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Validation of reaction mechanisms for gaseous and liquid fuels at technically relevant pressures by determining ignition delay times, laminar flame speeds and the product distribution of stable species

Improving efficiency, reducing pollutant emissions, and cost-efficient use of alternative fuels are among the main goals in developing advanced combustion technologies. CFD simulations of the combustion chamber are an essential tool in the development process. The basis for such simulations are validated chemical reaction mechanisms for conventional and alternative fuels. These mechanisms are provided by the chemical kinetics groups as one of their main tasks.

Validation by determination of ignition delay times

Three shock tubes are available for measuring ignition delay times under technically relevant conditions (see Figs. 1-3). Experiments can be performed at pressures up to 200 bar. Ignition delay times up to 25 ms can be determined by tailoring the driver gas. Two shock tubes are heatable, which means that low-volatility fuels can be studied. The ignition delay times are determined by OH* or CH* chemiluminescence measurements. Pressure profiles are always simultaneously measured as well. It is also possible to determine reaction progress variables and heat transfer to the wall.

Fig. 1: Heatable single-pulse shock tube for measuring ignition delay times at pressures up to 40 bar and for determining product distributions

Fig. 2: Heatable high-pressure shock tubes for measurements at pressures up to 200 bar

Fig. 3: Shock tube for measuring elementary kinetics data and ignition delay times of diluted mixtures at pressures up to 16 bar

Diagram of the validation procedure using the MPFR (Multiple Plug Flow Reactor) – CHEMKIN II extension for SENKIN

Further validation of the mechanisms is performed by measuring stable species in a single-pulse shock tube. A sample is taken after a defined reaction time. The product distribution is then analysed using gas or high pressure liquid chromatography. The soot particle size distribution is determined by a Scanning Mobility Particle Analyzer (SMPS).

Validation by measurement of laminar flame speed

The laminar flame speed s_l is defined as the propagation speed of a planar flame front perpendicular to its surface in a static mixture of air and fuel. It is used for the validation of reaction mechanisms. The laminar flame is of fundamental importance for modelling technical combustion processes because it enables the propagation speed of flames and the resulting energy release to be calculated.

One technique for the determination of laminar flame speed is the cone angle method (see Fig. 4). Premixed conical flames are stabilised above replaceable nozzle flame holders in the two high-pressure burners (see Fig. 5). The application of nozzles with different diameters allows determination of the flame speed as a function of the fuel/air ratio at high pressures and preheat temperatures. Gaseous fuels (e.g. natural gas, biogenic gases, fuel mixtures with high hydrogen content) and pre-vaporised liquid fuels are used. The maximum achievable pressures at a given preheat temperature are dependent on the flame stabilisation potential.

sl=vu·sin(α) vu Flow speed of the unburned fuel/air mixture α Flame angle
Fig. 4: Determination of the laminar flame speed sl of conical flames by applying the cone angle method.	Fig. 5: Typical stoichiometric laminar premixed cone flame at 200°C preheat temperature and a pressure of 20 bar using natural gas reference fuel.