Visualization of gaseous flows and mixing processes via „Tracer"-LIF
In combustion and other processes it is often desirable to experimentally visualize the mixing of gaseous flows. In gas turbines, chemical reactors, internal combustion engines, fuel cells etc., this process often happens under unknown environmental conditions of temperature, pressure, and gas composition. In the case that species within the mixture fluoresce upon excitation with a UV laser, laser-induced fluorescence, LIF, can be used to locally measure the species concentration in the flow - often in imaging configurations that yield two-dimensional concentration maps.
Figure 1: Turbulent mixing of gaseous flows, visualized with tracer LIF
However, in many cases such species are not naturally part of the gas mixture. In these cases, a suitable „tracer" is mixed to the respective gas component. This tracer should exhibit an evaporation behavior similar to the respective gas component (e.g., the fuel), and its absorption and fluorescence behavior should be well known to enable quantitative interpretation of LIF signal intensities. Typical tracer molecules for this kind of optical diagnostics are ketones (acetone, 3-pentanone) or aromatics (toluene, naphthalene, etc.). As an example figure 2 (left) shows the fluorescence spectrum of toluene after excitation with 266 nm from fourth harmonic of a Nd:YAG laser [1,2].
For extracting quantitative information out of measured LIF signal intensities (e.g., temperature or concentration distribution of fuel vapor with tracer additive in the probe volume) the photo-physical properties of the tracer must be known. This includes the dependence of the absorption spectra and the fluorescence quantum yield on temperature and pressure .
Figure 2 left panel: Emission spectra of toluene vapor after excitation with 248 nm (from a KrF excimer laser) at different temperatures.
Right panel: high temperature, high pressure flow cell for investigating tracer-seeded gaseous mixtures at temperatures up to 1200 K and pressures up to 10 bar, respectively. The cell has for optical windows for introducing laser beams and detection of fluorescence radiation.
At IVG an optically accessible high-pressure high-temperature cell is available for the systematic study of tracer-seeded gas flows in the pressure and temperature range between 1 - 10 bar and 300 - 1400 K. To avoid pyrolysis of the tracers at high temperatures the gas mixtures are constantly exchanged in a flow configuration. Figure 2 (right panel) shows the cell, the gas supply, and the electrical control systems. The gaseous flows (tracers liquid at room temperature are evaporated before mixed with other gases) are metered by electronic flow controllers.
Tracers are excited in the near UV spectral region by a Nd:YAG laser at 266 nm and fluorescence spectra are recorded using a combination of a spectrometer and a CCD-camera (see Fig. 2, right panel).
The left panel in Fig. 3 presents the pressure dependence of the fluorescence intensity of 3-pentanone when this tracer is mixed in three different carrier gases (nitrogen, air and oxygen, respectively). The right panel in Fig. 3 shows the behavior of the fluorescence signal intensity of toluene as a function of air pressure. One notices the strong decrease of the signal intensity with increasing air pressure due to the efficient collisional quenching of fluorescence radiation by the increasing concentration of molecular oxygen.
Figure 3 Left panel: dependence of measured LIF signal intensity of 3-pentanone on gas pressure in three different carrier gases (N2, air, O2, respectively).
Right panel: dependence of LIF signal intensity of toluene as a function of total air pressure .
1. W. Koban, J. Koch, R. K. Hanson, and C. Schulz, "Oxygen quenching of toluene fluorescence at elevated temperatures," Appl. Phys. B 80, 777-784 (2005).
2. W. Koban, J. D. Koch, R. K. Hanson, and C. Schulz, "Absorption and Fluorescence of Toluene Vapor at Elevated Temperatures," Phys. Chem. Chem. Phys. 6, 2940-2945 (2004).
3. W. Koban, J. D. Koch, V. Sick, N. Wermuth, R. K. Hanson, and C. Schulz, "Predicting LIF signal strength for toluene and 3-pentanone under engine-related temperature and pressure conditions," Proc. Combust. Inst. 30, 1545-1553 (2005).