A08: Optimization of combustible mixtures for the shockless explosion
Prof. Dr.- Ing. Neda Djordjevic (email )
Research assistant: Johann Vinkeloe (email )
Research assistant: Lisa Zander
Shockless explosion combustion (SEC) is a novel combustion concept aiming at achieving approximate constant volume combustion based on a quasi-homogeneous auto-ignition throughout the total volume of combustion mixture. Therefore, the process is dependent on fuel specific chemical processes during auto-ignition and thus very sensitive to local inhomogeneities in mixture composition and temperature. Relevant time scales, ignition delay time and excitation time, play a major role for the behavior in presence of such inhomogeneities. The goal of the present project is to support the realization of a robust quasi-homogeneous auto ignition in SEC through adjustment of the relevant time scales based on tailoring of combustion mixtures.
1st Funding period 2016 - 2020
- Determination of ignition delay time from a shock tube measurement
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Experimental Characterization of Ignition Behavior of Combustion Mixtures
During the second funding period, an experimental facility was built for measurement of ignition delay times of combustion mixtures at technically relevant conditions based on a shock wave method. A shock tube is essentially a very long tube closed on both ends and separated in a high pressure section and test section by a diaphragm. High pressure section is filled with a driver gas until the diaphragm ruptures due to the force caused by the pressure difference between the two sections. In this way a shock wave is created and propagates through the test section filled with the investigated combustion mixture. Ignition delay time is measured behind the shock wave reflected at the end wall. Behind the reflected shock wave test gas is brought instantaneously to a measurement temperature and pressure. Ignition delay time represents the time interval between the arrival of the reflected shock wave at measurement location near the end wall and the ignition of the combustion mixture detected from the pressure or chemiluminescence signal traces.
The high-pressure shock tube has a length of 12.05 m and relatively large inner diameter of 95 mm to minimize boundary layer effects. The design pressure is 400 bar. To facilitate investigations of liquid fuels or mixtures containing steam the test section can be heated up to 200°C. The design enables an operation in double-diaphragm mode and thus more accurate adjustment of the test conditions behind the reflected shock wave. The fast data acquisition with 10 MHz data rate provides highly time-resolved measurements. The high-pressure shock tube has been extensively characterized regarding the non-ideal effects. The measurement method for ignition delay times was validated for gaseous and liquid fuels based on the experimental data published by other research groups. The new research infrastructure facilitates a comprehensive characterization of ignition behavior of alternative fuels and combustion mixtures at technically relevant conditions.
Extension of Excitation Times of Combustion Mixtures
SEC is based on a quasi-homogeneous auto-ignition of combustion mixture and therefore susceptible to a premature ignition in local zones of increased mixture reactivity (so called hotspots). A premature auto-ignition in such hotspot causes a propagation of subsonic or supersonic auto-ignitive fronts or under certain circumstances, even a detonation wave can develop. The development of a detonation wave is a direct consequence of coupling between the auto-ignitive reaction front propagating along the reactivity gradient and the pressure wave resulting from the heat release. Detonation waves issuing from hotspots should be prevented in SEC. If combustion mixture exhibits relatively long excitation time i.e. characteristic timescale for chemical energy release, the development of a detonation in presence of hotspot is impeded. The excitation time can be extended by diluting the combustion mixture with carbon dioxide, exhaust gas of steam. However, the dilution effects also the ignition delay time of combustion mixture. These effects are experimentally quantified in the high-pressure shock tube and analyzed based on numerical and kinetic studies.
- 1D-simulation of reaction front propagtaion from a generic hotspot in a stoichiometric dimethyl ether/air mixture at 20 bar. Through the dilution with steam the development of a detonation can be suppressed and a quasi-homogeneous auto-ignition realised.
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- Ignition delay times of fuel blend containing dimethyl ether, hydrogen and methane (1:1:1) and pure components at 33 bar simulated with reaction mechanism Aramco 3.0
- © SFB1029
Fuel Tailoring for Optimization of Ignition Behavior
In order to realize a robust quasi-homogeneous ignition in SEC, a strategy to minimize the temperature sensitivity of ignition delay time was proposed in project A07. To achieve this, fuels with and without NTC-behaviour (NTC – negative temperature coefficient) are blended. The fuel blends are characterized regarding the ignition delay times at technically relevant conditions in the high-pressure shock tube. The influence of fuel components on ignition behavior and relevant parameters is investigated numerically in kinetic studies.
J. Vinkeloe, N. Djordjevic: Optimierung der Temperaturabhängigkeit von Zündverzugszeiten für pulsierende stoßfreie Verbrennung durch Brennstofftailoring, 29. Deutscher Flammentag, September 2019, Bochum
M. Rekus, L. Zander, J. Vinkeloe, N. Djordjevic: Influence of CO2 dilution on shock tube ignition delay times of dimethyl ether/air mixtures at high pressures, 9th European Combustion Meeting, 14 – 17 April 2019, Lisboa, Portugal
L. Zander, G. Tornow, Rupert Klein, N. Djordjevic: Knock control in Shockless Explosion Combustion by extension of excitation time, Active Flow and Combustion Control 2018, pp. 151-166, Springer, Cham.
C. Lhuillier, R. Oddos, L. Zander, F. Lückoff, K. Göckeler, C.O. Paschereit, N. Djordjevic: Hydrogen–enriched methane combustion diluted with exhaust gas and steam: Fundamental investigation on laminar flames and NOx emissions, ASME Turbo Expo, Charlotte, NC, USA, 2017 (GT2017-64885)
R. Oddos, C. Lhuillier, L. Zander, P. Habisreuther, N. Zarzalis, N. Djordjevic: Experimental and Numerical Study on Diluted Laminar Hydrogen-Enriched Premixed Methane/air Flames, European Combustion Meeting, Dubrovnik, Croatia, 2017