Dual-resolution Raman spectroscopy for measurements of temperature and twelve species in hydrocarbon–air flames

Abstract

This paper introduces dual-resolution Raman spectroscopy as a novel diagnostics approach for measurements of temperature and species in flames where multiple hydrocarbons are present. Simultaneous measurement of multiple hydrocarbons is challenging because their vibrational Raman spectra in the C-H stretch region are closely overlapped and are not well known over the range of temperature encountered in flames. Overlap between the hydrocarbon spectra is mitigated by adding a second spectrometer, with a higher dispersion grating, to collect the Raman spectra in the C-H stretch region. A dual-resolution Raman spectroscopy instrument has been developed and optimized for measurements of major species (N 2 , O 2 , H 2 O, CO 2 , CO, H 2 , DME) and major combustion intermediates (CH 4 , CH 2 O, C 2 H 2 , C 2 H 4 and C 2 H 6) in DME-air flames. The temperature dependences of the hydrocarbon Raman spectra over fixed spectral regions have been determined through a series of measurements in laminar Bunsen-burner flames, and have been used to extend a library of previously acquired Raman spectra up to flame temperature. The paper presents the first Raman measurements of up to twelve species in hydrocarbon flames, and the first quantitative Raman measurements of formaldehyde in flames. The accuracy and precision of the instrument are determined from measurements in laminar flames and the applicability of the instrument to turbulent DME-air flames is discussed.

Turbulent Combustion Modelling and Experiments: Recent Trends and Developments

“…Hence, a combination of Raman and Rayleigh scattering has proved adequate for capturing many of the major species of interest (see, for example, Refs. [47,50,51] for a review and some recent developments), which allows validation of sophisticated models that solve for many reacting scalars together. Our understanding of the structure of non-premixed and of sooting flames has benefited enormously from such techniques.…”

Understanding and Interpreting Laser Diagnostics in Flames: A Review of Experimental Measurement Techniques

“…These spectra may be theoretically computed and subsequently used to determine flame temperatures and compositions by scanning excitation wavelengths in laminar flames or, with more application to turbulent flames in hot and diluted coflows, collecting single-shot spectra at a point or line from Rayleigh-Raman (Carter and Barlow, 1994;Masri et al, 1996;Barlow et al, 2000Barlow et al, , 2015Barlow, 2007;Dunn et al, 2009;Magnotti and Barlow, 2017) or CARS (van Veen and Roekaerts, 2005;Oldenhof et al, 2010;Correia Rodrigues et al, 2015a). These techniques are both capable of high accuracy temperature measurements to within 1.5% (Magnotti and Barlow, 2017) and 2% (Roy et al, 2010), although Rayleigh-Raman simultaneously provides quantified measurements of mixture fraction and major species to within 10% (Magnotti and Barlow, 2017). Specifically, in turbulent flames, accuracies of 2% are possible in measurements of CH4 (Magnotti and Barlow, 2017), 3% of N 2 concentration, whilst CO 2 and H 2 O concentrations can be measured to within 6% and equivalence ratio ( ) measured to within 10% (Fuest et al, 2012).…”

“…These techniques are both capable of high accuracy temperature measurements to within 1.5% (Magnotti and Barlow, 2017) and 2% (Roy et al, 2010), although Rayleigh-Raman simultaneously provides quantified measurements of mixture fraction and major species to within 10% (Magnotti and Barlow, 2017). Specifically, in turbulent flames, accuracies of 2% are possible in measurements of CH4 (Magnotti and Barlow, 2017), 3% of N 2 concentration, whilst CO 2 and H 2 O concentrations can be measured to within 6% and equivalence ratio ( ) measured to within 10% (Fuest et al, 2012). Similarly, concentrations of O 2 may be measured to within 2% in laminar flames (Fuest et al, 2012).…”

“…Simultaneous measurements of species and temperature may be performed in "clean" flames using the previously described Rayleigh-Raman technique. Although this is a very well established technique (Masri et al, 1996;Nguyen et al, 1996;Frank and Barlow, 1998;Cabra et al, 2002;Dally et al, 2002;Barlow et al, 2005Barlow et al, , 2015Magnotti and Barlow, 2017), it suffers from the low signal provided by spontaneous Raman scattering, which is ∼1,000 times less than the corresponding, linear Rayleigh signal (Eckbreth, 1996;Frank et al, 2002). This promotes the use of high energy lasers, although short, high power laser pulses may result in "optical breakdown" (plasma formation).…”

“…This technique has not, however, been performed under MILD combustion conditions in flames which appear visibly lifted or exhibit a "weak-to-strong transition" in a continuous flame-front (Medwell et al, 2008(Medwell et al, , 2016Medwell and Dally, 2012a;Evans et al, 2016b;Ye et al, 2017). The Rayleigh-Raman diagnostic technique has been extended to one-dimensional data measurements, and extensive measurements of piloted jet flames have been performed to quantify scalar dissipation, length scales and flame-sheet orientation (Karpetis and Barlow, 2002;Barlow et al, 2005;Magnotti and Barlow, 2017), these have not been performed on flames in hot and diluted coflows. This is despite strong coupling between the burning state and heat release profiles of such flames to scalar dissipation rate (Oberlack et al, 2000;Özdemir and Peters, 2001;Ihme et al, 2012;Evans et al, 2016b;Ye et al, 2016Ye et al, , 2017.…”