Proper orthogonal decomposition of continuum-dominated emission spectra for simultaneous multi-property measurements

Abstract

A novel spectrum calibration method is proposed for high-accuracy emission spectroscopy to extract quantitative gas property information from the spontaneous photon emission spectra. This is challenging, particularly in the presence of broadband emission, which smears the characteristic local spectral features that are often used as property indicators in conventional emission spectroscopy. Proper orthogonal decomposition (POD) can reduce the dimension (ND) of the broadband spectrum profile by selecting a number (NM≪ND) of dominant bases that span the gas-property-dependent spectrum space. Each POD-decomposed spectral basis represents critical broadband spectral features that are sensitive to the gas properties of interest. A reduced-order model (ROM) employing the kriging method correlates the entire emission spectrum profile that decomposed by the bases to multiple gas properties. To provide a spectrum database to train the ROM, chemiluminescence was captured from a flat flame with varied diluent gas compositions (N2 and CO2), fuel-oxidizer equivalence ratios, and pressures (1–5 bar). Subsequently, the ROM employing an emission spectrum can predict the gas properties under high-temperature and high-pressure conditions where conventional sensors are inapplicable. The average relative prediction errors obtained using this new analysis method were approximately 3.2% and 1.3% for the pressure and equivalence ratio, respectively.