Quantitative measurements of OH concentration fields by two-dimensional laser-induced fluorescence

Abstract

Two-dimensional laser-induced fluorescence measurements (2-D LIF) of the hydroxyl radical distribution in a laminar, premixed methane/air flat flame at various pressures were performed. Instantaneous and time averaged concentration fields were determined for three different pressures (1, 5, 20 bar ). The 2-D LIF data were set to an absolute scale by a calibration with a novel, single pulse one-dimensional UV absorption measurement made simultaneously with only one intensified CCD camera. To compensate effects of beam steering a spherical lens was inserted into the sheet path after the flame. The measurements are compared with results from numerical calculations. PACS: 07.65; 42.30.Va; 82.40.Py Combustion processes are characterized by the complex interaction between different transport processes and numerous chemical reactions. To study the various processes and to validate numerical calculations, non-intrusive in-situ measurements of concentration and temperature are necessary. For this purpose several laser-based spectroscopic methods have been developed and successfully applied in recent years [1–3]. As most technical combustion systems are turbulent, multidimensional single-pulse diagnostics is required. Two-dimensional laser-induced fluorescence (2-D LIF) is a powerful technique to measure many of the chemically important species, and temperature in a flame, with high spatial and temporal resolution with single pulse capability [4]. For 2-D LIF a laser beam is formed into a light sheet by several lenses. The light sheet intersects the flow field in a twodimensional plane and excites the molecules or atoms. The occurring fluorescence is collected with a two-dimensional detector [5, 6] such as an image intensified CCD camera [7]. Concentration fields of OH radicals are of special interest owing to its important role in combustion chemistry and for comparison with results from reduced reaction schemes. Although qualitative results are sometimes valuable, often quantitative data are necessary. However, quantification is not a trivial problem, especially at higher pressures. Usually other spectroscopic methods, e.g. absorption spectroscopy [8–12] have to be used to calibrate LIF data. In the present paper we propose to combine planar LIF measurements with absorption measurements at any height above the burner surface (1-D absorption) on a single pulse basis. The calibration of the LIF signals at any axis in the plane by a separate determination of the absorption along that axis reduces strongly the influence of an unknown quenching rate, since absorption measurements are less influenced by quenching. One could consider the proposed method to be just a 1-D absorption measurement. However, the LIF measurements are needed not only to determine the length of the absorption axis but also the distribution of the OH radicals along the absorption path. 1 High pressure burner The flame under study was a methane/air flat flame stabilized on a water cooled sinter plate of 20 mm diameter. This burner, constructed following drawings of O.N.E.R.A. allows steady combustion within a pressure range between 1 and 40 bar . Gas velocity and mixture composition were controlled by mass flow controllers. Thereby mixing ratios λ (air to fuel ratio) between 0.5 and 1.3 are possible though most measurements were done for λ= 1 and λ= 1.1. Optical access is realized by four suprasil windows orthogonal to each other. Some major advantages of this type of burner are the very high stability of the flame, which allows time integrated measurements, and the fact that similar devices are available at several other groups like O.N.E.R.A. and DLR Stuttgart. So data of these research groups can be compared directly. 2 Laser and detection system To detect OH radicals using 2-D LIF several detection schemes have been published. Advantages and disadvantages of these schemes are discussed by Ketterle et al. [13]. The absorption of the (3–0) band in the A2Σ–X2Π system [14] is too weak to be measured with pulsed laser systems. Excitation