Determination of OH concentration and temperature in pressurized lean kerosene/air flames using simultaneous measurements of PLIF and absorption

Abstract

Planar laser induced fluorescence (PLIF) images of OH as temperature indicator and kerosene (Jet A1) were used to characterized the flame structure of a new lean pre-mixed prevaporized (LPP) injector at realistic test conditions. The measurements were part of a test series in the Single-Sector combustor (SSC) at pressures of 3 bar, 6 bar, and 12 bar. Flames with equivalence ratios Φ referring to the nozzle between 0.5 (lean blow-out) and 0.7 were studied. While the kerosene LIF provided qualitative images of the fuel distribution, absolute OH concentrations were determined by calibrating the OH LIF by simultaneous measurements of the laser absorption. Assuming equilibrium conditions in these lean high pressure flames temperature distributions were deduced from the OH concentration. The high temperature sensitivity of this method enables the statistical evaluation of the temperature images on the single-pulse basis. The measurement of OH LIF in technical kerosene/air flames leads generally to the problem of interference with kerosene LIF and to the problem of laser absorption due to the OH radical itself. To reduce the influence of the kerosene interference the exciting laser had to be tuned to a rotational line of OH at 283 nm with a high transi-tion moment, which increased the laser absorption problem. To overcome the difficul-ties the kerosene LIF (340-380nm) and the LIF of OH and kerosene at 305-325 nm were detected simultaneously by 2 cameras. A third camera measured the intensity distributions of laser light sheet in front of and beyond the combustor to determine the laser absorption. For these experimental conditions of lean high pressure flames this experimental set-up and a fast correction algorithm for laser absorption provides not only reconstruction of the OH distribution, but also the calibration of the OH LIF to the well known absorption line strength of the OH transition. The assumption of the pro-portionality of absorption and fluorescence only approximatively is correct (+- 15%). Exciting a temperature insensitive OH transition this measurement technique pro-vided the simultaneous determination of absolute OH concentration and of the quali-tative fuel distribution. In lean flames of an equivalence ratio Φ < 0.9 the equilibrium concentration of OH is nearly independent to Φ, but increases strongly with respect to the temperature. The temperature sensitivity is about 10 times higher as the sensitivity of the OH 2-line thermometry. Owing to this temperature sensitivity the spatial temperature distributions could deduced from the OH concentrations and a reason-able statistical evaluation of the temperature fluctuation is possible on single-pulse basis.