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Accurate measurement of post-flame temperatures can significantly improve combustion efficiency and reduce harmful emissions, for example, during the development phase of new internal combustion engines and gas turbine combustors. Nonperturbing optical diagnostic techniques are capable of measuring temperatures in such environments but are often technically complex and validation is challenging, with correspondingly large uncertainties, often as large as 2 % to 5 % of temperature. This work aims to reduce these uncertainties by developing a portable flame temperature standard, calibrated via the Rayleigh scattering thermometry technique, traceable to ITS-90, with an uncertainty of 0.5 % of temperature (k = 1). By suitable burner selection and accurate gas flow control, a stable, square, flat flame with uniform post-flame species and temperature is realised. Following development, the standard flame is used to validate two IR emission spectroscopy systems, both measuring the line-integrated emission spectra in the post-flame region. The first utilises a Hyperspectral imaging FTIR spectrometer capable of measuring 2D species and temperature maps and the second, a high-precision single line-of-sight FTIR spectrometer. In the central post-flame region, the agreement between the Rayleigh and FTIR temperatures is within the combined measurement uncertainties and amounts to 1 % (k = 1) of temperature.

An imaging Fourier-transform spectrometer in the mid-infrared (1850–6667 cm−1) has been used to acquire transmittance spectra at a resolution of 1 cm−1 of three atmospheric pollutants with known column densities (Q): methane (258 ppm·m), nitrous oxide (107.5 ppm·m) and propane (215 ppm·m). Values of Q and T have been retrieved by fitting them with theoretical spectra generated with parameters from the HITRAN database, based on a radiometric model that takes into account gas absorption and emission, and the instrument lineshape function. A principal component analysis (PCA) of experimental data has found that two principal components are enough to reconstruct gas spectra with high fidelity. PCA-processed spectra have better signal-to-noise ratio without loss of spatial resolution, improving the uniformity of retrieval. PCA has been used also to speed up retrieval, by pre-calculating simulated spectra for a range of expected Q and T values, applying PCA to them and then comparing the principal components of experimental spectra with those of the simulated ones to find the gas Q and T values. A reduction in calculation time by a factor larger than one thousand is achieved with improved accuracy. Retrieval can be further simplified by obtaining T and Q as quadratic functions of the two first principal components.

In this Letter, we present and experimentally validate the first direct hyperspectral dual-comb gas imaging system operating in the mid-infrared region. This method provides an unmatched combination of super-fine spectral characterization and high temporal resolution without the need for thermal contrast between the target molecules and the background. In a proof-of-concept experiment, the system has allowed us to perform precision hyperspectral imaging of butane in the 3.4 µm band with a time resolution of 1 s.

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Abstract. Nowadays, determining the temperature of flames is a challenging measurement in industry. The EMPIR project 14IND04 EMPRESS, in its WP4, address these temperature measurements. The Infrared Lab at the Physics Department of Universidad Carlos III of Madrid (UC3M) has developed a technique for measuring the temperature of a standard flame, in collaboration with the Centro Español de Metrología (CEM). An Imaging Fourier Transform Spectrometer (FTIR) has been used to acquire the emitted radiation coming from the flame and establish its temperature through several processing stages. This equipment has been calibrated with standard radiation thermometers and blackbodies at CEM.

Temperature measurement in flames is a challenging problem. Recently, hyperspectral imaging has demonstrated to be able to provide accurate temperature maps in a standard flame. However, hyperspectral imagers are expensive instruments, and the data analysis is laborious. Thus, a more simple approach to temperature imaging would be advisable. Since important and systematic differences exist in the low-resolution spectra of flames as a function of their temperature and chemical composition, it is in principle possible to retrieve these parameters by means of multispectral imaging. In this work, a standard flame, whose temperature and CO 2 concentration are known, is studied with an infrared camera in the MIR band (3 to 5 µm), provided with a six interference filter wheel. High-resolution emission spectra are calculated, using the HITEMP2010 database, as a function of flame temperature (T) and CO 2 column density (Q CO2 , measured in ppm•m), and integrated over the spectral transmittance profile of the selected interference filters. Measured radiances in each channel are compared to these simulated values and the absolute error is minimized at each pixel to retrieve values of T and Q, obtaining temperature and column density maps for the flame. Results are compared to the known values of the standard flame. First estimations of errors are found to be ∆T< 100 K and ∆Q CO2 < 400 ppm•m for flames with T∼2200 K and Q CO2 ∼3500 ppm•m. The possibility of reducing the number of filters and their effect on accuracy is studied.

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