Fan Beam Emission Tomography for Estimating Scalar Properties in Laminar Flames

Abstract A new method of estimating temperatures and gas species concentrations (CO 2 and H 2 O) in a laminar flame is reported. The path-integrated, spectral radiation intensities emitted from a laminar flame at multiple wavelengths and view angles are calculated using a narrow band radiation model. Synthetic data, in the form of radial profiles of temperature and gas concentrations, are used in these calculations. The calculations mimic measurements that would theoretically be obtained using a mid-infrared spectrometer with a scanner. The path–integrated spectral radiation intensities are deconvoluted using a maximum likelihood estimation method in conjunction with an iterative scheme. The deconvolution algorithm accounts for the self-absorption of radiation by the intervening gases, and provides the local temperature and gas species concentrations. The deconvoluted temperatures and gas concentrations are compared with the synthetic data used for calculating the spectral radiation intensities. The deconvoluted temperatures and gas species concentrations are within 0.5 % of the synthetic data. The deconvolution algorithm is expected to provide combustion researchers with an easy method of obtaining the radial profiles of major gas species concentrations and temperatures in laminar flames non-intrusively using a mid-infrared spectrometer with a scanner. * Corresponding author: jongmook@enurga.com Associated Web site: http://www.enurga.com Proceedings of the Third Joint Meeting of the U.S. Sections of The Combustion Institute Introduction Obtaining information on the instantaneous structure of turbulent and transient flames is important in a wide variety of applications such as fire safety, pollution reduction, flame spread studies, and model validation. Durao et al. (1992) has reviewed the different methods of obtaining structure information in reacting flows. These include Tunable Laser Absorption Spectroscopy (Hanson et al., 1980), Laser Induced Fluorescence (Crosley and Smith, 1983), Coherent Anti-Raman Spectroscopy (Eckbreth et al., 1979), and Fourier Transform Infrared Spectroscopy (Best et al., 1991), Laser Induced Incandescence (Dasch, 1984), and Emission Spectroscopy (Sivathanu and Gore, 1991) to mention a few. LIF and CARS can be used to measure temperatures and species concentrations with much higher spatial resolution than either absorption or emission spectroscopy (Durao et al., 1992). However, the accuracy of these two techniques in the presence of interference from soot radiation is an unknown factor. These techniques in addition require very expensive and bulky lasers, detectors and signal processing equipment. Therefore, they may not be suitable in drop tower or micro-gravity experiments, due to power, volume, and mass restrictions. LIF and CARS are ideally suited to measure concentrations of minor species and radicals, but for temperature and major gas species concentration measurements, absorption or emission spectroscopy is the more accurate and feasible technique. Absorption spectroscopy using either tunable laser diodes (Hanson et al., 1980) or FTIR (Best et al., 1991) can be used with deconvolution to obtain local gas species concentrations, soot volume fractions and temperatures in laminar flames. However, absorption spectroscopy requires a source, with corresponding alignment problems. The biggest advantage of absorption spectroscopy is that it can be utilized even in low temperature flows. In addition, absorption spectroscopy is relatively insensitive to temperature variations and is ideally suited to measure gas species concentrations (Zhang and Cheng, 1986). NASA/TM—2003-2123021