Figure 1 Schematic of Ir Led Test Stand The Visible Laser Was Made To Be Collinear with the LED and Was Used for Alignment

The results of high pressure transmission (Intensity I divided by baseline intensity Ic.) vs. cell pressure are given in Figure 2. As can be seen, for 5% methane the absorption coefficient for the IR LED is about an order of magnitude smaller than that for a 3.39 mm He-Ne laser. This demonstrates that fuel concentration is measurable with this instrument at pressures and air-fuel ratios found in lean premixed gas turbine combustors. This is an important and significant result, in that measurements of the fuel concentration can be performed at pressures typical of lean premixed gas turbines (ranging from 6 to 15 bars). Transmission from a 3.39 m m He-Ne laser would be too low for accurate concentration measurements for methane at these typical gas turbine concentrations and pressures for any pathlengh of around 1 cm or greater. It was also shown that the amount of absorption (absorption is one minus transmission) increases with pressure for a given fuel concentration.

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