe the impact of the temperature and species fluctuation on the chemistry by transporting two additional quantities (temperature variance and sum of species variances). Reference calculations will be presented and validated against the available PIV measurements. The influence of the combustion model and chemical kinetics mechanism used on the chamber performance is investigated and a detailed comparison is presented.

Experimental Study and Modeling of Autoignition of Natural Gas/Air-Mixtures Under Gas Turbine Relevant Conditions

The objective of this work was the improvement of methods for predicting autoignition in turbulent flows of different natural gas mixtures and air. Measurements were performed in a mixing duct where fuel was laterally injected into a turbulent flow of preheated and pressurized air. To study the influence of higher order hydrocarbons on autoignition, natural gas was mixed with propane up to 20% by volume at pressures up to 15 bar. During a measurement cycle, the air temperature was increased until autoignition occurred. The ignition process was observed by high-speed imaging of the flame chemiluminescence. In order to attribute a residence time (ignition delay time) to the locations where autoignition was detected the flow field and its turbulent fluctuations were simulated by numerical codes. These residence times were compared to calculated ignition delay times using detailed chemical simulations. The measurement system and data evaluation procedure are described and preliminary results are presented. An increase in pressure and in fraction of propane in the natural gas both reduced the ignition delay time. The measured ignition delay times were systematically longer than the predicted ones for temperatures above 950 K. The results are important for the design process of gas turbine combustors and the studies also demonstrate a procedure for the validation of design tools under relevant conditions.Copyright