High-Pressure Flames
Spectroscopic Investigation in High-Pressure Flames
Elevated pressures are a common feature in many practical combustion processes like IC engines and gas turbines. The development of spectroscopic techniques for high-pressure applications requires the consideration of variations of absorption spectra due to line broadening and -shifting, the increasing importance of collisional processes like energy-transferprocesses and fluorescence quenching. The influence of these processes on laser-induced fluorescence signal intensities needs to be studied in detail when quantitative concentration data should be derived from LIF measurements.
In collaboration with the DLR in Stuttgart and HTGL (Stanford University), spectroscopic investigation of high-pressure combustion processes were carried out in a high pressure burner. Experiments were conducted in laminar, stationary flames at pressures up to 80 bar (Fig. 1). The resulting data is used to validate and improve spectral simulation programs.
Figure 1: High-pressure burner at HTGL in Stanford (DLR design)
NO LIF schemes for IC engine application
For selective detection of NO a carefully selected combination of the excitation and detection wavelength is necessary. To cope with interferences by other species mainly oxygen in high-pressure applications, the detection scheme based on excitation with a tunable KrF excimer laser at 248 nm was characterized by measurements in stabilized high-pressure flames. Excitation-emission maps (Fig. 2) reveal all spectroscopic information necessary to select the best excitation-emission wavelength combination. In case of NO, the excitation laser is tuned to the O12-bandhead of the A-X(0,2)-transition where the fluorescence excitation spectrum of molecular oxygen has a local minimum. NO fluorescence emitted in the A-X(0,1)- and A-X(0,0)-bands at shorter wavelengths is detected which further minimizes the influence of interferring species.
Figure 2: Exciation-emission maps for different pressures. Exciation is around 248 nm. The vertical axis shows exciation wavelengths, the horizontal axis emission wavelenghts.
Excitation efficiencies depend on the spectral overlap of absorption spectra and spectral shape of the laser profile which can be calculated using simulated or measured absorption spectra of NO. Fluorescence excitation spectra around the NO A-X(0,2) O12 bandhead were recorded for various pressures and compared to simulated spectra. Measurements were carried out in lean (Ø = 0.9) methan/air flames doped with 400ppm NO at various pressures between 1 and 80 bar (Fig. 3).
Figure 3: Variation of NO Absorption spectra with pressure. Pressure broadening and spectra simulation is based on high-pressure LIF measurements.
These experimental data provide the background for the development of spectra simulation programs. For NO and O2 under practically relevant pressure and temperature conditions, the group has developed the LIFSim program (www.lifsim.com).
References:
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